Help with opamp test jig

I'm finished with some melcors and wanted to test them before I install them in any circuits. I'm also interested in experimenting with optimizing the components for least noise and better performance. What would be the obvious and not so obvious tests to run and have incorporated into the jig?

thanks

Mike

Perhaps the best way is to drop them in a mike-amp board, slightly modified so you can read (and limit) power supply current and easily change resistors.

Optimizing noise is VERY much about the specific circuit it will be used in. I'm not familiar with the Melcor, or what it is good for. But some audio amps are made for low noise power at fairly high (stepped-up) impedances, others are made for lowest noise voltage from existing low-Z sources (dynamic mikes, mix-buses).

Hi PRR,
thanks for the tip. Is it important to measure the DC leakage current on the output of the opamp if it's going to drive a transformer? Would it be wise to use a resistor in place of the tranny to start with?

mike

There are four ways to couple the Melcor to the x-former. One is direct coupled, one is cap coupled, and one is cap coupled with a resistor in paralell with the cap and the last is just a resistor. I tried all of these and found no sonic diferences so I would go for cap coupled as this will protect the x-former from dc current if you have an offset voltage at the output.

Experiment. The output iron is M6 which is more likely to be unaffected by dc.

I would mess with the feedback resistor/cap for low noise and amp gain.
That's the 47k/47 pf network that goes from input to output.

Thanks CJ,

I'll try the coupling cap. Question though, if I wanted to measure the DC offset and the current that would be traveling through the transformer, but didn't want to measure with the transformer connected, is it as simple as finding a resistor value that matches the primary of the output transformer and measureing the current running through it?

thanks again,

Mike

from Mike:

Question though, if I wanted to measure the DC offset and the current that would be traveling through the transformer, but didn't want to measure with the transformer connected, is it as simple as finding a resistor value that matches the primary of the output transformer and measureing the current running through it?

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For DC that would mimic the situation OK I think.
But I expect that the DC-offset (the voltage) wouldn't change that much if you don't have a spot-on valued resistor.

So the DC-voltage will more or less stay the same and by varying the resistor value (within range) you're just scaling the current (in an Ohms law like fashion :wink: )

As you already will know, note that 'DC-operating points' can be different (shifting) when the amp is oscillating. So connect a scope as well to verify for eventual oscillations.

BTW, what supply voltages (typ. & max.) are you all using with the Melcor 1731 ?

Bye,

Peter

HI clintrubber,

I'm going to start at +-18V for the rails and I'm going to see how far I can take it before something goes poof. Seriously though, I'm going to use Fabios PSU board, which is already built and allows me to adjust the rails. I hoping to find a point at which the opamp sounds it's best. Can I damage the output tranny by experimenting in this way? I don't mind making more melcors, but I do mind frying trannies. I did my best to match transistors and hopefully, I get good results. I won't have time to do any testing until later next week, but in the mean time, I'm hoping to put together a test jig that will hopefully help us all.

thanks,

mike

from Mike:

Can I damage the output tranny by experimenting in this way?

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Especially for testing, the resistor you mentioned in place of the TX is a good idea. And use that cap that got mentioned. If after all debugging it all seems OK then replace the resitor by the TX but keep the cap. If that still looks OK & remains OK and the VDC is low (say no more than 100mV, but I don't know these Melcors so far) then you might remove the cap but might also keep it - as mentioned, for safety.

but in the mean time, I'm hoping to put together a test jig that will hopefully help us all.

Click to expand...

Thanks for the info & good luck !

Bye,

Peter