Flame On!: Tragic loss of magic smoke

[anatech]

Hi Ken,
That SA-220 is as reliable as one of these gets with used outputs. Let me know how you like it. (how close it is to original).

I also still owe Chris for some the work he's done so far. tsk-tsk.

LOL, no worries there. My shoulders are too messed up to lift it right now. I may return it to a pair of power cords again (instead of the modified single cord). If you have a preference, mail them. Don't go nuts with the size or I will not be able to find a strain relief to fit.

How are the other amps?

-Chris

[peufeu]

This happened the day I decided to test if the short circuit protection of TDA7294 which they are so proud of in the datasheet really worked...



The amp was playing a test tone, something like 1V @ 1 KHz. When I made contact between the output wires, I saw a nice modulated spark which actually sang to the 1 KHz tone. This lasted for about 0.1 second then the chip exploded and caught fire (you can still see the flame residues on the heatsink).

When I saw that I dropped the wires, which touched the wire for the other channel, and...



No fuss ; LM3876 sounds a lot better anyway (and the protection works).

[anatech]

Hi peufeu,
From the looks of it, Counterpoint designed that too!

-Chris

[KBK]

I think I just answered my own question on the replacement of my Belles 450's 2SJ50 and 2SK135 for the higher voltage and Current 2SJ56 and 2SK176 pairs.

This website seems to be saying that they are drop in's.

http://www.sound-smith.com/fets/

Mine are a matched set of 16 of each. Apparently, that comes out to be a small fortune. Ouch. If these guys were selling them, they'd be $60.00 per matched pair. So..I'll be installing $500 worth or more ((2x) matched quads!) of these puppies.

Now for my big question. The "emitter's" on the original design are 0.33ohm. What about swapping that out to 0.2ohm? Does that screw things up? In my feeble recollection of amplifier realities, specifically on a MOSFET design, not exactly. Since I purchased the Caddocks 820's to do exactly that, over a year ago, I dearly hope so. Otherwise I will soon have 16 MP820's at 0.2ohm in the for sale area.


I want mo' better bass.

[anatech]

Hi Ken,
Try again. $145 a pair.

But, they are only worth that if someone pays it. They are getting sales, but not a high volume for sure.

Keep them for your Belles 450's. I know you like them.

-Chris

[KBK]

Double ouch. At their prices, that's about $1.2k worth of transistors. What a bargoon. I guess buying the 16 of each, matched by hafler themselves, off of ebay for a total of $250, was likely a good idea at the time.

The big problem with the Belles 450, is the rise of non linear distortion characteristics, that I have surmised, rise from the same are of design as the counterpoint's issues. Many amps have similar issues - I'm sure you are aware.

They use a single transformer for bothe predriver and power sections. In the case of the 450, it uses the main power PS as the power rail for the pre-dirver circuitry. No buffering of any kind, no seperation from the power section PS at all.

The story goes, as you may imagine, in terms of 'complaints' of it's sound, that it gets congested on 'busy' signals. It's not a great leap of understanding to figure out why. Load up the amp, rails sag, micro-minutely, and add a extra layer of distortion due to corrective behaviour from the feeback and overall functional situation in the pre-driver/control stage..

My solution, in the short term, (over 10 years, now ) was to drop 300kuf (0.3F) at about +/-85V into it, to stiffen those rails up a bit. eventually, I might try a small transformer for the pre-driver alone, and that might finally fix this puppy.

Now, I'll finally fire up one of my 220's this evening, and hopefully things will be uhm..copacetic. then put the ole' 450 back up on the block for more surgery.

[anatech]

Hi Ken,
Have you thought of designing a regulated supply for the front end with it's own power transformer? You could then end up with the same voltage, but regulated. Supply sag under load would then give you full drive capacity (only 2 ~ 3 V drop with lateral fets). You could then reduce your main supply caps in size.

I imagine this would really clean things up. Investment woiuld be less than some other mods I've seen people do. (like your huge caps at list prices - I know you probably got them cheaper).

Also, leaving the front end running eliminates all turn off and on thumps.

-Chris

[KBK]

That was my greatest worry, with 85 volt rails. DC offset at turn on, from the predriver turn on cycle. Right now, it's zero. So I'd have to leave it on (predriver) all the time. No big deal. IIRC, I've had a few amps that were done that way. However, I have no desire, whatsoever, to destroy the simplicity and connectivity of this circuit. I don't want even one single relay to enter the equation. I am aware of very few amplifiers that are as well thought out in terms of overall pathway.

The charging issue and phase angle aspect are not as bad as one might think, with about 20kuf per rail in smaller caps (and about 50uf of film caps), right at the outputs, besides the 47kuf big cans.

Time fo finally fix this puppy. One bit at a time, though. 0.2ohm emitters and the output fet swap first. I hate it when things go awry, due to 14 changes enacted all at once. Then you have no frikin idea where the problem originates from.

[anatech]

Hi Ken,

I hate it when things go awry, due to 14 changes enacted all at once. Then you have no frikin idea where the problem originates from.

I guess you got all excited and ahead of yourself. On a new mod, I generally try one thing at a time in case things go backwards a little. I was very careful with the updates to the Counterpoint SA-100. I am still in process. one thing at a time.

So take your time Ken.

You really ought to try the front end supply. All they needed to do was give some higher taps to the main secondary windings. I suggested a small transformer for this for simplicities sake. You could then use a timer for the main transformer. Little current draw on the from end should keep it up longer than the main supply, eliminating any turn off thump.

-Chris