Giovanni Stochino Power Amp

[steevo]

I have been testing my stochino amp that I built and never finished many years until recently when it was first published, and so far have destroyed a IRF9640 and a IRF640 mosfet transistor. I have been testing it using square and sine waves up 100khz the specs say the -3db power point is 120khz so it shouldnt have any problem with this. I was only drawing about 2.1 amps from recomended power supply and driving a 4.7ohm resistor load. The amp should deliver 120 watts into 8 ohms and probably more into 4.7ohms So there was plenty of capacity left. I was Luckily the mosfets did not destroy any of the eariler stages when they failed but its still annoying. The mosfets were faily well matched. Maybe I had a poor batch or that these are just poor and ill suited for this application. Has anyone had any similar experiences.

Steevo

[Holger Barske]

I don't think building a 120W/8R amplifier with a single pair of output devices is a good idea. And IRF640/IRFP9640 aren't an ideal choice, as TO220 devices can't dissipate very much heat.
You won't come even close to the 150 Watts the datasheet promises, no matter how big your heatsink is.

[jacco vermeulen]

The 1984 StoCChino* amplifier i recollect does state TO-220 devices, but has ~35Vdc rails.

The Stocchino 300V/uS design, printed in Electronics world in 1997, does 120W/8 but has 2 pairs of TO-220 irf640/irf9640

* CCh=k, Ch =g

(can't wait to read your lines on the DiS article, Holger. A shame the mag always arrives so late here)

[steevo]

The power amp circuit has 2 pairs of mosfet outputs, see the attached diagram.

[smithy666]

I successfully built this amp several years ago and have been using them without problems since. The only issue I had was when first setting the bias current I didn't let the amp warm up enough and then wound the bias current up too much, which then caused a catastrophic failure. You only ever do that once...
Second time around worked like a charm. I'm pretty sure I ran a sine wave through the amp right up to 120W output (I used the higher 58V driver rail voltage) without any problems (but not at 100kHz). The power supply is regulated 55-58V for the driver stage and unregulated 48V for the output stage.

I have a question - why were you testing at 100KHz with >100W output? If you wanted to test the bandwidth, why not just run a watt or 2 through? From what I've read of other designs, few would stand up to such a test either.

Good luck - well worth fixing, as these are great sounding amps.

[steevo]

If the amp has been designed properly then it should full fill its specification as with any well designed and engineered piece of equipment. You dont buy a car that claims 0-60 in 7 secs only to find that it blows up if when you try it or instead of 7 secs you only get 8 seconds. I was simply testing it thoroughly cheking its performance.



Steevo

[smithy666]

Steevo
I understand what you are saying, but how long were you driving the amp at this power before they blew? John Atkinson from Stereophile tests all amps by driving them at 1/3rd of their rated power for an hour (within the audio bandwidth), which he describes as the worst case scenario for an amplifier. Most are too hot to touch and ones with thermal protection shutdown. So while amp specs say 100W into 8 ohm, you can't possibly drive a 20kHz square wave at full power for any extended period of time, unless you have huge heatsinking and/or fan forced cooling. How hot did the amp heatsinks get? Maybe they went into thermal overload?

[janusz]

Hi,

IRFP640/9640 devices are not well suited for continuous high power application of that magnitude. Alex Soton reported problems with his Stochino amp running hot a few years ago on this forum. As I bought four boards of the amp as well we started to exchange ideas how to solve the problem.

Alex also intended to get rid of the NFB electros. It appeared that from the beginning both of us were somewhat suspicious about the use of IRFP640/9640 devices in this design. I suggested that IRFP240/9240 or SFH9240 devices would better dissipate heat.

Also the use of 2n5551/5401 input pairs created matching problems if the NFB electro was to be removed so we decided to use 2sa970bl/2sc2240bl instead. Alex managed to match their hfe within 1%, my batch allowed me to match them within 2-5%.

Anyway, after Alex IRFP640/9640s blew he increased gate resistors to 100ohm and used IRFP240/9240 pairs and all problems disappeared. The amp with 150mA bias was running cool, while before he could not go above 120mA.

As I had a few other more urgent things to do my boards still miss a few parts and amps are not yet running. Running the amp without NFB caps was less successful. Offset in one of his amps was almost acceptable at about 40-60mV but the other channel was above 250mV. I do not know if he has managed to fix that problem since then as he also had a few other things to do apart from experimenting with offset.

Most of his experience with Stochino amp is available on his site
www. soton.ac.uk

cheers,

[smithy666]

Hi Janusz
I saw on another post that you (or maybe is was someone else on the thread) was looking at a way of matching devices. Have you seen the Elektor circuit (Giesberts) for the power mosfet matcher? I am constructing this at the moment. It tests 2 x N channel and 2x P channel at the same time. Was published in Nov 93 issue.
Can you also tell me the link to Alex Soton's site?
Thanks

[janusz]

Hi Smithy,

Alex'es site is
www.soton.ac.uk
It's the addres I gave in the previous post but without that space.

Yes, some time ago I was looking for a good circuit to match Mosfets. I got one used by Nelson Pass, one from ESP and some other one. I did not see the one from Elektor so I'd be grateful if you could send me a scan of this circuit with some description. It sounds interesting.
my direct email is:
janusz.pradzynski@dpi.wa.gov.au

I hoped to have my first two Stochino modules finished by the New Year but first I had to fix my Yamaha c-65 preamp I bought on Ebay last year. It took me some time to fix all problems and when assemblig somehow I shortcircuited two resistors which were soldered vertically. If they were soldered with theother ends down nothing would have happened but in this case two emitters were shortcircuited and smal part of the board damaged by burning transistor. Hope to fix it this weekend. That will be no problem if the input transistors are still OK.

cheers,