Oscillation under load - diyAudio
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 3rd January 2006, 04:21 PM   #1
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
Default Oscillation under load

Hi,

I posted a question on this a few months back, it still remains an unsolved problem for me though.
I built a Quad 405-2 amplifier, with the well known mods done to it ( OPA134, MJ21194's, more capacitance)

Also an HT of 55v (5v above the original design).
It works very well without a load doing 80V pk-pk, 100w into 8 ohms.
But as the frequency goes up (roughly at 10khz and above) or as the load is reduced from 8 to 4 ohms the oscillation gets worse until the waveform becomes almost jagged. With oscillation in the order of several Mhz on positive cycles, crossover distortion becomes visible too.

I actually gave up on it before but have been away at university for a while and now want to get it working.

Somthing is wrong but i cant find out what, i have snubbersied, ferrite beaded, base stoppered but still cannot solve the problem.

Cheers
Craig
  Reply With Quote
Old 3rd January 2006, 04:57 PM   #2
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
ok i just got rid of the oscillation problem by putting 680nf in // with the speaker now it doesnt oscillate at all even under full power into 3 ohms.

I still get jagged waveforms on high frequencies.

I am sure that the capacitor solution is a terrible one but it works well enough and doesnt attenuate the signal within the 20khz range at all so im happy :P.

What might be causing the jagged sine waves?

Cheers
Craig
  Reply With Quote
Old 3rd January 2006, 08:02 PM   #3
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Craig,
A few things can cause that. PCB layout is one of them. Experiment with the compensation caps. If you run a frequency sweep, you may see some peaking.

Does the current increase as you sweep the frequency up?

-Chris
  Reply With Quote
Old 3rd January 2006, 11:47 PM   #4
clem_o is offline clem_o  Philippines
diyAudio Member
 
Join Date: Jun 2005
Location: Manila
Hi Craig,

Agree with Chris - could be the layout of the amp is causing instabilities. Could you post what the "jagged" waveform looks like? Doesn't have to be a true picture, even just a hand-drawn figure might help.

Also, try 'scoping the supply waveforms going into the OPA134, see if you can spot some niggles when the oscillation happens...

Cheers!

Clem
  Reply With Quote
Old 4th January 2006, 03:06 PM   #5
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
As i remember from last time i tried to repair this, the oscillation i get appears all over the PSU grounds and on the output at about 5mV pk to pk but gets worse at higher frequencies.

I am rebuilding the PSU and rewiring the lot today to eliminate any possibilities from this area.
I will try and do a sweep of the frequency response and get current readings from the supply as soon as its back in one piece.

The only problem i forsee in the sweeping is that i only have one ancient function generator and i think i need 2. Ill try and get some results today or tomorrow.

Also here i a picture of the waveform at higher frequencies with the oscillation also present.


Thanks
Craig
Attached Images
File Type: jpg waveform2.jpg (6.0 KB, 271 views)
  Reply With Quote
Old 4th January 2006, 03:17 PM   #6
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
Could you supply a scan/picture of the PCB layout ?

Regards

Charles
  Reply With Quote
Old 4th January 2006, 03:41 PM   #7
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Craig,

Did you have the usual 0.1uF + 10 ohms across the output? If not, try that, or even keep the 0.68uF but put a 5 or 10 ohms in series. It probably still gets rid of the oscillation without loading the amp in the audio range too much.
Did you put in an output series inductor?

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W S Maugham
Check out Linear Audio!
  Reply With Quote
Old 4th January 2006, 06:05 PM   #8
diyAudio Member
 
Join Date: May 2004
Location: Hertfordshire
Hi Craig,
I see you have been struggling with this amp for months now. This link is a brilliant bit of work on modding the 405 which may give you some info on how to solve the problem.

http://www.dc-daylight.ltd.uk/Valve...on/405_Qw_6.pdf

I would echo Janneman's thoughts on the zobel network but you might need to make the resistor lower than the 5 to 10 ohms he suggests. The standard 0.1uf and 10 ohms will not do much.

The waveform you have posted is classic slew rate limiting. I note from the above link that the 405 does not have good figures for its slew rate spec.. Perhaps you are expecting too much from the design. It would be worth noting the slope of the straight bit of the distorted sine waveform (in V/uS) and comparing that with the spec.
The presence of numerous inductors and capacitors for compensation shows that the design is touchy and any deviations from non-standard may be troublesome oscillation wise!
  Reply With Quote
Old 4th January 2006, 06:20 PM   #9
diyAudio Member
 
Join Date: May 2004
Location: Hertfordshire
sorry, got links wrong:

http://www.dc-daylight.ltd.uk/Valve-...n/405_Qw_6.pdf

http://www.dc-daylight.ltd.uk/Valve-...-405-mods.html
  Reply With Quote
Old 4th January 2006, 07:15 PM   #10
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
Thanks for replies everyone.
I have rewired the PSU and one channel alone with very short thick wire so thats fine now.

Here are current readings taken from the positive rail at various input frequencies;

10v pk-pk output into no load;
100hz = 89.7mA
1kHz = 89.8mA
17.5K = 89.4mA (first obvious signs of triangulation appear)
22.5K = 87.9 mA
60k = 86.3mA

with 5 0hm load, no input measured 86.5mA idle;
100hz = 90mA ( wave already looks bad)
10Khz = 89.9mA
20K = 89.2mA
25K = 88mA
30K = 87.3mA
60K = 86.5mA

As for the op amp supply it looks horrible even though it is transistor regulated, the input signal appears there with lots of oscillation all over it.

Jan yes i have the 0.1uf 10 ohm network in place, and have tried a few values of output inductor// resistor too but i still get the same trouble.

consort_ee_um thanks, i pretty much built my amp using tips from those sites they are very good.

I cant find my PCB layouts, i hadnt backed them up and probably deleted them a while ago (never thought id need them again..doh)

Thanks
Craig
  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
Power load, dummy load (pic) luka Power Supplies 43 9th February 2012 02:50 PM
Dynamic load line analysis using music and speaker load Michael Koster Tubes / Valves 0 7th March 2008 08:47 PM
T-Amp 5Hz oscillation rudiloos Class D 6 24th May 2006 08:27 PM
oscillation merkur Tubes / Valves 28 10th May 2006 12:19 AM
GC oscillation ? Dennis Hui Chip Amps 20 28th September 2004 10:26 PM


New To Site? Need Help?

All times are GMT. The time now is 11:34 AM.


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