Thread: PLZ pre-preamp
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Old 23rd February 2019, 01:28 PM
bikerhifinut bikerhifinut is offline
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Default Re: PLZ pre-preamp

Quote:
Originally Posted by Black Stuart View Post
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.
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