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ashok 22nd January 2010 03:10 AM

JLH Headphone Amp
How about this to start it off ?

The Class-A Amplifier Site - JLH Headphone Amplifiers

ashok 24th January 2010 04:36 PM

1 Attachment(s)
I spent some time doing some simulation of the JLH headphone amp. A Word file is attached.
I tried the amp with split supplies of +/- 12 V and a single supply scheme using a single + 24 volt supply and a +12 volt supply.
All seem to function fairly well. Distortion is low and I tried comparing them producing about 100dB spl on phones with 32 ohms/ 90 dB/mW and 300 ohms/94dB/mW. The better phones have higher sensitivity than what I assumed.

I have not made one but intend trying out one. Output impedance is very low at less than 0.2 ohms except the single supply unit which has a capacitor in series with the output and causes the Zout to rise with falling frequency.

The output stage has about 100mA through it and PSRR is just about 50dB. So a low noise power supply is required. A wall wart might not be suitable I think, unless you add a capacitance multiplier after it ?

Smeggy 26th January 2010 03:30 AM

hah, I probably read the article at the time, I used to get Hi-Fi News and Record Review regularly back then. Sadly I'm still not very clued up on electronics.

burbeck 26th January 2010 03:42 AM

i use the circuit2 version (same page), great sound, classA, runs approx 100mA bias per channel.

ashok 26th January 2010 12:41 PM

I'm going to try out the single supply version with an onboard supply to which a wall wart can be connected if not a transformer.
The single supply circuit performs pretty well on paper. I can compare it with my opamp / discrete output driver headphone amp. Basically an opamp ( OPA2134 ) driving a pair of TO126 devices through some TO92 devices. It's part of my DAC and can be used by itself.
Another advantage of the single supply circuit is that the output cap cuts off all dc to the phones. So it's pretty safe if anything breaks down. However the performance of the output cap itself is to be checked. No output cap is of course the best way ...but....

I simulated a 6922 tube circuit ( at 40 volts) with MOSFET follower and it performs poorer on paper. Wondering if I should rig it up to see how it sounds.........

ashok 2nd February 2010 06:31 AM

I finally got a pcb layout done. I've been trying to make a pdf file from the .brd file but couldn't figure out how. Didn't have much time to Google and check that. However I just managed to make a .tif file. Let me see how I can attach that.
I've made some slighty modifications to the original JLH circuit. It looks fine on simulation and stable also. I'll post that soon.
Does anyone know the inductance of a typical 32 ohm and 300/600 ohm headphone phone ? Couldn't find that data on several web sites.

Would like to know if anyone made a stock JLH single supply headphone amp earlier ? If so any comments ?

Now I've got to get back to work !


ashok 2nd February 2010 12:25 PM

Here is the modified circuit
1 Attachment(s)
I've attached it as a word file.
Note that the + ve going peak will always be about 2.6 volts less than the supply voltage. The negative swing goes almost down to 0 Volts. So with 12 volts you can get a maximum of 9.4 volts peak to peak. This is only possible if the output dc voltage is not at 1/2 Vsupply but slightly less than + 5 volts.

As shown you can get peak levels of about 110 dB which can be damaging to the ear . With more sensitive headphones it will be even higher. Note that a 45 mA quiescent current should be fine for most listening purposes. The transistors will need a small heat sink to keep them reasonably cool. With a 110 deg/C per watt rating in free air it means a 30 degC temp rise . Not Ok here with ambient as high as 35 deg C in summer !

I found an error in the pcb layout. Will have to include two resistors and capacitors that I missed out. That's going to delay things some more. Meantime I found my old Tube/MOSFET hybrid phones amp. It certainly will have higher distortion, but it will be interesting to see how it compares.

kenpeter 2nd February 2010 04:42 PM

1 Attachment(s)
I know where you can get six of those cheap...

Add a PNP and collector pulldown at the input,
to keep that first Schottky forward biased...

kenpeter 2nd February 2010 09:03 PM

2 Attachment(s)
74S04 might work better (more current) than LS?
LS is actually based upon older DTL style logic.

But S has got that strange TTL base - collector
junction thats sometimes forward biased. Didn't
know how transistor sim models might handle...

You see the JLH now?
There's a lot of Schottkys doing nothing in this
analog application. I left them for reality check...

ashok 3rd February 2010 04:35 PM

Hi Ken,
That looks interesting. Just goes to show that 'most' circuit configurations have already been tried long ago.
I've just corrected more errors on my board design. Added four smd parts as I could not fit them otherwise. Forgot about them when I started and I don't plan to move everything around and spend a lot of time on it. The amp will still work without them.
I like big pads and fairly wide tracks because they don't peel off when experimenting and soldering and desoldering several times.
Hope to get something ready by the weekend. It's a very busy month and so I cannot say when it will really get done.

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