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Old 15th February 2004, 04:26 PM   #1
diyAudio Member
Join Date: May 2003
Default Passlabs preamp as headphone amp- need help

Hi all-

I have been wanting a class A solid state amp for my headphones for a while now and have decided that it should be some kind of Pass design. I have looked through everything on the PassDiy page, but I do no want to run the thing on +/-60V. I would want a max of +/-24V, preferably +/-15. I have a few problems other than supply voltage, though...

My headphones are the Sennheiser HD580s, so they are 300-ohms. A simple MOSFET follower with unity gain wouldn't work unless I had a gain stage, and I really want to keep it ultra-simple with one active device per channel. So, it might work to use the MOSFET in common-source for a little voltage gain. However, I am not sure how much gain I would get. I usually use an amp with 21dB gain, which works out well. I looked at the BLS, but I don't have a soruce with balanced output, so it would defeat the purpose of having a balanced amp. I would prefer SE with just one MOSFET per channel (unless another is needed for CCS).

I bought an IRF510 from RadioShack for testing purposes... because I don't do a lot with discretes. I cannot get the thing to do much at all. It just will not produce sound, and if it does, it never gets loud or sounds very good. So, I am not exactly as optimistic about the ease of building this as I once was.

Could anyone point me towards a Pass design that would work really well in SE for a 300-ohm load? How might I modify it to run +/-15V or +24 or +30V rails? Thanks.
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Old 15th February 2004, 05:14 PM   #2
joensd is offline joensd  Germany
diyAudio Member
Join Date: Dec 2001
Location: Germany
I think that Nelson and others stated that the BLS sounds better than the BOZ even with unbalanced sources.
In the article it says the BLS can safely drive loads down to 600Ohms at the cost of some output swing so you definitely have to change some circuitry to get it to work with your headphones.
What about a BOZ followed by a mosfet follower?
A downscaled zen similar to Marcello´s zen headphone amp ?
Marcello´s you´d probably have to scale to suit your 300Ohm headphones as it is intended for grados.
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Old 15th February 2004, 06:55 PM   #3
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Join Date: Jun 2002
Location: Italy
Marcello´s you´d probably have to scale to suit your 300Ohm headphones as it is intended for grados. [/B]
Marcello's Zen hp amp works very well with 600ohm , I tried it for months until a year ago with a Sennheiser .

Now I enjoy the Bride of Zen with a Grado Alessandro Music Series One , 100db efficient and 32 Ohm impedance. The BOZ is a current source amp and any load is OK for it ,
the gain just go down if the load is smaller .
Anyway , with BOZ I tend to prefer the current source as a wirewound resistor , but I believe it is a matter of taste.

The third possibility is the aleph scaled down version for HP by , if I recall right , JWB .

Marcello's HP Zen needs at least 30 volts , but I would go for 60

Boz need 60 V

The JWB sort of lower voltage ...

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Old 16th February 2004, 09:30 PM   #4
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Join Date: May 2003
I put a little thought into it and re-read the Pass documents on the BLS/BOSOZ and SOZ. The BOSOZ would have enough voltage and current gain, but would have too high of an output impedence to run my 300-ohm cans well. The SOZ, however, has a much lower output Z, but runs very hot because it is intended to drive larger speakers, not headphones. So, if I tone down the SOZ quite a bit, would it run headphones well without another gain stage before it? Or should I run the BOSOZ first with a gain of 10 (20dB) or so and then the SOZ following that with a gain of about 2? Thanks all
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