741 op.amp works, tl 071 is oscillating? - diyAudio
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Old 7th February 2009, 11:11 PM   #1
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Default 741 op.amp works, tl 071 is oscillating?

What is wrong with this schematic, I found from some"Pakel ProAudioKit electronic"
I have made this amplifier and with the original Tl 071 I have oscillating problems which depends on position of trim. pot. for bias adjusting, but oscillation and distorsion are also present at any position.
With 741 this amp. working without oscillating ear test, without measurements.
I can post the original PCB of this amp. if it's needed.
Any suggestions are welcome.

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Here is my case :

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Old 8th February 2009, 02:09 AM   #2
jaycee is offline jaycee  United Kingdom
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Put 100nF decoupling capacitors to ground on the opamps supply pins, as close to the opamp as you can.
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Old 8th February 2009, 02:36 AM   #3
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Quote:
Originally posted by jaycee
Put 100nF decoupling capacitors to ground on the opamps supply pins, as close to the op. amp as you can.
Thank you for this advice, I will try that, and back here with the result, I really wish to see this amp. working with TL 071 cos 741 is to poor for the highs.
PS: Just my thought: Could the heat sinks on BF 469/470 I putted is big and perhaps too close to the Power transistors MJ 15003/15004?

Here is the printed circuit board

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Old 8th February 2009, 06:16 AM   #4
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I tried with 0.1 mfd caps connected from pin 4 and pin 7 of TL071 to ground , but this has no effect.
When the trim .pot is at min. bias , min. current of the power tr. then the oscillations are the strongest, when is close at the max. position osc. are quite gone , but than the bias is too high and heat of the power tr. are too high, but still I feel , hear that something is wrong with this schematic, according to the sound, very un sure and un stable.
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Old 8th February 2009, 08:02 AM   #5
Mooly is online now Mooly  United Kingdom
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Don't blame the opamp, blame the circuit design. Sorry I know you don't want to hear this but these type of designs are disaster stability wise. The low slew rate of the 741 is part of the sollution to it being stable.
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Old 8th February 2009, 08:12 AM   #6
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The leads to the outputs and drivers look like they may be a problem. I would add a 47R base resistor to each of the drivers and a 4R7 to the base of each output device.

I would add a 100R on the output of the opamp where it connects to the node of R6 and R7. I would add about 10pF from Pin 2 to pin 6 of the opamp.
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Old 8th February 2009, 08:51 AM   #7
jaycee is offline jaycee  United Kingdom
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Really, we need the parts list as well, what other transistors are used etc. We need the whole picture. djk's advice is good though.

Mooly - not neccesarily. Look at ACD's "Lynx" power amp - http://www.audio-circuit.dk/lynx/LYNX-v3-0-schem.pdf
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Old 8th February 2009, 09:01 AM   #8
Mooly is online now Mooly  United Kingdom
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Pins 2 and 3
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Old 8th February 2009, 09:28 AM   #9
djk is offline djk
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"Pins 2 and 3"

No, I was looking at the RCA handbook for their 100W and 300W amplifier using this same basic circuit, and they use a small cap (7pF~12pF) from pin 2 to pin 6, and the resistor on the output of the opamp (an FET type).

"Really, we need the parts list as well"

http://www.servimg.com/image_preview...=61&u=12272940
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Old 8th February 2009, 11:25 AM   #10
jaycee is offline jaycee  United Kingdom
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Between pins 2 and 6 will limit the feeback of the opamp at high frequency, thus stabilising it

Another question for zoranaudio, I presume you have T5 mounted to the main heatsink for thermal compensation?
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