Acoustic 370 output distortion problem

[prulup]

Hi, I have a problem with Acoustic 370, (230V version) it gives only 70W to 8 ohms or cca 110W to 4 ohms without the distortion / limitation of the signal. I checked the monitor outputs, nice sine signal with volume up to half. But the output is distorted with volume on cca 2-3, doesn't matter what load it has or even without the load - all the same. I disabled the limitation circuit - the same. I put the trim pot instead of the 680R and tried to bias it - the same with 10-30mV on the 1R resistors. BTW. how many mV would you recommend over these resistors to keep the amp in safe mode? On the scope the signal has no transient distortion, it is limitated and distorted more and more up to square signal. I checked all voltages, it is +- like shown in the schematics with one weird : on Q310 I have 0,7V, 44,7V, 44,3V . Only 0,4V BE voltage. (Q305 is 89V, 46,9V, 46,1V). THe sine signal is good up to the 270k resistor (R314). THe output voltage is exactly the half of the pwr voltage. I would be glad for any help, thank you very much!

[Dave H]

Is this it?

[prulup]

Is this it?

[ATTACH=CONFIG]50729[/ATTACH]

Yes, it is.

[prulup]

anyone?

[J M Fahey]

Your amplifier is working perfectly fine.
**all** amplifiers clip once sinewave reaches rail voltage.
Not sure what you think is "a problem".

[Dave H]

Your amplifier is working perfectly fine.

It's good to "see" you Juan.

Yeah, It could be fine. Here's the spec sheet. It says 205W but that's into 3.2 ohms not 4 which will make quite a difference and it's at 5% distortion. prulup, if you can't measure distortion turn it up so there's a bit of flat on the peaks of the sine wave and check it again.
It's distorting at 2-3 on the vol pot but what was the signal level? It only need 18mV on ch 1 (56mV ch 2) for full output with the vol on '10'



Edit: here's the rest of the manual. 370 Manual.zip

[Leo_Gnardo]

It's good to "see" you Juan.

Yeah, It could be fine. Here's the spec sheet. It says 205W but that's into 3.2 ohms not 4 which will make quite a difference and it's at 5% distortion.

I'll double & redouble that good to see you Juan! I hope you won't be so scarce in the future.

About that Acoustic, I've found their power figures to be a bit disappointing when amps are actually measured. Plus, with not much in the way of output transistor cooling, it's suicidal to run them at 2 ohms.

On the good side, it's refreshing to see their % distortion vs power specs closely match my old "rule of thumb." For every 5% increase in distortion, add 12% increase in measured power. Bingo!

[J M Fahey]

Thanks
First question worth answering in about 2 Months, so ...
That amplifier is working fine.
90V single rail means, after supply dropping under load (15/20%) plus transistor losses (4 or 5V *each* side) that guaranteed output voltage, 20V RMS clearly written by output jack, is realistic.
It might reach 21 or 22V RMS on a good day, but that would be a Bonus.

So 20V RMS means 50W into 8 ohms (so your 70W is perfect), 100W into 4 (so measured 110W is fine) and 200W into 2 ohm, which is the standard output you can get with that supply and transistors.

In fact my own B200 from the 70´s and contemporary with similar Acoustic amps was 200W RMS into 2 ohm (4 x 12" 8 ohm speakers in parallel) , +/-42V rails, 4 selected 2N3055H transistors.

That Acoustic can *easily* drive 2 ohm loads, and was typically used with 2 x 4 ohm cabinets or the huge 2 ohm 4 x 15" 408:


In fact head is rated down to 1.6 ohm load, go figure, so it´s useless to try them to put out more into 8 or 4 ohm which are the wrong impedance.

[nsubulysses]

I been through this, and have worked on dozens of 370s. might be worth perusing as far as output power or just to read about it -- https://music-electronics-forum.com/...ad.php?t=36989

a "normal" (meaning, works as it should) Acoustic 370 does 24 to 25V into 4 ohms for 144-156W at clipping. Probably varying just because of line voltage. The manual is BS because it's rated at 3.2 ohms (no one has a cab this impedance) and also 10% distortion (!) for 230W at 4 ohms. Let the good times roll for overrating power output.

http://mikesgig.com/wp-content/uploa...ual-410056.pdf

On an acoustic 370 it seems like 0-3 on volume knob are volume. 4 is usually the onset of distortion. beyond 4 is no more volume, only more distortion. Of course this can vary on how turnt up your tone and EQ settings are.

[nsubulysses]

Also, those old Acoustic 2x15s rule, but IMO the REAL cab to use 370 with, which blew my mind when I heard it compared to playing various other sunn/acoustic/peavey 2x15s over the years, is Acoustic 301, singe 1x18 that is the same size as an 8x10. Has a folded horn design so the speaker faces backwards in the cab.

After playing through an Acoustic 301 I had the feeling of wow, now I "get it" This is what an Acoustic 370 is supposed to sound like.

[nsubulysses]

Also, just guessing because thre are smarter people than me on the board, so let them chime in.

If the amp does not put out full power, could it be an issue with driver transistors Q305 and Q310. I have heard that when an amp will not put out full power into a load but all the power supply voltages are there, it means that it cannot develop the current necessary to drive the speakers.

I have my Acoustic 370 on hand right now and can check a few voltages if you want

[The Dude]

Yeahhhhhh! Good to see you, Juan!

[nevetslab]

And likewise from me....great to see yer presence.

I had to download the schematic and manual to get a close look at the signature on the schematic. Yup! Duke Aguiar. One of my many bosses at BGW Systems, who had been their chief engineer in the late 70's, and he had been one of their main engineers when Acoustic first got started, when they were on Jefferson Blvd in LA, before moving to El Segundo. My band used to borrow Acoustic gear belonging to the Doors as well as Sweetwater back in the late 60's...a bit before the 370 was built.

[g1]

Thanks for joining in Juan!

One more thing that hasn't been mentioned is that the spec. sheet in the manual says it is for 240V at 60Hz and there will be power losses at 50Hz (and of course if 230V rather than 240V).

[prulup]

It's good to "see" you Juan.

Yeah, It could be fine. Here's the spec sheet. It says 205W but that's into 3.2 ohms not 4 which will make quite a difference and it's at 5% distortion. prulup, if you can't measure distortion turn it up so there's a bit of flat on the peaks of the sine wave and check it again.
It's distorting at 2-3 on the vol pot but what was the signal level? It only need 18mV on ch 1 (56mV ch 2) for full output with the vol on '10'

[ATTACH=CONFIG]50784[/ATTACH]

Edit: here's the rest of the manual. [ATTACH]50785[/ATTACH]

Hi Dave, the measurements were done with the bit of flat on the peaks to simulate the 5% distortion. And the signal is the same with the 8 ohm load or with no load...