channel distortion

sriegel

Member
2001-11-24 1:24 am
hello all,
well, i finally got my aleph5 up and running, sort of. this is the wierd trouble i have;
i built the class A case/heatsink from 'lottusblossum's' web-site, i.e. the whole case is connected together.

i developed an intermittent static in one channel so i opened that side up to check it out and all measured fine. so i powered up and it worked fine. i "closed" it back up and tightened down the bolts, fired it up and there was that static??? only out of that one channel, the other is fine. and when i power down, it takes a long time for that channel to stop playing music.

i was thinking that maybe i was shorting out the output hexfets somehow, but can't figure it out and i don't know how to go about testing it to see.

any thoughts?

cheers,
scott
 
sriegel said:
... my amp sounds bad ...

Sounds like the biasing isn't right. Since Class A draws more or less the same current all the time regardless of output, the fact that one side takes a long time to "die out" seems to me like a lack of bias. Maybe a bad connection somewhere.

This would also explain the horrible sound you must be experiencing :eek:

Good luck
 

sriegel

Member
2001-11-24 1:24 am
Well, it's as i thought, sort of. when the heatsinks aren't touching the rest of the case, it works fine. turns off right away and the distortion is gone. so, do i have a comprimised sil-pad? it seems that maybe i'm shorting the outputs to the case somehow. does that seem like a reasonable culprit? what's weird is that it worked fine for ~ 3 days at around 3-4 hours each before this happened. and i didn't change anything during this time. any thoughts as to what would cause the insulating pad to fail over such a short time? they are kapton pads, supposed to be alright.

thanks for any input!
cheers,
scott
 

sriegel

Member
2001-11-24 1:24 am
hey all,
i replaced the insulators with mica and then electrically isolated that channel with electrical tape to be safe. everything seems fine, however, when powering up, there is still some static in that channel and the LED changes intensity, like there are voltage spikes or irregularities. this all seems to die out after a minute or two and all is fine.

any thoughts as to what would cause this weirdness?

cheers,
scott
 
Hmmm...
You ask for a reason, but <i>not</i> a good one. Why didn't you set me an easy task, like cleaning the Stygian stables?
Uh, well, okay, how 'bout this? One of my degrees is in geology and I find mica (biotite, muscovite, et al...) inherently more interesting than a man-made material.
Not good enough, huh?
Okay, well, from a tree-hugger's perspective, mica is a natural material that won't hurt bunny rabbits, box turtles, or kangaroos. The research on pandas isn't in yet, so they'll just have to take their chances.
Ah, fooey, go for the jugular...
I'm a naturally suspicious, cynical critter who feels that every new technology is guilty until proven innocent. I've heard too many people complaining about problems with 'em. In my view, if your cooling is so marginal that the 1 bazillionth of a degree difference in cooling between mica and Kapton is all that stands betwixt you and a puddle of slag that used to be an amplifier, you need bigger heatsinks.

Besides, you can see through mica...and we all know that transparency counts in audio.

Grey
 
static

Scott,

Always check the least obvious..

You mentioned you still get static after changing the insulation.
washers.. but disappears after a little while.

The problem could be heat or a fault mechanical connection

1. Have you metered the insulation for each hexfet (with hot and cold heats), there could be a problems with the mounting bolts on the hexfets.

2. Are the heat sinks and the chassis earthed to the mains plug ground(not the 0 volt ground which should only be in connection with the chassis at one point).

Test for zero ohms for the faulty channel. or tighten any bolts or other offending hardware.

I have found that if the heat sink is floating you get static in the output .

3. The power supply.

If the Led flickers either the positive rail or output are shifting for some reason obviously.

This could be due to loose screws in your power filter capacitors for that channel or a dry joint somewhere(I had this problem).

4. The fact that you only have this problem in one channel suggests that all things being equal it is not component failure if everthing was previous replaced.

(But not with standing this one of the capacitors (or zeners) could be routed so replace if all else fails.. capacitors are strange and they can be intermitently faultly and change with heat).

I hope these suggestions help at least solve the problem by process of elimination.

regards

macka