Mu-follower with ECL805 and output xformer - diyAudio
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Old 8th October 2010, 11:19 PM   #1
00940 is offline 00940  Belgium
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Join Date: Sep 2003
Location: Liège
Default Mu-follower with ECL805 and output xformer

I've quite a few parts lying around that I'd like to use for an headphones amplifier which would be useful for both high and low impedance headphones. Among those parts:

- A xformer with 330-0-330 @85ma and 6.3@4A secondaries,
- A pair of 25W 100V line transformers with 4, 8 and 16 ohms taps with an advertised bandwith of 30hz-20khz.

You'll find below what I'm considering for now... but if you have better ideas or improvements to suggest, I'm all ears.

Click the image to open in full size.

Obviously, the inspiration is coming from Alan Kimmel's article on mu-follower stages with pentodes.

If my calculations are ok, using the 25W and 16ohms taps of the line transformers, headphones with a 320ohms impedance should create a 8K load on the primary, with a 5/1 ratio. Using the 6W and 8ohms taps would convert a 32ohms pair of headphones into a 6.6K load. Such loads should be quite ok for a beefy mu-follower stage. Hopefully, as I'm not loading the xformers too much, I should also get a decent frequency response.

The xformer will give me 350VDC to play with (using an EZ81 for rectification), with current to burn. By running the pentodes at 30-35ma (the ECL805 can provide 75ma), I'll have plenty of current for low impedance cans. The concern is more the limited voltage swing on 350V (340V most probably after a choke). Sims give me (yeah, I know ) "only" 8.5Vrms at the output for 300ohms cans before thd figures start hitting 1%.

I've got a question still about the pentode screen biasing. Kimmel explains well how to set all the resistors values... but this part is more obscure. I've used what gave best results in sims but I've got a limited trust in the tubes models for such fine adjustments. If anyone can comment on this, thanks a lot !
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