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Black Stuart 19th February 2019 07:29 PM

PLZ pre-preamp
 
Specced by Pickering UK specifically for the top of the range Pickering 7500S cartridge. There was a Stanton equivalent.

Not at all sure I gave this cartridge my best shot when I bought it s/hand from our friend Ian. I know I never bothered to use the PLZ.

The PCB is silver traced not copper and very substantial. Powered by 2 x AA batteries = 3V. The caps and resistors are easily identifiable but the trimpot and 4 x regs are not.

So I contacted Vickers Hifi in Yorkshire who deal with the remaining Pickering UK products who informed me that they no longer had any info so I should contact QED who made the product for Pickering UK. I should have noticed the QED logo on the PCB.

So I contact QED via an enquiry form and sure enough they get back to me in 48 hours only to tell me that they didn't make the product. This simply doesn't make sense, I can't imagine any company letting someone else make a product and use their logo.

The trimpot was made by Piher a Spanish company. I wasn't at all sure what the info on the pot stood for so contacted the compan y by email - zero response (cabrons). The side of the pot states - 100 321 - can anyone enlighten me on what this stands for?

The regs. have zero info and measuring them doesn't make sense - remember the battery power is only 3V when 5V is normal.

All the resistors are either spot on or well within 1%, so just need to replace the 6 caps - vamos aver.

Richard 19th February 2019 10:52 PM

Re: PLZ pre-preamp
 
Hi Stuart,

Just replace the caps and batteries then and give it a listen. Without a circuit diagram and set-up info there's not a lot more you can do.

If it looks fairly simple you could try drawing it out and compare it to circuits on the web but most seem to use the 9V PP3 batt and we don't know what the pot is for. There were lots around 40 years ago. I've just checked eBay and see there's one of those Ortofon MCA76 head amps on for £200! I had one back in the day, mid 70's, mains powered and many pairs of transistors iirc, very quiet, I thought it was good but the flat earth press didn't like it :D

Specs of the PLZ amp are for flat x20 gain to feed into an existing RIAA phono input, so it's just a linear head amp, http://www.pickeringuk.com/plz.html

bob orbell 20th February 2019 07:03 AM

Re: PLZ pre-preamp
 
I have one of these, I did try it with my Kontropunkt, feeding it into Phono III MM, it worked OK but never had it working for long, as Richard said, it has no RIAA, just a simple step up. Bob

Black Stuart 20th February 2019 01:59 PM

Re: PLZ pre-preamp
 
Bob,
you didn't understand the function of the PLZ - you shouldn't have used it into a MM stage but into an MC stage. The PLZ provides 27dB of gain and it's low o/put is 0.33mV, same as my AT 33EV MC.

The PLZ was designed to be used with the Pickering 7500S LOMM - low output moving magnet cartridge. This like the Stanton equivalent combined the best of both the MM and MC designs.

Studying the comments on a few sites the PLZ has been used with success with many MCs but into the moving coil output. The one criticism is the resistive load is 100 ohm when many use 1K.

I never gave this cartridge a fair chance some time ago but now i will. These LOMM came along late in the day. I was on the point of buying either an AT ART 9 or a Soundsmith Zephyr and I might still but at little expense (cap renewal) I now I will.

I have a very under rated phono stage the Talk Audio MC3 I can easily configure it to suit the 7500S but as I have the PLZ I need to see what it is capable of.

Bob, are you capable of producing a diagram of the circuit? I would like guidance on changing the values so that I can try a 1K load.

bob orbell 20th February 2019 04:16 PM

Re: PLZ pre-preamp
 
Stuart, I think you are wrong, this is a step up amp for MC cartridges, so with that gain you do go into the MM side of the phono, I did and it was very loud compared to my step up transformers, they are 1-12, and no, I can't map out the circuit, sorry. Bob

bikerhifinut 20th February 2019 04:57 PM

Re: PLZ pre-preamp
 
1 Attachment(s)
I'm with Bob, Gain of 27db that means my ortofon MC would give a nominal 6mV give or take at the step ups output. Which is just about bang on for the average magnetic cartridge input, (somewhere between 2 and 5MV for most modern kit).
Or to put it another way, if it was for stepping up a VERY low output source into a MC input lets make a couple of assumptions and do the maths.
Assume 500uV ie 0.5mV for a fairly low sensitivity MC stage, divide by 27dB gain (22.4x) and you get a sensitivity of around 22 microVolts. Now thats so close to the natural noise produced by even the quietest metal film resistors for instance that I'd bet all you'd get is a very hissy noisy result.

If you can get into the guts of it ok and you think you need to alter the input loading then it should be possible to trace the wiring from the input socket to the loading resistor which will likely be the first resistor across the input. For an MC its usually somewhere between 50 to a couple of hundred ohms, many designs seem to fix it at about 150 ohms if they dont have adjustable loading.
You will have I hope the info for loading your low output moving magnet as I guess it may not be the industry standard 47k. The only low output magnetic I know of and have had practical experience of is the Grado moving iron job and that simply worked into a standard MC input, I think maybe it was 100 ohms? thats a function of the lower number of turns on the generator coil.
Incidentally, moving coil cartridges are, as a rule, very tolerant of loading resistance variations and their loading figures can often be regarded as a minimum figure. Certainly I can't hear any difference between 100, 150 or 300 ohms when I had a fiddle with my rega Fono MC's adjustable loading. The thing to watch with MC stages is that they could well have a higher loading capacitance than for MM.

I've attached an interesting little circuit that I fancy having a go at, it'll cost washers to build on a bit of veroboard and you can run it nicely off a couple of PP3 9V batteries in series.
Here's a link, they do a PCB kit too.
https://boozhoundlabs.com/collection...ono-kit-2sk170

Best regards

Andy.

bikerhifinut 20th February 2019 05:18 PM

Re: PLZ pre-preamp
 
Any chance of a photo of the insides of the PLZ Stuart?

I just had a quick google and the 7500 cartridge seems to be a LoZ low output 0.3mV Magnetic as you describe.
Advised loading 50 to 250 ohms so you have lots of latitude there, and no issues with capacitance as I'd expect with a low impedance source.

It should work into any MC input as well as anything, with my personal thoughts that it may not be too happy into a step up transformer, but then again I'm not keen on step up transformers anyway and would rather have a nicely designed non inductive, low noise solid state step up........... just like your PLZ in fact.

A.

Richard 20th February 2019 05:20 PM

Re: PLZ pre-preamp
 
Yes it's a MC head amp, designed to provide the correct cart loading and increase MC output to MM level, and then be plugged into a MM RIAA phono input.

Richard 20th February 2019 05:57 PM

Re: PLZ pre-preamp
 
Hi Andy,

That little circuit looks easy to build. Have you heard one? One of my earliest projects was a single transistor (BC109?) head amp circuit in one of the hifi mags of the day powered by a PP3 batt. I tried it against the Ortofon MCA76 and it was lively and gave enough gain but was noisy (hiss) and rough sounding. So would that modern Jfet be good enough (showing my ignorance of modern devices here :D) or would it just be another fun project capable of comparison maybe to the built in MC gain in some integrated amps?

bikerhifinut 20th February 2019 06:16 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by Richard (Post 89194)
Hi Andy,

That little circuit looks easy to build. Have you heard one? One of my earliest projects was a single transistor (BC109?) head amp circuit in one of the hifi mags of the day powered by a PP3 batt. I tried it against the Ortofon MCA76 and it was lively and gave enough gain but was noisy (hiss) and rough sounding. So would that modern Jfet be good enough (showing my ignorance of modern devices here :D) or would it just be another fun project capable of comparison maybe to the built in MC gain in some integrated amps?

No I haven't heard one richard. I forgot to ask a friend who has a stash of 2SK104 to flog me a couple last weekend, as i have a long term project on the go to build a hybrid silicon/valve MC stage.
It's actually an obsolete FET, but thats nothing unusual these days as most new stuff seems to be SMT stuff. I wouldnt like to speculate on its noise performance without the spec sheets etc as FET's can be noisier than BJT's. I know of at least one manufacturers solid state phono stages that use an FET front end and apparently the double FEts he uses are shockingly expensive but are of the few that are sufficiently low noise. Previously he used low noise BJT's for the commercial stuff.
I'd look upon it as a "fun" project Richard but you never know it might be up to snuff? I figure you could knock that circuit up using decent standard quality parts for around a fiver to a tenner excluding case and sockets assuming the 2SK104 is still available for a quid or so each. Last time I looked the prices varied enormously. There'll be an equivalent though and certainly if you can work with surface mount devices theres apparently quite a lot of stuff available.
I'm not constructing just now until the building works done here, hopefully June, but I'll be doing this circuit and I hope I can get some meaningful numbers out too when I put it on the scope.
Andy.

bob orbell 20th February 2019 08:12 PM

Re: PLZ pre-preamp
 
Well Andy, if you want one just ask. BOB

bikerhifinut 20th February 2019 08:48 PM

Re: PLZ pre-preamp
 
I hope we haven't got our wires crossed here Bob?

Richard was asking about that wee boozhound labs circuit, and i was replying that I hadn't heard it. At least not yet but when we get the house back to normal I'll be having a go at it as it looks easy enough to knock out on a bit of veroboard.

A bientot!

Andy.

bob orbell 20th February 2019 08:52 PM

Re: PLZ pre-preamp
 
Sorry Andy, I read it that you would like a PLZ as I have one in the draw. BOB

colin.hepburn 20th February 2019 10:09 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by bikerhifinut (Post 89197)
I hope we haven't got our wires crossed here Bob?

Richard was asking about that wee boozhound labs circuit, and i was replying that I hadn't heard it. At least not yet but when we get the house back to normal I'll be having a go at it as it looks easy enough to knock out on a bit of veroboard.

A bientot!

Andy.


Hi Andy/All
Beware if you build it that the 2sk170 JFETs are getting thin on the ground and costing abit plus you will need them matched You Could get them from Cricklewood Electronics but they won’t match them for you
https://www.cricklewoodelectronics.c...keep_https=yes

Greg. 20th February 2019 10:37 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by bob orbell (Post 89198)
Sorry Andy, I read it that you would like a PLZ as I have one in the draw. BOB

Where can I buy tickets?

bikerhifinut 20th February 2019 10:42 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by colin.hepburn (Post 89200)
Hi Andy/All
Beware if you build it that the 2sk170 JFETs are getting thin on the ground and costing abit plus you will need them matched You Could get them from Cricklewood Electronics but they won’t match them for you
https://www.cricklewoodelectronics.c...keep_https=yes

I have a source Colin, so I'd get em for nowt. Other FETs are available. They arent that dear when you look around, still available for 2 to 3 quid each which when you look at the price of a valve aint so bad.
Ah yes matching, well that's the price of a zero feedback design I guess.

bob orbell 21st February 2019 06:59 AM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by Greg. (Post 89202)
Where can I buy tickets?

Oh dear, have I spelt DRAW incorrectly, :(. Bob

colin.hepburn 21st February 2019 10:37 AM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by bob orbell (Post 89206)
Oh dear, have I spelt DRAW incorrectly, :(. Bob


Ha Ha did not see that one Bob:eek::D

Black Stuart 21st February 2019 01:43 PM

Re: PLZ pre-preamp
 
Error, yes it should be plugged into an MM stage.

Everything I've read on the 7500S is that a 1K load opens up the top end. I'd like to try 500ohm first and then 750ohm.

Zev audio has all the numbers on the 7500S.

The first resistor is 27ohm.

I will within a few days take some pics of the circuit.

No one who uses the PLZ said anything about it being noisy.

bikerhifinut 21st February 2019 02:52 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by Black Stuart (Post 89210)
Error, yes it should be plugged into an MM stage.

Everything I've read on the 7500S is that a 1K load opens up the top end. I'd like to try 500ohm first and then 750ohm.

Zev audio has all the numbers on the 7500S.

The first resistor is 27ohm.

I will within a few days take some pics of the circuit.

No one who uses the PLZ said anything about it being noisy.

The comment about noise refers to if it was being used to raise very, very low level signals to MC levels, that was all Stuart. I wouldnt expect it to be noisy if good low noise semiconductors and High quality metal film resistors were used. The battery operation certainly will eliminate any mains borne hash and hum issues, It's not such a bad idea when amplifying the tiny signals from MC and these ultra low output MMs.
Ok it may be worth metering the inputs to see what its shunted with, but I'd be wary if there's a sensitive FET or something directly coupled to it. It's easy enough to trace the input circuitry if you can see the PCB tracks. The load resistor is obvious by being connected across the input from hot to earth. An idea if you are keen to try experimenting with load resistors would be to leave the load resistor out of circuit on the board, (you could stick a 1meg in there if you are worried about the input not having any reference to ground, it'll have negligible effect on your loading shunt.
Then, you could make loading plugs up. Simply have the resistor of choice wired across the pins, I am sure somewhere out there there are RCA plugs that are terminated in an RCA socket so you can wire a load resistor or attenuator potential divider something like thos Rothwell things. They must get their plug bodies from somwhere. I'll have a sniff round the internet. otherwise its a fiddle on to make something out of a plug and socket, but it would be feasible I think if you keep things short and use some decent shrink tubing. If theres room to get a couple of extra sockets in beside the existing inputs you can wire an RCA in parallel with the input and make little loading plugs up to go in. That would be a neat way too. theres at least one commercial design that uses that technique.
27 ohms seems very low for a loading resistor, I'd expect more like 100 or 150.
That said, ortofon, for example, recommend loading at "Greater than" 20 ohms and I have never noticed any fundamental difference between the various loading values on my MC stage.
I could of course have cloth ears.

A

Black Stuart 23rd February 2019 11:40 AM

Re: PLZ pre-preamp
 
Andy,
it couldn't be a simpler circuit - 6 caps, 4 are smoothing caps, 2 regs. per input/output operating at less than 1V (batteries 2AA x 1.5V each. 1 earth 4 band R. 1 R per smoothing cap and 3 x Rs for i/put an o/put, all of these bar 2 are 6 band 1% as are the 27ohm i/put Rs.

All Rs are either still spot on or well within 1%. I havn't bothered measuring the caps as they are around 40 years old so will be replaced. The caps are 2 x Anodia (Italian) and 4 x Samar (Taiwan) - that's it.

This PCB is has silver tracks and very chunky at that. Can't find out anything about the ultra low voltage regs. The caps + postage are about £8, a cheap experiment that may well be worth the effort.

I still have an NOS Acutex M312III STR PZ to try - a straight ahead MM, lots of detailed info on these on Audiogon.

It definitely makes sense to experiment with these late in the day MM cartridges before shelling out serious money for the AT or Soundsmith.

bikerhifinut 23rd February 2019 01:28 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by Black Stuart (Post 89236)
Andy,
it couldn't be a simpler circuit - 6 caps, 4 are smoothing caps, 2 regs. per input/output operating at less than 1V (batteries 2AA x 1.5V each. 1 earth 4 band R. 1 R per smoothing cap and 3 x Rs for i/put an o/put, all of these bar 2 are 6 band 1% as are the 27ohm i/put Rs.

All Rs are either still spot on or well within 1%. I havn't bothered measuring the caps as they are around 40 years old so will be replaced. The caps are 2 x Anodia (Italian) and 4 x Samar (Taiwan) - that's it.

This PCB is has silver tracks and very chunky at that. Can't find out anything about the ultra low voltage regs. The caps + postage are about £8, a cheap experiment that may well be worth the effort.

I still have an NOS Acutex M312III STR PZ to try - a straight ahead MM, lots of detailed info on these on Audiogon.

It definitely makes sense to experiment with these late in the day MM cartridges before shelling out serious money for the AT or Soundsmith.

Are you sure they are regulators?
And I very much doubt that circuit is running at 1V or less, heck most silicon devices need around 0.6V across their junctions before they even conduct, hence the popular trick of using a silicon diode for a 0.6V drop for biasing and regulation. (2x1N4148 or similar in series under a 7805 reg and its a 6.2V reg, a trick used in PSU2 as I remember.)
You make no mention of the active devices in this circuit and I'm wondering if what you are identifying as regulators are , in fact, transistors? I'd expect this amp to use discrete, probably bipolar for good noise performance, transistors as there aren't many Op Amp chips around with low enough noise performance for MC applications. They do exist but at a price.
I'd be a bit wary of fixing what isnt broken also, what you call smoothing capacitors will be smallish value supply decoupling capacitors between stages, it's battery operated so the supply doesn't need smoothing. Electrolytics of decent quality, and I suspect they are before the "bad time" when there was a spate of dodgy chinese electrolytics for a wee while, last a lot longer than you might think.
If the coupling capacitors, and if 2 I suspect they will be DC blocking coupling caps on the output are non polarised electrolytic jobs, ie film types then again they'll likely be as good as the day they went in. If they are electrolytics then I'd replace em anyway with polyprop or similar, there are some good little uns around and at that working voltage you only need buy 6.3VDC jobs which means you can get some decent value ones in a small package.
Yeah well modern metal film resistors are so stable nowadays I'd expect them to stay in value pretty much for life. Especially in that application where they are running very cool.
Heres a thought, why not get a couple of potentiometers as a temp arrangement if theres room to do so without inducing external hum, remove the 27R loading resistors and connect the pots across the input. Use say, 2.2k and then you can twiddle and listen until you get the sound you like best. Measure the resistance with the meter and sub in a fixed resistor.
I don't know what effect making a small external box with a pair of phono sockets and short flying leads with phono plugs will have on the hum pickup off the TT leads but that could be a way of adjusting the loadings?
I take it you have listened to it already as is? And you aren't happy with some aspect of it?
Are they silver tracks or is it just a flow soldered board and what you are seeing is a solder plating on standard copper tracks?
A.

bob orbell 23rd February 2019 05:15 PM

Re: PLZ pre-preamp
 
Just took mine apart, there is no regulator although the circuit has 3 volts to part of it and about 2.5 to other sections adjustable, the PCB is copper solder wiped, not silver and the transistors look like FET's, but I may be wrong, all 6 capacitors are electrolytic, 4 off 10 v 470uF, and 2 off 10v 10uF . BOB

bikerhifinut 23rd February 2019 07:11 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by bob orbell (Post 89238)
Just took mine apart, there is no regulator although the circuit has 3 volts to part of it and about 2.5 to other sections adjustable, the PCB is copper solder wiped, not silver and the transistors look like FET's, but I may be wrong, all 6 capacitors are electrolytic, 4 off 10 v 470uF, and 2 off 10v 10uF . BOB

So the mystery potentiometer could be to trim the supply voltage?
The other thing that crosses my mind is are the 470uF supply bypass/decoupling capacitors or are they emitter bypass jobs? Did you trace the path to see how they are connected Bob?
The 10uF coupling caps could be usefully swapped for polyprops, as the voltage is 3V or less 6.3V jobs or better will be good and that gets the physical size down if using film caps. If Stuarts feeding a higher Z input as is usual on Valve amps then you can get away with a 4,7 uF poly or even 1uF into 50k will still be plenty, and they can be very small physically. I'm just a little bit phobic about electrolytics as coupling caps but I imagine using something of the quality of Nichicon FG or Sanyo Oscon etc with low ESR etc wont have a detrimental effect on sound quality.
There are small polywotsit films but I think at that capacitance they'll be too large to fit the PCB so its electrolytics. They seem to have lead pitches around 22mm which is a bit bigger than a 10uF electrolytic.
I'd love a photo with an idea of sizes, there must be some service info out there somewhere.

bob orbell 23rd February 2019 08:19 PM

Re: PLZ pre-preamp
 
The two 10uF capacitors are in the 3 volt power line Andy, the 470uF are connected to each transistor, but don't ask me which leg, I will see if I can post a photo, but for some reason my pictures are always too large to post, but I could email a picture if you wish. BOB

Black Stuart 24th February 2019 10:15 AM

Re: PLZ pre-preamp
 
Bob,
you have different caps to me - the 2 Anodia caps are 100uF/10V and the 4 Samar caps are 470uF/16V which is totally at variance with your values.

There is no mystery about the trim pot, that is what it is a Spanish Piher - no one seems able to state what the info relates to - 100 321. looking at trimpot definitions, these figures mean nothing.

I contacted Vickers Hifi who deal with the residue of Pickering UK products and whose address was the same as that company in it's heyday that QED made the PLZ. The PCB clearly shows the QED logo and other info but QED deny that they made it. I simply don't accept that any company would allow another to use their trade name.

The contact at QED did admit that they made a PLZ type product. As I understand it they were virtually the same. If the original info was available it would make life so much easier.

Re. voltages of the regs/transistors/ whatever - the highest V I measured was 0.626V.

Nowhere do I measure anything like 3V even though that is the battery supply

Bob - does your PCB clearly show, in the middle of the board the logo QED? - bottom left in vertical format MCA1?

Andy the caps are most definitely not bi-polar, they are electrolytics.

Why did the designer choose values that you say are not needed, it's not as if lower values were not available then.

I'll see if I can get some pics loaded.

bikerhifinut 24th February 2019 04:36 PM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by Black Stuart (Post 89241)
Bob,
you have different caps to me - the 2 Anodia caps are 100uF/10V and the 4 Samar caps are 470uF/16V which is totally at variance with your values.

There is no mystery about the trim pot, that is what it is a Spanish Piher - no one seems able to state what the info relates to - 100 321. looking at trimpot definitions, these figures mean nothing.

I contacted Vickers Hifi who deal with the residue of Pickering UK products and whose address was the same as that company in it's heyday that QED made the PLZ. The PCB clearly shows the QED logo and other info but QED deny that they made it. I simply don't accept that any company would allow another to use their trade name.

The contact at QED did admit that they made a PLZ type product. As I understand it they were virtually the same. If the original info was available it would make life so much easier.

Re. voltages of the regs/transistors/ whatever - the highest V I measured was 0.626V.

Nowhere do I measure anything like 3V even though that is the battery supply

Bob - does your PCB clearly show, in the middle of the board the logo QED? - bottom left in vertical format MCA1?

Andy the caps are most definitely not bi-polar, they are electrolytics.

Why did the designer choose values that you say are not needed, it's not as if lower values were not available then.

I'll see if I can get some pics loaded.

The only difference is the 100uf, the 470 is the same value however you look at it. The difference in working voltage suggests to me that whoever made it bought a different batch of parts in. 10V wgk or 16V wkg both are fine when the maximum voltage they'll likely see is 3V.
So I think the 100uF are just supply decouplers for each stage, 4 transistors so its a 2 stage amp in all likelihood. 100uF seems a more sensible value although 10uF would have probably been adequate too. Bob may have missed seeing a zero on the caps on his amp too, I'll bet the printing is tiny on the sleeve.
So its more than likely DC coupled, which in terms of signal purity should be a Good Thing.

Have you tried to measure the preset pot stuart? that will tell you what it's value is. It's still a "mystery pot" in as much as we don't know what function it performs in the circuit. It may even be a "balance" adjuster between stereo channels, I've seen this in some designs, which will compensate for differences in gain on each side. Piher pots are very common.

If you are only measuring 0.626V I'm surprised it works at all, the transistors will be barely switched on at that voltage I would have thought and its a waste of the 3V available. There could be an explanation if one or both decoupling caps for instance were short circuiting through age degradation.

I havent seen a piccie yet but It cant be a very complex board to trace through.
If the 470uf have their negative ends at the negative supply rail then you can be fairly certain they are bypass capacitors.

I'm assuming you have used it and it's known to work?

Bob or Stuart, if you can get me some Hi def photos of the board that i can enlarge considerably, both sides and showing the input and output etc I could have a stab at reverse engineering it. Also any chance of an I/D on the transistors type no? That will help to tell if FET or bipolar. I strongly suspect bipolars as they may have better noise performance at low signal levels and its only asking for a 22x gain which off 2 stages is hardly a huge amount.
I'm intrigued.
Andy.

Black Stuart 25th February 2019 10:29 AM

Re: PLZ pre-preamp
 
Andy,
the trimpot (that is only an assumption) measures 40.5K, which seems weird. It's a shame that Piher didn't respond to my request for a value explanation re. 100 321 but there you go.

I havn't given it a try as I have experienced what happens when a cap goes - a PSU cap took out at least 1 transistor in my first Kenwood KD990. Once I decided to try the PLZ after reading positive things about it on a few forums, I knew I would as a first action replace the caps.

Pics of the whole piece and the PCB should be online this afternnon.

Stuart

Black Stuart 25th February 2019 03:18 PM

Re: PLZ pre-preamp
 
1 Attachment(s)
Here is one picture
problem miniaturising the PCB picture?

Greg. 25th February 2019 09:00 PM

Re: PLZ pre-preamp
 
Stuart,

Sign up to tinypics. Just Google it. You can upload all your pics there and then select from the uploaded library what you want to convey on the WD forum. Size conversion is one click automatic. So easy. tinypics is the sister company of photobucket which we all originally used. Give it a a go.

Richard 26th February 2019 08:50 AM

Re: PLZ pre-preamp
 
Quote:

Originally Posted by Black Stuart (Post 89246)
Here is one picture
problem miniaturising the PCB picture?


Yes Q for transistor,
https://electronics.stackexchange.co...r-transistor-q

Black Stuart 26th February 2019 09:20 AM

Re: PLZ pre-preamp
 
Greg,
thanks for that, we spent at least 1.5 hours trying to get the pics small enough for WD.

Richard - time to order the caps, I've got so much work to do on my system but can't get away from ter garden, serious pruning of fruit trees and now making a potage of a chicken run before the weather turns. Then i can work on my speakers/PLZ/h/amp - it never ends.

Black Stuart 26th February 2019 04:14 PM

Re: PLZ pre-preamp
 
1 Attachment(s)
Hi, PCB picture.

bob orbell 26th February 2019 04:58 PM

Re: PLZ pre-preamp
 
I would try taking pictures of landscapes Stuart, :(. Bob

Black Stuart 27th February 2019 09:51 AM

Re: PLZ pre-preamp
 
Ultra high definition Bob.

bikerhifinut 1st March 2019 10:33 PM

Re: PLZ pre-preamp
 
I think I may have stumbled on the circuit.
I had asked a friend who is very knowledgeable on amplification and apparently the large power devices are used because of their base spreading resistance properties.
I was given a name of a designer, W Marshall Leach and I looked it up on the web.
I have attached the link.
All is revealed I think its a common base amplifier to this design. An examination of the PCB suggests this with the emitters commoned and the preset pot is configured as a variable resistance which could be in place of R2 in the published circuit to adjust the very low bias current quoted here as 125uA (0.125 mA).
I'd be loth to fiddle about with the 470uF electrolytics unless you are absolutely sure they havent upset the biasing. The 100 uF as you can see from the circuit are simply DC blocking coupling caps to the output.
Interesting comment too about certain designers grinding off the transistor markings as if their type was some sort of industrial secret when its been lifted from a design in the public domain!
OK stuart I think you've got something to go on there.
Try it with the existing caps in, you won't blow anything up with 3V across it. You've already prodded about with your DVM so its had a few volts bunged in places already.

http://leachlegacy.ece.gatech.edu/headamp/

Andy.

Black Stuart 2nd March 2019 10:21 AM

Re: PLZ pre-preamp
 
Andy,
thanks for that. Yes very frustrating not having any clue as to the transistors. You've explained why I measured such low current on those devices.

It will be interesting to hear what a LOMM sounds like via this dedicated pre-preamp.

Apparently the stereohedron stylus was developed to avoid patent problems with the shibata design AND is supposed to be better.

There are those who swear that this type of cartrdige combined the best of MM and MC and betters MC. Nice to know that it can be re-tipped by Soundsmith if it is that good.

Let's see!

bikerhifinut 2nd March 2019 01:19 PM

Re: PLZ pre-preamp
 
All cartridges, generally speaking, can be retipped by a competent technician. As well as Soundsmith there are people like Len Gregory and Dominic harper in the UK that offer cartridge overhaul and retipping services.
That's quite useful as at a price, you can have your stylus not only replaced but possibly upgraded to a better/different profile.
There are many opinions on stylus profile, and the most advanced and expensive don't necesarrily guarntee a better audible performance.
That stereohedron is simply a brand name for a line contact variation I think possibly a Microline. The Shibata was initially developed to play Qudraphonic LPs where the decoding information etc was up beyond the 20 khz region and so a stylus and suspension had to be developed to be able to trace those high frequencies.
Other really effective profiles are, for example, the Fritz Gyger types and the GygerS and II amongst others are used by Goldring in theoir cartridges. I happen to like the Gyger profile a lot as well as the Ortofon Fine Line which for me are every bit as good as the way more expensive Shibata profile which can be a bit edgy to my ears.
Other factors are things like tip and cantilever mass and stiffness. This seems obvious of course. And brings me to why the output of moving coils is so low. In order to have a manageably low mass and thus low inertia, the coil on the end of the cantilever has to be light and constructed of extremely fine wire with very few turns. This means that the magnet assembly around the coil has to be incredibly strong and the field highly focussed in order to produce a useable output voltage, which is still in the order of a couple of hundred microvolts or so. Moving coils have really only taken off since the development of very strong magnets using rare earth metal alloys etc, making them easier to design and manufacture.
So this brings us to the good old Moving magnet, for which I shall include variations such as moving iron and variable reluctance which are all basically variations on a theme. As you will know, a moving coil works by waggling a coil in a magnetic field to generate a voltage, and therefore by the same token if you hold the coil steady and "waggle" the magnetic field around the coil you will again get a voltage generated. The advantage here is that you can wind a nice big coil to get a more effiecient generation of volts at the cost of a smaller magnetic flux density as the magnet on the end of the cantilever has to be tiny in order to keep mass down, or the magnetic alloy in a moving iron design that alters the field from an external magnet and then induces the coil voltage. On balance however the advantage is an output some 10 to 20 times that achievable by a moving coil. And there are many very good cartridges indeed with high outputs.
The issue here though is the DC resistance and inductance of a larger coil will cause its own problems with matching and frequency responce, especially at the higher frequencies, this is why most magnetic cartdiges need a 47k loading and are very susceptible to capacity effects from circuitry and screened cables due to the RC and LC low pass filters created thus.
So there are a few very low output moving iron and moving magnet cartridges, basically all that is, is fewer turns on the pickup coil which brings the advantage of MC in terms of matching and less susceptibility to capacitance effects.
Magnet technolgy has benefited these designs to the same degree in that the inertia from tip and cantilever mass can be reduced which allows the fancy line contact stylus to actually respond at the high speed (ie frequency) demanded.
I've heard a Grado low output moving iron and very nice it was but I felt the responce was skewed at the frequency extremes. There are articles out there that suggest we should alter the RIAA response filtration for MM and MC/LOMM because of the effects of electrical loading and ability to trace higher frequencies more effectively. It's more than just that simple step up which really only has to be linear from Lf to HF.
Loading in MC is also a very contentious subject, for the most part a MC cartridge is unaffected by capacitance issues, which is a good thing and also usually anything above 25 ohms depending on the DC resistance etc of the coils, which is usually never more than 4 ohms or so on the better carts, will not have any material effect. I havent noticed my Ortofon MC cartridges to ever be fussy about DC loading. I can't speak for other brands. I suspect the improvement I hear with MC is down to the reduced masses being accelerated in the groove and so we hear less mistracking and a better detail, combined with a much reduced interaction with the various capacity, resistance, and inductance related effects in the electronics and wiring. The same result may be reasonably expected with low output magnetic types for the same reasons.
A long post Stuart but maybe explains the reasoning?


Andy.

Black Stuart 2nd March 2019 03:21 PM

Re: PLZ pre-preamp
 
Andy,
quite by chance I read something very similar on Aduigon which triggered my memory that I had bought a Pickering LOMM years ago and though I had liked it, see a post on WAD I had never bothered to use the PLZ that came with it.

All cartridges need a good phono stage to really shine and a good compatible arm, get this combo right and vinyl is worth the work you have to put into to get the optimum performance.

I wish I could go back about 30 years with the knowledge i have now minus the 'received wisdom', which mostly turned out to be b/s.

The signal from a cartridge, MM,MI or MC is very low level, any interference, any magnetic field which is stronger will alter that signal conductance. Long ago I rejected using any kind of shield on phono or any i/connect cable. The only cable that should be shielded is a power cable, I f that is done I have never noticed any interference.

I was still using commercial i/connects when I last listened to the Pickering 7500S, which all totally disregarded the stone cold fact that air is the best dialectric.

My Kenwood KD990 was designed from the ground up by engineers with open minds and they determined that the arm cable should be very high purity solid silver. This cable is solderded to a piece of tag board and the phono out cables are soldered to this.

As I have 2 x KD990s and the first only cost me £120 (it was dropped in transit but still worked) I intend to open up the arm on this deck so that there will be one continuous run of cable from cartridge tags to the PLZ or if I don't like what I hear to my MC3 phono stage, which I really do like.

What a shame these LOMM only came along at the wrong time Andy. The Ortofon/Soundsmith design has a huge advantage over MC which you have mentioned. The advantage of the Soundsmith cartrdiges also has a very real economic advantage they can be 'renewed' for a very reasonable price unlike MC.

I was very wary of MCs since production has been very variable. The ART 9 when properly made has a very good reputation but just a few years ago there were many being sold that were far from accurate in their alignment.

I never really liked the Goldring sound, I have an NOS G1000, forgot all about it. I have heard that Ortofon have very strict production protocols, hence no bad reviews on quality.

I shall give the PLZ a try and if I like what I hear, then around £8 for new caps to the same specs is a small investment.

There was an American who worked specifically on stylus research on V/E and he did a whole thread ob the various stylus profiles, their effect on vinyl wear and how well they tracked and relayed the information.

bikerhifinut 27th June 2019 10:47 AM

Re: PLZ pre-preamp
 
Further down the line and I have now got the new living room up and running on the first floor.
So I put a pair of AA cells into the PLZ that bob O kindly lent me to evaluate and plugged it into the MM input of the Rega Cursa preamp, I will try it in the WD Phono3 at some point too.
Well, it's fine as one would expect. My observations were that its susceptible to hum fields so keeping it well away from mains transformers is essential, as one would anyway when dealing with 300uV signals. The other observation is that it's just as noisy, if not a tad noisier than the Valve Phono3 and considerably noisier than my off board dedicated rega MC stage. I'm not surprised really as we are talking designs more than 30 years apart here. That said, at normal to loud listening levels it's inaudible under the Vinyl Roar etc.
Sound quality wise it's a tad brighter than using a pair of Ortofon T5 step up transformers, (Made by Sony and jolly useful little inline devices) and I'd be happy using it if it was all I had, there's nothing really not to like about it. I wasnt worried about leaving the original electrolytics in place, the worst that could happen is that they might have dried out over time but I think its unlikely and their service life in view of the never more than ambient temperature and weedy 3V Battery supply will likely be decades before any measurable drop in performance. Switching it off revealed a quite long period where it was still amplifying as the 470uF decoupling caps discharged so again these are good signs.

Andy


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