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Old 25th August 2010, 02:25 AM   #1
Brit01 is offline Brit01  United Kingdom
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Default 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.
18-20mA??

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.

Strange.
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Old 25th August 2010, 03:44 AM   #2
piano3 is offline piano3  United Kingdom
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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.
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Old 25th August 2010, 03:51 AM   #3
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Matching transformer would be fine to add.
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Old 25th August 2010, 03:52 AM   #4
Brit01 is offline Brit01  United Kingdom
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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.
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Old 25th August 2010, 09:11 AM   #5
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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?
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Old 25th August 2010, 12:54 PM   #6
Brit01 is offline Brit01  United Kingdom
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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.

Last edited by Brit01; 25th August 2010 at 01:00 PM.
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Old 25th August 2010, 02:44 PM   #7
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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....

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

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

Last edited by Alastair E; 25th August 2010 at 02:50 PM.
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Old 25th August 2010, 03:09 PM   #8
Brit01 is offline Brit01  United Kingdom
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Maybe I got confused with the load.

These phones are standard small headphones. Easiest to drive.
300??

Why does the SRPP with 6N6P's perform flawlessly?
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Old 25th August 2010, 04:17 PM   #9
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Hmm...

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!
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Old 25th August 2010, 04:23 PM   #10
Brit01 is offline Brit01  United Kingdom
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Quote:
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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