Mosfet mods

hihopes · 23rd September 2007, 04:07 PM

I have really tried to sort this out without asking for advice, but I don’t know where to turn any more. I am doing everything I can think of to linearize/stabilize the mosfets in my hybrid amp. So far I have done the following:-1) Fiddled with gate stopper values. 2) Put caps right on the drain pins where the power supply is connected. 3) Output inductor 4) Zobel There has been a lot of improvement from these mods, but there is still a slight distortion apparent in certain passages with complex waveforms (FWIW, the biggest improvement I noticed was due to the PS bypass caps).
Can anyone suggest anything else I can try?

unclejed613 · 23rd September 2007, 08:16 PM

what kind of MOSFETS are they?
what are you driving them with?
class A or AB?
gotta schematic?

Leolabs · 24th September 2007, 03:09 AM

Lateral or Hex???

hihopes · 24th September 2007, 08:09 PM

Exicon 10n16 and 10p16 lateral mosfets. Single supply rail, class AB I think.

traderbam · 24th September 2007, 08:51 PM

Many possibilities.

Quote:

there is still a slight distortion apparent in certain passages with complex waveforms

Can you describe the distortion?

hihopes · 24th September 2007, 09:52 PM

The distortion is like a kind of haze/buzz around piano notes. It varies in intensity according to the frequency of the note. It is not a loud/obvious distortion. You have to put your ear near the speaker to hear it. It is also not noticeable most of the time, but there is a particular passage of piano with a lot of reverb that I am using as a reference to test my mods. (It isn't the recording. My Hart Linsley Hood amp which uses the same mosfets doesn't produce the haze in this passage)

diy4 · 25th September 2007, 01:10 AM

This is crossover distortion caused by a mis-biased AB output stage.
Piano brings this out more than anything.
can you post a schematic? Voltage readings would help too.

traderbam · 25th September 2007, 07:29 AM

As diy4 suggests, need to see the circuit.
I've also seen oscillation cause haze/fuss on certain types of sound. Piano notes have fast edges and this can set it off, especially into capacitive loads. Don't put a cap across the output as a test unless you've got some way to protect the output stage from melt-down.
If it is instability then there are many potential causes, some of which have nothing to do with the schematic. Ground paths are a notorious cause.

hihopes · 25th September 2007, 08:48 PM

I will see what i can do for a drawing with voltages. In the meantime I can tell you it is a single supply rail, so has DC blocking output caps. (Phillips 4700uF bypassed with 0.47 polyprops) I have put a zobel, 10r and .15uF polyprop, after the caps and before the inductor. I may remove the zobel but i am just giving it a chance to settle first before making a decision on that.
I am also thinking that it may be some sort of ringing that i am hearing which is being triggered by complex waveforms. If it is crossover distortion, then why is it so seldom audible?

hihopes · 27th September 2007, 06:57 PM

Thanks for the input so far. My clumsy fingers and unfocussed mind have struck. I have blown something (probably a transistor). I forgot to drain the caps before working on it and shorted a cap into earth. Will post again when I can fix it and measure it.

23rd September 2007, 04:07 PM	#1
hihopes diyAudio Member Join Date: Sep 2004 Location: Cape Town	Mosfet mods I have really tried to sort this out without asking for advice, but I don’t know where to turn any more. I am doing everything I can think of to linearize/stabilize the mosfets in my hybrid amp. So far I have done the following:-1) Fiddled with gate stopper values. 2) Put caps right on the drain pins where the power supply is connected. 3) Output inductor 4) Zobel There has been a lot of improvement from these mods, but there is still a slight distortion apparent in certain passages with complex waveforms (FWIW, the biggest improvement I noticed was due to the PS bypass caps). Can anyone suggest anything else I can try?

23rd September 2007, 08:16 PM	#2
unclejed613 diyAudio Member Join Date: Dec 2006	what kind of MOSFETS are they? what are you driving them with? class A or AB? gotta schematic? __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: http://www1284177414881.v-dc.net/

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24th September 2007, 03:09 AM	#3
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Lateral or Hex???

24th September 2007, 08:09 PM	#4
hihopes diyAudio Member Join Date: Sep 2004 Location: Cape Town	Exicon 10n16 and 10p16 lateral mosfets. Single supply rail, class AB I think.

24th September 2007, 09:52 PM	#6
hihopes diyAudio Member Join Date: Sep 2004 Location: Cape Town	The distortion is like a kind of haze/buzz around piano notes. It varies in intensity according to the frequency of the note. It is not a loud/obvious distortion. You have to put your ear near the speaker to hear it. It is also not noticeable most of the time, but there is a particular passage of piano with a lot of reverb that I am using as a reference to test my mods. (It isn't the recording. My Hart Linsley Hood amp which uses the same mosfets doesn't produce the haze in this passage)

25th September 2007, 01:10 AM	#7
diy4 diyAudio Member Join Date: Jun 2004 Location: .	This is crossover distortion caused by a mis-biased AB output stage. Piano brings this out more than anything. can you post a schematic? Voltage readings would help too.

25th September 2007, 07:29 AM	#8
traderbam diyAudio Member Join Date: Jan 2002 Location: Earth	As diy4 suggests, need to see the circuit. I've also seen oscillation cause haze/fuss on certain types of sound. Piano notes have fast edges and this can set it off, especially into capacitive loads. Don't put a cap across the output as a test unless you've got some way to protect the output stage from melt-down. If it is instability then there are many potential causes, some of which have nothing to do with the schematic. Ground paths are a notorious cause.

25th September 2007, 08:48 PM	#9
hihopes diyAudio Member Join Date: Sep 2004 Location: Cape Town	I will see what i can do for a drawing with voltages. In the meantime I can tell you it is a single supply rail, so has DC blocking output caps. (Phillips 4700uF bypassed with 0.47 polyprops) I have put a zobel, 10r and .15uF polyprop, after the caps and before the inductor. I may remove the zobel but i am just giving it a chance to settle first before making a decision on that. I am also thinking that it may be some sort of ringing that i am hearing which is being triggered by complex waveforms. If it is crossover distortion, then why is it so seldom audible?

27th September 2007, 06:57 PM	#10
hihopes diyAudio Member Join Date: Sep 2004 Location: Cape Town	Thanks for the input so far. My clumsy fingers and unfocussed mind have struck. I have blown something (probably a transistor). I forgot to drain the caps before working on it and shorted a cap into earth. Will post again when I can fix it and measure it.

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