The Crescendo woes again

[RaviRao]

Hi Everyone, Great to be part of this forum at last... I have a request to anyone that can shed some light on the current issue with my Crescendo amp.
1. The amp was constructed by me way back in 1989!!
2. Has been working till last month!! Dec 2007
3. Now I get a heavily distorted sound with a lot of DC on the output --around 20 Volts without the LS connected of course.
4. I have already blown one channel of my Klipsh horns and do not dare connect these back till i fix the problem.

I know this is really insufficient info to suggest anything ...but is there anybody out there who might have some suggestions to go about fixing this.... Ive looked for the scematics and alas they are nowhere to be found. The MOSFETs are already history I beleive and so Iam really open to suggestion on the next set of actions to take.......HELPjavascript:smilie('')

[EC8010]

I've moved your post because it might get more replies here. I expect you've blown up one half of you output stage at the very least.

[RaviRao]

How did you guess!! I have since started looking for MOSFETS to replace them with. Any ideas ? I have also dug up some mods people have done to this amp over the years and the one with the opamp to correct for the imput offset looks very neat.

Where do I go from here........

[linuxguru]

The Crescendo was prone to a lot of instability even when it worked - it was prone to oscillate under a lot of conditions, including reactive loads. Your FET(s) may have blown as a result, or for other reasons including output shorts, electrostatic damage to the gate oxide, etc.

The original (Hitachi?) FETs are probably going to be hard to find, so you may have to modify it to use commodity FETs like the IRF540/9640 instead. Search the forums for info on how to go about this.

Just BTW, just about any entry-level gainclone/chipamp today works more reliably, has better specs, sounds better than a Crescendo, and costs a small fraction of the Crescendo. You may want to take a look at the TDA7294 or the LM3886.

[pjp]

If you are near bombay, visha/vega sell the 2SK1058/2SJ162 for around 350 rupees.

linuxguru, you have any objective evidence that a TDA7294 is better than a crescendo ? I'm in the process of building one ...

[Leolabs]

Sounds like a thermal drift to me due to heat problem and all the DC operating points starting to drift and cause all the stupid problems.

[RaviRao]

Thanks for your inputs linuxguru. Just for your info the Crescendo once stabilized worked like a charm for around 19 years without any instability problems whatsoever. I think that goes to say something. Any how I have spent some time on the amp and have seen the following .....DC drift was one of the reasons as the zener diode D1 was showing around 4.7 volts and the damping cap across it C8 looks completely open( tested with a DMM). The result was a blown MOSFET and the worst part is it took my 12" woofer( Klipshhorn) with it...amazingly my tweeters survived!!!

Now I am looking at Lateral Mosfet of the type pip has suggested!! Thanks pip I shall check these guys out and try to procure the devices.

I am currently looking to see what mods ,if any,will allow me to ressurect the Crescendo's again. Currrently my thinking is to gut the internals and make a brand new MOSFET if I get the time!!!!

[dobias]

Gentlemen,
I hope you aren't discussing the Audire Crescendo amplifier I just purchased on eBay.
Please describe what amplifier design you're talking about.
dobias

[RaviRao]

Dear Dobias,

We are talking about the Crescendo /Mini Crescendo MOS Fet amp described in the Elector Magazine way back in 1984 or thereabouts. I can send you the schematics if you would like to see them to be sure. Just let me know you want them.
Regards
Ravi Rao

[dobias]

RaviRao,
Thanks a lot for your response. I'm relieved to know you weren't referring to my Audire Crescendo. No, I don't need any info on the component... I wouldn't know what to do with it. While I can troubleshoot electrical, I am a raw beginner when it comes to SS electronics. My last contact with electronics was over 50 years ago with tube circuits.
dobias