Yamaha A-S700 Amp Resistance Problem

3601 Views 3 Replies 3 Participants Last post by Conz1995, Oct 28, 2014

Conz1995

Discussion Starter · Oct 27, 2014

I bought a yamaha A-S700 amp at the end of 2012. About a year ago i tested the resistance coming out of the amp with a multimeter. The resistance of the left channel was around 0.9 ohms. But the resistance of the right channel was sitting at around minus 7 ohms. I tested the resistance again this week and the right channel resistance has dropped to minus 10 ohms. Does anyone know what might be causing this?

Even with this discrepancy the amp has never cut out, has never produced distorted sound or had trouble powering the speakers up until about a month or so ago when i noticed a small amount of distortion and a faint and randomly occurring sort of crackling noise coming from the right channel when the amp is up loud. The right channel heat sink seems to heat up quicker and stay a bit hotter than the left. Is this distortion, crackling sound and extra heat the result of a component beginning to fail? what should i do with the amp? Is it ok to keep using it? I can't bring it back as the store i bought it from (peats world of electronics, Parnell st. Dublin, Ireland) has since closed down.

(The amp has been powering a pair of Bose 251's since day one, and recently a pair of pioneer S-P01-LR as well)

Any help would be greatly appreciated. Thank you

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mollie

· #2 · Oct 27, 2014

Conz1995 said:
I bought a yamaha A-S700 amp at the end of 2012. About a year ago i tested the resistance coming out of the amp with a multimeter. The resistance of the left channel was around 0.9 ohms. But the resistance of the right channel was sitting at around minus 7 ohms. I tested the resistance again this week and the right channel resistance has dropped to minus 10 ohms. Does anyone know what might be causing this?

Even with this discrepancy the amp has never cut out, has never produced distorted sound or had trouble powering the speakers up until about a month or so ago when i noticed a small amount of distortion and a faint and randomly occurring sort of crackling noise coming from the right channel when the amp is up loud. The right channel heat sink seems to heat up quicker and stay a bit hotter than the left. Is this distortion, crackling sound and extra heat the result of a component beginning to fail? what should i do with the amp? Is it ok to keep using it? I can't bring it back as the store i bought it from (peats world of electronics, Parnell st. Dublin, Ireland) has since closed down.

(The amp has been powering a pair of Bose 251's since day one, and recently a pair of pioneer S-P01-LR as well)

Any help would be greatly appreciated. Thank you

I don't know this amp, but if it is a class a, b or ab output stage you could look for the following problems which can occur.
1) It could be a problem with the relay contacts of the protection circuit which is in series with the speaker output terminal.(bad contact)
2) it could be a problem in the protection circuit itself, i have seen sometimes a bad electrolytic condensator in this circuit.(look for short sleeve)
3) It could be that the DC output voltage on the bad channel is a lot higher as from the other channel. (check / adjust output offset voltage)
4) It could be that the socalled bias current in the output stage is low.
I hope this info is giving you some help on this issue.
Succes anyway.
mollie

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lcaillo

· #3 · Oct 28, 2014

Resistance measurements with a multimeter on amp outputs are meaningless. You can measure voltage with a multimeter on outputs.

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Conz1995

Discussion Starter · #4 · Oct 28, 2014

Mollie, thanks for your suggestions. Don't have time to look inside the amp at the moment but will check all your suggestions in a few days.

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