SVT-7 Pro dead and free

[tedmich]

Local C list free section gave away Ampeg SVT-7 Pro that is totally dead, no nothing on power up.

first off, fan header is disconnected on main board, so it got HOT, a 3 wire fan with a safety shutoff would be a good idea...

sure enough Infineon SMPS IC (ICE2A165) looks scorched



And R46 (removed) measures way more than 1 ohm

Also over by the funky bare metal 0.02 Ohm resistors there is a cooked resistor and film cap (C16)



schematic says resistor (R115) should be 22 ohms but it measures 54 ohms, bands are long gone...

gotta clean up board



several people think the tube heater causes SMPS chip to overheat, and recommend replacing with ICE2A265 with heatsink. Later versions supposedly got rid of the optos associated with the burned R115/C16


Lots of problems on these units, people pay $700 for this ticking time bomb? jeesh...

[Richard]

What do you think of the class-d amps? I'm a little skeptical of anything that light.

[tedmich]

They are a compromise, huge power, light weight but engineering wise they are balanced on a precipice, so many things can go wrong if the parts spec'd are a little bit too cheap or the design/mfg are too sloppy fast and untested. On many of them the end user is an unhappy beta tester, you're rolling dice, caveat emptor.

[Richard]

Thanks, I kind of got that impression but wasn't sure. I've read somewhere that the power ratings are measured in short bursts.

[tedmich]

Well every amp Mfg is guilty of that, all the wattage inflation, very few use 20Hz-20kHz true RMS power output

Mackie made fun of it,

but probably uses it too now

[Enzo]

very few use 20Hz-20kHz true RMS power output

Well, yeah, instrument amps don;t need remotely that much bandwidth.

[km6xz]

The SVT-7 is a well made amp, with solid construction. That said, it is complex in the power supply, the main defect seen by shops, because there are actually several switch mode power supplies there and they cascade in turn on in sequence so a number of feedback and protection schemes can halt turn on at any of those supplies. Getting one for free is a great deal, even if the whole board was replaced you would be ahead.
Class D amps can't be compared directly to RMS rated amps and that is not a bad thing really. For a long time power was measured for RMS over a wide range of frequencies but that really is not how they are used or how we listen to sounds. Music or speech is nothing like RMS and headroom is limited on AB amps compared to the high peak to average amplitude ratios of normal program material.
Class D are not great at generating high RMS values over a sustained period but in actual use, it is not even even a factor since a peak is usually short duration in sound reproduction/production. So basically the old much ridiculed Music Power rating is more realistic in quantifying the sensation of power of sound....if every brand used the same version of Music Power or Peak Music Power.
There are standards for measuring and specifying power and the one for digital power amps uses a 4 cycle pulse and interpolated to 1 second for equivalent RMS power. For moderate crest factor music it translates well to perceived power. But for highly compressed music/sound, the burst value underperforms.
A big problem has developed in the imbalance between speakers and power amps. It is very cheap now to build a 2000 watt Class D amp but very hard and very expensive to build a speaker system that can handle 2,000 watts. The only way to do it is make drivers very low efficiency so lower SPL. Matching a typical bass cabinet rated at 400 watts with any of the modern Class D, less than 10 lb, monsters is a disaster waiting to happen. Unless a new technology of moving air is invented, the mismatch between affordable drivers and affordable amps has pretty much reached a practical end point. Any bass or full range pa system buyer who is looking for louder levels by adding power is a fool and does not understand what the hell they are doing. Higher power requires lower efficiency so SPL might even be reduced. Besides, high SPL is killing live band work, few clubs or bars can tolerate the extreme levels and still sell drinks or keep customers coming back. For any other reason other than ego, why the very loud sound that prevents most bands from getting gigs and prevents most clubs from ever hiring live bands? The typical musician who claims they are pros but have no idea of being professional in earning an income. They have no business plan or clue on how to make themselves hirable. TOO much ego and not enough consideration of listener experience or the tough row to hoe by the club owner.

[tedmich]

The Ampeg Pro series amps are wondrously complex products and (when they work) they are a rare combination of high performance for low dollars.

Compare this amp in price (and weight!!) to John Champions 1kW tube amp


with 20 x 807 tubes, even this beast has CONSIDERABLY fewer components than the Ampeg and their additive failure rate is tiny compared to the Ampeg's couple thousand SMD components (chosen for lowest possible price) slapped together by machines in a couple minutes.

The wonderfully engineered, amazingly complex Ampeg, that represents the highest possible performance/price ratio, will necessarily have a quite high failure rate, and woe be the tech expected to fix it in a timely and cost effective manner!

[Enzo]

But it doesn't have a quite high failure rate. We hear of some, but what we don;t hear about is the tens of thousands of them that just work day in and day out for years on the road in the hands of professionals. The internet is not representative of any product line.

The pick and place machines and automated soldering that goes into the boards in these things is the same process that is used to make that smart phone you carry everywhere. Those work. And that flat screen TV on your wall? Same thing. When was the last time the computer in your car broke? Made the same way.

[tedmich]

OK OK I'll admit to being seduced by "Internet Induced Spurious Magnification"!

They may well have a tiny failure rate but the few seeking a fix are remarkably sad and pitiful in their (largely wasted) efforts!
One was a guy at Talkbass who went straight from NAD chest thumping into "Wha happened??" lamentations that took nearly 40 pages!

[Enzo]

Sure, and I a while back had some "simple" little Laney amp I had been poking at for over 10 years off and on until I finally figured out its problem. it happens to everyone. A complex circuit like this can become devilish to repair sometimes. The internet is like the service counter at Best Buy. If you hang out there, you only see what breaks. No one brings in or gripes about all the ones that work.

[Tom Phillips]

...The internet is like the service counter at Best Buy. If you hang out there, you only see what breaks. No one brings in or gripes about all the ones that work.

In a similar fashion I often remind people that not everything was higher quality in the good old days. I was in an antique furniture store once when someone remarked that the furniture was better quality "back then." That may he partially true but they totally forgot to consider that the good stuff survived and most of the old crappy stuff ended up in the dump.

[Justin Thomas]

Like all those "golden-age" amps with corrugated cardboard cabinets?
(Cough-Lectrolab-Cough) for example...

Justin

[Leo_Gnardo]

Hey I swapped out my VibroChamp for a LectroLab when the prices on the little Fenders went kookoo. (Paid a month's mortgage with the VC, not bad!) LectroLab's doing just fine thanxveddymuch, and that's authentic vintage cardboard, by the way.

[Justin Thomas]

Hey, I didn't say they SOUND bad, I just doubt most didn't survive their first backyard barbecue or frat party gig, after the cabinets dissolved when the first thunderstorm or beer pitcher fell on them! Personally, I love the little things, but darn, don't pull the chassis more than once, or all the screwholes will strip out!
The one I got to work on (R500-B) sounded amazing with an efficient and decent-powered speaker hooked up!

Justin