ZCA need help!

darijus · **Posted:** 22 Jul 2010 4:33 pm

Hello guys!
I'm trying to make a ZCA Mosfet amp and have a problem. I can't get 1/2 supply voltage across Mosfet. Supply voltage ~23V, around Mosfet ~1V. Mosfet is totally cold and load resistors became extremely hot in few seconds. 100K pot does not regulate voltage anyway. I check the scheme 4-5 times - it's seems everything ok. My head become squared and I don't know where problem can be...
Help! :hot:

P.S. Sorry for my English :blush:

dtsup1 · **Posted:** 22 Jul 2010 11:30 pm

It looks like you have connected the 100k pot wrongly.
Show us some pics of your wiring.

darijus · **Posted:** 23 Jul 2010 6:25 am

100K pot one side contact to GND, other to supply after load, center - 1M to Gate and to 33uF positive "leg". Load is two resistors Vishay Dale 30R 50W. Nothing connected to input and output. Even if I disconnect Gate from scheme, load resistors became very hot in few seconds.

KrammeAcoustics · **Posted:** 24 Jul 2010 3:11 pm

darijus wrote:

100K pot one side contact to GND, other to supply after load, center - 1M to Gate and to 33uF positive "leg". Load is two resistors Vishay Dale 30R 50W. Nothing connected to input and output. Even if I disconnect Gate from scheme, load resistors became very hot in few seconds.

It's difficult to see how the 100k trimpot is mounted. If you've adjusted ex. 5/95 %, instead of 95/5 %, you'll have to turn the trimpot opposite. ( NOTE: the numbers in % are only examples, not real values ).

tombethe · **Posted:** 24 Jul 2010 4:21 pm

Hey Darijus, Hej John,

is it possible that you have connected the trimpot's output to the MosFet's Source?
I'm not sure, if the picture shows a junction between the trimpot tap and the source tap on the picture...

Please see:

Attachment:

zca-scheme.jpg

It seems that the trimpot tap is shortened to ground (via the source pin) and the bias can't be adjusted.
Please check this out... The trimpot should also get very hot so...

KrammeAcoustics · **Posted:** 24 Jul 2010 4:42 pm

tombethe wrote:

Hey Darijus, Hej John,

is it possible that you have connected the trimpot's output to the MosFet's Source?
I'm not sure, if the picture shows a junction between the trimpot tap and the source tap on the picture...

It seems that the trimpot tap is shortened to ground (via the source pin) and the bias can't be adjusted.
Please check this out... The trimpot should also get very hot so...

Hi Tom

I guess the jump on the line ( red arrow ) in the schematics, was ment like the line ( blue arrow ) i've added in the picture below ( if not you're right Tom ).

dtsup1 · **Posted:** 25 Jul 2010 3:35 am

KrammeAcoustics wrote:

If you've adjusted ex. 5/95 %, instead of 95/5 %, you'll have to turn the trimpot opposite

This seems to be the same issue I faced. Very fine tuning was required to get the voltage to 12V. You still need to readjust the bias again after your system as reached thermal stability. Once adjusted, no more adjustment needed, even if you choose to use speakers of a different load. (4 or 8 ohms).

Best solution is to use a multi turn trimpot instead of a pot so it can be adjusted more easily.

darijus · **Posted:** 25 Jul 2010 5:55 am

Sorry, this is my trimpot construction

KrammeAcoustics wrote:

I guess the jump on the line ( red arrow ) in the schematics, was ment like the line ( blue arrow ) i've added in the picture below ( if not you're right Tom ).

Yes, I connect (2) strip over 1M to Gate.
I turn trimpot 100% to one and to another way - it doesn't make any changes.

KrammeAcoustics · **Posted:** 25 Jul 2010 6:52 am

darijus wrote:

Sorry, this is my trimpot construction

Yes, I connect (2) strip over 1M to Gate.
I turn trimpot 100% to one and to another way - it doesn't make any changes.

Did you measure components before soldering them together ?

Are resistors and trimpots correct values ?

It's not for sure that the values are what they intended to be. A salesman could have given you wrong components ...just a thought :idea:

Anyway ...i'm always measuring components, before installing them, just to be sure nothing should go wrong.

mwhouston · **Posted:** 25 Jul 2010 7:42 am

I don't want you to draw the schematic. Because the cct. works. There are hundreds of ZCAs in the wild.

I want you to hand draw all the componets on a piece of papers. Then look at your build and trace every wire and draw THAT trace on the paper from component to component. No crappy block schematics, a hand, wire by wire link-up. Scan the image and post it on this forum.

Have you swapped the FET??

ZCA need help!

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