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Old 12th July 2009, 07:45 PM   #1
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Unhappy JLH Headphone amp

Hi all,

I'm building a JLH headphone amp, circuit1 in this article-

http://www.tcaas.btinternet.co.uk/jlhphones.htm)
I'm wondering if anyone here has any experience with it.

So far one channel is built but not working . I'm pretty sure that the circuit is ok. I've substituted BD139 and BC547/557 transistors.

Power supply is from a 24V DC unregulated wal wart to a standard LM317 regulator circuit. This is tested and working fine, giving 24V DC out.

Now this is where I think the problem is and I'd appreciate some advice. I used a circuit snippet from the Chu Moy article at headwize (fig 3).

http://headwize.com/projects/showfile.php?file=cmoy2_prj.htm] http://headwize.com/projects/showfil...=cmoy2_prj.htm[/URL]

This is to give a split supply, +/-12V in this case. Unloaded it looks fine but hooked up to the amp I get something like 13.5V on one side and 9.5V on the other.

I think the impedences are interacting and throwing the voltages out of kilter. Any workarounds for this or else I need to start again with the power supply.

Also- the diagram shows no cap on the output- won't this put DC on the headphones?!
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Old 12th July 2009, 09:40 PM   #2
Nisbeth is offline Nisbeth  Denmark
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I think the JLH-amp draws more power than the voltage splitter can supply, hence the imbalance - in this case 2x12V is not the same as 24V. You need a "proper" split supply with a dual-wound or centre-tapped transformer to make it work. If you only have a single transformer (note: must be AC) you can use a voltage doubler circuit (example here: http://sound.westhost.com/project05.htm)

/U.
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Old 12th July 2009, 10:35 PM   #3
jaycee is offline jaycee  United Kingdom
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As said above, it's not going to work with a simple potential divider to power it, especially as it's class A. You need to make a split supply using LM317 and LM337.

You'd also want a wallward with a fair bit of current. It's probably going to be better to use a transformer. A 30VA unit should do. Get a 2x15VAC one and that should work great.
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Old 13th July 2009, 07:37 PM   #4
jaycee is offline jaycee  United Kingdom
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Since as you have no email button, im curious, where about in Norwich are you? You can e-mail me privately if you wish. It'd be nice to know someone local with the DIY passion too
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Old 13th July 2009, 11:14 PM   #5
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Hi,

thanks for the responses. I was hoping not to have to build a "complex" power supply, partly for ease and partly for lack of parts and a small budget. Guess I'll have to have a rethink!
I'd like to hear any comments on the performance of the circuit if anyone has anything to say on that too.
Jaycee, I'll email you.
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