[tooler]

I've got my Szekeres amplifier on breadboard right now. It's the standard configuration except that I do not have the diodes in place. C2 is 2200uF. R3 is about 500k. I am using Rg = 220 with the leads of the resistor right next to the gate.

The amplifier sounds fine for just over one minute. At that point, it produces a high-pitched wah sound for no apparent reason. The music still plays in the background. Sometimes the noise goes away and comes back. At first I was using tiny heatsinks and I figured the MOSFETs were just overheating. I've added some large ones and the amp still makes that noise without the transistors getting very hot.

Could this have something to do with the large capacitors charging up the voltage? It takes about that long for Vds to settle to within 5% of V+/2, which is 7.1V. Vds is not well matched between channels: it's about 0.5V off.

Any ideas?

 

[rickcr42]

heya tooler

I thought this baby was working man ?

and yup , sounds like an oscillation problem

does the mosfet get hotter at the same time as the sound craps out ?

did you connect the 0.1 uf cap at the mosfet source ?

if so try paralleling another

and BTW-the 220 ohm resistor needs to be RIGHT AT the gate or it is useless.

and I mean right at the gate pin

let me know how things turn out

 

[Tomo]

Hi,

Just for a record, mosfet's heatsink lug is electronically active. Which means, you have to be very careful to isolate the heatsink from mosfet "stones."

I have the "wah" when I physically touch the wire before the gate stopper resistor (Rg).

Probably some miswiring. I would go over it again.

I use 2200uF per channel. So no worries about that.

T

 

[jeffreyj]

Quote:

Originally posted by tooler I've got my Szekeres amplifier on breadboard right now. ... Any ideas?

I agree with rickcr42 that you have an oscillation problem, and with his advice, as well. However, I think it is useful to add that the gate "isolation" resistor is often replaced with a simple ferrite bead. Another suggestion would be to connect a 10-100k resistor directly from gate to source.

Oh, and btw, the Vgs threshold is poorly controlled in any flavor of FET so gate bias voltages will be anything but equal! If the bias voltages are within 20-25% of each other then I wouldn't worry about it. If the spread is more than that, well, you might want to hand-select pairs to get something more reasonable

 

[tooler]

Quote:

Originally posted by Tomo Just for a record, mosfet's heatsink lug is electronically active. Which means, you have to be very careful to isolate the heatsink from mosfet "stones."

The MOSFET has to be isolated from the heat sink even if the heatsink's not touching anything?

 

[rickcr42]

from the master

http://www.passdiy.com/projects/zenv5-5.htm

 

[Tomo]

Yes,

I sparked the heatsink to chasis by mistake and completely burnt out one of the mosfet.

If you refer to the specification sheet, you will see that metal tab is actually an electrode. Convenient when board space is restricted, but inconvenient when mosfets operates high power and requiring heatsinks.

T

 

[tooler]

Help me understand here: I realize that the metal tab is an electrode, but if it's screwed to a 2"x2" heatsink that isn't touching anything, how would that effect anything?

I have some insulation I can try, but I don't realize how a hunk of metal sitting in the air attached to that electrode would mess anything up.

 

[Budgie]

The heatsink provides a large surface area, which will increase the capacitive coupling to other parts of the circuit. The insulator disconnects the fet from that capacitance.

 

[guzzler]

and there's always the chance it could touch something!

g

 

[rickcr42]

Quote:

Help me understand here: I realize that the metal tab is an electrode, but if it's screwed to a 2"x2" heatsink that isn't touching anything, how would that effect anything?

it should not as long as the critical parts are in direct contact with the mosfet

i run a 3X4 heat sink and have no adverse effects but the before mentioned gate resistor and psu bypass cap are hard wired direct to the mosfet

 

[rickcr42]

Quote:

and there's always the chance it could touch something!

especially if you are like me , never happy with what i have and always screwing around to "improve" the sound

so unless you are going to place the heat sink on a non ferrous surface (non metal/non conducting) it is way easier to isolate the mosfet from the heatsink than to isolate the heatsink from the chassis

 

[tooler]

Solved one problem, now I have another.

I am using a rickcr42 dual-mono power supply (LM317 I think, it's been a while) and didn't have the second power supply hooked up. I was running both audio channels from one power supply channel. Oops.

Unfortunately, the second power supply channel introduced some crackling into the audio channel that it is hooked up to. I can swap PSU1 and PSU2 and the crackling noise follows the faulty supply to the new audio channel. After the capacitors charged up, I noticed that the faulty PSU only gets up to about 13.6V and the good PSU gets up to 14.2V (the correct voltage). It does this when connected to either audio channel.

Both power supplies go to 14.2V when disconnected from the audio circuit. I put a single resistor across the faulty power supply leads to see if it had a problem with a load, but it went to the correct voltage. Is this a poor way to test a power supply under load?

 

[tooler]

The crackle occurs a few seconds after I turn the power supply on. This is without the music turned on, but with everything hooked up.

 

[rickcr42]

Quote:

I am using a rickcr42 dual-mono power supply (LM317 I think, it's been a while) and didn't have the second power supply hooked up. I was running both audio channels from one power supply channel. Oops.

not a bad thing if you limit the amp to say6 250 -400 ma per ch




just for chuckles and giggles

disconnect the psu from the amp and connect an LED across each polarity (with the proper resistor of course)

measure from common to each polarity across the leds and see if the supply is putting out the correct voltage