Ampeg Micro CL distortion

[fredcapo]

The music shop took in an Ampeg a Micro CL bass stack on trade. Of course did not fully test it. It has an overall muddy raspy tone at all volume levels, and,just read similar complaints on various bass player forums...some said it was an acknowledged factory problem. It's not the cabinet. Did not check but some geezer said headphone out was clean. Doubt if I could get any info out of Ampeg. Anyone know about this or any suggestions? Thanks.

[Jazz P Bass]

Have you tried a different speaker cabinet?

[fredcapo]

Yes sir. The cabinet sounds great with my micro VR head ( I play bass). And the Micro CL head sounds raspy with VR cab. Also headphone output is clean. As stated above, there is enough talk on various forums to suggest there is some type of design defect or premature failure.

I don't have a schematic or have not devised a plan to isolate the cause of the distortion.

Thanks.

[J M Fahey]

Try both side by side going back and forth and also plugging same cabinet into one or the other, simply to make them work in a very similar way.

If you confirm that the CL head works significantly worse than the VR, * at all power levels, even bedroom and garage* , then no doubt it's a head problem.

You might make a simple YT video showing that, play a few bars with one, then with the other, go back and forth 4 or 5 times so our ears become accustomed and notice differences better.

Now if bedroom level works fine in both and CL cr*ps at higher levels while VR stays clean, maybe that's nthe nature of the beast, 100W vs. 200W .

Many say "but that's just a negligible 3 dB difference" .

Maybe so, but countless times 100W has been crushed by the drummer, and 200W , "only 3 dB more" does the opposite, so in that case you sould really have nothing to "repair".

[MarkusBass]

Did not check but some geezer said headphone out was clean.

It seems to me that this is the basic check you should have performed. It will tell you whether the problem is related to the preamp, or the power amp.

Doubt if I could get any info out of Ampeg.

Why do you doubt it? If you ask them, they will send you the schematic of the amp (especially that you are from the US).

Anyone know about this or any suggestions?

Based on your very brief description there is chance that bias in the power amp is set incorrectly. I'm only guessing but you can check it directly having the schematic and performing some measurements. In the meantime, you can post a photo of the power amp board.

Mark

[fredcapo]

Yes did the side by side swap instrument and cabinet comparisons, high and low gain. This particular CL sounds fizzy, leaves a fuzzy, raspy artifact as the note decays. Sounds a bit better in the -15 db input. Headphone output uniformly clean. Did not try aux out yet. I will call Loud for support...I was unnecessarily dismissive above...guess was thinking Stl music...

[Leo_Gnardo]

Based on your very brief description there is chance that bias in the power amp is set incorrectly.

What I'm thinkin' ^^^. I hope you have a scope, load resistor(s) and sine wave generator. You'll see in an instant if it's crossover distortion due to insufficient bias.

Another possibility is a shorted/leaky limiting transistor in the power amp. Also easy to see with the oscillator/resistor/scope rig.

One last: if there are rail fuses in the amp maybe one has gone open circuit.

[Jazz P Bass]

There are many possibilites to the cause of the 'fizzy'.

Try to narrow it down to the power amp or preamp.


FX return will bypass the preamp.
Aux in will bypass the tone controls.
FX send will bypass the power amp.

Micro-CL_OM.pdf

[fredcapo]

Thanks for the manual w block diagram...I was not expecting that the headphone jack comes directly off the speaker leads... Padded headphone out sounds fine, but speakers sound raspy distorted... Is that diagram correct?

Will try things suggested when I get to the shop tomorrow. Thanks guys.

[MarkusBass]

Thanks for the manual w block diagram...I was not expecting that the headphone jack comes directly off the speaker leads... Padded headphone out sounds fine, but speakers sound raspy distorted... Is that diagram correct?

I think it's correct. Headphones disconnect the speaker. The problem can be caused by failed headphones jack, or failed speaker. If none of them is the case, the problem may be caused by failed power amp, which works correctly with high impedance of the headphones but fails to work correctly with low resistance of the speaker. This may be still incorrect bias, or more complex failure of the power amp.

Mark

[fredcapo]

Markus - I have not opened up the unit yet. But thinking it would be possible for headphone jack to open the speaker leads, yet take a signal from another point in circuit...thanks
Got the expected response from Loud...

Unfortunately, schematics for our current products are ONLY available to our authorized service centers or distributors. There is no acknowledged defect with these amps that I am aware of.
All the best,

[blindboybenton]

Hi Fredcapo

Did you ever fix this amp ?. I have the same amp with what seems to be the same fault. I was holding out for a schematic, but it aint been forth coming
go to bite the bullet and get started on mine

BBB

[fredcapo]

BBBenton...have you fixed your Micro CL? I wasnt ready to invest the time to break down mine to troubleshoot and put it on the shelf. Easier to just use my Micro VR. Seems to have been a problem enciuntered by several. Some folks said it was the speakers, but not in my case.

[DrGonz78]

Here is the schematic for the amp... I would test all power supplies and then trace the signal all through the amp to see what I could find. Perhaps soldering imperfections can be found on R129. Got to start at the start again.

[fredcapo]

Doc. Thanks. Now I can open it up. Have noticed the distortion sounds different with different basses...some times ok, others totally useless. Will report back in a week or so.