works great with lm741, almost with half of TL072???

[njkmonty]

Ive fabbed a board using LM741  running off +/- 13v  and works great.

out of curiousity I fabbed the same board again, (mainly to practice using eagle)   but instead replaced the Lm741, with smd TL072 (using only half)

It works perfectly except, I can hear a very slight audible buzz, (that other family members missed until I pointed it out to them)  and mainly heard on lower frequencies.
(this occured on all 3 boards i did.
i Traced the buzz back to the ic area right at the beginning of the circuit with an audio probe
I was wondering if anyone can see if I did something obvious in my conversion?
or if i should go back to LM741 in smd format?

[Scruffie]

Did you leave the other half of the TL072 floating? If you did, that'd explain it.

[njkmonty]

thats how i did it?

[Scruffie]

That's fine, in that case i'd hazard a guess it doesn't like driving that cap to ground straight on its output, I see what you've tried to do there but that's not its purpose on the original chip.

[njkmonty]

the lm741 version  works perfectly,

its the Tl072  (without the cap)    that has the issue

[amptramp]

The TL072 has much higher frequency response than an LM741 so high-frequency oscillation could be the culprit.  Add a small capacitor across the 470 K resistor to add feedback lead.  22 pF should be OK for a rolloff of 15400 Hz.  The stage gain of 48 can lead to oscillation.

[njkmonty]

cheers I was thinking about that, was going to try a 100 pf     , but will adjust to advice,  will report back

[amptramp]

100 pF would give you a rolloff at 3388 Hz.

See here to zero in on a value:

http://www.muzique.com/schem/filter.htm

[njkmonty]

It made a slight improvement. ( 25pf)
Even more with 100pf.  But still there

[ElectricDruid]

Could it be power noise finding its way into the input? The PCB layout will be different because of the different chip, and if it's SMT, you've only got a very tiny bit of PCB between the -V supply and that sensitive +ve input.

It might not be the different chip that's the problem, it might be the different layout.

Tom

[njkmonty]

good point!

going to do another layout and see what results i get