JLH problems

[serosmaness]

Hello everyone. I am wondering if one of my regulators are bad?
I have + 22v and -22v untill I connect the power supply to the amp. When I turn the amp on I still have +22v but the neg goes to 0v and then goes to -5v when I switch the amp off.
I have been checking the amp board for the last 3 days. I can't find any faults.
Thanks Steve

 

[Panelhead]

Hello everyone. I am wondering if one of my regulators are bad?
I have + 22v and -22v untill I connect the power supply to the amp. When I turn the amp on I still have +22v but the neg goes to 0v and then goes to -5v when I switch the amp off.
I have been checking the amp board for the last 3 days. I can't find any faults.
Thanks Steve

Sounds like the neg rail reg is shutting down. Try checking the impedance to ground from the power input with the power supply disconnected. If something is shorted it should show up hitting it with a VOM.

George

 

[serosmaness]

I checked for a short on the amp board without the ps connected between the Neg and ground and it showed open. With the PS connected it showed around 3K .
Thanks for the reply.
Steve

 

[Geoff]

Hi Steve

No answer for you as yet (too little information on which to work) but a few questions.

What regulators are you using and in which circuit arangement?

Which version of the JLH have you built and what is your intended quiescent current?

Have you set the quiescent current control preset to give the minimum Iq prior to conecting the power supply?

Have you tested the power supply using a dummy (resistive) load?

Geoff

 

[serosmaness]

Hi . I am using the circuit on page 4 fig 2 The Final Circuit without the feedback cap C-4. I am using the lm338t regs. I have the iq set low. I read only .114v acrossR10
I found Q.2 shorted to the heatsink via the screw. Now I have +22v and Neg 22v but the - side drops out after a few seconds but when I have the -22 I have 7v at the speaker output.When the -22 drops out I get 22 volts out.



Thanks Geoff

 

[serosmaness]

I have it working so far. I will set the quiescent current at 2 amps.
Serosmaness.

 

[Geoff]

I have it working so far. I will set the quiescent current at 2 amps.

Steve

Glad to hear the good news. I was just about to reply to the post you made yesterday, but now you have saved me the trouble.

As a matter of interest, what was the cause of the problem?

Geoff

 

[serosmaness]

Thanks Geoff. I eliminated the 7 volts with the dc bias pots.
The regulator was dropping out because I had the Iq set to high and I had not put a heatsink on the regulator.

Is .115V drop across r10 .33ohm enough Iq. Does this equate to a Iq of 3.4 amps?
Thanks again.
Steve

 

[Geoff]

Is .115V drop across r10 .33ohm enough Iq. Does this equate to a Iq of 3.4 amps?

0.115V across R10 would give an Iq of 0.348A which is far less than the 2A you are seeking.

 

[serosmaness]

Thank you. I thought that I was off by a decimal point so I asked.
I think that I will need a much bigger heatsink since the one that I am using gets warm with .34a Iq.
Thanks
steve

 

[chris ma]

Size matter

Hi,

If you are aiming for 2A the heat sink should not be less than 2 pound in weight per output transistor in order to keep the heat sink below 60 degree C at room temperature of 25C without using a fan.. do over kill here and good luck with your project...

Regards,
Chris

 

[serosmaness]

Thanks for the info.
If I loose a rail I will end up with 22v at the speaker?
Thanks again.
Serosmaness