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Old 1st February 2018, 06:37 AM   #11
RobLK is offline RobLK  United States
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I was just about to say the same thing about removing c2. I would also remove the center connection that joins c17 and c12 to r23 and r15. You should get about 160ma with the 3.3 ohm resistors you are using. It is possible you might get the 160ma's back if you remove the connection. In Wayne's design that connection is used for dc offset but since you use a transformer there is no dc. You could use a jfet op amp to auto adjust the offset, connecting it between the output and the r23 and r15 junction.
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Old 1st February 2018, 06:48 AM   #12
EUVL is offline EUVL  Europe
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A DC servo will spoil its simplicity and elegance, IMHO.




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Old 1st February 2018, 09:32 AM   #13
EUVL is offline EUVL  Europe
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This is what I would do if I were to build myself.


Patrick

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Old 1st February 2018, 09:47 AM   #14
RobLK is offline RobLK  United States
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Oh, I see just trim r6 and r15 at first I missed it. Blackdod said his bias current fell to 34ma when he used 20k resistors. Why would this cause this problem?

Last edited by RobLK; 1st February 2018 at 09:53 AM.
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Old 1st February 2018, 10:15 AM   #15
EUVL is offline EUVL  Europe
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Because he was still using 10k for R15 & R23.
This results in only 5V bias for the Fairchild MOSFETs which is of course not sufficient.
In changing R6 & R24, you must also change the values of R15 and R23 by the same amount.
Unless of course you use Toshiba 2SK2013 / 2SJ313, which require much less bias voltage.

But you can readily test all these in LT Spice.
Then you have a good idea what happens by changing which parameter, before you solder anything.


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Old 1st February 2018, 10:51 AM   #16
RobLK is offline RobLK  United States
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Oh! Thanks Euvl, I'm glad I askI missed that in his first post. I skipped over it to fast an just assumed he would change them all to 20k.
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Old 1st February 2018, 11:32 AM   #17
RobLK is offline RobLK  United States
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Well Ok Euvl I simulated this circuit in Ltspice with a dc servo and the dc offset is a few hundred nanovolts. Thd is .011% at 2vpp and 1khz into 30 ohms. But I have much to learn about Ltspice so I don't know how accurate these results are.

Last edited by RobLK; 1st February 2018 at 11:34 AM. Reason: added info
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Old 1st February 2018, 05:18 PM   #18
EUVL is offline EUVL  Europe
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The distortion spectrum will be dominated by that of the transformer.
So unless you have a means of modelling that, the rest is IMHO irrelevant.


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Old 1st February 2018, 08:29 PM   #19
blackdod is offline blackdod  Europe
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Quote:
Originally Posted by EUVL View Post
This is what I would do if I were to build myself.


Patrick

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I would like try your idea this weekend and jumper C2 too. Probably C2 only add distortion but for me is funny done this kind of tests.

Answer for Rob; For check it, I only changed R6 and R24 for 20K and I was still using 10k for R15 & R23
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Old 1st February 2018, 08:33 PM   #20
RobLK is offline RobLK  United States
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Even if the distortion is wrong it is still a very useful tool. When I simulated xrk971's lender headphone amp the distortion readings were very close to his measurements. Then I tried his amp with a mosfet and a larger depletion fet at the output and I got some ridiculously low readings. Under the same conditions it showed 0.00028% thd. the model for the p channel mosfet was from bob cordell and the depletion device model was from the manufacture, it is a sic device. I also tried it using dual supplies with no cap on output with same results.
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