Help With Zen Amp

[vdi_nenna]

I rebuilt my Zen amp, but I am having some problems.

I brought up VR1 and I heard a soft pop and a bit of smoke came from R1, then R1 went cherry red. But before that , as I was turning up VR1 I did hear some sound and it became distorted. The other channel played ok as I ran to cut the power.

I guess something is drawing too much current, but why didn't the fuse blow?

Any help or ideas?

Thanks,

Vince

[azira]

Well, first questions:

What wattage and size are you using for R1?
What amperage is your fuse and where is it (are they) located?

--
Danny

[vdi_nenna]

Hi Danny,

R1 is .22 and is 3watts. The fuse is 5amp slo-blo. on the opposite side of the switch.

I set up R1 for 2 amp draw.


Vince

I just realized the amp needs fast blow fuses.

[azira]

Your chosen R1 is going to draw 3 amps. The Vbe of the BJT is going to be much higher than .44 volts, more like .65 volts.

But at 3 amps, your current dissipation is going to only be I^2 * R = 9 *.22 or 1.4 Watts, so your resistor should still hold.

At 5 amps. you'd have 25 * .22 = 5.5 Watts.

And 4 amps you'd have 16 * .22 = 3.52 Watts disspation.

Somewhere between 3 and 4 amps you would max out your resistor and cause it to burn up. This is probably why your fuse did not blow.

I would suggest that you replace R1 (pref with a 5-watt device but 3 can still work) and monitor the voltage accross it as you raise Vr1. If the current goes above 3.5amps, shut things down and debug your current source circuitry.

--
Danny

[moe29]

P1 may be mounted incorrectly.

If not turned all the way counter-clockwise you can fry R1 quickly.

so replace resistor and check P1

[vdi_nenna]

Thanks guys!

Danny, I should start the VR1 from the CCW position or 1 O'clock, right??
Originally, I only wanted to draw 2 amps per channel.

V~

[moe29]

R1 should be 0.33 ohm, i've used 0.47 ohm and it works

[azira]

Originally posted by vdi_nenna
Thanks guys!

Danny, I should start the VR1 from the CCW position or 1 O'clock, right??
Originally, I only wanted to draw 2 amps per channel.

V~

Ah, moe29 brings up a good point. What you SHOULD do is measure between the GND leg of VR1 and the wiper. One setting should make it register 0-ohms between the wiper and GND and the other will measure the full value of VR1. The one you want is 0-ohms. If you have it mounted wrong it's not a problem as long as you start it in the right direction.

Originally posted by moe29
R1 should be 0.33 ohm, i've used 0.47 ohm and it works

.33 Ohm for a 2-amp bias, .22 Ohm for a 3 amp bias. It's all about max power dissipation, NP has mentioned that if you use lower rails you're free to increase the bias for the same dissipation and slightly better performance.

--
Danny

[Blues]

If you decide on 2A bias your fuse should be rated 3A 120V or 1.5A 220V. Fast blow for now that your amp is on the test bench.

If your fuse is 5A 220V thats a whopping 10A capability on a 120V circuit...explaining R1 burning before your fuse. My guess is your mosfet D-S has shorted out drawing more current than R1 can take.