NMOS 350/500 by Quasi

ID_ · 10th January 2011, 05:12 PM

Hello to all.

I decided to build the mentioned amp, becouse the one other that I got blew.

The schematic can be seen here

So I fallowed the construction guide and assembled the amp. Everything seemed to be ok. As I vas setting the bias I noticed, that the current was increasing with the temperature increase of T6,7,9 and 10 (they are mounted on common heat sink). I'm worried about this becouse of thermal runaway. When the temperature of this common heat sink settled the bias was steady, but heat sink was extremely hot to the touch. I've let the amp run about 1 hour. Everything seemed to be ok, except the small common heat sink teperature. So I decided to test the amp with some loudspeaker. I connected everything and there was some noise comming out of speaker (not humm). So I disconnected the input (I tought maybe it's the faulty cable) and then it happened. As soon as I disconnected the input I noticed smoke and the noise went crazy (I almost crapped myself

). So I instantly turned the amp off.

The resistor R20 lost his smoke. I measured every transistor and fet! They are all ok. Just the R20 blew!

I checked and rechecked every connection and whole PCB assembly before I wired the thing together. All was OK!

I was running the amp on +- 81V!

I can't find the error, so maybe someone can help me!?

I have no idea what have gone wrong!

Tekko · 10th January 2011, 05:55 PM

I have a very similar circuit running on 50V rails and its stable. However VAS and its current source are not mounted to a heatsink and the drivers doesent have a heatsink.

The drivers get hot but not burning hot.

Its possible that 81V rails are just too much since it was designed for 75V rails.

jackand08 · 10th January 2011, 06:10 PM

You say that T6,7,9 and 10 were on a common heatsink but for the temperature compensation of the bias to work properly, it is also important that T8 is mounted on the MAIN heatsink with the output transistors. This is just a suggestion as you doesn´t say anything about T8 in your first post.

Also, are you sure that your amp really does have an adequate heatsink for use with higher voltage rails. Rails of a higher voltage will increase dissipation in the output transistors and may result in the idling temperature being quite a bit higher than expected.

As Tekko says, it may well just be that +/-81V is pushing it too much for an amp designed to work on +/-75V. Remember with +/-81V rails, it is a total difference rail to rail of around 162V which certainly isn´t a low voltage.

ID_ · 10th January 2011, 06:35 PM

I'm aware of this voltage difference! Quasi said in some other thread that the amp was designed to run at maximum +- 85V.

Yes, I forgot to mention, that T8 is on the main heat sink!

moschfet · 10th January 2011, 06:39 PM

Hi ID,

Quote:

I connected everything and there was some noise comming out of speaker (not humm)

Noise are signs of oszillations. The louder the more.

Good old problem

Have you got an oscilloscope?

ID_ · 10th January 2011, 06:52 PM

I have osciloscope- maybe some advice on how to make a measurement.

The FETs do not come cheap and I do not want to smoke them!

Oh and just to mention, I took Quasi's PCB design, so in general it should work!

moschfet · 10th January 2011, 07:08 PM

Quote:

I have osciloscope- maybe some advice on how to make a measurement.

using an oscilloscope

Quote:

Oh and just to mention, I took Quasi's PCB design, so in general it should work!

Why, are there many who build an amplifier with this layout?

ID_ · 10th January 2011, 07:29 PM

Few people including this forum have mentioned that they've build the amp yes.

moschfet · 10th January 2011, 07:47 PM

Can you show me the layout?

ID_ · 10th January 2011, 08:16 PM

Here are the PCBs and layouts.

10th January 2011, 05:12 PM	#1
ID_ diyAudio Member Join Date: Jun 2006	NMOS 350/500 by Quasi Hello to all. I decided to build the mentioned amp, becouse the one other that I got blew. The schematic can be seen here So I fallowed the construction guide and assembled the amp. Everything seemed to be ok. As I vas setting the bias I noticed, that the current was increasing with the temperature increase of T6,7,9 and 10 (they are mounted on common heat sink). I'm worried about this becouse of thermal runaway. When the temperature of this common heat sink settled the bias was steady, but heat sink was extremely hot to the touch. I've let the amp run about 1 hour. Everything seemed to be ok, except the small common heat sink teperature. So I decided to test the amp with some loudspeaker. I connected everything and there was some noise comming out of speaker (not humm). So I disconnected the input (I tought maybe it's the faulty cable) and then it happened. As soon as I disconnected the input I noticed smoke and the noise went crazy (I almost crapped myself ). So I instantly turned the amp off. The resistor R20 lost his smoke. I measured every transistor and fet! They are all ok. Just the R20 blew! I checked and rechecked every connection and whole PCB assembly before I wired the thing together. All was OK! I was running the amp on +- 81V! I can't find the error, so maybe someone can help me!? I have no idea what have gone wrong!

10th January 2011, 06:10 PM	#3
jackand08 diyAudio Member Join Date: Oct 2008 Location: England	You say that T6,7,9 and 10 were on a common heatsink but for the temperature compensation of the bias to work properly, it is also important that T8 is mounted on the MAIN heatsink with the output transistors. This is just a suggestion as you doesn´t say anything about T8 in your first post. Also, are you sure that your amp really does have an adequate heatsink for use with higher voltage rails. Rails of a higher voltage will increase dissipation in the output transistors and may result in the idling temperature being quite a bit higher than expected. As Tekko says, it may well just be that +/-81V is pushing it too much for an amp designed to work on +/-75V. Remember with +/-81V rails, it is a total difference rail to rail of around 162V which certainly isn´t a low voltage. __________________ Jack "Near enough is good enough, so good enough is best"

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10th January 2011, 05:55 PM	#2
Tekko diyAudio Member Join Date: Jan 2005	I have a very similar circuit running on 50V rails and its stable. However VAS and its current source are not mounted to a heatsink and the drivers doesent have a heatsink. The drivers get hot but not burning hot. Its possible that 81V rails are just too much since it was designed for 75V rails.

10th January 2011, 06:35 PM	#4
ID_ diyAudio Member Join Date: Jun 2006	I'm aware of this voltage difference! Quasi said in some other thread that the amp was designed to run at maximum +- 85V. Yes, I forgot to mention, that T8 is on the main heat sink!

10th January 2011, 06:52 PM	#6
ID_ diyAudio Member Join Date: Jun 2006	I have osciloscope- maybe some advice on how to make a measurement. The FETs do not come cheap and I do not want to smoke them! Oh and just to mention, I took Quasi's PCB design, so in general it should work!

10th January 2011, 07:29 PM	#8
ID_ diyAudio Member Join Date: Jun 2006	Few people including this forum have mentioned that they've build the amp yes.

10th January 2011, 07:47 PM	#9
moschfet diyAudio Member Join Date: Jul 2008 Location: Hannover	Can you show me the layout?

10th January 2011, 08:16 PM	#10
ID_ diyAudio Member Join Date: Jun 2006	Here are the PCBs and layouts.