JFET input, MOSFET VAS, LATERAL output = Perfect!! - Page 25

GregH2 · 11:34 AM

Well done danspy. Very glad you got it going. Sounds nice huh?

Oh dear, it looks like I jumped the gun with my comments about stability...

I went home today and wired things up exactly as they were before with the only difference being that I added a 1uf cap across the bias resistor.

Turned it on, decided to scope the output before adding a load. Terrible rail to rail oscillations somewhere in the MHz range. This time no amount of poking and prodding the components would get rid of them. I don't think could make an oscillator work this well if I tried!

Hmm, added a 150pf miller cap....no good.

Hooked up a 6R resistor to output. Oscillations gone...hmm maybe an output filter would help?

Added a rc filter to the output with a 2W 10 ohm resistor and 100nf cap. Removed 6R dummy load and turned it on. 2W filter resistor started to smoke...sheesh, there's some serious rf going through that 100nf cap.

So what do you think? When I'm lucky the amp won't oscillate. Sometimes it turns on fine, other times I get rail to rail oscillations. If a low z load is attached it seems fine. Seems ok with a speaker attached.

How disappointing!

Guess I need to drop the olg even more and think seriously about some compensation...

Boy does it have some bass though!

danspy · 13th May 2011, 11:53 AM

this one want smoke that quick.

And who run,s an AMP without a Speaker?

metal · 13th May 2011, 11:58 AM

come on... An amp should not oscillate whether a speaker is present or not.

GregH2 · 13th May 2011, 12:13 PM

Danspy, do you have a scope? Yours might be oscillating too. What do you think of the sound?

GregH2 · 13th May 2011, 12:16 PM

Quote:

Originally Posted by metal

come on... An amp should not oscillate whether a speaker is present or not.

My sentiments exactly.

mikelm · 12:22 PM

Fast amps do need extra attention regarding stablity considering cct design & layout but in my opinion it is well worth the effort.

Personally I would not just add more pF to the VAS Miller position - for me that would be a last option and perhaps just a very small value.

Other options available are 1) what I did in my spice cct above and 2) taking HF feedback from the VAS stage ( I will be playing with this in spice some time soon ) and also loosing OLG with more VAS degeneration.

sorry I can't join in with the fun just now

mike

mikelm · 13th May 2011, 12:31 PM

swordfishy - if a 6 ohm load fixed it - then perhaps 0.1uF + 6ohms would be better ? ? ? or even 4ohms - might be worth a try.

When I was experimenting in spice I found that with capacitive loads the miller cap actually made things worse - which is why I removed it.

danspy · 13th May 2011, 01:25 PM

Quote:

Originally Posted by swordfishy

Danspy, do you have a scope? Yours might be oscillating too. What do you think of the sound?

No i have no scope, and if it Oscillate,s and i don`t hear it and the Output,s don`t get hot it don`t upset me.

I wold say it Sound,s Groovy an very musical.

GregH2 · 03:03 PM

Yeah, musical is the word...if I had to criticise one thing it would be a lack of treble. However I haven't had a chance to audition it with decent speakers yet, just some bottom line JAMO bookshelves. I also have nothing in the way of supply rail bypassing which can't help.

Anyway, mikelm I managed to move the board around a bit and get it to behave for a few minutes. Here is the 20kHz square wave performance at nearly 40v p-p across a 7R load. What do you think?

By the way, when it's not oscillating the DC offset is stable to within a few mV.

By the way no 2....the rise and fall times aren't quite as impressive as they were yesterday as I have upped the gate stoppers to 500R in an effort to stop oscillation. However, the ringing has entirely disappeared which is very good!

mikelm · 13th May 2011, 03:07 PM

With a 20khz square wave like that I don't think there is any fear of lack of treble !

it looks like it's soaring way past 100khz

13th May 2011, 11:24 AM	#241
GregH2 diyAudio Member Join Date: Oct 2009	The world's best oscillator.... Well done danspy. Very glad you got it going. Sounds nice huh? Oh dear, it looks like I jumped the gun with my comments about stability... I went home today and wired things up exactly as they were before with the only difference being that I added a 1uf cap across the bias resistor. Turned it on, decided to scope the output before adding a load. Terrible rail to rail oscillations somewhere in the MHz range. This time no amount of poking and prodding the components would get rid of them. I don't think could make an oscillator work this well if I tried! Hmm, added a 150pf miller cap....no good. Hooked up a 6R resistor to output. Oscillations gone...hmm maybe an output filter would help? Added a rc filter to the output with a 2W 10 ohm resistor and 100nf cap. Removed 6R dummy load and turned it on. 2W filter resistor started to smoke...sheesh, there's some serious rf going through that 100nf cap. So what do you think? When I'm lucky the amp won't oscillate. Sometimes it turns on fine, other times I get rail to rail oscillations. If a low z load is attached it seems fine. Seems ok with a speaker attached. How disappointing! Guess I need to drop the olg even more and think seriously about some compensation... Boy does it have some bass though! Last edited by GregH2; 13th May 2011 at 11:34 AM.

13th May 2011, 11:53 AM	#242
danspy diyAudio Member Join Date: Jul 2007	this one want smoke that quick. And who run,s an AMP without a Speaker? __________________ 知る者は言わず、いう者は知らず。

13th May 2011, 12:19 PM	#246
mikelm diyAudio Member Join Date: May 2003 Location: England	Fast amps do need extra attention regarding stablity considering cct design & layout but in my opinion it is well worth the effort. Personally I would not just add more pF to the VAS Miller position - for me that would be a last option and perhaps just a very small value. Other options available are 1) what I did in my spice cct above and 2) taking HF feedback from the VAS stage ( I will be playing with this in spice some time soon ) and also loosing OLG with more VAS degeneration. sorry I can't join in with the fun just now mike Last edited by mikelm; 13th May 2011 at 12:22 PM.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
10 Watt Single End, JFET input - Lateral N-MOSFET output	lineup	Solid State	82	30th July 2009 06:23 PM
Lateral MOSFET, How do I calculate power output	synonymous	Solid State	4	18th December 2007 06:54 AM
Calculating VAS current for mosfet output	bogdan_borko	Solid State	21	11th April 2006 03:57 AM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

13th May 2011, 11:58 AM	#243
metal AVR & PIC C Dev. diyAudio Member Join Date: Jan 2004	come on... An amp should not oscillate whether a speaker is present or not.

13th May 2011, 12:13 PM	#244
GregH2 diyAudio Member Join Date: Oct 2009	Danspy, do you have a scope? Yours might be oscillating too. What do you think of the sound?

13th May 2011, 12:31 PM	#247
mikelm diyAudio Member Join Date: May 2003 Location: England	swordfishy - if a 6 ohm load fixed it - then perhaps 0.1uF + 6ohms would be better ? ? ? or even 4ohms - might be worth a try. When I was experimenting in spice I found that with capacitive loads the miller cap actually made things worse - which is why I removed it.

13th May 2011, 03:07 PM	#250
mikelm diyAudio Member Join Date: May 2003 Location: England	With a 20khz square wave like that I don't think there is any fear of lack of treble ! it looks like it's soaring way past 100khz

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read