What went wrong with my amp? Bass driver strange movement.

[chengs]

Hi, I am electronically challenged. But I would like to know what is the problem and how to fix it. At least I know what to say to a tech if I need one. Here is what happened yesterday:
I turned on my Aronov IC-70 (60W 4x12AX7 4x6550 P-P duo-mono intergrate amp)for a listening session this afternoon, I noticed there were some movement of the bass drivers on both right and left speakers. They move in sync slowly moved in and out a few times and then stopped, start moving again after 30-40 second (sometimes longer, sometimes shorter). There is no pattern when the drivers move and they move somewhere between 5 to 20 seconds. The drivers' movement is quite big (in and out about 2/3 inch) I thought the cones are going to fall out. It went on for an hour then I turned it off. It did not seem to affect the sound. The drivers moved even when volume is turned all the way dead silent. The drivers moves even when there is no input signal. I tried all different inputs and they still move. I feel like the amp was trying to adjust something and had to do it every so often. Or, there is very strong low frequency (0.3 to 0.5 Hz) going through or generated by my amp and sent to my speakers. I know this only happened today, because I don't put the grils on my speaker and I am picky about how my system works. Help!!
What might be going wrong? Tubes? 12ax7 or 6550? Caps? Coupling or power supply? I only know this much. Why both channels move in sync? Should I take my amp out of my system? Thanks.

[EC8010]

Your problem is known as "motorboating" and it's a power supply design fault. By the sound of things, your amplifier is right on the verge of instability, so it could be anything changing very slightly that caused your present problem. In other words, changing the slightest thing might make the problem appear to go away.

Dealing with the real problem, however... You'll need to post a circuit diagram and we can offer suggestions.

[Brian Beck]

EC is probably right that this is a power supply impedance problem at low frequencies; although I have also seen feedback amps with regulated power supplies that were only marginally stable (see case below). This can be due to excess phase shift from too many inappropriately selected coupling capacitors within the loop. Add an output transformer to the mix and you’ve added even more low-end phase shift. So it doesn’t have to be the fault of the power supply alone, but it often is. Sometimes motorboating it is an unfortunate combination of both: RC filtered power supplies and coupling cap selection.

If you only measure from 20Hz to 20 KHz, you might miss this kind of thing. Also, if you only used small two-way speakers, the very low frequency motion might not be so visibly obvious, even while the motorboating is wasting amplifier power and heating voice coils.

Years ago, I owned a classic Audio Research SP-3A preamplifier. During a routine check out, I hooked a scope to the output (it might have been the phono out or the line out – I don’t recall). I chose the DC input setting on my scope. I had set the DC line to the center of the display and I had set the volts-per-division sensitivity for the expected 1kHz sine wave output. When I connected the scope, the trace disappeared altogether. No trace visible at all, nothing. Perplexed, I zoomed the display out to a much larger setting of volts-per-division, and there was my little 1KHz sine wave riding on top of a very low frequency “sine wave” of gigantic amplitude (moving at a mere fraction of one Hertz). It took a long time for the signal to damp down to where the 1KHz sine wave could be seen in the originally-scaled display. This ARC model used a crudely regulated supply but it may have had RC decoupling for the phono stage as well. This kind of problem can be seen in other classic vintage preamps and amps too. If your low-frequency cartridge-arm resonance happens to be near the low frequency peak in the response of one of these amps, expect some unpleasant interaction. BTW, simulation is pretty good at catching these things early in the design phase, although the proof is in the testing and listening. I routinely start simulation frequency spans at fractions of one Hertz to catch these things.

If your musical source material were to plunge momentarily into the infra-bass region (a bass drum "thwack"), or if you introduce a switching transient by changing input sources, for example, you can trigger this behavior, which may take a while to subside.

As EC says, please post a schematic so we can help.

[cerrem]

Check if you have any DC offset voltage moving about on the voice coil, speaker terminals.....
Many amps send the feedback from the secondary of the output transformer to a cathode in the first stage, (global feedback)..
The DC at the cathodes will offset the speakers if it is jumping around due to bad or incorrect tubes...warm-up ..ect..

Chris

[richwalters]

Originally posted by chengs

I turned on my Aronov IC-70 (60W 4x12AX7 4x6550 P-P duo-mono intergrate amp)for a listening session this afternoon, I noticed there were some movement of the bass drivers on both right and left speakers. They move in sync slowly moved in and out a few times and then stopped, start moving again after 30-40 second (sometimes longer, sometimes shorter). There is no pattern when the drivers move and they move somewhere between 5 to 20 seconds. The drivers' movement is quite big (in and out about 2/3 inch)

Hi, As others mentioned could be problems in power supply decoupling. The fact that both channels do the same is a clue.

Did you make this amp from a kit ?

As Brian mentions the loop feedback time constants v.s phaseshift at the low frequency end could be so high (i.e too much NFB below output tranny cutoff) that phaseshift i.e positive feedback is trying to get in at the LF end. IF you made this amp from a kit, check the value of the interstage coupling capacitors.

If this is the problem, the solution is simple:- reduce NFB at the lower end by reducing the interstage capacitor values. We need a schematic.

richj

[Wavebourn]

Hi freq oscillations may cause motorboating as well. For example, some extra ground point.

[chengs]

Hi,
I do not have the schem to show, but I do have a block diagram that is attached.
Should I stop listening to that amp right now? It does not seem to affect the sound, except for the visual threat.
I would like to know more about what to do to fix the problem(s).

[richwalters]

Ummm......that's thrown it... preamps is also on the same chassis as main amp. Problem could be in preamp section. I'm tempted to take all the tubes out of the preamp and switch amp on and see if instability is still present. An easy way ? Maybe See what other forum users mention.
You say you are "electronically challenged". Are you up to using a DVM on high voltage and looking inside a chassis ???

richj

[chengs]

I don't know much about electronic and don't feel comfortable poking around inside the amp. But I do have a friend who can help me if knowing what to measure or do. So, if you all can tell me what to do or how to do, we will try our best. Thanks.

[EC8010]

You really need to find a circuit diagram. Without one, even an experienced engineer would find it hard going poking around inside.