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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.


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