Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 22nd April 2004, 12:53 PM   #281
diyAudio Member
 
Join Date: Apr 2003
Location: Southern Tier NY
Default Absolute DC Offset

It took me about a 10 days to get the abs DC offset right.
It initially seemed that I couldn't get it to stabilize under 1.5 volts. At present, it operates as follows
Turn on about 3.5 Volts offset
1 hour 1 Volt offset
2 hours or more +/- 100mV offset

I think in my case it takes quite a while for the amp to reach thermal equilibrium because of its size. The heat sinks will reach temp in about 30 minutes but the rest of the amp takes awhile.

Scott
  Reply With Quote
Old 22nd April 2004, 04:27 PM   #282
jwb is offline jwb  United States
diyAudio Member
 
jwb's Avatar
 
Join Date: Mar 2002
Location: San Francisco, USA
Send a message via AIM to jwb
Protos: it will depend on the operating temperature of your amp and how long it takes to warm up. With mine, the DC offset starts at 2V cold and goes to zero within about 15 minutes. But my amp is fan-cooled and warms up pretty quickly. I didn't like that power-on offset so I am playing with a DC servo design. The serv o eliminates the adjustment altogether.
  Reply With Quote
Old 22nd April 2004, 04:58 PM   #283
jwb is offline jwb  United States
diyAudio Member
 
jwb's Avatar
 
Join Date: Mar 2002
Location: San Francisco, USA
Send a message via AIM to jwb
Default For Rob Dingen

I'm posting this response here for Rob Dingen, because his email doesn't work correctly. His question was about how to very accurately match FETs for both Vgs and tempco. I said:

First I made a high-precision current source, which provides the same
current regardless of temperature. I hope you'll excuse the extremely
bad drawing:

Click the image to open in full size.

Secondly I attached the FETs to be tested to a very large heatsink, to
test their Vgs when cold, and then I attached them to a heater at 30C to
test them when hot.
  Reply With Quote
Old 22nd April 2004, 07:13 PM   #284
diyAudio Member
 
Join Date: Jan 2003
Location: Netherlands'
Hi jwb

Thanks for the schematics.
You also match the IRFP240 that way?
And after which time do you measure?
I build a +/- 30v powersupply and a current source for 12.5 ma.
I used a half of a dip8 socket and place the 9610 in and wait 5 min and measure vgs.
After 4 min the vgs is getting stable and the voltage drop is getting very slow.
I had my old E-mail address so I changed it.

Rob
  Reply With Quote
Old 22nd April 2004, 07:28 PM   #285
diyAudio Member
 
Join Date: Apr 2003
Location: Southern Tier NY
Default tester

jwb

I like the current source idea. When I matched mine I was the current source adjuster using my variable power supply. A little tedious.

I tested at two currents for the 244's. 200 and 600mA. Started at 200 for 30 seconds, measured, and then jumped to 600 mA for an additional 30 seconds and measured. I tried to keep the cooling between fet tests similar to get decent results. I think by doing it this way I rejected alot of parts out of my original supply that would have been happily matched.

To get an absolute temperature reference I have been thinking about using boiling water as a temperature source and then clipping the FET under test to a copper bar that sits in the water. This way the fet doesn't get wet and it quickly will reach a temp of 100C. (I know about the altitude effect) In this mannor the juntion temp is closer to real life operating conditions.

Any thoughts on that?
  Reply With Quote
Old 22nd April 2004, 11:44 PM   #286
Coulomb is offline Coulomb  England
diyAudio Member
 
Coulomb's Avatar
 
Join Date: Jul 2002
Location: Ancaster, Ontario
Here is thought you are all nuts!!



Anthony
  Reply With Quote
Old 23rd April 2004, 12:04 AM   #287
jleaman is offline jleaman  Belgium
Banned
 
Join Date: Apr 2002
Send a message via AIM to jleaman
Quote:
Originally posted by Coulomb
Here is thought you are all nuts!!



Anthony
Were all nuts but at least we use them : O ) TEE HEE HEEE
  Reply With Quote
Old 23rd April 2004, 07:10 AM   #288
hifiZen is online now hifiZen  Canada
diyAudio Member
 
hifiZen's Avatar
 
Join Date: May 2001
Location: Mountain View, CA
protos:

Wow, I haven't experienced anywhere near that much thermal variation. At most, I'm seeing +1.4V to 0V from cold to fully warmed up. Do you have the MacMillan resistors installed (R46/R47 on my schematic)? With my prototype channel, nulling the absolute DC did nothing for the differential offset (actually made it a hair worse, as I recall).

I have an idea for a simple two-transistor servo which could be connected to the existing Aleph-X circuit to null it's absolute DC offset (see attached image). Basically, this circuit just adds and subtracts current directly from the main CCS, thus affecting the absolute DC offset. A pair of equal resistors forms a voltage divider between the outputs, thus removing the differential-mode signal, and a cap forms a low-pass to help further attenuate stray signal voltages at the servo diff pair input. R1/4/44/45 must remain to keep the open-loop absolute DC gain low, though the MacMillan resistors would be rendered redundant and could be removed entirely. I provided a trimpot to control the overall current through the servo diff pair. This can be trimmed so that when operating at normal temperature, the servo diff pair sees roughly equal currents on each side, or as needed to prevent servo saturation when the amp is cold and coming up to temperature. With the servo installed, VR2 becomes useless for adjusting DC offset, but can instead can be used for additional control over current balance in the servo circuit.

BJTs should be low noise types, and I haven't done any calculations, since it's still just an idea I scribbled down on a post-it. In practice, some emitter degeneration resistors may be needed for the servo diff pair. In fact, the servo should have a low-noise CCS of it's own for best results, but you get the idea here.
Attached Images
File Type: jpg absdc_servo_idea.jpg (11.0 KB, 1681 views)
__________________
- Chad.
  Reply With Quote
Old 23rd April 2004, 07:36 AM   #289
hifiZen is online now hifiZen  Canada
diyAudio Member
 
hifiZen's Avatar
 
Join Date: May 2001
Location: Mountain View, CA
SGregory:

In theory, if you used deionized water, you could completely submerge the DUT, since pure water is supposed to be non-conductive. I've never tried it myself, but with a constant-current source feeding the DUT like in jwb's pic, the worst that could happen is the water would short out the device, and you'd get a voltage reading of 0V Vgs... no harm done. For the diff pair, I would not heat the water to boiling temp, though I understand your motivation to do so, since it provides reliable temp regulation of the water, and gets the silicon close to it's actual oprating temp. In practise, the diff pair will be quite a bit cooler. Perhaps an aquarium heater would be the way to go?

I would make one small addition to jwb's circuit: a momentary-off switch to short out the LEDs. This way, the current source only turns on when you press the button, allowing you to do very quick momentary tests while the silicon remains normalized to the water bath temp. Rail-to-rail opamp would be needed, or maybe a discrete circuit.
__________________
- Chad.
  Reply With Quote
Old 23rd April 2004, 01:24 PM   #290
protos is offline protos  Greece
diyAudio Member
 
protos's Avatar
 
Join Date: Mar 2002
Location: Athens+Addis Ababa
Hi Chad,
I guess an op-amp, as jwb suggested, would also work here.Would the ZTX450 work here as well?It should be ok no?
I just don't get how a small variation in current at r24 could cause such a huge shift of DC offset.After all we are talking about a "constant" current source?
Can somebody explain please?
What about the thermal drift ?I guess the current source and diff pair pass more or less current at different temps but I am seeing several volts for very few degrees.As I said before , the moment I lift the lid of the amp to adjust anything the DC offset may change up to 3v in about 20 secs without touching anything else so I can't even adjust it properly.
Of course my front end is running at up to double the bias which might explain a wider variation than some others ......but still it's too much of a change for me to be really comfortable with.
It's a bit like trying to balance on a surfboard with one foot in the air.( I have been trying to find a picturesqe analogy).
  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



New To Site? Need Help?

All times are GMT. The time now is 07:43 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