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  #1  
Old 19th February 2019, 07:29 PM
Black Stuart Black Stuart is offline
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Question 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.
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Old 19th February 2019, 10:52 PM
Richard Richard is offline
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Default 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

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
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Old 20th February 2019, 07:03 AM
bob orbell bob orbell is offline
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Default 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
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Old 20th February 2019, 01:59 PM
Black Stuart Black Stuart is offline
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Default 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.
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Old 20th February 2019, 04:16 PM
bob orbell bob orbell is offline
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Default 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
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  #6  
Old 20th February 2019, 04:57 PM
bikerhifinut bikerhifinut is offline
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Default Re: PLZ pre-preamp

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.
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File Type: jpg Boozhound-Labs-JFET-MC-Pre-Preamp-Schematic.jpg (26.9 KB, 11 views)
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