Thread: PLZ pre-preamp
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Old 24th February 2019, 04:36 PM
bikerhifinut bikerhifinut is offline
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Default Re: PLZ pre-preamp

Originally Posted by Black Stuart View Post
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.
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