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transconductance amp

kevin gilmore

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11 hours ago, FallenAngel said:

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.


As far as I know inputs should be shorted when setting bias, offset. or connected to something that has no DC on it-  not floating.

I connect a 10kR across the in + & -    then can hang scope probes and signal on them   


true current source amps need a load, so when you test, add the jumper for higher current gain and use 50 ohm load.

to check a BJT ... one way is to measure voltage--- base to emitter---VBE

Edited by congo5
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1 hour ago, congo5 said:

I know it's marked and easy to populate

It only takes one thing to send it into a bad place

one part or connection can cause voltages to go crazy all over the place

Do not use this for preamp duty!

don't ask me how i know

I do wonder if it could be used for the CAST Inputs>>>>

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On 26/10/2016 at 3:01 PM, mwl168 said:

To second congo5's point, the input should be shorted when you test and adjust DC offset. This is something I often forget to do too. 

I've read that part a couple of times and it confuses me completely. What do you mean "adjust DC offset"? There are no trimpots on this amp. Where does adjustment come in?

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  • 1 month later...


first post, long time reader, please don't be harsh with me, I read the forum rules a couple of times.

I have Stax SR-2170 system for two years, very happy with it.

However, I received an Audeze LCD-2 as christmas present (god bless christmas!), but...I don't have a proper amp for them.

I thought this is a great opportunity to start my first DIY project (this transconductance amplifier).

But, I have a few (probably dumb) doubts about the design.

Kevin said there's no way to make a balanced/bridged transconductance amp, but looking at some commercially available products, I've found this:

Bakoon AMP-51R speaker amp: ("Maximum power output: 100 watts in stereo, 250 watts in mono")


Audio GD NFB-1 headphone amp (fully balanced, class A, with 4 pin xlr headphone output)


I mean, is something wrong with those transconductance amplifiers or a design of this headphone amp with a 4 pin XLR output for the headphones can be done?


Thanks for your attention, and please forgive my ignorance.

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5 hours ago, kevin gilmore said:

for sure the audio-gd is NOT a transconductance amp. I have one. diamond buffer output.

and If the bakoon is bridgeable for double the power then it cannot be a transconductance amp.

Thanks Kevin!!!

Mentioned the Audio GD stuff because I read this some days ago:


Kingwa mentions he uses the Current Conveyor Technology on his designs, like Krell and also Bakoon ("SATRI" circuit mentioned), my doubts were generated because of that.



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3 minutes ago, kevin gilmore said:

current mirrors used as voltage gain stages is very different from high power versions used as output stage.

as far as I know bakoon is one of the very few that make transconductance headphone amps.  It really works well only with very flat impedance headphones like ortho's.

(Read with Narco's character voice):

Gracias Patrón!

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  • 2 years later...

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