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Brit01 25th August 2010 02:25 AM

MU follower - distortion
First attempt at mu follower for headphone amp.

2 x 6N6P.

Voltage on bottom anode is 100V - led bias at about 2.0V across it.

top cathode has 120 ohm resistor (1.9V-16mA) follower by 10K with bootstrap of 220K.
Small bypass cap connected to bottom anode.

Turn is up a little a it distorts quite a bit.
32 ohm phones.

For me the voltages and currents look ok.


piano3 25th August 2010 03:44 AM

32 ohms is a pretty impossible load for a valve. I think you would need a pretty special circuit with lots of feedback. Think about the problems of designing OTL amps.I think EC8010 probably knows a lot about this kind of thing judging from some of his posts.

Wavebourn 25th August 2010 03:51 AM

Matching transformer would be fine to add.

Brit01 25th August 2010 03:52 AM

Isn't 32 ohms the impedance of the smallest headphones??

Mt SRPP drove them really well. And I expect the 300 ohm phones such as Sennheiser.

FYI- I have a 220uF output cap.

revintage 25th August 2010 09:11 AM

If I understand your schematicless description right, you have a voltage drop of 160V over the 10k. So with 100V over the upper tube you need just over 350V B+. Do you have that?

Brit01 25th August 2010 12:54 PM

I have 325V.

Where am I going wrong? Do I need to lower or increase the upper cathode resistor or the 10K plate resistor?

The mu follower should have lower distortion than the SRPP.

Alastair E 25th August 2010 02:44 PM

I think that the load impedance you're trying to drive is way too low for that valve....:(

32 ohm is VERY low impedance for a 6N6p....:rolleyes:

Try a higher impedance headphone, around 300 ohm....

Or you could replace the top valve with a MOSFET a la Allen Kimmel....

Brit01 25th August 2010 03:09 PM

Maybe I got confused with the load.

These phones are standard small headphones. Easiest to drive.

Why does the SRPP with 6N6P's perform flawlessly?

Alastair E 25th August 2010 04:17 PM


You say, 'small and easy to drive'....

What is the actual Impedance of these phones...?

That type used fro MP3 players etc are usually below 50 ohms, and not a good load for any small Valve circuit!

Brit01 25th August 2010 04:23 PM


That type used fro MP3 players etc are usually below 50 ohms

Big error on my behalf. I thought they were easier to drive.
Come to think of impedance I couldn't have been so wrong.

Yes small maxell phones.

The SRPP worked great with them. Quite superb. Guess I'll go back to my original design.

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