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QED A240CD Blows Fuse on Powerup
QED A240CD Blows Fuse on Powerup
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Old 24th January 2018, 11:34 AM   #91
rhiollyrubgt is offline rhiollyrubgt  United Kingdom
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Join Date: May 2010
All done. Thanks such a lot for all your help. Sounds great!
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Old 7th March 2018, 01:00 PM   #92
sensai is offline sensai
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Join Date: Nov 2014
Default QED 240 SA, problem with pre-amp

Hi. I have two of these amps, a 240 CD (upgraded with SA board) used as a pre-amp as one power channel is faulty, and a 230 which is used as a stereo power amp.

1. The 240 has gone faulty, the volume is now very low with dull sound, but equal on both channels. PSU rails are about right. It's the later version which does not have a buffer IC (next to the volume pot). Just wonder where to make a start?

2. For last 3 weeks I've been driving my Quad 57s with the 230 as it sounds clearer and sharper than many expensive amps (including valve) I have tried over the years; why I don't know, but it does! Same thing with the 63s I have. However, I have just discovered that the manual does not advise driving electrostatics.

Last night I decided to lube the 230's pots and sockets and check the bias. I found that it was around 63mv, so I corrected it to (I presume) 16mv. The heat-sink is now cool as opposed to "very warm" before, but the sound is
now duller and has lost its sharpness.
Any comments welcome
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Old 8th March 2018, 05:15 AM   #93
Ian Finch is offline Ian Finch  Australia
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Join Date: Apr 2010
Location: Coffs Harbour, NSW
When we overbias an amplifier, as the 63mV measurement across the output transistor emitter resistors suggests, we force it to remain in class A operation up to a higher output level than required to minimise distortion. At low volume levels, it might even appear to perform like a class A amplifier. As you find though, QED amplifiers weren't designed for class A operation or the nasty load that Quad electrostatic speakers are.

There may be something else at play here with the way you have used 2 amplifiers to keep your system working but that would need a closer look and probably bench testing with appropriate instruments to see what goes on. As there are already unresolved problems there, you should fix the A240 before trying to change anything with the A230 and a professional repair may be best. If that seems expensive, just consider what it costs to buy new and what problems you'll be facing all over again with untested fleabay gear from liars looking to offload their own problems.

Meantime, I suggest you beg, borrow but not steal a much larger integrated amplifier in good condition and listen to your speakers with that for a few hours and see what you think. It may even be that its the sweet class A sound you really seek.
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Old 8th March 2018, 08:28 AM   #94
sensai is offline sensai
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Join Date: Nov 2014
Thanks Ian for the comprehensive reply. You've confirmed my suspicion about the biasing, especially as the heat-sinks begin to get hot! I am fortunate in that a friend regularly trades on ebay so I get the opportunity to try various amplifiers etc and hear the the effects on frequency responses that the Quads impose on them (bearing in mind that older amps can have deteriorated from their original specs).

I use moderate volume levels in a small room 16ft long, so yes the power being drawn will be small. Prior to using the 230 my Sony 1130 (although not class A) drives the Quads very well. I also hear that amps that are specified to larger currents into low impedances can also be suitable.
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Old 8th March 2018, 01:14 PM   #95
Ian Finch is offline Ian Finch  Australia
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Join Date: Apr 2010
Location: Coffs Harbour, NSW
The TA1130 amp probably produces the current necessary to drive the Quads comfortably at your sound levels and if you still have it, that could do for a reference amplifier, if has had at least one full service including electrolytic capacitor replacements, since it was built. If it's still battling along with >40 year old main power supply caps, it's high time you did something about that amplifier too.

Compare again, now that you have the A230 bias sorted and see what you think. If you find the sound is still unacceptable in your QED setup, I don't think it will be just a case of the wrong type of amplifier but there is going to be a problem there - perhaps it needs capacitor replacement or there may other components such as semis that have died too.
First things first.
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