Ampeg Micro CL distortion

[cjenrick]

$$ on headphone jack>jumper that jack.

[fredcapo]

I am hedging on my statemant that the headphone output is clean...tried again and it sounded bad same as speaker. Anyway until i put the signal generator and scope on it, this is all imprecise talk...

[Familyortiz]

+1 on testing all of the supplies. Simple to do and all you need is a multimeter.

[fredcapo]

Thanks. the schematic doesnt show voltages, though the zener is rated 15v. Im seeing +\-48 at bridge, +15.5 and -16 at the zeners though -14.5 on the case of c20 the same node...

I dont have any values shown on schematic for power amp stage.

[fredcapo]

Bringing this up again. The speakers are good and the board voltages look ok. So put the scope on the output with power resistor instead of speaker, and applying sine wave to input with scope ch 2 monitoring input on r24. Sinewaves look fine but...

I am suprised to see the sinewave on r24 change amplitude as i raise the master gain control on the amp, and when the output clips, it seems that the input clips... i know the op amps use feedback, but i did not think from the output stage...though i did not consude where the gain control is located...

I will make some detailed input output voltage measurements to try to determine if the clipping is premature.

[The Dude]

It looks to me like VR3 is the master volume. It looks to be a dual pot. I don't see any way changing the master level SHOULD change the input level at R24. This assumes your amp matches the schematic Gonz posted in #14. Does it? Maybe you have a grounding issue? Also, you might use the send/return jacks to determine if the clipping sound is coming from preamp or power amp.

[fredcapo]

Thanks Dude, correct, flipped probe...yes the cable to my dummy load flipped the red wire to ground.

OK, so setup is correct now. The speaker output is clipping asymetrically (on the positive side) at around 40 v p-p.

I am also seeing what looks like intermittent high frequency fuzz on the input signal. Didnt see it last night when the scope ground reference was flipped.

I will work backward to determine where the clipping starts, though seems likely in the output stage.

NB schematic looks like a match . Pots are all single. The pot vr3 shown on the aux in stage prob should be labelled vr5.

[J M Fahey]

I vaguely remembered the Micro CL and now rereading the schematic I confirm it:
to begin with, a **TERRIBLE** design.

I remember reading it for the first time, maybe a couple years ago, and mumbling: "this is NO Ampeg at all" .
Not even close, lots of errors which not even a beginner should make.

Guess Loud Technology (current asset holders) Marketing Dept. thought "a small SVT lookalike will sell well", they contacted some Asian OEM manufacturer and said: "oh, donīt worry about what you fit inside, just copy SVT looks".
Not kidding.

1) the choice of input mixing resistors is *ludicrous* :
2k2 and 10k? .... gimme a break!!!!
22k and 100k at the least, to match standard Bass pickups.

"Hey !!! but I tested it with the bench oscillator and it works fine"
Sure thing, bench oscillators routinely drive 600 ohms, many even a small speaker.
Guess what, Musicians use Musical Instruments, not bench gear.

2) the gain structure is all wrong: lots of input gain, straight driving active tone controls which also provide gain, and *first* gain/volume control *after* that: a recipe for input clipping which will sound farty/buzzy at all levels.

Itīs the classic gain structure used on old (say 70`s) Home Audio preamps, where volume control was between preamp and power amp, after everything else, including tone controls.
Not a bad choice *there*, since record cartridge, Tape, FM Tuner, Tape Recorder levels were basically normalized, but a poor choice for wildly varying Musical Instruments and players.

Yes, *some* instruments will work fine, the weaker ones, or those used "flat" or played with a light touch.

3) the power amp is basically fine, a classic simple design, will provide stated 100W with no effort, but it has no limiter, a standard element today on all SS Bass amps, from Fender DeltaComp to Peavey DDT to Crate/Ampeg own optical compressor, so playing at Rock Drummer levels will make it buzzy/farty clip.

Another proof it is not a Crate/Ampeg design, which would have included their trademark Led/LDR limiter which they use on all Bass amps, from 10W to 600W .

4) since you already have this, what to do?

a) use sensible input resistor values, as I suggested 22k and 100k.

b) since you are at it, add a 2200pF or 4700pF cap in parallel with 100k resistor (Active input)

c) lower input stage gain by 6dB , this should make amp usable, either replace R6 with 47k or solder 100k in parallel with the current one.

And since input mixing resistors now will have a usable value, IF a strong instrument still overloads it, plug it in the Active input ... even if itīs passive (but loud).

Funny thing, it was clearly meant to be sold on looks and maybe not actually even used for rehearsal (with a drummer that is) but just a glorified home practice amp, I remember the Ampeg Ad stating something like: "how would you like to have an SVT in your Living Room?"

[fredcapo]

Juan. Thank you for your knowlegable and generous input. I will follow your advice. Yes, it does seem to sound better depending upon the instrument used. I did not trace yet, but you are suggesting the clipping is in the preamp?

So, regarding the output stage, the rails are +\- 48 vdc...what is the max p-p output voltage to exoect with a clean input signal? (I will try driving the power section direct).

[Jewellworks]

Juan,
I hope this thread is still active. It's been a few YEARS...
Im primarily a guitar player, but I have a Bass and I have a Micro CL. I've also been a little disappointed with the CL, as I can only turn it up about halfway before the bottom end starts farting out. This has been the only place on the internet that describes what can be done about this, other than getting some other amp.
I'm understanding the modifications you listed, but some things aren't clear...

a) use sensible input resistor values, as I suggested 22k and 100k.


I checked the values and found a 2.2k and a 10k resistor near the input Jack's. (R24 and R18 respectively) are these the correct ones to replace with the 22k and 100k?


b) since you are at it, add a 2200pF or 4700pF cap in parallel with 100k resistor (Active input)


This is the NEW 100k I'm replacing, correct?
(I ask because R6 is already a 100k resistor, and your next statement says to replace it)


c) lower input stage gain by 6dB , this should make amp usable, either replace R6 with 47k or solder 100k in parallel with the current one.


Any clarification would be most helpful.
Thanx again

[J M Fahey]

Juan,
I hope this thread is still active. It's been a few YEARS...
Im primarily a guitar player, but I have a Bass and I have a Micro CL. I've also been a little disappointed with the CL, as I can only turn it up about halfway before the bottom end starts farting out. This has been the only place on the internet that describes what can be done about this, other than getting some other amp.
I'm understanding the modifications you listed, but some things aren't clear...

a) use sensible input resistor values, as I suggested 22k and 100k.


I checked the values and found a 2.2k and a 10k resistor near the input Jack's. (R24 and R18 respectively) are these the correct ones to replace with the 22k and 100k?

Yes, make 2k2>22k and 10k>100k .

b) since you are at it, add a 2200pF or 4700pF cap in parallel with 100k resistor (Active input)

Yes, I mean in parallel with the 100k resistor you just added.
Play it safe and stay with 2200pF only, 4700pF maybe too much of a good thing.

c) lower input stage gain by 6dB , this should make amp usable, either replace R6 with 47k or solder 100k in parallel with the current one.
Any clarification would be most helpful.

Itīs literally that.
Current gain is set by (R6/R18)+1 = 11X
Not bad at all if you had a volume control just there, or a *passive* Tone Control ... but you have an active one, so boosting Bass can add extra 10X gain and create farting mushy Bass which can NOT be corrected lowering volume, since itīs *after* that mess.

Lowering R6 to 47k halves gain and makes amp more usable.

And NOW input attenuation works with a real world instrument (instead of a Test Oscillator) so worst case you plug into the attenuated input.

Not dissing Ampeg in general, but this particular design is clearly made by a no clue "regular" Engineer, ZERO experience on Musical Instrument design.

I have also seen lat Marshalls which "are NOT Marshalls" if you catch my drift.

But lately brands get bought by investors who want returns, have no clue about this, just mimic *looks* and order stuff from generic OEM makers .... letting them "deal with details" such as design

[bsco]

Excellent info Juan.....I haven't seen one of these amps before but I could in the future...now I know why I joined this forum in the first place......the info and tech help here is something that could never be taught in an electronics course....this is a fantastic site guys and I hope it will be here for many years to come......

[Jewellworks]

Thanks for the clarification.
One more thing...
since you are at it, add a 2200pF or 4700pF cap in parallel with 100k resistor (Active input)
Yes, I mean in parallel with the 100k resistor you just added.
Play it safe and stay with 2200pF only, 4700pF maybe too much of a good thing.

Isn't the cap a high pass filter? Wouldn't I want it on the 22k as well?? If not, I'm not sure what it's doing... ??

Thanks again

[J M Fahey]

Yes, itīs an old Ampeg trick, they call that input "bright".
It works well on the attenuated input, not so much on the other.
After all, itīs a passive circuit.

Check old Ampeg circuits (thing early 70īs or so).
SS of course, although maybe some Tube amp also had its own version.

[Jewellworks]

UPDATE:
I followed the instructions above and replaced resistors and added a Cap, and I'm here to say that my amp is much more usable now, and doesn't fart out on the bottom end anymore. However, I did make 1 variation to Juan's recommendations. Instead of swapping the R6, 100k resistor for a 47k, I put in a 62k. The 47k took a little too much life out of the amp, while the 62k kept it LOUD with plenty of bottom end punch, with just the right amount of distortion, but not getting poopy. At least it works very well with my Bass, and that's what counts.
So glad I found this thread. Thanks alot guys