Using Gilbert Cell for Volume Control - 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 30th August 2003, 06:30 PM   #1
kyrgeo is offline kyrgeo  Greece
diyAudio Member
 
Join Date: Dec 2002
Location: Serres
Default Using Gilbert Cell for Volume Control

I have never seen it done before, but I don't see a reason why it shouldn't be possible.

The Gilbert cell is a fully balanced four-quadrant multiplier circuit, that is extensively used in RF telecom as a mixer.

There are two inputs, and the output is simply the multiplication of the two input signals, times the gain constant. In this case, one of the inputs would be your balanced or single-ended input signal. The other input would be a DC voltage generated with the volume pot. For zero DC input voltage of the volume control, you get maximum attenuation.

Using this circuit, the audio signal will not pass through the volume pot. Another advantage is that the input impedance of the amp is much better defined, since impedance variations due to the volume control are eliminated. Inverting configurations are now easy to handle, since there is no more variation in loop gain as the volume is changed.

The only problem I see is noise. Any AC noise on the volume control input will be mixed with the input signal, giving all sorts of intermodulation products. However, this should be relatively easy to filter out (now that I think of it, a large capacitor after the volume pot would also act as a "soft start" circuit, and the volume changes would be smooooth), and also using large emitter resistors in this input to keep the gain low should help.

Any opinions on which input should be used for the audio signal and which for the volume control?

Comments are welcome!

Kyriakos Georgiadis
Attached Images
File Type: gif gilbertcell.gif (4.1 KB, 488 views)
__________________
Pan metron ariston
  Reply With Quote
Old 30th August 2003, 07:58 PM   #2
diyAudio Member
 
Join Date: May 2003
Location: Colorado
Hello -

I believe this is the standard way that a VCA (voltage-controlled amplifier) is implemented. For instance, look at:

http://www.analog.com/UploadedFiles/...786ssm2164.pdf

the data sheet for an SSM2164. On page 7 there is a schematic that is very close to what you've drawn.

Hope this helps,
Charles Hansen
  Reply With Quote
Old 30th August 2003, 08:37 PM   #3
jcarr is offline jcarr  United States
diyAudio Member
 
Join Date: Jul 2002
Location: Tokyo, Japan
kyrgeo:

I also considered doing something like a Gilbert multiplier attenuator, but haven't tried to pursue it in earnest, mainly because time is finite and I already have too many other priorities. The following thread includes some weblinks to other patents and reading material that you may find interesting.

Attenuator based on I/V converter

hth, jonathan carr
__________________
http://www.lyraconnoisseur.com/, http://www.lyraaudio.com
  Reply With Quote
Old 30th August 2003, 08:55 PM   #4
Bricolo is offline Bricolo  France
diyAudio Member
 
Bricolo's Avatar
 
Join Date: Nov 2002
Location: Grenoble, FR
can this design be used for multi channel?
I mean, tying the bases of the transoistors connected to the pot all together (1 pot for as many channels you want)
  Reply With Quote
Old 30th August 2003, 10:20 PM   #5
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Default 4 quadrant multipliers

Sorry to dampen things,

Before digital, this was the standard way to mix and fade video. I will warn you that making several channels track correctly is a difficult job, and requires periodic tweaking.

Cheers,
  Reply With Quote
Old 30th August 2003, 11:27 PM   #6
diyAudio Member
 
Join Date: Aug 2003
Location: Swampscott MA
Default Vca?

Hi K.G.
I don't remember my VCA stuff that well but let's have a go anyway. It looks like V1 is your signal input and V2 is your DC control voltage. Like anything else in engineering there is no free lunch. The VCA will give lower noise than the standard ladder attenuator, but the distortion at greater than 10db of attenuation has been found audible by many. DC feedthrough may also be a problem.
For further research I would suggest going to the THAT Corp website and take a look at their equivalent schematics.
BTW, I was thinking of using a VCA for a headphone amp.
Dan Banquer
__________________
I was an audiophile until I found out what they were doing in the recording studio.
  Reply With Quote
Old 31st August 2003, 12:56 AM   #7
diyAudio Member
 
Terry Demol's Avatar
 
Join Date: Apr 2002
Location: *
Default Re: Using Gilbert Cell for Volume Control

Quote:
Originally posted by kyrgeo
I have never seen it done before, but I don't see a reason why it shouldn't be possible.

The Gilbert cell is a fully balanced four-quadrant multiplier circuit, that is extensively used in RF telecom as a mixer.

There are two inputs, and the output is simply the multiplication of the two input signals, times the gain constant. In this case, one of the inputs would be your balanced or single-ended input signal. The other input would be a DC voltage generated with the volume pot. For zero DC input voltage of the volume control, you get maximum attenuation.

Using this circuit, the audio signal will not pass through the volume pot. Another advantage is that the input impedance of the amp is much better defined, since impedance variations due to the volume control are eliminated. Inverting configurations are now easy to handle, since there is no more variation in loop gain as the volume is changed.

The only problem I see is noise. Any AC noise on the volume
control input will be mixed with the input signal, giving all sorts of
intermodulation products. However, this should be relatively easy
to filter out (now that I think of it, a large capacitor after the volume pot would also act as a "soft start" circuit, and the volume changes would be smooooth), and also using large emitter resistors in this input to keep the gain low should help.

Any opinions on which input should be used for the audio signal and which for the volume control?

Comments are welcome!

Kyriakos Georgiadis
You ave just created a VCA, and it will work as a vol control.
For more info on this type of circuit searc for MAT04 and
download data sheet, there is a good example.

VCA's are used widely in audio and infact every channel has one
in consoles like SSL which LOTS of our music is mixed through.
They are used mainly for compressors (which are also used
on 99% of recordings) and fader automation systems.

Unfortunately they have a reputation of being sonically pretty
coloured and have a bad name because of this, however the
harmonics are reportedly pretty benign (low order mainly
second). Bottom line is this circuit won't come close to a high
quality pot or stepped attenuator if transparency is the goal.


Cheers,

Terry
  Reply With Quote
Old 31st August 2003, 02:00 AM   #8
diyAudio Member
 
Join Date: Jul 2002
Location: Santa Maria CA
Default VCA

FYI

Here are links to VCA ICs manufactured by National Semiconductor:

LM3080 Operational Transconductance Amplifier [Discontinued]:

http://www.national.com/pf/LM/LM3080.html#Datasheet

LM13700 Dual Operational Transconductance Amplifier with Linearizing Diodes and Buffers:

http://www.national.com/pf/LM/LM13700.html



Dave Kimble
  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
Closed Cell Vs. Open Cell Foam-Which For Damping Loudspeakers? kelticwizard Multi-Way 25 10th July 2013 03:55 AM
Volume control on DVD player remote. Lose resolution at low volume? Circlotron Digital Source 2 10th June 2011 07:20 PM
i need preamp schematics w/o tone control or volume control hernanstafe Parts 7 30th January 2008 03:12 AM


New To Site? Need Help?

All times are GMT. The time now is 04:04 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