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Hearinspace 10th October 2009 07:26 PM

Steve Bench's LR Phono Stage - Cathode Bias Circuit Question
 
3 Attachment(s)
Would anybody care to take a crack at why Steve put R10/R23 into this circuit? Steve himself went off the grid a couple of years ago and is no longer reachable for his comments. I heard from one person who built a version of it using the Sowter Transformers that it was to put the bias diode into a more linear operating range but if that's true , why not do it with the first stage as well?
Any thoughts on the design as it is or suggestions for variations or possible improvements etc. to explore would be much appreciated.

Thanks

Sheldon 11th October 2009 11:42 PM

Quote:

Originally Posted by Hearinspace (Post 1946834)
Would anybody care to take a crack at why Steve put R10/R23 into this circuit? Steve himself went off the grid a couple of years ago and is no longer reachable for his comments. I heard from one person who built a version of it using the Sowter Transformers that it was to put the bias diode into a more linear operating range but if that's true , why not do it with the first stage as well?
Any thoughts on the design as it is or suggestions for variations or possible improvements etc. to explore would be much appreciated.

Thanks

That's an interesting one. The voltage figures for the respective cathodes don't add up. If it were done to bias up the diodes, then I don't know why it would skip the first diode in the string. Maybe to bias up the adjustment pot, or to lift the filament a little.

Sheldon

Hearinspace 12th October 2009 12:29 AM

No they don't add up. I've just been going through the curves for the diodes and the D3A and they don't really work out right. In Christian's schematic the pot is noted as being a 20Hz. adjust but with from 24mA up to 40mA going through the shared leg that pot is also needed for trimming the bias.
I'm going to build a simple version of it with AC heaters to start so no R10. We'll see how it works.

I don't know if you've seen his original article or not . Here's the link

Hearinspace 19th October 2009 04:26 AM

Calculate new Network Compensation in LR RIAA
 
Steve Bench's original LR Phono Stage design uses the LL1623 as the output transformer which is followed by an LR filter stage with resistor R2 in parallel with the L (labeled transformer compensation resistor in Christian Rintalen's schematic - also linked in the top post this page). If building this with the 1660 instead of the 1623 how would the compensation resistor value be adjusted? Would it be the difference in source resistances (seen looking back from the secondaries) added to the resistor?

av-trouvaille 16th July 2013 09:43 PM

The next European Triode Festival 2013 (ETF) challenges the participants for a shootout with a tubed LR phono stage. Is the design of Steve Bench of 2005 still state of the art, or have new insights occurred? I wonder whether Hearinspace can share his experience. Maybe, others also have advanced by simulating or building a new design.

Hearinspace 17th July 2013 12:16 AM

yeah sure, I can draw up what i have done and post it. I kind of wandered off into doing my own quick and dirty differential phono amp with single stage rc eq. which I like a lot. It sounds more live than the very earthy LC filter but I'd guess that changes to the LC phono stage's power supply could have a big influence on the sound.
I can tell you too that component choices make important differences - sometimes impossible to name in any other way than to say that the sound went from being good to being something that you like. Fine tuning pays off.

Hearinspace 18th July 2013 03:59 AM

OK, I took a good look at the versions I did and neither of them have anything unusual or technically complex in their design. I built them both using a VERY simple PTransformer >Full wave bridge rectifier> LCLC > Bench Circuit.

I didn't even use DC heaters. I put the heaters of each channel on its own 12.6VCT power transformer wiring each 6.3V heater so that it was out of phase with its mate, the idea being that perhaps there would be some small noise cancelation. I also biased the heaters to something like 35VDC through a simple bipassed divider from the B+ supply .

The rest of the circuit is as was in the original design posted above though now I recall I had to play with the cathode bias diodes a little.

The other thing was LF oscillation as the circuit powered up and down. It seemed perfectly stable once it reached its steady state operation. but without a mute it would fart its way up the scale when turned on and then fart its way back down when it was switched off. I guess this might have been aided by the cathode positive feedback but that's only a guess. I haven't tested it yet.

As I mentioned above, I got sidetracked by an urge to try a simple all in one go rc eq stage but now that I've done it succesfully I'll probably have another look at debugging the LR stage. My problem is theory. Like I don't know any ! So I pretty much have to reinvent every wheel I want to turn by building it first. PITA really . . . . .


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