PLZ pre-preamp

[Black Stuart]

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]

Originally Posted by Black Stuart

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]

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]

Originally Posted by bob orbell

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]

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]

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]

Originally Posted by Black Stuart

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]

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]

Here is one picture
problem miniaturising the PCB picture?

[Greg.]

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.