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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Headphone amp OTL

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I need to know the output impedance of attached schematic? also I need to know if I have to change some resistors & caps values to use 6SN7 in place of 12SX7?



  • head_amp_schem 6SN7 + EL34 OTL.jpg
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Joined 2013
Please see attached. You should increase the value of output capacitor if you use low impedance hp.


  • el34 hp output impedance.png
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  • el34 hp output impedance -2.png
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  • el34 hp FFT-1.png
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Joined 2013
Thank you, do you now anyway to improve the amp?

Well the noise level is very low so that is very quiet as claimed. EL34 can output 120V at 1.5% THD if bias @ half HT (+125V), then if only 1 to 10V is required to drive the hp. that is still very good bias point. Second, gain of 6SN7 stage can be reduced and also bias EL34 @ about 1/2 HT, so what gain do you want so I can try to come up one meet your need.

A suggestion : to use the double triode 6SN7 in SRPP mode rather than to put 2 triodes in parallel.

This tube is perfect for that as its µ is constant when Ip > 5 mA. It will give very low harmonic distorsion.

Polarization points should be revisited but maybe Koonw can give a try with Spice.

B+ for SRPP can be reduced introducing a RC network in the B+ line.

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Joined 2013
Here is SRPP sim you requested, please have a look. Unable to get srpp stage distortion lower than 0.1%, and noise floor looks like higher.

Edit: Sorry noise level is no difference based on 20k load


  • el34 hp srpp-1.png
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Well done, Koonw !

0.1% is very low and excellent in my point of view.

In sims, noise floor depends on the sim parameters.

Real noise floor will be higher, no matter the stage, because of the real PSU.

So I think it is a good circuit Merlin el mago can try. Not too much components to change.

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