[Tedro]

I am working on completing a PPAv2 and I have run into a problem. I get the OP Amps powered up and the buffers powered up but when I play anything into them the output is distorted and is very low, like I am over driving the AMP but not putting out enough power. I have been playing around with it and did notice that when I disconnect one of the power rails from the Buffer, the sound comes out loud and clear, like normal. I have built similar buffered headphone amps before but this is my first time using the BD139/BD140 output combo and I am disappointed in myself that I am failing at this device since it is very popular. I have great respect for all of you builders out their and would like to know what to look for as in problems are test that I can do to find out what may be the problem. Thanks in advanced.

-Ted

Fake Edit: What is the minimum voltage that I can test the buffers at before I apply the full voltage to them, what would be the maximum as well?

 

[tangent]

Quote:

Originally Posted by Tedro I get the OP Amps powered up and the buffers powered up

You've set the bias point, then? What to?

Quote:

What is the minimum voltage that I can test the buffers at

Oh, 6V, perhaps?

But that's really chasing the wrong thing. If you've built it wrong such that it will destroy itself with 30V, it'll probably destroy itself with 6V, too. This isn't one of those projects where testing at a lower voltage and ramping it up to the target voltage makes much of a difference.

Quote:

what would be the maximum as well?

I'd imagine that the op-amp's supply voltage limits kick in before the buffer's.

 

[Tedro]

I really appreciate your swift response.

Quote:

Originally Posted by tangent You've set the bias point, then? What to?

Well I believe it is around 30mA but it I adjust it up or down it still does not make a difference. I should be measuring the output (in mA) of the drain on Q4 for the bias current, right? I will check to make sure what I am getting across the output resistor as well. Would lowering the value to the output resister decrease the output distortion?

Quote:

Originally Posted by tangent Oh, 6V, perhaps? But that's really chasing the wrong thing. If you've built it wrong such that it will destroy itself with 30V, it'll probably destroy itself with 6V, too. This isn't one of those projects where testing at a lower voltage and ramping it up to the target voltage makes much of a difference. I'd imagine that the op-amp's supply voltage limits kick in before the buffer's.

I understand what you are saying but for the sake of testing I was only using a +/-12 power supply before I start playing with it at +/-24 volts that I am overbuilding for this amp.

-Ted

 

[jar]

Quote:

Originally Posted by Tedro I understand what you are saying but for the sake of testing I was only using a +/-12 power supply before I start playing with it at +/-24 volts that I am overbuilding for this amp.

What Opamps are you using that can tolerate 48 volts? The only one I know of that is decent for audio is the OPA604. Why do you think you need such a high supply voltage?

 

[Tedro]

Update:

I found the problem and I am embarrassed to say that I had my output transistors rotated the wrong way. I corrected the problem and everything is bronze. Another problem though, how do I fix or what do I need to look for if I have an high DC offset? If I measure the ground and the output of the buffers should give me the dc offset right?

_ted