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Messages - Kaz Kylheku

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Amplifier Discussion / Re: Safety Tips
« on: July 15, 2011, 06:43:47 PM »
(Updated 11/02/09)
-It's always a good idea to use a hold-down footswitch when probing around a live chassis. This way your foot will release the electricity in case of a shock. Harbor freight sells these switches for like $10. (Two kinds are available, make sure it's the hold-down kind not the on-off kind.)
[ ... ]

Please add to the list...

Sure: when live probing, instead of, or in addition to such a foot switch, plug it into a GFCI outlet.

The foot switch is basically a part of a "human GFCI": your nerves are the current detector, and your reaction is the interruptor.

If I had to bet, my money would be on the speed and sensitivity of the GFCI.

Of course, with the foot switch you can react to any type of problem. E.g. some component suddenly getting too hot (without tripping the main fuse, since perhaps there is no global overcurrent situation).

By the way, I should be able to use electrolytics for C1 and C2.  I was just being general when I put those in.

The original voltage divider (whose top resistor we are splicing out) has one at the bottom, below the 1K resistor, indicating that the feedback termination point provides DC.  I.e. even if the output stage of the amp swings deeply negative, that does not cause the much smaller sampled feedback voltage to go sufficiently negative to offset the DC and reverse-polarize the cap.

When I get home, I will be sure to measure what that DC level is.

Hi all,

I'm trying to add a damping control pot to my Alesis 100 amp (one channel for now; if it works, then the other channel too).

I have reverse engineered enough of the amp to find the feedback path. The feedback goes through a 39K:1K voltage divider to feed 1/40th of the output stage voltage.

I'm going to rip out the 39K resistor and patch into there. The 39K resistance will be replaced with 100K to reduce the voltage feedback a little bit, so the current feedback is more significant (without having to use a larger current sense resistor that makes more heat!)

I plan to use a 0.22 ohm, 5W current sense resistor, and use a 50K pot.

The schematic is attached.  The terminals on the left are where we patch in place of the 39K resistor. The terminals on the right represent where we patch into the speaker return circuit and obtain ground.

Any advice is kindly appreciated.

Hey everyone,

can anyone rationalize why in the Alesis RA-100, two parallel 18 gauge hookup wires go from each power amp board to the speaker out terminal, but from the return terminal to ground, there is just one 18 gauge wire (per channel?)

Amplifier Discussion / Re: What causes a rectifying diode to fail?
« on: April 20, 2011, 03:26:35 PM »
One thing that puts a load on the diodes in a rectifier is the filter capacitors, during power up.  The capacitors are initially empty, and a surge of current flows through the rectifier to fill them up.  Sometimes you may hear a 60 Hz "twang" when you turn on some power supplies. That's the rush of AC being drawn to fill up the filter caps.

The bigger the filter caps, the harder the full-wave bridge rectifier has to work to load them up on power up. (Some people think that the filter caps are like the amplifier's testicles: the bigger the better).

A leaky capacitor will also add to the workload of the rectifier diodes. Of course any kind of short circuit condition anywhere in the device.

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