Tube DIY Asylum: Latest version of my 7788/7721 LCR 600Ohm RIAA Stage by Thorsten Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by Thorsten (M ) on October 2, 2005 at 15:11:26

Folks,

Since the 2002 version of my Phonostage I have made a few changes. I thought it may interrest some.

Here the 2002 signal circuit (per channel):

and here the powersupply (one for two channels):

Originally I had RC circuits for the cathodes, but the "semi-fixed bias" shown sounded better.

But over time the big Electrolytic Capacitors bugged me, just on principle and because of course some signal current must flow that way.

In stages I worked my way along.

I figured I could change the Interfaace between input stage and RIAA to direct coupling and allow the second stage to be gridleak-biased (I'm quite fond of using that).

So, a 600 Ohm Load (a little higher actually to compensate the internal impedance of the input Valve) then direct coupling to the RIAA as the S&B 600 Ohm RIAA Network can witstand the kind of DC offset present here (others like Tango etc. may very well not) and then the second stage in gridleak bias. Worked out mighty fine too.

That left the input stage biasing.

After I recalled that the standards for cutting microgrove record specified no more than 25cm/s (and rarely is any more even that much cut) and that few if any MM cartridges (or MC Cartidges plus stepup) produce more than 10mV @ 5cm/S I figured that the largest amount of signal we are likely to see is around 50mV.

So, I asked myself, HOW BAD can the gridcurrent REALLY be if we operate the input valve zero bias with such a small signal (average 2.5 - 5mV near max with extreme peaks to 25 - 50mV). Tested it out, the answer was - not that bad at all, it seemed a lot more than 50mV are tolerated just fine.

So, here the current revision of the signal circuit with all changes applied:

The improvement in sound quality over the original is not huge, but appreciated. The upgrade may be worthwhile for any who have build a copy and of course for anyone considering to build one.

Ciao T

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