DC offset like crazy... OPA541- HELP! - diyAudio
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Old 19th June 2003, 11:19 PM   #1
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Default DC offset like crazy... OPA541- HELP!

OK, a friend needed an amp, so I volunteered to throw one together for him. I did so- my first GC. Worked right off, sounded ok. Chips got really hot... (REALLY) so I heatsinked, and tried again. The sound is accurate, but I kept hearing a fuzzy sound in the background. DC offset. Like no other. close to 40mV, which I am sure cannot be good for a 4-8ohm speaker.

I am using OPA541, with +/-25V@4.7A rails. How am I getting this DC? How can I fix it? Add a cap on output?

Please help....

Thanks
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Old 19th June 2003, 11:30 PM   #2
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To be honest, I wouldn't worry too much about a 40mV offset: that's pumping a mighty 0.04^2/8 = 0.2mW into an 8 ohm driver! And if you're experiencing a noise then it must be AC, not DC. It could be oscillation so check with a scope. If you don't have a scope then try a 12v car indicator lamp (the flashing turn signal) with 0.1uF in series - the bulb should light if any oscillation above about 100KHz is present.

If you're running the chip in inverting inverting mode then a resistor equal to Rf from +ve input to ground should reduce or eliminate any DC offset. Some say this hurts the sound though....

Nice one,
David.
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Old 19th June 2003, 11:37 PM   #3
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Ok *phew* i was over-reacting.

Believe it or not... nothing was added, nothing was changed, and now it sounds great. I guess the chips just needed to warm up a tad. (really warm... finger-burning hot) It actually sounds quite nice now... I am sure he will like it... cost me around $20. hehehe of course, I lose a transformer to him for this.... and I still need decent heatsinking (just a piece of 1"wide aluminum now) and an enclosure.

Well... I think I need to eat. I have a headache.
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Old 20th June 2003, 01:14 AM   #4
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[QUOTE]Originally posted by daatkins
[B

If you're running the chip in inverting inverting mode then a resistor equal to Rf from +ve input to ground should reduce or eliminate any DC offset. Some say this hurts the sound though....

I am waiting on Plitron to send the transformers to try an inverting GC. But the resistor on the non-inverting input was in my beginning circuit. Do this really mess up the sonics.

George
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Old 20th June 2003, 02:12 AM   #5
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I'm not sure what you mean George: if you're building an inverting amp then the input resistor is on the -ve input, in which case a resistor from the +ve to GND will remove any DC offset. Personally, I don't think a resistor from +ve to GND will affect the sonics, but I haven't tried this so I can't comment. Any differences will certainly be small though.

If you're building a non-inverting amp then the signal will go straight into the +ve input. There maybe a resistor to ground in this configuration but this sets the input impedance. To remove DC offset from a non-inverting amp requires a capacitor in series with the resistor from -ve input to gnd. The value of ths cap will affect the low frequency response and the quality of this part is important- cheap electrolytics are to be avoided!!

Nice one,
David.
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Old 20th June 2003, 03:10 AM   #6
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Well, now that I am not in such a panic...

I used a non-inverting cirtuit... 2.2uF input coupling cap, 22k input resistor, gain=23 (1k and 22k). 1000uF, 50V per rail, bypassed by .0022uF cap. After 2-3min. warmup (for some reason it HAS TO warm up) it sounds quite good. If you run it before it is warm, it sounds kind of fuzzy and distorted, but cleans up nicely. yellow LED, +/-25V supply@4.7A. I built it in a shoebox, to match my shoebox midbass horn speaker (which it powers to VERY loud levels, with tremendous bass-- now would you thinik that that bass came from a 3" woofer? no... but it does )

Anyway thanks for the help. If I had a digi-cam, I'd take a pic ot show my uhh, handywork, if you'd call it that.
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Old 20th June 2003, 11:08 AM   #7
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If the chip is getting that hot, you may well have some oscillation going on. Does the OPA541 have built-in thermal protection?
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Old 20th June 2003, 04:11 PM   #8
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I am pretty sure it does... It seems to work just fine when heatsinked. I don't know...
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Old 20th June 2003, 05:55 PM   #9
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Quote:
I am waiting on Plitron to send the transformers to try an inverting GC. But the resistor on the non-inverting input was in my beginning circuit. Do this really mess up the sonics.
Not in my experience Panelhead. I'm using that arrangement in my LM3875 inverting GCs and it sounds just as good as with the wire from non-inverting input to ground BUT with 0mV offset instead of around 28mV.

And let's be honest - it's no work at all to try both arrangements!
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Old 21st June 2003, 06:12 PM   #10
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Quote:
Originally posted by Nuuk



And let's be honest - it's no work at all to try both arrangements!

I am planning on only trying it with the resistor to ground from the noninverting input. My speakers have extremely fine wire in the voice coils and no crossovers.
A little dc could damage something if used long term. But I hate to think that the direct grounding may yeild better sonics and I am too chicken to try it.
But I read another post where the comment was the buffered input sounded better because it reduced the amount of ground hash feeding into the input. This is very possible. Some think it sounds worse, some the same, and others better. Sounds like you cannot go wrong no matter how it is hooked up.
I will be glad when the transformers arrive from Plitron. I also am going to try driving balanced which makes this a non-issue. The use of the DVR134 looks simple, cheap, and reportly sounds great.

George
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