fender acoustasonic 30dsp has low output power

[turbo_burt]

This one has me stumped - It sounds good, until the volume goes up. Thats when it produces a horrible clipping sound, unlike a unit that is intended to amplify an acoustic instrument. So far I have been unable to squeeze more than a 12.9VAC (21 watt) sine wave from the output, this also seems to be the limiting number for headroom. And with a guitar as the signal source, it gets worse, especially playing a chord.
Following through the circuit - Everything looks clean up to the begining of the power amp section (TP36). The output device (LM3876) seems to be symmetrically clipping at 13VAC into 8 ohms. The power rails +-26 volts unloaded, each drop a few volts when the amplifier is at its full output.
The clipping is very audible through the tweeter, although no clip hash indicative of oscillation was seen on the scope.
Of the schematics I have revision A & B, both use identical power supplies and output sections. Version A is capable of 17.7volt output and Ver B only gives 12.7volts, I would assume this is the one on my bench.

Do you think the output amp LM3876 has failed somewhat, and needs replacement. Or is too much being expected out of this whole amplifier?

Fender Acoustasonic 30 DSP (Revs A and B).pdf

[Enzo]

The drawings have a number of errors. The rails are drawn as -21 and +26, very unlikely, especually since they are pulled from the same transformer winding. Over in the powr supply section they say 21v for both even though the +21 has a +26 arrow leading from it. And the +15VDC shows 12.7VDC as a rest point. Unlikely. I am very suspicious of that 17.7VAC output spec. 17.7VAC means a peak voltage of over 24v. That is very difficult to do with 21v power rails.

Look at the test points through the circuits. The version with the 17v output shows 1.6v of signal; going into the PA, while the 12v version shows only 0.56v there. Look right center for the summing stage, U10b. The same signals are there as at the PA in. Now follow back to the signal sources for that summer. The mic channel puts out 400mv in one version and 1.4v in the other. Instrument channel? 250mv versus 43mv. And DFX output? 190mv or 350mv, take your pick. SO clearly the two schematics were not going to give the same results. What is correct and what not? Good question. I see no difference through the instrument channel to explain the differnce between 40mv and 250mv.

On the other hand, maybe the 21v is wrong and 26v is right. The plain vanilla Acoustasonic 30 - no DSP - shows 26v rails with the same power transformer. 17.7v out into 8 ohms is 39 watts, the amp is rated for 30 watts.

And also keep in mind that those output specs are what they expect under the test conditions specified, such as 20mv input signal. It doesn't really specify that is all the amp is capable of.

But enough of all that. Throw another IC in there and see if it makes a difference.

[turbo_burt]

Thank you very much Enzo for such a detailed review. A replacement output device will be obtained, and hopefully then the full advertised thirty watts will be delivered.

[turbo_burt]

The IC (LM3876t) was replaced, and the output power is the same as before - 21 watts / 12.9 volts.
Other modifications I tried - A massive capacitor for bypassing the power supply, and also removed the PA input protection diodes (D15 & D16). Neither of these changes did anything & I dont know what else to try besides a higher voltage transformer - expensive & difficult to find one that fits the chassis cutout.
In the manual, is this what is meant when they say "product specifications are subject to change without notice"?

[Enzo]

It means exactly that. It means that just because this schematic says such and such a resistor is 10k ohms, that doesn't mean they won;'t change it to 22k ohms next production run. It means the schematic is largely correct, but don;t count on it to be 100% identical to your amp no matter what. And it can also mean that they might change the ratings to make a product more reliable. AN example might be Mackie changing the woofer in the SWA1501 to 16 ohms so it won;t b,low the amp up. It also results in lower power than advertised power.

[valleywide2002@netzero.net]

Were you able to solve the issue with this amp?

The IC (LM3876t) was replaced, and the output power is the same as before - 21 watts / 12.9 volts.
Other modifications I tried - A massive capacitor for bypassing the power supply, and also removed the PA input protection diodes (D15 & D16). Neither of these changes did anything & I dont know what else to try besides a higher voltage transformer -ch expensive & difficult to find one that fits the chassis cutout.
In the manual, is this what is meant when they say "product specifications are subject to change without notice"?

I have the same amp with the same problem on my bench right now. Were you able to locate the source of the problem? Please let me know.
Thanks, Mike valleywide2002@netzero.net

[turbo_burt]

After being unable to get any more electrical power out of that amp, I replaced the original eight inch speaker with one that is more efficient. While that created an audible increase in loudness, it wasn't a huge difference.

[cgoepel]

Post a reply to the thread: fender acoustasonic 30dsp has low output power

It means exactly that. It means that just because this schematic says such and such a resistor is 10k ohms, that doesn't mean they won;'t change it to 22k ohms next production run. It means the schematic is largely correct, but don;t count on it to be 100% identical to your amp no matter what. And it can also mean that they might change the ratings to make a product more reliable. AN example might be Mackie changing the woofer in the SWA1501 to 16 ohms so it won;t b,low the amp up. It also results in lower power than advertised power.

Hi Enzo - I have an Acoustasonic 30 DSP on the bench. I am suspicious of the CA3080 limiting amplifier. On my unit in for repair, the volume sounds like it’s being limited; no distortion just simply not too loud.
On the Fender DSP amps, I go to the forum first before opening the unit up and look up the symptoms to help me troubleshoot. Regards, Charlie G - SevermaPark Maryland

[Jazz P Bass]

If you feel that the CA3080 is the issue, desolder it & remove it from the board.

If the signal comes up good, there you go.

Personally, I think they overdid it with that ic.
If the amp distorts, turn it down.

[ThisLifeILead]

i have the same DSP on my bench and i cant find anything wrong. One of the 8" speakers had a rubbing voicecoil so i replaced it. but its still not as loud as it used to be.... generally ICs totally fail so im hesistant about replacing them but they are cheap.

[Raymon]

Fender Acoustasonic-30DSP clipping distortion -- solved?

i have the same DSP on my bench and i cant find anything wrong. One of the 8" speakers had a rubbing voicecoil so i replaced it. but its still not as loud as it used to be.... generally ICs totally fail so im hesistant about replacing them but they are cheap.

I have this same problem and have also replaced the LM3876 IC which puts out about 17.6V p-p AC before it clips -- power supplies are + and 26VDC solid -- (very little drooping with large signal)

Anyone fixed this yet? Can anyone explain why the output IC can't seem to get
closer to the supply rails? Shouldn't it be able to go to at least +/- 5V of the rails
which would give a 40Vp-pAC signal?

Thanks for looking....

[nickb]

I have this same problem and have also replaced the LM3876 IC which puts out about 17.6V p-p AC before it clips -- power supplies are + and 26VDC solid -- (very little drooping with large signal)

Anyone fixed this yet? Can anyone explain why the output IC can't seem to get
closer to the supply rails? Shouldn't it be able to go to at least +/- 5V of the rails
which would give a 40Vp-pAC signal?

Thanks for looking....

Using a 1Khz sine wave, measure the output voltage RMS (TP37) just prior to any clipping and then measure the VRMS at TP36 and report. This will tell us if the problem is before or the power amp or not. They should be in the ratio of around 17.6:1.63 i.e. a gain of 10.8.

[Raymon]

Using a 1Khz sine wave, measure the output voltage RMS (TP37) just prior to any clipping and then measure the VRMS at TP36 and report. This will tell us if the problem is before or the power amp or not. They should be in the ratio of around 17.6:1.63 i.e. a gain of 10.8.

I do get this gain ratio however, I did measure with Vp-p as stated. The RMS voltages would be correspondingly less, but the ratio the same. Line out is clean. If I should be able to get 17.6 VRMS
with no clipping then the power amp is at fault because I only get 16-17 Vp-p before the clipping.
TP36 is clean (no clipping) all the way to max (several volts Vp-p ) past the 1.63vrms mentioned.

Thanks

[nickb]

Acoustasonic_30_DSP_Schematics.pdf

I do get this gain ratio however, I did measure with Vp-p as stated. The RMS voltages would be correspondingly less, but the ratio the same. Line out is clean. If I should be able to get 17.6 VRMS
with no clipping then the power amp is at fault because I only get 16-17 Vp-p before the clipping.
TP36 is clean (no clipping) all the way to max (several volts Vp-p ) past the 1.63vrms mentioned.

Thanks

Edit - I thought I might be looking at the wrong schematic. Take a look at the attached - pages one and two have different gains for the power amp based on the indicated TP36/37 measurements. Yet they have the same values in the gain setting network. Both cannot be right. Looking at the values I would say the gain should be around 20 meaning the numbers on sheet 2 are the more believable.

So, can we do that again, please?

(1) What is the highest unclipped Vpp on TP36?
(2) With the output just short of clipping the gain should be 12.7/.56 ~22.


If you are only getting a gain of ~10 (from post #13 above) then take a close look at the muting cct. Measure the voltage across R101 and divide by 10 to get the current in mA. It should be at least 0.5mA. Also the voltage at pin 8 should be -2.6V.

If the current is correct but the voltage is not, check C102 for leakage. Otherwise take some DC measurements around Q5 see what s going on.

[Raymon]

Still can't find problem with thiis Acoustisonic 30 DSP (mute circuit??)

[ATTACH]37537[/ATTACH]

If you are only getting a gain of ~10 (from post #13 above) then take a close look at the muting cct. Measure the voltage across R101 and divide by 10 to get the current in mA. It should be at least 0.5mA. Also the voltage at pin 8 should be -2.6V.

If the current is correct but the voltage is not, check C102 for leakage. Otherwise take some DC measurements around Q5 see what s going on.

Thanks for the reply and sorry for the delay in getting back-- been very busy with other repairs but....
.... I think you mean R121 according to my print.... in any case the voltage on pin 8 is -3.1Vdc and
across R121 is -8.1V so greater than the .5ma you mentioned into the mute pin.
Any other ideas??

Thanks for your consideration,
Regards,
Raymon