• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

SRPP preamp - Rk calculation

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Reporting back the results

I finished the project couple of weeks ago. Results were quite interesting. I made two output stages to compare. Same tubes, same power supply, same operating point, same parts quality, - so the comparison was quite "objective". One stage was CCDA, which I really liked from my engineering point of view. I was almost certain that it would be the winner. The second was "SRPP-like" (credit goes to this guy http://www.lampizator.eu). In reality, this "SRPP-like" is simply a common cathode amplifier with a second valve acting as a current source. See images of actual used schematics of both.

Listening tests brought unpredictable results. CCDA sounded Ok. The "SRPP-like" design sounded AMAZING, against the prediction, so it's left in as a final solution. I tried increasing B+ voltage to 200V and adjusting the biasing, - no noticeable sound difference.

I trust my ears. After many years of listening to high end audio gear, I should say that this design, taking the signal directly from TDA1541 DAC output pins, delivers unbelievable sound.

Thank you all for ideas and suggestions!

I'm back enjoying the music... :)

TubeStageCCDASchematics.JPG . TubeStageSchematics.JPG
 
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DmitryKulakov said:
The second was "SRPP-like" (credit goes to this guy http://www.lampizator.eu). In reality, this "SRPP-like" is simply a common cathode amplifier with a second valve acting as a current source.
That is, not an SRPP - just an active load. It therefore misses the two features of an SRPP which makes it an SRPP: reduced output impedance, balanced push-pull operation for the optimum load impedance. As it happens you needed neither of these, so you didn't need an SRPP. However, as the SRPP is regarded as being a good thing for audio (expecially in China) it helps popularity if something which is not an SRPP is called 'SRPP-like'.
 
I know of SRPP advantages. However, SRPP only performs well if it's balanced with the load, which makes each particular implementation the load impedance specific. This is what my initial question was about, which was ignored :)

Anyways, as I said, I'm happy with the sound, which is all that matters...
 
analog_sa said:
So, you like an srpp better than a cathode follower? You are not alone
Not sure if you were talking to me, but preferring an SRPP to a cathode follower is a bit like preferring a 747 to a bicycle. Both are vehicles, but they have very different properties and so are best used for different purpose.

DmitryKulakov said:
This is what my initial question was about, which was ignored
No, your question was answered by referring you to Merlin Blencowe's website.
 
I forgot to mention that it's the lower circuit I was referring to where the output is taken from the grid instead of the cathode synonymous to common collector where Av is less than 1 but current gain is high...in essence a buffer. I'm totally a newbie in tubes.

The original poster is using the grid as output??
 

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Take the output from the upper cathode and you have SRPP.

Take the output from the lower anode and you have an active load on a grounded cathode stage.

If it is driving a high impedance then there is no practical difference. You don't get, but don't need, the particular advantages of an SRPP - lowish output impedance and reduced distortion when driving the optimum load.
 
The original poster is using the grid as output??

You have your answer already. Thank you guys.
I just wanted to add that in my final solution, that I keep enjoying, I put 82 Ohm input grid resistor instead of 91 and brought the voltage to +190V. Other than that - as per the initially posted image.

Enjoy! :)

BTW - here's what that CD player looks like (the tube stage is at the right with the power supply on the left).
Inside:
An externally hosted image should be here but it was not working when we last tested it.


Outside:
An externally hosted image should be here but it was not working when we last tested it.
 
Thanks for the explanation, DF96, appreciated.

Hello DmitryKulakov,
I'm pleasantly surprised you chimed in since you're the original poster! I'm going through the same journey with the TDA1541 and exploring the vastly available options for I/V output stages.

Your circuit intrigues me as I am slowly getting into tube output stages for which I accidentally stumbled on this thread. It appears similar to Lampizator output except your output is taken from the grid instead of cathode.

I appreciate your response, thank you!

Btw, the two images posted did not show, can you try reposting, thx!
 
Exploring TDA1541 was quite a journey. I modified two CD players before I built the on on the image, that I'm totally happy with (I hope it shows up now).

The tube output stage was one part of the equation. If you want to get the best performance, I would recommend having few other updates:
1. Clean dedicated PSU for TDA.
2. Get read of 7220 and use another re-clock board instead.
3. Use film SMD capacitors for TDA (like ones from Digikey - code:pCF1132CT-ND).

Regarding the #2 - I experimented with different schematics and then found a brilliant and elegant solution developed by a guy on eBay (seller's name "alexhifi1"). The board is called "NOS Femto Jitter clock re-clock for SAA7220 in TDA1541A based CD players and DAC". It's $99 but is worth every penny. Contact the guy and he will send you the one even if there is none on eBay.

Regarding the #3 - I created a separate small PCB that is mounted right underneath of TDA1541 and soldered directly to its legs. A much better solution that trying to figure out how to place better capacitors on the top. I can send you the schematics and PCB design for this, if you're interested.

There are other updates but the three above + tube output are most rewarding.

Also, the schematics of the output stage is exactly as it was on the Lampizator web page. I still have an image somewhere of his initial hand drawing. I just showed the tube differently. The right part of the tube on my image corresponds to to the upper portion of the tube as per Lampizator.

Good luck!
 

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WOW!! Yes, the images showed up and is absolutely fabulous!!

Thanks for all the suggestions as I really want to get the tube output stage going. Two circuits I did was a major disaster - really newbie I am!!

Hopefully, with your clear schematic, I can get that going, thanks again!
 
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