Single-ended input to balanced output, no DC offset - 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 13th October 2005, 03:51 AM   #1
diyAudio Member
 
LineSource's Avatar
 
Join Date: Sep 2003
Location: SiliconValley
Default Single-ended input to balanced output, no DC offset

Has anyone designed a circuit that can translate the single-ended outputs from my active crossover into a balanced output with no +/- DC separation to drive my power amp. I'm trying not to use blocking capacitors in this signal path. The crossover circuit is on the same PCB as the power amp driver circuitry. The power amp has a DC servo, but this cannot null the outputs from two opamps if the +IN and -IN have a fixed DC separation plus drift. My search in diyAudio has yielded typical two opamp buffers which generate a few millivolts of non-symmetric and variable drift voltage separation which translates to 100ma of bias current at the amp's bridged output.
  Reply With Quote
Old 13th October 2005, 04:08 AM   #2
diyAudio Member
 
xplod1236's Avatar
 
Join Date: Dec 2003
Location: On the couch
Send a message via AIM to xplod1236
Try the drv134 from TI.
  Reply With Quote
Old 13th October 2005, 04:11 AM   #3
Account Disabled
 
Join Date: Aug 2004
Location: Earth
Hi LineSource,

Your quest of no signal path C's is commendable. However, I may be missing the 'drift' here. You are wanting to run a bridge amp sub, correct? You say the amp servo can only track one, but each power amp channel should have it's own dedicated servo so nulling it's output. So if you run an inverted signal from your LF x'over out it will invert the DC offset and drift being fed to the other cannel which is on side of the bridge out while the other goes the opposite direction, amounting, as you say, to 100's of mA Iq. But each output is independantly servoed for NULL at it's output so there should be no DC across the bridged load because both ends are corrected to DC=0.

Am I missing something, or is this a non-issue.

Cheers,
Greg
  Reply With Quote
Old 13th October 2005, 01:35 PM   #4
diyAudio Member
 
LineSource's Avatar
 
Join Date: Sep 2003
Location: SiliconValley
xplod, thanks for the BB pointer. I simulated a similar circuit using a discrete opamp (diff JFET Pass crossover clone ckt) and noted about +/- 12 mv from the sum of the balanced outputs(attached). Maybe discrete circuits cannot track as well as the Burr Brown IC with laser trimmed resistors and single die thermal tracking. I keep wondering if a good discrete transistor solution is possible.


ampguru, the amp uses one complementary differential folded cascode input stage with balanced inputs to drive both +/- bipolar outputs, so there is only one servo amp. In this topology, if the +/- balanced inputs have a DC sum on top of the signal, instead of just a balanced tracking DC offset, the outputs drive a DC current into the speaker.

Are DC blocking capacitors always required on balanced input amps?
Attached Images
File Type: jpg singlebalanced.jpg (46.7 KB, 549 views)
  Reply With Quote
Old 14th October 2005, 07:16 AM   #5
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
you could try a pseudo balanced output from the pre to feed the balanced input stage of the power.
The pre single ended output is normal but you add a complementary shunting resistor from cold output to ground after the extra line source resistor.
The balanced input sees identical source impedances on both lines and thinks a balanced output is driving it.
Look up the Jensen website to see the circuit. It is two extra components and an XLR socket. They show the in line caps to block DC but you could try without if you are confident that the pre is NOT putting out a DC offset and that your power can correct itself for its own DC offset drift.
By the way you will need to select the balanced source resistors very accurately try for +-0.1% or better. A little bridge measuring circuit using your DMM will easily achieve this. A DMM on it's own will be right at it's limit of resolution
__________________
regards Andrew T.
  Reply With Quote
Old 14th October 2005, 07:25 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Default DC blocking caps

Hi,
DC blocking between pre and power is always required unless the designer has the chosen to use another method of eliminating the output offset. This would be rare and could only apply to a matched pair of pre & power.
Either the pre must have a DC block or the power must have a DC block. Most equipment has DC block at both the input and output resulting in the two RC time constants compounding themselves. In the worst case if both had the same turnover frequency then the overall Bass turnover would move up one octave and all the phase angle errors would also move up one octave.
There is one thing to look out for that is often omitted;- A shunting resistor (1M or higher is generally OK) at the RCA connector to bleed DC on the blocking cap to earth (on both the pre and the power) to help eliminate that loud pop (or worse) if you ever try to hot plug in RCAs without switching off.
__________________
regards Andrew T.
  Reply With Quote
Old 15th October 2005, 03:05 AM   #7
diyAudio Member
 
LineSource's Avatar
 
Join Date: Sep 2003
Location: SiliconValley
Attached the Burr Brown drv134 buffer internal ckt diagram.

My design is all discrete, with JFET opamps used in the crossover.

A discrete circuit for single-ended-output from crossover to balanced-input on amplifier that nulled any DC offset created in the generation of the +/- balanced drive would help me and all of the SuperSym_X amp designers as well.

Let the transistors fly.........
Attached Images
File Type: jpg burrb.jpg (41.7 KB, 506 views)
  Reply With Quote
Old 15th October 2005, 04:19 AM   #8
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Default Re: Single-ended input to balanced output, no DC offset

Quote:
Originally posted by LineSource
Has anyone designed a circuit that can translate the single-ended outputs from my active crossover into a balanced output with no +/- DC separation to drive my power amp. I'm trying not to use blocking capacitors in this signal path.
A couple of balanced input transformers will work and save you a potful of components.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 15th October 2005, 04:25 AM   #9
diyAudio Member
 
macka's Avatar
 
Join Date: Dec 2001
Location: Australia
I thought is was standard practise for at least one capacitor before the power amp input to prevent damage to the loudspeaker....try a Auricap.

Remember virtually all recordings known to man pass through countless capacitor coupled stages..so one more...the most important can't hurt.

macka
  Reply With Quote
Old 15th October 2005, 11:10 AM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi Sy,
your suggestion for an input transformer will not avoid the DC offset from the Pre. I think the DC current will upset the transformer core with some effect on the electrical output that feeds the power amp (early saturation and probably other distortions).
The tranny only provides a balanced input which he already has.
If I have misunderstood the workings of an electrically balanced input and a transformer coupled balanced input then please correct me.
__________________
regards Andrew T.
  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
WTB: Single Ended Output Transformers Zedguy Swap Meet 2 2nd August 2009 02:05 AM
Aleph P 1.7 Single Ended Input Question kilowattski Pass Labs 1 28th April 2004 06:06 AM
Preamplifier balanced output, single input olvrkrg Pass Labs 12 3rd November 2003 05:30 PM
Converting single ended amp to balanced boyelroy Solid State 15 10th November 2002 04:51 PM
Does it Make Sense? Balanced to Single Ended Super_BQ Pass Labs 3 3rd October 2002 09:01 AM


New To Site? Need Help?

All times are GMT. The time now is 05:10 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