Speaker pulsating with or without any input signal. What causes that?

[Kenrod]

Yamaha MS60S powered monitor speaker.
Here's a link to YouTube video
http://youtu.be/7KmBalFIsYw

[J M Fahey]

You sure no input signal of any kind?
Un[lug everything connectedn to inputs, and set any controls present (volume?) to 0 .

If you still have such very low frequency vibration, the amp is unstable and oscillating, that's often called "motorboating".

If it has an SMPS (switching type power supply) it might also be unstable.

In any case it's way beyond what a home user can fix.

[Kenrod]

It's not doing it with the original speaker. I had pulled it because the foam surround had busted all the way around and I was trying a spare 8 ohm speaker I had.
I may swap the spare speaker back in and see if it repeats.

[Kenrod]

I put the replacement speaker back in and it does pulse with no input signal and all gains (3) turned down to zero.
Maybe it can't pulse with the original speaker because the support is shot?
This thing still sounds good, just needs a speaker, unless there's something else going on.

[J M Fahey]

Maybe, just maybe you pulled some connector or a similar action while pulling the original speaker.

Just a shot in the dark, but look inside with good light and glasses if necessary, carefully pull every connector you see, dust it with a clean brush or blast of compressed air and reseat firmly, you might have lost some ground cnnection.

It's hard to suggest anything else without any kind of data.

PS: did you reconnect the original speaker, just to test your theory?

Measure carefully voice coil DC resistance , maybe replacement does not match original.

Absolutely worst case, get a refoam kit and repair the original speaker.

[Kenrod]

Anything is possible, but I jut lifted the speaker out of the hole far enough to disconnect the leads every time.
I've now tested with the replacement speaker (8 ohm Peavey) twice and it pulses both times. I've listened to the original speaker 3 times now. No pulsing detected. Original speaker is stamped as 7 ohms, 70 watts.

[J M Fahey]

‘How often have I said to you that when you have eliminated the impossible, whatever remains,however improbable, must be the truth?’

Sherlock Holmes Quote

So although there is no visible reason for that, the Yamaha monitor works *only* with its original speaker and none else.

So get the proper refoam kit, watch a couple YT videos, do it, and replace original speaker.

Here's one:
https://youtu.be/71ExFMuhqxM

[Kenrod]

It "works" with the replacement speaker, but it also pulsates.
I've seen the kits. I suppose I'll give it a shot.
I have the schematic for this amp. I've replaced the low EQ pot in the past. It's worked fine until the complaint of the sound degrading and I find the foam surround broke loose from the basket.

[Jazz P Bass]

Late to the party here.

It makes no sense that only the replacement speaker should 'pulsate'.
(the fact that the Yamaha speaker has a ruined surround may mask the fact that it too moves in & out. How can you tell?)

I would have a good long look at the power supply when this occurs.

It sounds like the amp is 'motorboating'.
Which may be caused by one of the split supply rails.
Maybe a bad main filter capacitor.

Schematic link: http://music-electronics-forum.com/t15051/#post119676

STK output ic: stk4036.pdf

[Tom Phillips]

I put the replacement speaker back in and it does pulse with no input signal and all gains (3) turned down to zero.
Maybe it can't pulse with the original speaker because the support is shot?

Correct. The speaker with the broken surround cannot respond physically to the very low level, low frequency signal that the amp is producing.

This thing still sounds good, just needs a speaker, unless there's something else going on.

Yes. This is a clue that something is going wrong in the amp. It may or may not get worse but it isn't right.

[Kenrod]

Maybe, just maybe you pulled some connector or a similar action while pulling the original speaker.

Just a shot in the dark, but look inside with good light and glasses if necessary, carefully pull every connector you see, dust it with a clean brush or blast of compressed air and reseat firmly, you might have lost some ground cnnection.

It's hard to suggest anything else without any kind of data.

.

I'll do this while i'm in there.

[Kenrod]

Late to the party here.


It sounds like the amp is 'motorboating'.

yeah, after Juan mentioned this i had to go research it.
Although i've heard of it, I've never dealt with it. It looks like ground issues, or capacitance issues may be the source.
I only worked on this preamp section in the past, but after pulling the speaker, i've now seen the power section. I'll consult the schematic for test points, and start there, unless you guys tell me something specific to check first. Again, willing, but unexperienced in oscillation problems.

[J M Fahey]

I have the schematic for this amp.

Oh, having it would have saved time.

It "works" with the replacement speaker, but it also pulsates.

Well, sort of

It makes no sense that the replacement speaker should 'pulsate'.

But it does so let's try to find an answer

(the fact that the Yamaha speaker has a ruined surround may mask the fact that it too moves in & out. How can you tell?)

Well, not heard but should be visible, with front grill off.

Maybe a bad main filter capacitor.

Mmmmmhhh, it does it or not depending on speaker, but still the same capacitor.

I'd focus, instead, on what could make the amplifier (I think of it as a "black box" , not as "parts") become unstable.

Haven't seen the schematic yet nor a description or the user manual but suspect it might have some kind of motional feedback circuit, to track and compensate for cone excursion.

Philips used what amounts to a piezo disc glued to the (rigid) dustcap wired as an accelerometer, but it can be roughly simulated by using the own speaker generated back EMF , maybe also detecting current through the coil (with a small series resistor) , in this case lesser current means higher impedance and indicates resonance.

Or: even with the best damping possible, it's still nominal because you always have an important voice coil wire resistance in series; I read somewhere about generating negative output impedance at the amplifier to compensate it.

Of course, both possibilities: motion sensing or better than usual damping *may* need careful tuning, probably to one very specific speaker only, and others might make the system unstable, both cases involve some kind of positive feedback.

Not all amps are simple Voltage sources , or even plain or mixed current sources like some guitar amps.

[Kenrod]

YOU-SHOULD-HAVE-ATTACHED-IT-TO-YOUR-QUESTION
You find it fun sending us on a wild goose chase?

Very sorry about that. It wasn't my intention. I started with a somewhat general question about the issue. I posted from my phone; the schematic is on my laptop.
I do thank Jazz P Bass for posting it this morning.

That's not "working"
Or if you think it is, just give it back as is to the owner and tell him : "it works!!!"

That's a little harsh, and not my intention. I'm here because i noticed an irregularity and want to verify it's and issue and solve it the best way possible.
It was reproducing music pretty well; "working", but I immediately noticed the pulsing speaker when i paused the music.
Perhaps working, but not working correctly. Depends on how you slice it i suppose. When i picked it up, the owner said it wasn't working.
The amp hadn't failed, even the original speaker in bad shape was working. The replacement speaker behavior was perplexing though, leading to my original post.

Let's think, instead, on what could make the amplifier (I think of it as a "black box" , not as "parts") become unstable.

Haven't seen the schematic yet nor a description or the user manual but suspect it might have some kind of motional feedback circuit, to track and compensate for cone excursion.

Philips used what amounts to a piezo disc glued to the (rigid) dustcap wired as an accelerometer, but it can be roughly simulated by using the own speaker generated back EMF , maybe also detecting current through the coil (with a small series resistor) , in this case lesser current means higher impedance and indicates resonance.

Or: even with the best damping possible, it's still nominal because you always have an important voice coil wire resistance in series; I read somewhere about generating negative output impedance at the amplifier to compensate it.

Of course, both possibilities: motion sensing or better than usual damping *may* need careful tuning, probably to one very specific speaker only, and others might make the system unstable, both cases involve some kind of positive feedback.

Not all amps are simple Voltage sources , or even plain or mixed current sources like some guitar amps.

I wonder about the speaker being very specific to the overall design of the amp.
I thought the speaker a little odd when i first pulled it, for 2 reasons.
1) it was stamped "7 ohms" in my experience, i usually see impedance given in multiples of 4 ohms.
2) the magnet arrangement, it appears to have the "normal" magnet, and then another attached right to the bottom.


At this point, I'll replace the foam surround (didn't know that was such a reasonable option) and do/check what ever is suggested here in the meantime.

[J M Fahey]

Very sorry about that. It wasn't my intention. I started with a somewhat general question about the issue. I posted from my phone; the schematic is on my laptop.
I do thank Jazz P Bass for posting it this morning.

Oh sorry if I was rude, you made my head think too much before my morning coffee.
And be certain that this *mistery* really makes anybody think too much, since it's , well, so .... "mysterious" .

I'll have a looong look at the schematic but I suspect some negative impedance or motion feedback trick , a regular high damping SS amp does not care much about what it drives, as long as over dissipation does not kill it.
Sorry again

Is this some kind of Recording/Studio monitor?

I guess some kind of tremolo effect must have been quite evident.

I wonder about the speaker being very specific to the overall design of the amp.
I thought the speaker a little odd when i first pulled it, for 2 reasons.
1) it was stamped "7 ohms" in my experience, i usually see impedance given in multiples of 4 ohms.
2) the magnet arrangement, it appears to have the "normal" magnet, and then another attached right to the bottom.

Same here, didn't worry much about the 7 ohm label, I've repaired Guitar speakers with as low as 5 ohms DCR, a classic Eminence trick to "boost efficiency" by an extra dB ; not too audible but sure looks good on the datasheet or ad, but might be a subtle cue: "just don't replace this with any random speaker you have lying around"

As of the extra magnet , it does serve 2 purposes:
1) it does "focus" more of the available magnetic force towards theuseful magnetic gap, where the voice coil works, is roughly equivalent to rising main magnet diameter by 20% or so.

2) the main reason: that redirected flux stays inside the magnetic structure, not annoying nearby stuff, *specially* TVs and PC monitors, so you'll find such "double magnets" in TV and PC speakers.

At this point, I'll replace the foam surround (didn't know that was such a reasonable option) and do/check what ever is suggested here in the meantime.

I bet that will cure it.

In any case, I'll study the circuit, such speaker wizardy usually has an adjustment trimmer somewhere.
Some reverb circuits which feed back a little of delayed signal to increase its depth also have such a trimmer pot.
See Champ 12 for one example, some Gibson amps too.