ART SLA-1 Power AMP Distortion

[ido1957]

I'm getting volume through my both of my channels of this power amp. But one channel is fuzzy and distorted. Other channel clean as a bell. I've swithed around the lines from the board and the speaker cable and it looks like one channel may be toast. I would have thought that the whole channel would just cut out though. It's just a s loud as the other one but distorted.

Any ideas as to what part(s) might be faulty? Ever had this problem?

I may have to buy a new one of these - fixing it will probably take too long (have to ship it somewhere probably) and cost more than a new one.

Edit - Just checked and it's an SLA-2 power amp....

 

[Zaphod B]

If memory serves (been a long time!) and not knowing anything about this specific power amp, I'm guessing that this may be indicative of a failed power MOSFET or something similar. There may be one device that's locked into saturation and it's distorting everything else in the signal path.

Lt. Bob and ggunn (I think) have current electronics experience - hopefully one of them will check in.

 

[ido1957]

This was a gift 2 1/2 years ago. The warranty is 3 years . Hopefully the retail store (Long and McQuade) still has some record of the sale on computer . They had a record of my 2000 tube change/purchase so they should. God knows how long it will take to fix it though. The last fix (my ADAT) took 6 months. It's off to the store tomorrow for return to the warranty/repair center

 

[mjhamil]

From your description it's probably in the input stage if you still have volume, hopefully a noisey control or input connector. 6 months? You might be able to arrange a loaner...

 

[ido1957]

So L & M tested it for 8 hours with no problems meaning I have to find the problem still.

I had lots and lots of crackling yesterday for the first 1/2 hour through the floor monitors. Now that's different problem - the original problem was a distortion artifact as the sound dissipated. I might be overloading my GK temporay amp though. It went away after a while so I just don't know....

Now I'm leaning toward the outputs on the Alesis board, or possibly a cable although I thought I tested all of them. Damn, wish it was like a lightbulb where it would just burn out and be easy to find/replace..

 

[ido1957]

Finally got to retry the SLA-2 amp the music store repair tech said was not broken.

I had two weekend gigs so I just got my floor monitors back in the basement. They run off the SLA-2 power amp that was giving me the trouble.

I just finished setting everything up again, hooking up my board to the amp to the monitors.

When I first hooked it up the amp was in bridge mode. Pan left - sound out of both speakers, pan right everything died. That took a while to figure that out...after fiddling with every cable . Repair tech left it in bridge mode....thanks buddy .

So I set the amp back to stereo mode and I'm playing my Yamaha keyboard through the board for testing...single notes and or chords.

%#$@^ The same distortion comes out of one speaker. Kind of a fuzzy noise on top of the signal and lasting a second or two as the signal dissipates. So I'm thinking my main outs are fried....the amp is working or so the tech says right?

I try the alternate main outs (there's two on this board), same distortion.
I try different RCA's from the board to the amp, same distortion.
I try running my tascam tape deck signal from the board, same distortion.
I try a direct RCA from my keyboard to the amp, same distortion.
It's clean into Channel one (either channel from the keyboard). It has the fuzz when I plug either L/R into Channel 2. If I switch speaker cables the distortion comes out of the other speaker.

So I'm now convinced, the amp has a bad Channel 2 as I originally stated.

Interesting thing though, when I played a song from a CD Boom Box direct from headphone out to the amp, you couldn't really hear the distortion. There was too much going on that the distortion was buried. But when you send a keyboard direct it clear as a bell on one side and obviously distorted on the other.

So to ensure I am not audibly hallucinating, I'm getting mjhamil over to check this out. Once he's had a chance to confirm it, it goes back to the shop.

I'll tell them to use a simpler signal to test it. They probably blasted their boom box through it like I did and couldn't hear the distortion.

This is the whole point of this post so hopefully this will help anyone who encounters the same issue.

It is a sweet amp when it works so I'm looking forward to getting this fixed......

 

[dementedchord]

it definately is not the output stage... when they blow there's no output in 99% of the time... ya gotta wonder what the tech checked it with... in my experience even a bad tech might not be able to fix it... but they generally can tell if it's working reasonably well...let's see... is it balanced ins??? maybe he was driving it with unbalanced and the unused input section is toast???? possible but a dual opamp rarely blows just one half.... but atleast now you understand why the biggest part of a repair bill is time.... it may just be a $2 part but which $2 part???

 

[ido1957]

it definately is not the output stage... when they blow there's no output in 99% of the time... ya gotta wonder what the tech checked it with... in my experience even a bad tech might not be able to fix it... but they generally can tell if it's working reasonably well...let's see... is it balanced ins??? maybe he was driving it with unbalanced and the unused input section is toast???? possible but a dual opamp rarely blows just one half.... but atleast now you understand why the biggest part of a repair bill is time.... it may just be a $2 part but which $2 part???

From the website:
Input Impedance 44k ohms (bal), 22k ohms (unbal)
From the manual:¼” INPUT JACKS
The ¼” Input jacks are wired as follows: Tip is positive, Ring is
negative, and Sleeve is ground. This connection can be used for both
balanced or unbalanced connections.

Yeah the labor gets you... As this is still under warranty I'm glad I'm not paying for the time spent to trace the fault.

 

[dementedchord]

actually i did think of a way that it could be in theoutput stage and not blow the whole output... it coulda opened an emiter resistor without taking a mosfet device... but that shoulda sounded pretty nappy on the bench under any circumstance as you'ld get only one side of the wave form...

 

[ido1957]

mjhamil came over and brought a scope with him.

He scoped the speaker outs on the two channels and we found a big difference between channels. Channel 1 is a smooth small compact wave line whereas channel 2 has a wave that extends way beyond the norm. He took a few digital photos which he will give to the music store/repair tech so they can see the problem.

The trick was to use the keyboard (Middle C) and just strike the note. The waves in the attached pictures are from the keyboard note. You can see the width of the distorted channel is way different. The noise/distortion is also clearly audible over the speakers.

We did try a test signal CD using 50Hz and 2K. The noise was not audible on the speakers, but the wave was still distorted (being consistent still had that wider range of noise).

mjhamil thinks it could be in the pre-amp. We didn't open it up because that would void the warranty. So it's back to the shop tomorrow with the pictures....

 

[dementedchord]

that amp is oscilating... does it do it with nothing connected???? may be a pre-amp isssue...

 

[ido1957]

that amp is oscilating... does it do it with nothing connected???? may be a pre-amp isssue...

When there's nothing connected the scope is a flat line. Here's a pic of the scope at "ground" as mjhamil advises. mjhamil said it was probably pre-ammp related. It's a week before they can look at it, so I'm told. Using my GK for a power amp in the interim - it works but it's not great....

 

[ido1957]

Well, the repair tech just ordered the service manual so I guess he's gonna try and fix it, rather than send it to the factory for a swap (or repair). Too bad, I would have liked a nice new unit lol . But it probably would have been a refurb. My unit is pretty minty clean too (maybe a little dust) but it's never been racked or on the road. You never know with a refurb....

 

[dementedchord]

actually i've never undferstood why people shy away from refurbs... bench QC on them should be 100%... anyhow hope it works oput for you...