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Archive for January, 2014

24
Jan

PMPO – Peak Music Phantom Output

Having a signal generator on the test bench helps with trouble shooting some of the audio circuits I assemble from time to time, but it would be nice to have a signal tracer as well. As in audio signal tracer.

Initially I had the idea that I’d assemble a token amp around one of those low power TDA chip amps. However when I started to cost out the plan with a case, pots, parts and so on, I thought I’d better check out the “other” source.

Of course, what better source for mislabeled widgets than the eBay supported off shore market. I didn’t need anything with a whack of output power, actually didn’t even need stereo inputs.

I immediately found a plethora of these styles of “iPod” amps for the low cost of about two Starbucks lattes:

IMG 4499

This thing is a MA-170. Whatever that’s supposed to infer. Digital Cinema Sound or DCS (darn cheap sound). Looking at the photos of the rear I found it looked like most of those car stereo power amps:

IMG 4500

Both the back of the amp and the eBay listing claimed this was a 2 x 100 watt amp. Which, incidentally, you could run off a 12VDC 2A wall wart. There was no familiar and standard RMS rating, just this PMPO moniker.

PMPO stands for Peak Music Power Output. It’s supposed to be a measurement of the peak power you could get from the amp in ms (milliseconds). Which is a pretty useless figure because even the shortest musical note lasts longer than the allotted PMPO measurement time. So, write down any number you like. It doesn’t mean squat any way.

You can also lump PMPO in with PEP (peak envelope power), amp ratings in IEC (Behringer used to do this a lot). Folks, if it doesn’t say R.M.S., you have no idea what it is.

If you’ve ever done anything with car stereo power amps or probably any amp at all, you’d probably already know that a 12V 2A supply isn’t going to generate 200 watts of anything. Except maybe warmth when it goes up in smoke.

Of course I knew this going in, but hey, I wanted a budget amp, so that’s exactly what I expected. And got.

When the box finally showed up, it listed the actual specifications on the side of the box. It clearly said, “2 x 4 Watts RMS”. Obviously a slightly smaller, and more realistic number than the 2 x 100 claimed elsewhere.

I contacted the seller and suggested their listing facts were incorrect. No, I didn’t outright say they were lying, I was actually quite nice and gently suggested they update their listing with the accurate information. To my efforts I received a reply that the box was “just any old box” they used for shipping and had no bearing on the contents. Considering the artwork and specs printed on the box were an obvious match for the contents, ya, they were lying.

Therefore, as Dave Jones from the EEVBlog says, “don’t turn it on, take it apart.”. I did.

IMG 4501

Removing the front splined knobs was the first trick. I swear someone must have pounded them on with a sledgehammer. I have a special tool for removing knobs on guitars without leaving scratches and there was zero clearance under these knobs to even use it. I pried the crap out of them with a screw driver then took off the four front mounting screws.

IMG 4502

You can just see an LED to the left of the volume pot, it lights up the translucent ring to show power is on and maybe, smoke to follow…

On the extreme right edge you can see the TAD chip “clamped” to the wall of the case.

IMG 4503

Slotted holes to allow for some “wiggle” room of course. You have to unscrew those two screws, let the clamp plate fall inside the housing to get the thing apart. Because the TDA chip doesn’t fit tight against the casing of the box, they put an aluminum spacer in there. Something else to contend with when you reassemble it.

IMG 4504

Looking at the PCB, there are as many jumpers as there are components. The 4558D op amp for preamp/tone controls, and the TDA chip on the right edge. That stuff that looks like silicon grease on the TDA chip isn’t. It’s silicon. Off shore construction seems to have found more uses for hot melt glue and silicon than Red Green has for duct tape.

IMG 4505

If you had to scratch build this circuit, it would cost more than buying the whole thing as is. So in that respect it’s kind of nice.

However the fact remains that the TDA is a TDA7266sa chip. An obsolete chip and its data sheet says 2 x 7 watt. Even the data sheet doesn’t say 7 watts RMS, PEP or PMPO. Nope, just 2 x7 watts. So you roughly figure RMS is about .707 times the power, well, surprise. You get 4 watts.

When I pointed all these facts out to the eBay seller, well, apparently I am invisible. I am quite happy with the price I paid and the product I got. But trying to pass a scooter off as a Harley is never going to work for me.