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Old 22nd March 2007, 10:19 PM   #1
Dusk is offline Dusk  United States
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Smile Opamp/mosfet power amplifier

Hello Quasi or anyone who can help

This about that opamp/mosfet power amplifier you turned me on to....for use as a guitar amplifier.....it's at post12 and#17......that is i did all the changes you pointed out in post#17 but i used a LM741CN instead of the TL071 the 741 worked........i could not find the TL071 around here....anyway i wanted to upgrade to a NE5534 it has a faster slew rate than the 741....anyway when i put in the NE5534 and tried to bias the amplifier like you said with a 100 ohm resistor in each supply line i used a meter across the 100 ohm it read 45 volts.......i tried to set the P 1 trim pot to 10 volts but it.....45volts did not decrease.......please tell me what iam doing wrong orwhat do ineed to change in the circuit.....componet that is........i put the 741 back and bias it to 100ma with no problem....please tell me what i need to ajust within this circuit...the post is: op-amp based mosfet power amplifier for guitar rig

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Old 25th March 2007, 04:18 PM   #2
Albertb is offline Albertb  United Kingdom
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You almost certainly have chronic oscillation. This would be because of the change from the 741, which is very restricted in its bandwidth, to the NE5532, which is a much better device but needs special compensation to specifically prevent this problem. It is so much better in demanding applications that it is almost too good for a guitar preamp, which is a very undemanding task, (in the strictest electronic sense not the tonal one).

I would advise that, if someone who knows has told you to use a TL071 then get one. They are a direct replacement for your 741 and perform much better all round plus, they are cheap and plentiful. The TL072 is a dual version and the TL074 is a quad version of the same opamp. Maybe these would be easier to find locally?

I don't know where you are in the world but I can't imagine you would have a problem getting them. Just look on eBay and you may find a few. You could also try online suppliers like RS or Farnell who would have them but the postage adds on to the price unless you are making a bigger order.

If they had your address by PM, maybe someone you know here could post a couple on to you.
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Old 26th March 2007, 04:45 AM   #3
unclejed613 is offline unclejed613  United States
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it is probably oscillation, most likely caused partly by the use of a device with such a high slew rate. the tl071 also has a higher slew rate than a 741, but i don't think it's as high as a 5534. the slew rate can be controlled by adding compensation capacitance on pin 5. the higher the capacitance, the slower the slew rate (a data sheet spoof once referred to it as "slow rate").
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Old 26th March 2007, 10:55 AM   #4
quasi is offline quasi  Australia
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Hey Dusk, long time no read.

Yes you should plug in a TL071 in as suggested. I would have thought this op-amp would have been good enough for a guitar amp.

Anyway back to your issue. When you put the TL071 in you will have to re-adjust the bias.

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Old 27th March 2007, 10:44 PM   #5
Dusk is offline Dusk  United States
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Smile op amp based guitar power amplifier

Hello Quasi and all who replied

Yes, Quasi it has been some time.........i will break down ane order some tl071......but i think they have the same slew rate as the NE5534? This amplifier has great deep bass response even with the lowly 741.........as long as you don't over drive it out of the output mosfet ...SOA .....im using IRF240 AND IRF244 in it......their current rating are close 12&14 amp....i think anyway thanks to all who.......helped out with my problem.


i know this is far from HI-FI but in guitar land it works

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