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Old 26th April 2005, 02:56 AM   #1
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Default My chip amps don't travel well...

I took along two chip amps to a friend's for an amp comparison. Of course, both misbehaved!

Both went noisy - in my system AC hum measured at the speaker terminals is less than 0.3 mV on all channels of the two stereo amps. I didn't have a chance to measure at the friend's place, but my guess is that it was at least 10 times higher than this with the quietest of the two amps (his speakers are around 94db/w/m sensitivity - so 3mV is easily audible at the listening position.) The other amp had around 10 mV hum as a guess as it was substantially louder. The two amps are very different in terms of grounding - one is completely floating from the safety earth (it's on a piece of wood so it's inherently unsafe) and the other is in a heavy aluminium chassis with the circuit ground attached to the safety earth through 15 ohm NTC thermistors. The latter was the quieter of the two.

The second problem is that DC offset increased dramatically. One of my amps only measures 1-2 mV on both channels at home - at the friend's this same amp was 250 mV. The other amp measures 20-40 mV and increased to over 100 mV in my friend's system.

It is interesting that these problems persisted even with no input to my amps. So could it be that the speakers / speaker cables caused these problems? I'm not using a Zobel on the output, or a series resistor / inductor. Maybe the amps didn't like his silver foil speaker cables? I wouldn't have thought a problem with oscillation would manifest itself so obviously (or have any impact on the DC offset.) I didn't notice either amp running unusually warm - but we didn't play them for long because they didn't sound all that good.

I couldn't measure any DC offset from the output of my friend's DAC (and I shouldn't as it has a valve output stage with coupling caps.) Other amps did not exhibit these problems (an AKSA, and a Sonic Impact digital amp) in the same system.

Any thoughts?
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Old 26th April 2005, 03:35 AM   #2
homer09 is offline homer09  Canada
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gainclones are picky with speaker cable, too much capacitance and things get real ugly. 47labs couples their gaincard with 24 gauge single strand speaker wire for a reason.
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Old 26th April 2005, 04:20 AM   #3
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Quote:
Originally posted by homer09
gainclones are picky with speaker cable, too much capacitance and things get real ugly.
This I know, but have not directly experienced. I'm not fond of jumping to conclusions, however, and would like confirmation / corroboration that these symptoms fit the cause rather than just assuming they do.
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Old 26th April 2005, 04:53 AM   #4
lgreen is offline lgreen  United States
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Very interesting. I have measured mine many places and the DC offset was always within 1 mV everywhere. My travel problems were due to poor construction. I thought DC offset was usually an artifact of the chip, not the quality of the power. And, like you, I've got a NTC in line with the AC ground prong. KT60 or something like that.

One thing that you can do if no one here is able to help is to have your friend transport his speakers and cable to your place, and see what the measurements and hum are with that setup.

Inverting or non-inverting?
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Old 26th April 2005, 05:04 AM   #5
Aghead is offline Aghead  Canada
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sounds like RF, noisy fluorescents or something nearby.

what are you using for the mains cable?
litz CAT 5 (solid) is a cheap option which will drop your jaw if you haven't tried it - both a filter and more.
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Old 26th April 2005, 05:30 AM   #6
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I suspect either RF interference (especially on the wooden one. I also like mine to have Zobels, input resistors with a small cap to remove RF, small value bypass caps at the chip pins, and sometimes I put a smallish cap accress the feedback resistor to bring the gain down at ultrasonic freqs. I also try to filter the mains if at all possible.) or construction quality. On some of my easlier homebrews, my construction wasn't up to par, and sometimes wires would rattle lose when I move them around. I doubt different speaker and power wires would make a difference

Did you just suggest making a 120 volts AC power wire out of CAT5 netowrk cable?!?!??!? sounds crazy and unsafe. I just stick with the generic computer power cords, and use the cat5 whenever I need some generic low voltage wiring that isn't going to be carrying an *** load of current.
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Old 26th April 2005, 05:52 AM   #7
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Quote:
Originally posted by Dr. Photon
or construction quality.
I checked that the amps hadn't suffered from the trip - they still work perfectly in my system.
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Old 26th April 2005, 06:15 AM   #8
lgreen is offline lgreen  United States
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Quote:
I need some generic low voltage wiring that isn't going to be carrying an *** load of current.
Never heard it put that way before, you crack me up!
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Old 26th April 2005, 07:28 AM   #9
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Jeff

I would suspect the foil cables - in some designs they can become quite capacitive. An interesting experiment would be to measure the offset and hum without the speakers connected.

Is it possible your friend's speakers are a lot more sensitive and hum becomes more audible
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Old 26th April 2005, 07:46 AM   #10
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Quote:
Originally posted by analog_sa
Is it possible your friend's speakers are a lot more sensitive and hum becomes more audible
They are some 6db more sensitive than mine, but 0.3 mV is *not* going to be plainly audible at the listening position - which is the measurement I get at home. I didn't have the opportunity to measure at my friend's, but I know it was much more than 0.3 mV.

I know what 10 mV of hum sounds like - I've built quite a few SET valve amps.
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