Help needed: EQ module oscillating

Hi folks. Hoping someone can lend a hand troubleshooting the following:

I have two Traynor line amp / EQ modules racked and powered for use in the mastering room here (they're useful for their saturation characteristics in the low end). Recently one channel has begun to oscillate at a very low frequency, below 5Hz I would say. Can't hear it, but I can see the speakers moving.

This seems to only happen with the LF eq pot set to flat, or toward the CUT side. I can stop the oscillation by moving it toward the BOOST side. It only starts to happen as music or other LF content is played through it, and the oscillation will then remain after the music is stopped. It will not start on its own, it seems.

Here's the module schematic, as well as a schematic of the (rather crude) power supply:

http://www.archivemastering.com/lab/YVM6_eq_schematic_May2006_small.jpg

http://www.archivemastering.com/lab/Traynor_PS.jpg

I replaced C2, C3, C9, and C10, as well as the final 330uF cap in the PS, hoping that would solve the problem, but it remains. My meager troubleshooting skills are tapped. Any suggestions at this point would be welcome. Thanks a million!

Best,

- J.

This does sound like a power supply decoupling problem, and I think you did the right thing in replacing C3 and C10. Is the power supply itself common to both modules? As an experiement, you could try briefly jumpering the rail at the top of C10 and then C3 across to the same point on the other module. If the oscillation stops or both channels then oscillate, it gives you some pointers.

One possibility is that a ground connection has gone high impedance, so it's worth checking all the ground points on the faulty module.

If it's not the power decoupling or the grounds, then you have to look for the bias conditions of the transistors having shifted, possibly due to component ageing. You're well placed to do this as you have two identical modules. Measure the volts to ground at the 4 salient points of each transistor pair of the oscillating module and compare with the good one. You will need to use a high impedance multimeter for this. From what you say about the behaviour of the bass control, the fault is likely to be in the second stage.

Boswell, thanks for your help!

[quote author="Boswell"]Is the power supply itself common to both modules?[/quote]

Yes, individual wires run from the 30v output of the PS to point H on each module. I'm going to try your suggestions and report back.

Best,

- J.

[quote author="Boswell"]As an experiement, you could try briefly jumpering the rail at the top of C10 and then C3 across to the same point on the other module. If the oscillation stops or both channels then oscillate, it gives you some pointers.[/quote]

Ok, tried the above, and the faulty channel still oscillates, but the good channel does not. (Same as always). I also tried removing the good module entirely, and the bad module still oscillates. So the problem is defintely in the bad module only, or perhaps has something to do with the power supply.


[quote author="Boswell"]One possibility is that a ground connection has gone high impedance, so it's worth checking all the ground points on the faulty module.[/quote]

All ground points checked out ok.


[quote author="Boswell"] Measure the volts to ground at the 4 salient points of each transistor pair of the oscillating module and compare with the good one. [/quote]

Everything measures ok on the oscillating module on the bench. (it was not oscillating when I measured it). I haven't yet checked the voltages while the module is oscillating.

I also rebuilt the ps slightly. Removed the voltage doulber and replaced it with a bridge rectifier. This brought the voltage going into the regulator down to a more sensible 39 V (from 70+ previously).

So, short story - the problem remains. Any further hints? Would a failing transistor exhibit such a problem? Anybody know where to find some TI597 transistors, or a suitable substitute? I'm terrified of changing these modules too much, as they sound absolutely incredible as is (when working!).

Thanks,

- J.

[quote author="J.LaPointe"]
Everything measures ok on the oscillating module on the bench. (it was not oscillating when I measured it). I haven't yet checked the voltages while the module is oscillating.[/quote]

I would think it critical to measure the module while its acting up in order for the measurement to have any relative conclusion. I would measure the module under the normal working conditions hooked up to the regular power supply. you might also consider hooking up the bad module to a different power supply if you have an appropriate one handy. If it oscillates on a second supply, it might be safe to assume your supply in the rack is ok and focus on tracking down whats happening inside the module.

dave

Good, so you've established that it's unlikely to be the power supply. Yes, you need to compare voltages between channels when the faulty one is oscillating, or, at least, the change of voltages in the faulty channel between the oscillating and non-oscillating conditions.

In terms of what the fault could be, it's most likely to be a passive component (resistor or capacitor), but it is possible that one of the transistors is drifting sufficiently far out of spec that it upsets the d.c. conditions in that part of the circuit. Note that the bass/treble controls have no d.c. path through them, if you include C9 (the output coupling capacitor) in that reckoning.

If you can get the module to oscillate on the bench, I would try jumpering another 4n7 capacitor across C5 and C6 in turn to see if the oscillation stops or changes frequency. It would be useful to find out which components are involved in the time constant that is generating the 5Hz.

One other thing occurred to me: are these modules interchangeable in the rack unit, and if so, does the fault follow one module when you swap them around? The reason for asking is that whatever the output stage drives can influence the feedback to the tone control circuit.

[quote author="Boswell"]One other thing occurred to me: are these modules interchangeable in the rack unit, and if so, does the fault follow one module when you swap them around? The reason for asking is that whatever the output stage drives can influence the feedback to the tone control circuit.[/quote]

Boswell, thanks again for your help! I decided to go back and take a look at how the modules were used in the original mixer.

(see http://www.archivemastering.com/lab/YVM6.gif)

Turns out they're happier looking into a series resistance. I played around with resistor values to find the lowest noise and flattest frequency response, and settled on 200 ohms on the output. The oscillation is gone, and all seems well. I guess the bad module was more sensitive to this than the other. Both are working well now. Thanks again.

Best,

- J.

I'm glad you were able to suppress the oscillation, but I'm intrigued: what are you driving with these modules that does not have a resistive input?

[quote author="Boswell"]I'm glad you were able to suppress the oscillation, but I'm intrigued: what are you driving with these modules that does not have a resistive input?[/quote]

The modules were driving (at various times, all with the same oscillating behavior):

- a Manley Massive Passive (transformer input)
- Pendulum ES-8 (transformer input)
- Crane Song HEDD

For me, the addition of the one resistor almost seems like too simple of a solution, and I'm not even sure why it works. Any ideas?

Best,

- J.

For me, the addition of the one resistor almost seems like too simple of a solution, and I'm not even sure why it works.

Click to expand...

I think it goes back to what I said in a previous post, that the feedback to the tone control circuitry can be affected by what the output of the module is driving. Putting a resistor in series with the output helps to decouple the feedback from the load. However, it almost certainly means that you will get an attenuation at the output which you will have to account for in your system gain calculations.

All the units in your list of ones that you had driven from the module should have looked resistive, even though some had transformer-coupled inputs. It is possible that the fall-off in the coupling properties of the transformers in the low Hz range could cause the primaries to look inductively reactive at those frequencies, and this could have upset the stability of the module's output stage.

[quote author="Boswell"]Putting a resistor in series with the output helps to decouple the feedback from the load. However, it almost certainly means that you will get an attenuation at the output which you will have to account for in your system gain calculations.[/quote]

Higher value resistors led to more attenuation, and roll-off of highs, hence eventually settlinng on the (relatively low) 200 ohms, which cost me about .2db gain.

Thanks again for your help.

Best,

- J.

Glad you found a workable solution.

I find it strange that circuits like this are still used in mastering work these days. You mentioned that you make use of the saturation charcteristics of these modules - can you explain a bit more?

[quote author="Boswell"]I find it strange that circuits like this are still used in mastering work these days. You mentioned that you make use of the saturation charcteristics of these modules - can you explain a bit more?[/quote]

I've had the modules for a while, but it's only recently that I've begun to convert them into something useful for mastering, so it's yet to be seen if they will actually be that useful in the long term. They only get used once in a while, on certain "problem" sources. For example, because of the way they saturate in the lows, they do a good job of adding some satisfying thickness to all-digital productions. You can turn up the low eq, and it will smooth out an uneven low end as it boosts, while also rounding off annoying high freq transients. They are no good for a mix that's already pretty hi-fi sounding - these are rescue tools for harsh mixes or otherwise difficult to tame problems. I do like the dead simple high and low eq though. I've thought about building a really clean line amp with that eq, but for now I've reproduced the curves in the Algorithmix Red eq, and it doesn't get much cleaner than that.

Best,

- J.