Mark Bass hates bass

[drewl]

So I replaced the blown MOSFETs in a Mark Bass combo head II.
Amp works fine on bench with sig gen slamming at least 400 watts at any frequency.

Cranking a bass guitar into it and it trips the shut down circuit, with speakers or into a load.

Even at about half cranked it shuts down with a bass, but can put out full volume with a signal pumped into it.
????????

[The Dude]

Well, just don't play bass guitar through it.

But seriously, how does your sig gen output level compare to the bass output level? Are they at least similar?

[Enzo]

Bass tends to be real spiky, scope the bass signal compared to your test signal.

[drewl]

I just cranked my gen to overload the preamp and the amp is screaming 550 watts no problem.

Pluck a bass note hard and it will trip at half volume.


I'll try some higher rated MOSFETs we have at work Monday and see if that helps.

[glebert]

I have a Little Mark 250 head that had a leaky cap on the part of the circuit that detects excessive voltage on the output,.so it would immediately go into protect mode, but when you powered down you could hear signal for a moment. Don't know what your amp has to sense the output, though.

[Enzo]

KInda blew off the question though. You pluck a bass string and there is a large transient, the note itself isn't very loud.

I would observe that your output devices seem to be working if they can put out 500 watts. If I read you right, the amp is shutting down - triggering protection. It is not blowing out MOSFETs each time, right? Not sure how heftier ones would help.


That is the point of 1500-2000 watt bass rigs. Not to be real loud, but so all the transients will come through. It gives a real effortless sound. Doesn't lop off the peaks. Play through a 100 watt amp the same way and it is like a Datsun towing a travel trailer. It can do it, but it sounds like it is working real hard to pull it off.

[drewl]

Sorry Enzo, didn't see your response before I posted.

yes, it is triggering the protection, not blowing the devices or anything else.

[g1]

Have you checked for oscillation? Maybe not doing it with sig. gen. due to grounding, whereas bass is floating?

[drewl]

Bass into amp with a grounded load shows no oscillation.

[glebert]

Do you have a schematic? I know that they are kind of stingy with those. I can send you a snip of what I have for the protection circuit to see if it similar to yours if you need it.

[Jon Snell]

Is it the amplifier or power supply that is shutting down?
Little Bass Combo has a switch mode power supply.
You don't give the model or year of manufacture so guessing a bit here.

[drewl]

I'll have to post schematics Monday, they're on my work computer.

SMPS stays up, relay disconnects and red LED inside turns on.

[J M Fahey]

I think I know what´s happening.

Only protection visible is speaker DC protection.



Speaker out is DC integrated using R68 1M and C29 1 uF which give you a textbook perfect time constant of 1 second.

If you have DCv for more than 1 second, relay turns speaker OFF.

Generator sinewave is "perfect" symmetrical by definition, zero DC component, triggers nothing, lets >500W out with no problem.

Real World Bass guitar signal is NOT symmetrical at all, can and will have a "DC component" just because of that.

As soon as integrated DC across C29 reaches 2.5V or so, whatever triggers T19 or T20 2N7002 MosFets, relay switches OFF

Suspect C29 dried up, in principle replace it (you need a bipolar because DC can be positive or negative) but worst case "upgrade" it to 2 uF

Suspect drying up because as Enzo would say: there are hundreds of MarkBass amps out there and that very one left Factory and has been used since.

[drewl]

That's what I was going to check like glebert mentioned.

Thanks.

[drewl]

Looks like the layout of this one is a little different.
It's the model CMD 102P.

So you're saying C29 is a bipolar, it had one of those little silver electrolytic caps in there.

Tried a 1uf poly cap with no change.

Also the one end of C29 is not connected to ground, but to the +12 protect line.