JLH Class A headphone amp from eBay...setting bias/output help needed - diyAudio
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Old 20th May 2012, 03:44 PM   #1
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Default JLH Class A headphone amp from eBay...setting bias/output help needed

JHL Class A headphone amplifier pre-amp kit diy JLH AMP | eBay

I purchased this kit for about $20 with shipping and all, just now got around to putting it together. It came without any build instructions and attempts to contact the seller have left me empty handed for answers.

As you will notice on the pictures it has a VR on each channel...i assume for setting bias or output or something...i have requested instruction on how to set this amp up regarding this VR but again, no response...

If anyone can tell me how to make the appropriate adjustments on this amp I would be greatly appreciative of your assistance.

Thanks in advance!
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Old 20th May 2012, 04:15 PM   #2
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The trimmers are likely for biasing the output transistors. Turn the board over and measure the resistance of each with your DMM. There is probably a sweet spot, but without more information I would just make them both equal in the middle of the range. If the trimmer is 100 ohms, for example, set them both to 50.

When you power up, you need to measure the current on each channel. Without a schematic, it's impossible to say where. This is often done by measuring voltage across a resistor and using Ohm's Law to get current. You will need two DMMs to balance the channels, each measuring voltage (current) on one of the channels. Once the amp has warmed up, you can start tweaking the trimmers. Usually one side will run a bit "hotter" than the other. If the output transistors are on the same heatsink, they will better share the excess heat and stabilize easier.

You want to make sure the transistors equalize at a reasonable quiescent current, certainly no more than 2A. If there is too much power applied or the bias is too high, you can get thermal runaway. You might try the Class A Website for info on this amp.

Again, a schematic would be helpful, as well as some information about your power supply.
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Old 20th May 2012, 05:17 PM   #3
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I buy kits from a few Ebay vendors and don't have the problems with schematics. These days, most arrive as attachments to the invoice and only a few dispatch with the kit. Leading sellers now post them in their full website descriptions so why not look at the sites of the bigger names, since these guys usually sell the same boards or even whole kits as everyone else. Look on the board to see who designed it, as guys like LJM are members here so ask around.
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Old 20th May 2012, 06:17 PM   #4
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The Class-A Amplifier Site - JLH Headphone Amplifiers
Believe it or not, first I found this article above and then five minutes later the seller sent me a message confirming what the article states close to the end...it is to adjust DC offset to as cose to zero as possible...

This guy at eBay seems to be a new seller, i too buy lots of these types of kits and generally have no issues receiving detailed instructions...he doesnt appear to have a website like some of the others.

Thanks again!
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Old 26th August 2012, 10:15 PM   #5
adason is offline adason  United States
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sorry to revive the old thread...
I too have bought the same headphone amp, just out of curiosity, and after assembly I am having some difficulties.
When I put 15-0-15 ac on it, it starts smoking. At closer inspection, the resistors R1 and R2 feeding the output transistor heats up too much. I used variac to power it slowly, and it can be had at 4-0-4 ac with those resistors warm. However, I do not know what else to do, there is not bias adjustment, and all the parts seems ok and in correct place, checked it twice...
Any suggestions what shuld I do to make it work? Should I use higher wattage resistor? Or lower resitance? The output transistors do not get hot, only those 5 ohm resistors. I attached schematics. Any suggestions? Thanks, ed
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Last edited by adason; 26th August 2012 at 10:25 PM.
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Old 27th August 2012, 03:27 PM   #6
adason is offline adason  United States
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anyone?
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