Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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
Reply
 
Thread Tools Search this Thread
Old 30th May 2011, 10:41 AM   #531
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
Default Near perfection....finely tuned

Hi All,

Well I just tried a few more things and now we're really getting somewhere.

First, I increased the input drain resistor to 1k to bring the current down to 4.5mA. All good, no adverse sonic affects.

Second, I added the filter cap to the input bias as lineup suggested. Not really any sonice affect, but clearly good practice.

Third, I added a grounding resistor to the input.

Fourth, I tackled the capacitive load overshoot and ringing square wave issue. I tried three things:

1) I wired a 4.7nF cap from the VAS to the output as was suggested here by Sonnya. This caused the amp to oscillate so I quickly removed it and aborted the idea.

2) I tried a 150pF miller cap. Made no difference.

3) I was so keen to try Hugh's idea of a cap from the VAS to the JFET source that I improvised and used 3x 150pF polypropylene in series for 50pF. Perfect. Now no capacitive load under 100nF makes any change to the 20kHz square wave that is worthy of mention. A 1uF cap will still make it do some funky things, but better than before.

It may be the placebo effect, but I would suggest that it sounds better too...

So ladies and gents, here is the current circuit. It is very good indeed. I think a change of VAS fet may make it even better!
Attached Images
File Type: jpg Image 009.jpg (97.4 KB, 570 views)

Last edited by GregH2; 30th May 2011 at 10:47 AM. Reason: Had Hugh's capacitor value wrong...it was 47pF!
  Reply With Quote
Old 30th May 2011, 11:12 AM   #532
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Mike,

It's a pleasure to hear your kind rantings despite my best regards...... everyone has a point of view, and it's an intolerant fellow who can't handle that!

SWF,

I'm really pleased phase lead has had the desired effect. You'll be pleased to know John Linsley Hood loved this method of stabilising amps, and always suggested that it was best on its own, without a Miller cap if you could avoid it. Congratulations, you are certainly in good company now!!

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com

Last edited by AKSA; 30th May 2011 at 11:15 AM.
  Reply With Quote
Old 30th May 2011, 12:11 PM   #533
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
Thanks Hugh, did JLH write any books? If so I need to get one!
  Reply With Quote
Old 30th May 2011, 12:16 PM   #534
mikelm is offline mikelm  England
diyAudio Member
 
Join Date: May 2003
Location: England
Send a message via Yahoo to mikelm
JLH always advised a bit more stabilisation rather than a bit less precisely because it made it sound a bit better

he wrote few books

http://www.amazon.co.uk/John-Linsley-Hood/e/B001HD0TGE

check this out

Last edited by mikelm; 30th May 2011 at 12:18 PM.
  Reply With Quote
Old 30th May 2011, 12:40 PM   #535
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
Hugh, mikelm, all,

With the 50pf of compensation outlined in post 531, the square wave is now clean up to 100nf of capacitive load. I would expect that further compensation would improve things further. What are the pros and cons of adding more capacitance here?

Thanks for your input.

Btw, mikelm that link did not work, but I'll go search for jlh at amazon.
  Reply With Quote
Old 30th May 2011, 12:42 PM   #536
mikelm is offline mikelm  England
diyAudio Member
 
Join Date: May 2003
Location: England
Send a message via Yahoo to mikelm
here they are:

Valve and Transistor Audio Amplifiers
The Art of Linear Electronics [1993]
Audio Electronics [1998]
  Reply With Quote
Old 30th May 2011, 01:45 PM   #537
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
Thanks Mikelm, I'm going to order what I can find of those.

Here is the current 20 volt peak to peak, 20kHz square wave with 7.27R and 100nF... Anything below 100nF looks much the same. 1uF is pretty messy, but a vast improvement on yesterday. Need I pursue this issue further or are these results good enough?

I have dropped the VAS current to 15mA as the devices were getting quite warm with no heatsink.

Edit: I posted the wrong photo. This is shot is actually with 75pf of compensation, but 50pf looks pretty much identical
Attached Images
File Type: gif new 20khz.gif (35.2 KB, 412 views)

Last edited by GregH2; 30th May 2011 at 01:57 PM.
  Reply With Quote
Old 30th May 2011, 02:28 PM   #538
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Hello swordfishy and mikelm

I have made an official version of Fetzilla

Eventhough people are free to build how they like
this is the one version I recommend people to build.
It is based on swordfishy's published amp, with super-good sound.
What is remarkable is that his amp is stable without compensation cap.
This is because of IRF9610 lower gain
but also emitter degeneration of VAS IRF9610 with one 49.9 Ohm capacitor.

This version below should be equally stable, sound good
and not give any headache with oscillation.

Fourier analyse at 1 kHz shows nice spectrum
with H2 and H3 sticking out. This is how we want it.
I be back with further tests later.

swordfishy
Could you setup this official version
should I be happy. Do it only if you want to.
Attached Images
File Type: png u_fetzilla_6b_official.png (54.2 KB, 387 views)
__________________
lineup

Last edited by lineup; 30th May 2011 at 02:45 PM.
  Reply With Quote
Old 30th May 2011, 03:12 PM   #539
mikelm is offline mikelm  England
diyAudio Member
 
Join Date: May 2003
Location: England
Send a message via Yahoo to mikelm
I would be interested to find out if this version with the CCS to the neg rail sounds as good as the the version swordfishy has built.

Personally I was so happy to see the critical FB point not connected to a power rail - thus with no possibility of power rail noise polluting the delicate signal . . .

. . . but if it sound the same or better when we do have a CCS to the neg rail I can implement exactly the same anti DC offset measures as Tim Andrews did which would be just great.

mike
  Reply With Quote
Old 30th May 2011, 03:54 PM   #540
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
hello mikelm
I'd be happy if you tested my circuit in real life.
swordfishy tested DC-linked version, with no big cap.
he was not happy DC-offset problems, like Hugh suspected.
I think you need not test the DC-linked version without cap.
It is simply not good and safe.

But the official version I posted now should not cause any problems.
I think it might sound good, too.
I suspect the VAS with IRF9610 is working wonders.

Only thing I forgot is one 2.2uF cap across the Gates of output.
Like in diagram below:
Attached Images
File Type: png u_fetzilla_6b.png (22.1 KB, 535 views)
__________________
lineup
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


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


New To Site? Need Help?

All times are GMT. The time now is 03:52 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2