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In Reply to: RE: e80cc operating points in SRPP posted by Triode_Kingdom on August 26, 2023 at 14:47:17
Very good.
I measured NF at three different frequencies because I don't know how accurate my multimeter is at the midrange frequencies
I'm getting the following NF:
60hz 6db
500hz 9.5db
1000hz 11.5db
But also just learned that I can get an additional 6db of voltage (and possibly better sound) from my Schiit Gungnir by using pin 2 and 3 of the XLR rather than the 'summed" RCA...
So, considering the current Gain is just fine, I can probably get the extra gain from the XLR out on my Dac and increase the NF another 6db.
But what about all these designers that swear that NF is bad, and they just leave it out completely- even on a SE kt88?
Also- why would Audio Note design this amp with the SRPP if it wasn't optimal...
Ok, truth be told, this amp was originally a 12ax7 SRPP with an additional non-gain 6sn7 buffer stage for the input... but the buffer stage was not needed and I dont like the sound of 12ax7/5751... let alone the 12at7. 12au7 and 6sn7 are better but even less gain, so the e80cc seemed logical... and please beleive me when I tell you- it IS enough gain...
But I'll take your advice- I'll look into increasing the NF.
I think the pain point for me right now is that the 'ideal' load for this SRPP with e80cc is 10k. Which is a factor of 22 less than the original design of 220k, but the 220k was no good for the kt88... i beleive it was maybe degrading my cathodes...
So now increasing the coupling cap: but also I use a sub, and have it all digitally crossed over (80hz) before my dac (i use two dacs- one for main and one for sub) but I'm fine with a -3 point at 50hz, because my tube amp does the highs and the rolloff is fine, and so as long as i dont need full-range, seems .47uf works...
Follow Ups:
"But what about all these designers that swear that NF is bad, and they just leave it out completely- even on a SE kt88?"
It's inconceivable to me that any educated designer would do that. The distortion products that result from this practice are highly invasive and easily measured. It's really not appropriate except for a guitar amplifier.
"Also- why would Audio Note design this amp with the SRPP if it wasn't optimal..."
You would have to ask them. All audio companies bow to marketing pressure, and SRPP was the rage for years. It was never intended for this use though, and it doesn't provide push-pull functionality with these circuit values. If you search, you'll find this has been discussed at length here on the Asylum and elsewhere. I'll just add that I have seen situations in which SRPP provided very good distortion numbers, even though it wasn't "properly" loaded. Perhaps AN made measurements and confirmed some benefit in this application.
TK, can you comment on the idea of removing the power tube cathode bypass cap? And would the resulting local NF from the cathode resistor(s) tame some of the power tube's intermodulation distortion?
I'm not sure what happens with a pentode or a beam power tube but with a triode the plate resistance goes up and the relationship between the load impedance (output transformer) and the tube's output impedance gets unfavorable and HD goes up.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Can you speak a little more about the needed load resistance when running without the cathode bypass cap? Without this cap would I need a transformer with more resistance or less resistance?
I'm asking because my transformer is already a bit short on primary resistance. If removing the cap makes this situation better that's good but if it makes this low impedance trefo situation even worse then the cap is definitely staying in until I can get higher primary trefos.
I calculated my needed trefo primary using the equation Z=Va^2 / Pa and for the e80cc I get 8k. But my trefo is 2.6k because it was calculated for the HT of 425 and not the 585 I'm running...- but someday I'm ready for the Hasimoto with the 7k primary when I can afford it...-
But the irony now is nit that I optimized my SRPP (today) the 6sn7 sounds so much thicker and musical than the e80cc and I'm gonna go with it. So, now optimizing for it. And going to give up on e80cc - it's sound is just too intense and bringing for prolonged listening
Without the cathode bypass cap there will be "local current feedback" applied to the tube and that increases the plate resistance of the tube. The output transformer would need to have a higher primary impedance to keep the relationship between the output impedance of the tube and the input impedance of the transformer (the load impedance for the tube) happy. Otherwise the load line (for the tube) will be rotated towards the vertical. That will lower the gain of the stage and increase the harmonic distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I measured NF at three different frequencies because I don't know how accurate my multimeter is at the midrange frequencies
I'm getting the following NF:
60hz 6db
500hz 9.5db
1000hz 11.5db
I've never seen a multimeter exhibit such a wide variation in readings. Even cheap models that aren't "true RMS" should be flatter and more linear than this. Can I assume you re-verified the before and after numbers at each frequency? Feedback does vary with frequency of course, but these numbers are extreme, especially the 2dB difference between 500Hz and 1kHz. I think you will need to determine exactly what's happening with this before the numbers will be meaningful.
Yes, three separate measurements with and without NF.
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