Carvin BX micro Bass MB210

[nosaj]

Any one have any experience with Carvin BX micro Bass MB210? This one seems to have a hiss in the background when an instrument is plugged in. Do not have a schematic it looks to be a class d amp?

Notice it is loaded with capxcon capacitors but that shouldn't contribute to hiss. A lot of SMD components.

Nosaj

[Leo_Gnardo]

This one seems to have a hiss in the background when an instrument is plugged in.

Lots of amps have a mute circuit post preamp, which undoes the mute when something's plugged into the input jack. Marshall seems to have started it with the JCM800 series in early 80's. I call it a "blame shifter"; if you don't know there's a jack operated mute, you blame whatever's plugged into the amp. Unless the hiss is really objectionable, I'd say forget it, par for the course.

[nosaj]

I spoke with the guy today he says it lost more than half it's volume an would make weird noises when playing the E or D string? None of which I get at this time. I'm gonna put it on a load and warm it up some and see what if any issue presents itself.

The loss of volume in my thinking puts me back to power supply caps need more investigating.

nosaj

[Leo_Gnardo]

Class D amps run off of switching-mode power surprise, if filter caps aren't up to snuff, noises could be hi frequencies from the SMPS modulated with the bass. Why it happens there (owner's place) and not here (your workbench) who can tell. The only such amp I was ever able to fix had a multipin comb connector sitting caddy-wampus on its mate, off by about 20 degrees, installed that way at the Friendly Fender Factory. Squared up the connector & all was well, cheap & fast. Open & look, you may have something that simple. Let's hope so. Look for easy obvious stuff first.

Beyond that I am out of my depth, let's hear from any class D experts. Anybody there?

[Enzo]

How would bad filter caps result in loss of half the volume?

Isolate the problem.

Plug a guitar into it to activate the input, but turn the guitar volume control to zero. Still hiss? Assuming it does, which controls affect the hiss in ANY way? I mean do they control the volume of the hiss or the tone of it? Does a master volume turn the hiss off completely?

Class D refers ONLY to the power amp, so if the preamp is hissy, then the class D-ness is not involved.

[nosaj]

Mv turns hiss off completely. treble knob can minimize it some. Parametric hi mid freq knob can minimize it some. meter on the output 8 ohms 1k signal in can only get 30- 32 v on the output. That means 7.5 watts right?
Contacted Carvin to see about a schematic, waiting to hear back.

Thanks, nosaj

Found schematichttp://carvinimages.com/schematics/8045300revh.pdf

Will check rails first then report back

[Leo_Gnardo]

31V across 8 ohms, more like 125 watts. P = Vsquared divided by R. What do Carvin claim for this amp? While your poking the rails, look for mis-seated multipins or any other out of wack items, burnt resistors, blown fuses, whatever.

Besides ordinary pre amp hiss, preamp "could" be picking up stray hi-frequency oscillation from the SMPS which may manifest as audio hiss. A scope on the output would reveal either "white/pink noise," par for the course with preamps, or a constant very high frequency wave - leakage from the power surprise. If it's regular random noise, can accept it as-is or go on the chase squirting preamp components with cold spray & maybe one will reveal as the culprit. If hi frequency wave, I'm at a loss. One would hope the manufacturer provides enough shielding between SMPS & audio, and/or appropriate filtering of DC to the power amp/ preamp. If not, what to do about it. Stumped.

Sure would be a hit if you could compare this amp with another identical one, see if it's a characteristic of the build or if this one's a troublemaker. Is there a Carvin owner's website? Ask there if there is. At Marshall, Fender, Vox, Epiphone there's sometimes answers to owners questions from other owners that help sort whether your problem is out of the ordinary, or to be expected.

And you could call or email Carvin, but I expect they'll be as usual unwilling to help with schematics or advice. "Send us the amp and we'll make it work like new," the s.o.s. Sometimes they do, sometimes the amp comes back with no significant improvement. You pay shipping both ways whether it gets fixed right or not. The customer experience - varies - customer to customer. Good luck with that.

[nosaj]

200w at 8 ohms. Will scope the output and try to take a pic.
Most things on this amp are smd. I'll check connectors then hook it up for measurements.

Thanks,
Jason

+15 rail 15.8
-15 rail -15.4

V+ 64.8
V- -64.9
So going by the schematic i would say the power supply is ok.



Would you say a noticeable difference in customer perception going from 200w to 125w( person says it's not as loud as it once was) Pretty dang loud to me.

[Enzo]

Any control that affects the noise is AFTER the noise source along the signal path. COntrols that have no effect are either before the source or not in the path. If the parametric and treble affect it, then I'd expect before R41. Does contour affect the noise? At this point excess hiss I would suspect from either IC13 - "A13" in their drawing - or the three JFETs in the preamp before it.

[Enzo]

The difference is only a couple decibels at most. besides, unless he was running it flat out, it wasn't producing max power anyway.

[Leo_Gnardo]

FWIW there is a Carvin Farcebook page but it looks like all ads from the company in the main section. There is a visitor post section as usual. May be worth a blink & you could post a query there. If others experience the same amount of noise then it's part of the soup. Preamps ain't perfect. They all hiss to some extent. It may even be the owner's choice of gain structure. Some players whip up the volume control on the amp, barely crack open the volume on their instrument & run things that way. If that's the case, gotta expect hiss.

TDPRI also has a Carvin Corner. You can join free, look around, pitch your question there:

The Direct Connection..Carvin Amp owners! - Telecaster Guitar Forum

Enzo's suggestions in post 9, good as always!

Good luck, I'm outta ammo, beating hasty retreat, but will keep a watch here. Can always learn more especially if you find a solution.

[nosaj]

Which transistors are the outputs? I looked up the IRFS4615 thinking it might be the outputs then looked up the datasheet. Just trying to figure out reasonable what kind of out to expect from this amp. I cleaned all the the cable connections jacks and pots and still not getting more than 30 32v on output. Not getting any odd noises on any notes like the guy said. So I'm kinda at a loss until I can get him to recreate the issue. Just wondering about power output at this point.

nosaj

[g1]

Your supply levels are +/- 65V.
In a perfect world, that would allow 130V peak to peak at the output, which would be about 45VRMS, so around 250W at 8 ohms.
The supply won't hold up at full load so the +/65VDC will drop somewhat.
The number you said before, 200W at 8ohms sounds like a reasonable expectation.
If you haven't already, quadruple check all your test & measurement gear, load, etc.

[g1]

Also, the built in compressor could be doing something. Can it be defeated? Otherwise, you may need to test with a "burst" signal.

[nosaj]

Also, the built in compressor could be doing something. Can it be defeated? Otherwise, you may need to test with a "burst" signal.

It can be turned all the way to the left which states off but there is no click.
I will try another meter and see if it is in line.

What is a burst signal?

Thanks, nosaj