Differential filter circuit: biasing problem - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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
Thread Tools Search this Thread
Old 1st May 2009, 11:45 AM   #1
Alex M is offline Alex M  Europe
diyAudio Member
Join Date: Apr 2001
Location: Hampshire, UK
Default Differential filter circuit: biasing problem

I have now almost finished building a differential active crossover, with solid-state circuits for the low-pass leg, and the SLCF buffers from Allen's RTP5 preamp for the high-pass leg.

I have just done some frequency response measurements, and although the high-pass and low-pass filters appear to be doing their job well around the crossover frequency (550Hz), I was surprised to see a large peak in the high-pass response at about 80Hz.

I can't see any obvious passive time constants as high as this (2ms). After some thought, the only way that I can imagine such a low-frequency resonance could arise is from the biasing circuit for the buffers, which keep the grids at 150V (half the 300V B+). Here is the circuit:

Click the image to open in full size.

The circuit here is fed from another SLCF buffer. The second-order high-pass filter (which I have marked in red) is made up of the 20.5K and 41K resistors and the 10n capacitors, and the 41K resistors meet at a virtual earth which is held at a DC potential of 150V.

Can anyone see what might be going on here, and is there a better way to bias the grids?


  Reply With Quote
Old 1st May 2009, 06:47 PM   #2
Alex M is offline Alex M  Europe
diyAudio Member
Join Date: Apr 2001
Location: Hampshire, UK
Further tests reveal that the resonance at 80Hz is actually common-mode (it disappears when I measure across hot and cold outputs, while the signal doubles). I find that very puzzling.

Here is a frequency response plot for one channel of the crossover:

Click the image to open in full size.

The low-pass response is near perfect; high-pass shows the peak and also a difference between hot and cold which may connected with the phase changes (though I would have expected a sharp null somewhere...).

  Reply With Quote
Old 4th May 2009, 11:38 AM   #3
Alex M is offline Alex M  Europe
diyAudio Member
Join Date: Apr 2001
Location: Hampshire, UK
Default Differential filter: problem solved

I've now fixed the problem, simply by shorting across the 1M0 resistors between the virtual earth point and the 150V bias divider, thereby removing the differential action of the filter circuits (although there is still a LTP at the input to ensure that a differential signal is passed to the filters).

I completely fail to understand why such a differential filter should be unstable, however. On paper it should work fine. Might it be that the filter requires a higher degree of balanced antiphase inputs than I am supplying? I still can't imagine where the 80Hz time constant would come from.

  Reply With Quote
Old 5th May 2009, 01:42 AM   #4
diyAudio Member
Michael Koster's Avatar
Join Date: Jan 2008
Location: Eureka, CA
Hi Alex,

That all sounds familiar.

I worked on a differential active filter about a year ago and ultimately
decided that the component matching problem would be too much
trouble. I asked myself why it needed to be differential or balanced
and couldn't justify it. I'll use transformer coupling for input and output
anyway which will work fine with my balanced XLR studio interconnects.

I decided to work on SE circuits to reduce the noise and linearize the
transfer function and came up with a SE augmented cathode follower.
The PSRR is improved by the MOSFET bootstraps and CCS bias. For
my purposes at least, this gets me to my goal of a great active line
level crossover sooner with less risk.

I have another version with cascoded MOSFETS everywhere but in
the end decided to simplify it to this for the first try.


Attached Images
File Type: png afbb.png (15.8 KB, 227 views)
  Reply With Quote
Old 5th May 2009, 09:44 AM   #5
Alex M is offline Alex M  Europe
diyAudio Member
Join Date: Apr 2001
Location: Hampshire, UK

Thanks for your response. The primary reason for going for a differential crossover was because I already use, and am very happy with, the Vacuum State RTP3 preamp I built two or three years ago, and my next project will be a 300B amp with a differential input.

Your buffer circuit looks rather like mine, although you have clearly put effort into optimising the current sink and bootstrap elements. Transformer coupling is an elegant solution. The one thing that has kept me away from using transformers in my system, apart from the cost and real estate, is the thought of all that copper. I have had one short experience with an OTL amplifier, next to a similar amp with an output transformer from the same designer, and heard a huge gain in transparency. All the same, I understand that there are plenty of people out there who would rather use a whole pile of transformers than a single capacitor.

My crossover, while on my workbench, is being fed from a single-ended signal generator, and I suspect that the differential input stage isn't quite perfoming as it should: the plot I posted has all the traces normalised by the passband amplitude, and in reality my circuit has a few dB of imbalance which I need to sort out. I still don't see why two single-ended filter stages which are working correctly should have such a response anomaly when they are used differentially.

Now I am running the filters single-ended, their responses seem to be matched to within a fraction of a dB (and also very close to the target LR4 curves), which is very pleasing. I matched components to within 1%, which seems to be enough in this case.

  Reply With Quote


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
Differential stage biasing choice zigzagflux Tubes / Valves 3 26th April 2008 01:42 AM
how to set current in G Pimm's latest self-biasing CCS circuit? jarthel Tubes / Valves 0 31st October 2006 11:36 PM
gary pimm's latest self-biasing CCS circuit jarthel Tubes / Valves 13 6th October 2006 04:16 AM
HELP! Differential Input Circuit mtnickel Solid State 33 26th September 2004 09:13 PM

New To Site? Need Help?

All times are GMT. The time now is 01:00 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2