As mentioned previously, I’m rather fond of SAE A502 amplifiers. Enough so that as I’m starting to think about biamping or triamping my system, I’ve started shopping for one or more A202 amps, the A502′s 100W little brother. And I found one on eBay, and it arrived this week.
I set it up in my stereo to drive the center channel speaker (for now), got everything connected, and turned it on. Pop. Turned it off. Pop. On. Pop. Speaker off. Pop. Speaker on. Pop. Amp off. Pop. No input signal at all, and pop pop pop.
SAE amplifiers don’t pop. They have relays specifically to keep startup and shutdown pops from making it to the speakers; so (I figured) I must have an unwanted DC component to my output. I ran this idea past Ron and he agreed that’s what was happening, so I measured the DC voltage at the speaker outputs with no input signal:
|Channel||DC Voltage||Channel||DC Voltage|
Huh. Looks like it’s time to adjust the DC offset.
Interestingly, I’m only using one amplifier channel for my center-channel speaker, and I happened to plug it into the left amp channel — the bad one. Had I plugged it into the right, I wouldn’t have noticed this problem for a long time, until I started using both channels. And that might have been when I set it up as a treble amp to drive tweeters, which might have blown out from the DC voltage. That would have made me cross.
Why is DC on a speaker output a bad thing? Well, each driver (the individual cone assemblies that mount in a box to form what we call a “speaker”) has an electromagnetic coil of wire in it. Current through the wire attracts and repels the back of the driver toward and away from a permanent magnet, causing the driver to move in and out, move air back and forth, and make sound.
The coil of wire also gets hot. The fine wire used in driver coils has a much higher resistance than the large wire between the amplifier and the speaker, so the coil is where the majority of the amplifier’s power is dissipated. Some of the amplifier’s electrical energy is turned into mechanical energy to move the driver, but much of it is turned into heat. Too much heat on a fine wire turns into a bad thing. And extra DC current doesn’t make sound; it just heats the wire.
Not much, to be sure. In my case,
.64V / 8Ω = .08A
.64V * .08A = .0512W
which isn’t much for my 150W speakers to handle. But it doesn’t do any good . . . and it pops when I turn the amp on and off.
This small DC output voltage comes about from an even smaller unwanted DC voltage in the preamplification stage, faithfully amplified through the power stage and delivered to the outputs. Because of imperfections in semiconductor manufacturing and the way preamplifiers are designed, this DC offset is virtually impossible to design out. Instead, preamp stages have a DC offset adjustment potentiometer, to adjust that particular preamp’s DC offset back to 0V (or very close).
Inside the A202
This morning I opened the A202 to make the adjustment. I was curious to see how much it looked like its big brother, and the answer is, not very much.
The power transformer is similar, and I recognize some other components and circuits, but the layout is completely different. This makes sense, given the smaller physical size of the A202, and its smaller power demands. The A202 is laid out asymmetrically, with power transistors on the amp’s right side (this picture is taken from the back, so our left) and the power supply on the amp’s left, rather than with the power supply sandwiched between two sets of power transistors.
The preamplification stage is on this smaller PCB located at the back of the amp between the large heatsinks. The DC offset pots are clearly visible and easy to get to at the back edge of the amp. (Thank you!)
The speaker terminals (barely visible in the upper picture) are an odd design, rather than standard binding posts, and my voltmeter probes aren’t quite long enough to make contact. So I inserted wires into the speaker terminals and used gator clips to connect to the voltmeter. I powered up the amp and carefully turned the pots until the DC output was as small as I was able to get it. I was disappointed that I couldn’t get the right channel all the way to 0V (on my meter), but 5mV isn’t bad.
|Channel||DC Voltage||Channel||DC Voltage|
I actually first made the adjustment with the amplifier cold, and then realized how silly that was. After letting it warm up for twenty minutes or so, one channel had already drifted up to .025V, so I was glad to have remembered to go back and fix it.
Any time you perform service work, you should always clean whatever you’re servicing to the best of your ability; it makes a huge difference to the customer’s perception of your work (even if the customer is you). Ron cleans and polishes VCRs he repairs; good auto mechanics clean the entire area around whatever they’ve replaced (and run your car through a car wash if they’re really clever), and I wipe down cases of PCs I repair for friends and family.
This amp was no different. The inside was full of dust, so I took it to my brother’s house and used his air compressor to blow it out.
The outside was more interesting. I don’t know what the former owner did with this amp, but the case looks like it had a fight with a parking lot and lost. Lots of dings, nicks, and scrapes.
I felt around the edge of the front panel for any dings with sharp edges, and carefully filed them flush with a fine file. Then I use a black permanent marker to fill in every shiny spot. It’s not a perfect match for the original finish and you can easily tell the difference from up close — but you can’t tell the difference from across the room in a normal listening position, and that’s well worth the effort.
About half of the model number on the front panel is scraped off, and I’m not sure how to repair that. It’s white (or near-white) paint, probably silk-screened on, and I think I’m more likely to make a mess than an improvement. I’d welcome experienced suggestions on how to touch that up.
Putting It All Together
I reassembled the amp and reinstalled it, and the popping sound is gone. Even with my ear directly in front of the speaker, I don’t hear any noise there — just the relay clicking in the amp itself.
Here’s what my stack looks like now. It’s in an old PA cabinet on which I mounted soft-wheeled castors for convenience and to protect my softwood floor. The cabinet has rack rails, but the amps aren’t mounted — just stacked. I need to get nylon shoulder washers to protect the amps’ front-panel finish before I’m willing to rackmount them.
From top to bottom,
- my Sony surround processor / preamp sitting on a rack shelf
- a rackmount, surge-protected power strip (love this!)
- my original A502 amp, running the main speakers
- the A202, running the center speaker
- the A502 with replaced relay, running the rear speakers
- another A502 that I bought working, running the subwoofer
I have two more broken A502s waiting to be repaired, plus a couple of other related projects, so there should hopefully be more along these lines soon.