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transconductance amp


kevin gilmore
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  • 1 month later...
4 hours ago, mdr30 said:

Is the amp sensitive to pulling the headphone plug while playing?

not for me.

you should check your socket with a meter and see if you can get it to short while inserting/removing plug

Thanks for the correction! was thinking of the current feedback amp

Edited by congo5
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  • 4 weeks later...
  • 1 month later...

So, I think I screwed up somewhere, but would like to ask advice before ripping things apart :)

I built two of these with sligthly different gain settings. Both sound brilliant with HE500, but when trying out higher ohm cans on them (HD800, HD650), it's like the whole tone curve tilts a little bit, bass goes up, highs drop.

It's the same effect on both amps, so my question is, did I systematically screw both up  (created a filter effect somewhere due to erratic grounding perhaps?) or is this is to be expected for current drive of higher ohm cans? 

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  • 3 months later...

Just completed this amp running from GRLV at +/- 20v, powered it up and see 5mV DC offset on one board and 2.7v on the other :( 

Where should I be looking to try to debug it?

I measured voltages across a bunch of the resistors and they certainly don't look similar between the two boards. I double checked the output transistors and they look to be installed properly. The resistors also look to be the same on both boards.

IMG_20161025_190043.jpg

Edited by FallenAngel
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10 minutes ago, Pars said:

Check all of the transistors. Include the SMT ones on the bottom. Something is out of place. Or blown.

Double checked every transistor, all are correctly placed and none get warm.

I don't know where to start replacing them and from experience, TO92 are a pain to remove and I've damaged boards before trying.

Any suggestions on where to start or which are likeliest to fail?

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8 minutes ago, Pars said:

TO92s are easier than hell to remove. Heat all 3 pins up at once. Ohm them out.

Did you verify all transistors with the low offset, presumed working board? Sure you don't have a PNP or NPN swapped or wrong?

I have not found that to be the case and have a few boards in the garbage to show for it.

I tested both board and one works perfectly. Unless I am really blind, a possible scenario, the transistor are correctly placed.

It troubled me that redoing the soldering raised the offset a little, making me think something really did burn up before.

I'm just uncertain of which to replace. I should have extras around, I think.

Any suggestions of where to start? I think smc as those are still on my table.

Thanks by the way.

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I can't tell from the pictures so gonna ask.

are the inputs shorted or connected when testing?

is there a 50R or so load connected to the outputs?

what is the voltage on pin6 of servo, good one and bad?

did you measure voltage base to emitter of all transistors?

its all bipolar so they should all be around .5v that will tell you if they are working.

and since the board is marked its easy and its low enough voltage to be safe.

still be careful not to short any pins.....

good luck, its probably not much wrong.

 

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22 minutes ago, congo5 said:

I can't tell from the pictures so gonna ask.

are the inputs shorted or connected when testing?

is there a 50R or so load connected to the outputs?

what is the voltage on pin6 of servo, good one and bad?

did you measure voltage base to emitter of all transistors?

its all bipolar so they should all be around .5v that will tell you if they are working.

and since the board is marked its easy and its low enough voltage to be safe.

still be careful not to short any pins.....

good luck, its probably not much wrong.

 

Inputs are not shorted, nothing connected.

No load on putout while testing

+9V good, -17V bad pin6 to ground on opamp

Not yet measured voltage base/emitter

I don't know what the .5v refers to

I know it's marked and easy to populate which is why I'm surprised I messed up somewhere. :(

 

Update: I replaced the 4 SMC transistors, no change.

 

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1 hour ago, kevin gilmore said:

flipping the power supply is likely to cause many bad parts. especially the opamp.

Yeah, I kind of figured that. :(

Not sure why the "good" board all of a sudden got offset issues though.

I personally really hate having to fix boards with major issues so I'll probably rebuilds a new set while salvaging the parts that I could.

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