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Old 5th Feb 2020, 10:44 am   #1
cathoderay57
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Default IC Pre-amp Oscillation

Morning all,

I have built an IC pre-amp with active filter to use with my Quad II PAs and also a home constructed EL34 push-pull stereo amp. I do have a Quad 22 valve pre-amp, but wanted to try out the homebrew pre-amp on the Quads. The circuit is a combination of 2 projects published in Babani Press BP24 "52 Projects Using IC741" by Rudi & Uwe Redmer ©1975 Babani Press. I do have written permission to reproduce the slightly modified circuit in hand-drawn form (see below). I did not use individual 741 ICs but instead I used a TL074 which has 4 op-amps on the same chip, using 2 for the high-gain magnetic cartridge pre-amp (one for each stereo channel) and 2 for the active filter channels. The circuit is built on prototyping board with copper "islands" as opposed to long strips like Veroboard. There are 2 long strips within the board that serve conveniently as supply and earth rails. For a power supply I am running at 30v DC with offset resistors as shown in the circuit diagram such that the outputs of the op-amps float around +15v DC. The issue of concern is that, while the audio performance sounds excellent and the treble and bass boost and cut work faultlessly to my ear, when I had a look at the output waveform on the scope I noticed something peculiar. For an input signal I used a 400Hz tone from the AF output of an Advance E2 sig gen, set almost at minimum output. The signals at the output of both high-gain preamp stages were very clean with no evidence of clipping. I then transferred the scope to the output following the active filter stage, with bass control set at minimum. With treble set at minimum the outputs were clean but on advancing the treble to the mid (flat) position I noticed that the 400Hz audio appeared to be modulated onto an HF sinewave signal somewhere around 10MHz (that's the frequency limit of my scope). Advancing the treble towards boost the modulation reduced to the lower half of the waveform and at full boost disappeared. See scope pics below. I wondered if it was breakthrough from the RF output of the sig gen but, as I change RF frequency on the sig gen, the 10MHz-ish frequency at the output doesn't change so it looks like the oscillation is being developed within the active filter part of the circuit. Any ideas as to what is happening and how or whether I should bother to do anything about it? Clearly the PAs are well capable of filtering it out and so audio is unaffected. Cheers, Jerry
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Old 5th Feb 2020, 11:42 am   #2
Ted Kendall
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Default Re: IC Pre-amp Oscillation

100nF between the supply rails as close to each chip as possible is a wise precaution.
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Old 5th Feb 2020, 11:51 am   #3
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Default Re: IC Pre-amp Oscillation

Stick a few hundred ohms from the output of the last amp to the output socket (take feedback from the driven end of this resistor NOT the output end)

The amplifier will have its stability margin degraded at high frequencies by the capacitance of the shielded cable to the power amp.

David
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Old 5th Feb 2020, 12:16 pm   #4
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Default Re: IC Pre-amp Oscillation

Quote:
Originally Posted by Ted Kendall View Post
100nF between the supply rails as close to each chip as possible is a wise precaution.
I'd try this first too. Op amps are famous for this kind of instability and more recent designs nearly always use decoupling capacitors fitted as close as possible to the IC supply pins. Stray capacitance due to layout and wiring is the usual cause of these issues. I've a feeling that the use of screened cable might be creating more problems than it's solving.

Alan

Last edited by ajgriff; 5th Feb 2020 at 12:33 pm. Reason: Missing apostrophe
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Old 5th Feb 2020, 2:56 pm   #5
cathoderay57
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Default Re: IC Pre-amp Oscillation

Many thanks for the suggestions. I will give them a try and report back. Cheers, Jerry
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Old 5th Feb 2020, 10:02 pm   #6
cathoderay57
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Default Re: IC Pre-amp Oscillation

Hello again, I tried the 100nF power rail decoupler at the chip to no avail. The tips about my (mis)use of screened cable were bang on the money. I had actually used coax to connect the inverting input of the second op-amp to the junction of the 10k and 3.3k resistors so I began to wonder if the resulting coax capacitance was upsetting things, particularly since the oscillation came and went when the treble pot was adjusted. I first disconnected the earth from the coax braid and the amplitude of oscillation dropped by 50%, so I then replaced the coax to the tone controls with single wires (kept as short as poss to reduce hum pick-up) and in addition took up the suggestion of putting a stopper resistor in the output chain (I used a 470R in series with each output, before the 330nF output coupling cap). Those 2 mods completely solved the problem. On the Quads the hum level is virtually inaudible even at high volume and bass settings, so I am very pleased with the result. Thanks again for pointing me in the right direction. Cheers, Jerry
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Old 5th Feb 2020, 10:31 pm   #7
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Default Re: IC Pre-amp Oscillation

The 741s are internally compensated. If the op amps you are using do not have internal compensation, then you probably need to do it externally. It's years since I did any of this stuff, but I think that you need to ensure that the gain is less than one at the point where the phase shift causes the feedback to become positive. As I say, I'm very rusty on this.

Roger

Last edited by LeakyGrid; 5th Feb 2020 at 10:32 pm. Reason: spelling
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Old 5th Feb 2020, 10:56 pm   #8
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Default Re: IC Pre-amp Oscillation

The 741 is a rather slow and somewhat noisy opamp, but it was the main player back when that Babani book was written. The TL074 is a later generation part and is significantly faster, and has internal compensation. In general, it out-performs the 741 in most ways, but the faster speed makes it significantly touchier about stray phase lags introduced by stray capacitance and driving cables is a weak point of many faster opamps.

It's a well known problem and people who've been bitten by it just take suitable precautions as a matter of good housekeeping. People who haven't been bitten just don't think about it. You've put yourself firmly in the former camp, now.

Things are always clearer when you've fixed them

David
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