Simple subwoofer buffer/summer circuit - 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 9th March 2012, 07:49 PM   #1
hpp3 is offline hpp3  United States
diyAudio Member
 
hpp3's Avatar
 
Join Date: Feb 2012
Default Simple subwoofer buffer/summer circuit

I have a powered subwoofer (Sony; please don't laugh, it's just for the workshop) and would like to add a line out to it from a chipamp I'm building (ESP preamp to 2x TDA2050s).
I've read the Rane article "Why not Wye?" about adding a passive summer to your inputs and built a stereo-to-mono unit in a small Radio Shack project box.
It works well, even considering the 27 dB crosstalk (not that I'd notice down here in the 'shop).

However, that got me thinking...
How about an active summer that wouldn't load the inputs AND avoid the crosstalk issue?
I wanted it simple with a minimum of parts so it could easily be added as a small PCB to a commercial unit or built into a diy board, and high-impedance to avoid loading the inputs as much as possible.

Here's my circuit; it's 2 FETs (for high-Z input; low loading) wired source-follower (for low output Z and maintained phase) with shared source resistor.
Works well in LTSpice simulation, and I wanted to run it by you folks before I blow something up with it.

Click the image to open in full size.

Questions? Comments? Suggestions?
Thanks.
-Eddy


P.S. No low-pass on the output because my powered sub already has a crossover.
  Reply With Quote
Old 9th March 2012, 09:32 PM   #2
tvrgeek is offline tvrgeek  United States
diyAudio Member
 
Join Date: Dec 2009
Location: Md
I would go to 100uF NPE on the output to lower distortion.
  Reply With Quote
Old 10th March 2012, 02:13 AM   #3
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
Try your sim again...My impression was that it would have bad intermodulation problems...I did a tran sim, with 0.5 volts of 100 Hz in one input, and 1 volt of 1 kHz in the other...the results were quite non-linear...
  Reply With Quote
Old 10th March 2012, 04:54 AM   #4
hpp3 is offline hpp3  United States
diyAudio Member
 
hpp3's Avatar
 
Join Date: Feb 2012
Hmm... hadn't thought of that.
So without a low-pass going IN, I'd likely get intermodulation artifacts dirtying up the output, right?

Just sim'd it, very interesting output that I don't remember getting before. Did you get something like this?

Click the image to open in full size.

Blue=1kHz / Green=100Hz / Red=Output
I imagine it'd do something different with actual music, rather than reference signals, but still that looks scary.

I also sim'd the passive mono-to-stereo circuit from the 'Why not Wye' article, and it looks like you'd expect; a 1000Hz signal modulated by 100Hz.
Maybe I should either live with the crosstalk, or come to grips with the fact that an active stereo-to-mono circuit just isn't as simple as it seems.

Last edited by hpp3; 10th March 2012 at 04:59 AM.
  Reply With Quote
Old 10th March 2012, 05:06 AM   #5
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
yes...your sims look like mine. You wouldn't have to add a lot to make it much more linear, and cut the crosstalk...I'll keep with your theme, just change it a little bit...(see enclosed pic). This version inverts, take output at the common drains through a coupling cap.

Alternatively, You can also use 1 opamp, and make a summer...owing to the virtual ground, it wouldn't cause crosstalk between the channels.
Attached Images
File Type: png summer.png (49.0 KB, 388 views)
  Reply With Quote
Old 10th March 2012, 05:44 AM   #6
hpp3 is offline hpp3  United States
diyAudio Member
 
hpp3's Avatar
 
Join Date: Feb 2012
Your idea worked out well in the simulation, i would just swap 5k's at sources and a 10k for the drain so the DC operating point at the drain is ~1/2 the voltage (give or take).
Drop the voltage to 12V, and it's 5k all 'round.

I had originally went with the source-follower circuit because it would give low output impedance and doesn't amplify; that's what the powered sub is for.
This configuration has a higher output impedance and just a touch of gain; not much at those values and without bypass caps at the sources, but it's there, and needs to be taken into consideration.

I also didn't want to go with an op-amp because it was a more complicated circuit, and there was still the issue (so I thought) of cross-talk.
Can you recommend a simple op-amp circuit?

Or maybe I just need to wire these things up and give it a listen.
I doubt I'm going to blow anything up, and prolly learn a thing or two in the process.
  Reply With Quote
Old 10th March 2012, 10:21 AM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Common drain (=source follower) does not invert.
Common source does invert and amplify.
You can use common source to invert and sum at it's input.

Just set the gain as low as you can and still maintain stability.

edit:
Now I am having second thoughts!
Does a single transistor in common source/emitter sum at it's input?
__________________
regards Andrew T.

Last edited by AndrewT; 10th March 2012 at 10:23 AM.
  Reply With Quote
Old 10th March 2012, 10:32 AM   #8
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
Here's a summer, based not on an opamp, but a 1 transistor version thereof, which I like to call a "slop-amp". It doesn't have as much gain as an opamp, but works in a similar fashion. Left and Right are the outputs of left and right preamp signals...you'll see that the crosstalk (left signal appearing on the right undriven output) is -73 dB. The output of the summer is at the collector of the transistor
Attached Images
File Type: png slop_amp_summer.PNG (37.4 KB, 357 views)
  Reply With Quote
Old 10th March 2012, 10:54 AM   #9
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Its far simpler to make an entirely passive mixer
driven from the output of the chip, its 3 resistors.
Say 2x1K and 50R.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
  Reply With Quote
Old 10th March 2012, 04:29 PM   #10
hpp3 is offline hpp3  United States
diyAudio Member
 
hpp3's Avatar
 
Join Date: Feb 2012
A passive mixer was what I learned and used from Rane's "Why not Wye" article, and I must admit it did work well enough for the workshop system.

BUT... the closet audiophile in me that just knew there was 27dB of crosstalk going on, --even if I couldn't hear it--, was always nagging me, so I thought I'd try to come up with a simple active solution because that would eliminate crosstalk and loss; of course, at the expense of a large degree of simplicity, but also allowing a low-impedance output which is, in my mind, always a good thing.

So, to clarify, I'm trying to design the simplest circuit possible that not only sums to mono for my sub, but isolates the two inputs.
Djoffe, your first circuit seems to fit the bill; it matches the type of signal that would come out of passive mixing, but with maximum isolation.
Perhaps an improvement would be adding a third FET or even a transistor as a source/emitter follower to buffer the first stage and provide low-impedance output for good measure, and it would only require two more parts.
I'll post a circuit and sim results later today...
  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
Summer is here - little tubes go in Simple SE w5jag Tubelab 12 7th July 2011 09:35 PM
Simple Volume/Buffer! amplifierguru Solid State 37 6th September 2010 07:41 PM
Simple Input Buffer Adam M. Chip Amps 17 1st May 2010 01:37 AM
Simple summer vacation amplifier :) Onnimanni Solid State 2 12th July 2009 05:09 PM
Good but simple subwoofer chip amp. circuit suggestion Alvin Chip Amps 2 17th January 2004 02:04 PM


New To Site? Need Help?

All times are GMT. The time now is 07:40 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