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Old 16th March 2013, 04:41 AM   #11
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Quote:
Originally Posted by Ciu View Post
Thanks for this - it looks very good in simulation with JFETs instead of Triodes, and an opamp instead of a chipamp. Basically, a solid-state mini-Aikido for driving heaphones.
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Old 16th March 2013, 08:23 PM   #12
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Thanks for comments guys. I know it isnt "doing alot" but like i said a simple first valve project which allows me to utillise my balanced mixer outs.
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Old 19th March 2013, 03:34 AM   #13
Fenris is offline Fenris  United States
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Originally Posted by Ciu View Post
Hello Fenris

JR Broskie wrote " we must also increase the 520-ohm feedback resistor to ensure that the solid-state amplifierís gain exceeds the triodeís mu. In the example shown above, the feedback resistorís value has been increased to 100k, which results in a gain of 100 for the solid-state amplifier "

The triode 6dj8 has mu value of 33, do you know the reason why JRB has choosen a very hight gain of 100, and not ~40 - 50 ?
Thanks
The 100K keeps too much current from flowing from the ouput of the opamp to the cathode of the tube. The ideal cathode load would be infinite, with a 100K plus the 1K in parallel with the 1M and 10K, it's an overall cathode load of 91K. The CCS keeps sufficient current flowing through the tube to operate it in the linear range. You might be able to change the 1K to 2K and the 10K to 20K. The high opamp gain may be to maintain a good phase margin, or could be overdesign to ensure that the tube gain is never more than the opamp gain.

I'm building one of these now and am about 95% done, but won't be able to finish for about another 7 months due to life.
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Old 24th March 2013, 11:40 AM   #14
Ciu is offline Ciu  France
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Originally Posted by Fenris View Post

I'm building one of these now and am about 95% done, but won't be able to finish for about another 7 months due to life.
Thanks for detailled explanations !
Which chip did you choose ?
LM3886 ? seems to be better about distorsion
LM3875/LM3876 ? seems to be more appreciated about soundings

R.C.
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Old 24th March 2013, 12:05 PM   #15
Fenris is offline Fenris  United States
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I'm using the 3886 due to its more rugged construction and the fact I had a few in my parts bin. The tube (a 7370 type) should dominate the sound since it's responsible for voltage amplification.
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Old 24th March 2013, 05:48 PM   #16
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Originally Posted by Ciu View Post
With the LM3886 and 3875, high gain and feedback resistors invite oscillation -- with the LM4702 driver chip, there's a pin for compensation.

I simmed the circuit in your post and you'll have to compensate it just a bit around the LM3886.
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