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RH84 question

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#21
Ok point taken -- I do, however, find the bass a little anemic through the phono stage. I've A/B'ed the same songs on LP and CD...

Is there a change I can make to that phono EQ to help out the low notes? Or will it just be squashed by the NFB?
 
#22
> I've A/B'ed the same songs on LP and CD...

LP cutting has a problem with deep bass. CD is agnostic to frequency. LPs and CDs are NOT the same recordings. (Also of course mastering tastes have changed over the decades.)

> bass a little anemic through the phono stage.

Have you built a reverse-RIAA network and verified your preamp?

FWIW: I was going to point out the changes from RCA's values to this "Based upon" plan. However they both sim incredibly accurate, +/-0.3dB. Likely errors would be weak tubes and wrong values. Also of course unfortunate tonearm resonance.
 

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#23
PRR said:
_I_ would say 2W 400V would be ample for most folks, but these R-C networks DID burn-out a fair amount, and it's not Rayma's money, and 5W 600V is not THAT expensive in DIY-context, no dissent there. And you do seem to be sweep-testing, not just blaring the ballgame, so you "could" put all 5 Watts of power into 20KHz and stress the R-C far beyond any radio would.
Click to expand...
It would seem easier to just put a small mica pf value cap across the feedback resistor. Is the Zoebel network a better solution than bypassing the fb resistor?
 
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#24
> cap across the feedback resistor

My mind had wandered back to radios with naked pentodes and no NFB loop.

You are right: the NFB taming should be explored.

There "is" a problem. The OT resonates at the top of the audio band. Makes a 2-pole response, and there is always a 3rd pole lurking. So NFB stability is not assured. The Zobel tends to damp the 2-pole and *may* shift things.

However this project seems to be polishing a cotton purse into a silk purse. It can be improved, but not trans-mutated.
 
#25
PRR said:
> cap across the feedback resistor

My mind had wandered back to radios with naked pentodes and no NFB loop.

You are right: the NFB taming should be explored.

There "is" a problem. The OT resonates at the top of the audio band. Makes a 2-pole response, and there is always a 3rd pole lurking. So NFB stability is not assured. The Zobel tends to damp the 2-pole and *may* shift things.

However this project seems to be polishing a cotton purse into a silk purse. It can be improved, but not trans-mutated.
Click to expand...

PRR, sorry to reply so long after your post, but i got busy with other issues with my amp and now need to revisit this rising high frequency response and how best to tame it.
The amp is also a Schade FB SE 7591 with a 12at7 driver. It was rescued from a Montgomery Ward console amp from the "good old days". So, i need to tame the Hi frequency and wrapping a pf value silver mica cap around the 100k fb resistor is easy enough to do. According to 2A3 for the el84 Akai amp he recommended 47pf. Is this value also ok for this 7591 powered amp? Thank you for replying
 
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