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Old 11th December 2005, 09:27 PM   #1
Katapum is offline Katapum  Europe
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Default Output stage instability

Hi all,


I built a new DAC output stage. It has sockets to fit different opamps. So with OPA604AP output stage works ok but with LM6181 and AD811 I listened a big bzzzzzzzzzzzz and they heat a lot.

Any comments or suggestions are very welcome (and sorry my English).

Katapum
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Old 11th December 2005, 09:31 PM   #2
Katapum is offline Katapum  Europe
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A picture


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Old 11th December 2005, 10:50 PM   #3
jcx is online now jcx  United States
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circuit looks ok, but needs high speed prototyping, also the .1 uf may be too big, for >>10 MHz I'd use 10nF or even 1nF to keep srf freq high

maybe you could use a socket on a clearance-hole ground plane perf board but for best performance with really fast op amps "dead bug" technique works best

you need Linear Technology an47 "High Speed Amplifier Techniques" - its really big, long download time

search linear.com for: high speed amplifier techniques

"dead bug" proto technique - start with a bare copper clad pcb:
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Old 11th December 2005, 11:53 PM   #4
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Hi

I would try a 10pf to 22pf cap across the Rf resistor.

If you have a CRO check for any instabliity on the O/P.
Also 47R on the o/p may need increasing to 100 Ohms
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Old 12th December 2005, 04:54 AM   #5
jcx is online now jcx  United States
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Quote:
Originally posted by The Saint

I would try a 10pf to 22pf cap across the Rf resistor.
...
that would be wrong with the decomp high speed amp and the current feedback amp, it would guarantee oscillation


passive i/v is going to be noisey, using ultra low noise op amps like AD797 or LT1028 would be better than the cheap LP record noise levels you'll get with the opa604

any dac i/v stage needs filtering, non-OS designs especially, sin(x)/x droop needs eq and images are bad because they generate imd that folds down into the audio band, the starting place here is paralleling the i/v resistor with a Cap, then follow up with 2nd order filter stages aligned to create a odd order filter with the correct frequency response

a Cap in parallel with the i/v R also soaks up dac glitch energy that could generate distortion in th op amp input - esp if you go to bipolar input for lower noise
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Old 12th December 2005, 05:47 AM   #6
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sorry didn't check to see what type of opamp it was
Perfectly right of cause this would cause oscillation
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Old 12th December 2005, 07:15 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
I echo previous poster, add a 10nF ceramic directly across pin4 to pin7.
On a slightly different tack.
What voltage have you got before the regs?
32Vac secondaries are very high for 7815 & 7915.
A split 2*16Vac would be OK.

What is it feeding? The 470nF output cap will become a bit small into low impedance loads.
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Old 12th December 2005, 08:06 AM   #8
boholm is offline boholm  Denmark
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A 47-100 ohms on the output of the opamp may also help.
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Old 12th December 2005, 09:21 PM   #9
Katapum is offline Katapum  Europe
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Hi,

Quote:
I echo previous poster, add a 10nF ceramic directly across pin4 to pin7.
Before i wrote post #1 i tried it with a 100nF MKT and didn´t work.

Quote:
32Vac secondaries are very high for 7815 & 7915. A split 2*16Vac would be OK.
i only have a 2x32Vac transformer (no centred tap).

Quote:
passive i/v is going to be noisey, using ultra low noise op amps like AD797 or LT1028 would be better than the cheap LP record noise levels you'll get with the opa604
What’s the “best” opamp to my dac output stage?

(I only have in my place: OPA604, LM6181, AD811).


So the new schematic will be:


Is it ok?


Katapum
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Old 12th December 2005, 10:06 PM   #10
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Katapum, you are comparing voltage-feedback op-amps with current-feedback op-amps, and exchanging them on the same circuit. They are totally different.
I would use a series input resistor (820R would do), BUT you are using 820R for feedback on the LM6181, with a 52.25x gain, and National recommends lowering that resistor to 500R for higher gains (~25x and up).

Also, some op-amps don't really like high dc-offset on their inputs.
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