Can a DOA be tested by itself?

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Ptownkid

Well-known member
Joined
Feb 1, 2005
Messages
4,256
Location
Ajax, Ontario, Canada
Can you test a DOA with nothing more than a power supply, function generator and a scope?

I was under the impression that you could feed an opamp a sine wave and see it at the output assuming you were powering the opamp.

Am i out to lunch, or is my SGA just not working right?
 
Minimum circuit would be a buffer: tie output to the inverting input. Number of resistors involved: 0.
(Assuming this opamp is unity-gain stable and IIRIC it is)

OK, let's add one resistor between signal ground & non-inverting input to keep the DC-operating point OK when the signal source is removed.

Bye,

Peter
 
[quote author="Ptownkid"]ok, you two have me confused[/quote]
No we didn't :cool:

What's unclear ?

The minimum circuit is the non-inverting buffer:
* use sym. supply
* tie output to inverting input (short these two connections so to say)
* put a resistor (say 10k) between ground and non-inverting input
* measure the DC-Voltage on the output w.r.t. ground
* if that DC-voltage is less than say a few tenths of mV say "operating point seems fine" and "let's now proceed"
* apply testsignal from signal source or whatever to the non-inverting input (keep the 10k)
* look/listen at the output signal (should have same level as input signal)

Bye,

Peter
 
[quote author="Ptownkid"]or should I do this...

http://www.bcae1.com/opamp.htm[/quote]
That's the one :thumb:

opamp2.gif


The inverting config. is about as simple, up to you which one you'd like to try first.

Enjoy,

Peter
 
Ok, let's get one step further.

How can we thoroughly test an DOA?

For example, I've some (30 :shock: ) 990 kits comming my way. I've also bought 2 John Hardy 990's for comparison.
Is there a simple test set up to see whether my DIY 990's are in the same league as the John Hardy ones?
 
[quote author="radiance"]Ok, let's get one step further.

How can we thoroughly test an DOA?

For example, I've some (30 :shock: ) 990 kits comming my way. I've also bought 2 John Hardy 990's for comparison.
Is there a simple test set up to see whether my DIY 990's are in the same league as the John Hardy ones?[/quote]
That's a nice move, getting two JH's for comparison !

Maybe you should define the goal you're aiming at.
They obviously should be biased fine, don't overheat, realize adequate DC- & AC-specs etc, but I'm not sure how many 'special details' are considered during assembly & part selection. And of course also dunno how much a present day JH-990 differs from the original DJ-incarnations of the 990.

The various remarks & details in this thread got me thinking about the very same thing:
http://www.groupdiy.com/index.php?topic=8888&highlight=990c
... as in: OK, fine, we have those kits now, but how close will we be getting ? We might be missing out on those inside-info tweaks etc, but I'd want to add right away that we'd probably better not worry too much about this. If we want the official 990's let's buy these then. Life's too short to worry whether our DIY's will be exactly identical. I mean, I might have gone great lengths to accomplish that but won't longer do so; there's simply too much of that stuff going on that can get you sidetracked. But OK, that was about getting or not a perfect 'clone'. I don't know how far you would want to go. You stated 'simple test'. I wouldn't think a simple test would be able to reveal the subtle differences. Or who know you might just compare by listening. Or by comparing in the way Wayne recently described when swapping out opamps in a desk (IIRIC it was checking/listening for the residue when two channels are subtracted)

About getting a nice DOA whether it's a 990 or not is another topic though. I recall a thread that Samuel has started about an opamp-test-jig. You might find more details there.
And there are various articles around about opamp testing.

Enjoy,

Peter
 
[quote author="clintrubber"]. Or by comparing in the way Wayne recently described when swapping out opamps in a desk (IIRIC it was checking/listening for the residue when two channels are subtracted)
[/quote]

Yeah, that sounds like a good idea. First compare the two John Hardy ones....they might differ a bit, who knows. Then just swap a Hardy out and a DIY 990 in.

Also, I don't want them to be exacly the same (although it would be nice :grin: ), just in the same league. I mean, we're making/buying DOA's because they're supposed to be better (sounding) than the IC equivalent right? When I've finifhed a 990 I just want to test it to see wheither this is thru or not.
And by simple test I mean, a test with a osciloscope,signal generator, multimeter and a DAW with decent (rme adi pro) converters but NOT an Audio precision Thingy.......

I'll do a search for that opamp-test jig and keep an eye on this thread :cool:
 
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