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Old 18th February 2003, 09:10 AM   #1
pundip is offline pundip  Australia
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Default opa541

I have have a few OPA541 opa amps in the T03 cases. Looks like a pretty powerfull amp from the datasheets. I am thinking of making a cheap and nasty amp from these. Has anyone constructed an audio amp based on opa541 and does anyone have any schematics etc on how to construct and audio amp with the opa541. Any advice would be helpfull
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Old 2nd March 2003, 06:57 AM   #2
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Default a big problem

Well... I didn't notice that there was a thread on OPA541. Unfortunately, the second post in this thread, which is this one, is not an answer but a problem.
I tried OPA541 (in TO3 package) as an inverting amplifier following the circuit shown below. I replaced 10k with 22k in order to decrease gain, and I didn't use a volume pot. Input was directly connected to 2.2uF capacitor. Power suppply was +/- 25V. The current sense and output drive (pin 1 and 8, respectively) were tied together to give maximum current limit.
Now, here is a big problem. When the input signal is low, I can hear (low) sounds from speaker. But, when the input signal level goes over a certain level, the output shows "spikes" of DC voltage close to 25V. Could anybody explain why this is happening? Thanks.

Circuit
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Old 2nd March 2003, 07:25 AM   #3
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Hi Jazz2250!

As you have defined a closed loop gain of 10...are you sure that the OPA541 is stable in a gain of 10 configuration?

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Jorge Santos
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Old 2nd March 2003, 11:18 AM   #4
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Default Re: a big problem

Quote:
Originally posted by JAZZ2250
Well... I didn't notice that there was a thread on OPA541. Unfortunately, the second post in this thread, which is this one, is not an answer but a problem.
I tried OPA541 (in TO3 package) as an inverting amplifier following the circuit shown below. I replaced 10k with 22k in order to decrease gain, and I didn't use a volume pot. Input was directly connected to 2.2uF capacitor. Power suppply was +/- 25V. The current sense and output drive (pin 1 and 8, respectively) were tied together to give maximum current limit.
Now, here is a big problem. When the input signal is low, I can hear (low) sounds from speaker. But, when the input signal level goes over a certain level, the output shows "spikes" of DC voltage close to 25V. Could anybody explain why this is happening?
I was having a look at the data sheet, and I think you have to put a resistor between pin 8 and pin 1 before joining them together. Something like 0.1/3watt or so. Have you tried that and see if that doesn't solve the problem?

Something I didn't know about this chip is that it has a FET input. On the other side it's very expensive.

The TO3 case is isolated, so you can heatsink it without any insulator in between.


Carlos
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Old 2nd March 2003, 05:49 PM   #5
PMA is offline PMA  Europe
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OPA541 is not the best solution for hi-end amp. You should not use higher gain than 2x - 4x. The circuit has low GBW (1 MHz) and there is not enough feedback for higher frequencies, this resulting in rapidly rising distortion at higher frequencies.
I would recommend this circuit as a car or basement amplifier, no hi-fi.
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Old 2nd March 2003, 06:08 PM   #6
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Default OPA 541

HI Jazz2250!

Have you put the recomended zobel network betwen output and ground!

Aka 5,6 Ohm - 0,068 uF

Regards

Jorge Santos
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