Quad 405-2 advice

[vintageampguy]

Thanks, I'll check that out!

[dave walsh]

Thanks for this useful thread. I have a "retired" 405 which developed switch on "thump" several years ago. Forum members advised not using it and seeking help! There seemed to be a consensus that the "crowbar" based protection circuits were somewhat too pro-active, with the risk of a cascade failure! I think that a very large Blue capaqcitor on the chassis was considered suspect? Don't get me wrong. I don't pretend to understand either the theory or practise but I believe there was a dedicated repair kit for this problem. I aim to get someone experienced to take a look eventually

Dave W

In a way, it reminded me of some of the late model VCR's that were so sophisticated that they would shut down at the drop of a hat, when older Tapes ran ok on less 'advanced' models!

[Jez1234]

Only capacitors on the chassis that spring to mind are the smoothing caps.. which are large... and the STC branded one's they used (probably others too) are certainly blue.

We had a switch off issue at Musical Fidelity with the big mosfet amps, A370 etc, as if there was anything much in the way of a difference in the capacitance of the smoothing (12 x 22,000uF per channel) then one rail going down before the other would cause a gentle whistle for a while. The "cure" was a resistor across the higher capacity side of the smoothing caps... I didn't consider it an issue personally as it was just what was happening with the last few volts on the smoothing caps, NFB no longer working, maybe 20mW's of short wave radio whistling sounds etc and of no threat to speakers but management insisted on the cheapest quickest "cure" possible

[Radio Wrangler]

That's the direct solution to switch-on and switch-off thumps. Have a well balanced, carefully biased amplifier design without any capacitive bootstraps that can operate and have feedback control right down to low power rail voltages, then get the power supplies (positive and negative rails) to track reasonably well both on charge-up and run-down directions, with similar dv/dt.

David

[fetteler]

I wonder if, on the 405, delaying the power to the op amp zenner supplies until the rails have settled down might improve things?
It would be relatively easy to test.

Steve.

[vintageampguy]

I replaced the old filter caps with new Kemet ALT22A153DE063. They were the same size as the old caps, so fit real nice. The old caps tested ok, with capacitance very close to rated capacity, and ESR at .05 & .06.
The new Kemet's rated at 15kuf, tested about 13.5kuf, 10% down from capacity, a bit disappointing, but within their 20% spec. The ESR was about 0.01.
Amp measurement wise has noticeable improvements:

• Amp now outputs rated power >100 watts before clipping (about 94 watts before)
• Clips more evenly, top, bottom, two channels
• THD measurement is now around .01 at full power, was very high before
• It appears the large turn on thump is gone, although there is still a small turn off thump
• There is still a bit of hum (L channel) and hiss (both channels) when ear very close to speaker

[Radio Wrangler]

Originally Posted by vintageampguy

The new Kemet's rated at 15kuf, tested about 13.5kuf, 10% down from capacity, a bit disappointing, but within their 20% spec.

Not surprising at all The capacitor manufacturing industry through improved process control can now make capacitors to much tighter tolerances than before. Their bean counters have noticed this and exploited it to make capacitors within the trad tolerance bounds that are promised, but biased to the low capacitance side to save a little on material costs. Tiny saving per part, but multiplied by their turnover means more bonuses for them.

15kuf =15mf but that risks confusion because of people using mf for microfarad!

David