Sunn Alpha 112R combo

[gbono]

I'm working on one of these and have a few question.

The board has several mods or maybe they were factory after thoughts. R137 and R108 have aluminum electrolytics caps in series - decoupling? Also what is the purpose of all the inverters on the input of the reverb pan?

Anyone have a picture of a chassis they can send?

I believe Loudthud had written a brief explaination about the differences between the beta and alpha preamp designs but I can't find it.

The amp has the original driver but the VC is rubbing - going to drop in one of these http://www.parts-express.com/pe/show...number=299-407

[Enzo]

We may think of an inverter as a functional logic block, but it is after all nothing more than something which puts an output on the end in response to an input on the other. It is essentially a little push pull device. Parallel a couple of them for current handling, and feed them audio, they will drive a load with it. We see that here and there. For that matter I have an old Laney something or other here that uses a 4069 CMOS IC as a preamp stage.

Factories modify things all the time. They discover that an amp has undesirable charachteristics under some circumstances that they never planned for, so they make a change. If they added series caps to those two resistors, I'd assume it was to block any DC oproblems that might occur between ICs.

[loudthud]

You have the rev B schematic, those extra components are on the rev C schematic. The one I have is built on a rev A circuit board. Like Enzo says, the caps were addded to fix some kind of DC offset problem and some RF susceptibility. Just like the Beta amps, 2.2uF non-polar electrolytics abound. I suggest you replace them all.

[loudthud]

More attachments.

For some reason the link to the speaker is not working. Is the stock number correct?

[gbono]

Thanks for the photos very useful - the parts express driver part number is 299-407. Looks like a good fit though I noticed that the original driver has a tab to ground it to the amp?

[loudthud]

Yes, Sunn used a ground wire to some speakers for some unknown reason. I found that Eminence speaker. It should work just fine. Stick with an 8 ohm load. The power amp in the Alpha combos does not have short circuit protection.

Summary of Changes for Alpha 112R from rev B to rev C
Caps to add:
C133 5pF in parallel with R107
C130 2.2uF NP in series with R108
C129 33pF in parallel with R109
C132 10pF in parallel with R154
C131 2.2uF NP in series with R137

Resistors to change:
R150 was 220K, now 18K
R149 was 4.7 meg, now 330K
R151 was 750K, now 39K
R148 was 10K, now 8.2K
R157 was 10K, now 12K
R131 was 10K, now 47K
R138 was 100K, now 220K

R119 should be 4.7K (hard to read on shcematic)

Keywords for search: Sunn Alpha 112R schematic loudthud

[gbono]

Thanks for the list of component changes. I have the amp working now. I did have to replace the PNP output transistor and while I was doing that I just replaced the drivers (TIP series TO-220) and output transistors (now On Semi MJE15022/23 TO-3). Reset the bias on the output transistors using the trimmer pot and monitoring the voltage across the .33 ohm resistors = 6mV.

The remaining issue is random noise. Every so often you can hear a "motor boating - tearing fabric" kind of noise that's fairly low level but annoying enough. None of the controls effect this noise, cleaned all jacks and pounding on the chassis doesn't help. I might have seen this noise on the bench with a scope but it's very random. Suggestions?

[loudthud]

If none of the controls affect the noise, it's probably coming from the power amp circuit after the Power Amp In jack. I would install sockets for all the IC's so you can substitute them without having to remove the PCB. Any chance the noise could be coming in from the power line? Switch the amp off when you hear the noise. If it cuts off immediately, it's coming from the power line. If it fades out, something inside the amp is the source. Always hard to find the cause of these type problems

[gbono]

Okay it is fixed. I did a bit of a shotgun repair on this amp. Replaced all the ICs with sockets, all the NP capacitors, all the bias/drive BJTs and MJ15022/23 to replace the damaged output transistors. Also replaced the zeners in the +-15V supplies since they appeared to be generating/contributing to the noise problem. Quiet as a mouse pissing on cotton ;-)