automatic volume control design - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: June 04, 2012, 06:24:10 pm
content: as I have not been able to find a basic circuit of the net I thought I would start to play around myself. Attached is a PDF of my scrawling so far. Any comments welcome. The general idea is a pre-amp stage which goes through a voltage divider. A voltage divide is made up of a resistor and a MOSFET, the MOSFET has the gate-controlled by a DC voltage which is being rectified by the output of the preamp stage. The RC filter on the output should give it the necessary dampening to stop it in effect creating a constant voltage output (in the event the plan could really react that fast).

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 04, 2012, 09:00:37 pm
content: Not sure why you could not find anything, or maybe I don't understand exactly what you are trying to do.http://www.google.com/search?q=agc+circuit&tbm=ischAs you can see most are based on a feedback.

author: Simon
author_rank: Unknown
timestamp: June 04, 2012, 09:03:12 pm
content: well we had a similar discussion before and people were telling me that most of what we found on the net was not very good. my idea is to create something rough and ready, more of a self controlled pot than a full of auto gain control. if I get it to work I may use it on a surveillance mic

author: Simon
author_rank: Unknown
timestamp: June 04, 2012, 09:18:19 pm
content: a lot of those circuits are part of radio receivers.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 04, 2012, 09:27:20 pm
content: Quote from: Simon on June 04, 2012, 09:18:19 pmYes, maybe this is a better link:http://www.google.com/search?q=audio+agc+circuit&tbm=ischIt looks like the majority is based on an opamp with a feedback controlling the amplification, like this:There must be a reason why they do that...

author: DrGeoff
author_rank: Frequent Contributor
timestamp: June 04, 2012, 09:33:15 pm
content: Sounds a bit like a FET compressor. Lookup 'compressor/limiter' and you will find many examples. Very common in audio production.

author: codeboy2k
author_rank: Super Contributor
timestamp: June 05, 2012, 06:33:13 am
content: Some time ago I thought about using a true RMS converter on an output, to produce a DC voltage (the RMS value), compare that to a set RMS value (the desired output level) and feed that back to the gain control of an amplifer at the input section. It would work really well for complex audio signals (or AWG outputs)I never built it, but would it work? maybe it's overkill..I just drew a block diagram and included it.

author: codeboy2k
author_rank: Super Contributor
timestamp: June 05, 2012, 07:04:20 am
content: doh! .. never mind. and I thought I was brilliant.. But it's been done alreadyI'll save my brilliance for another day.I just found an AGC using a true RMS convertor inside an old Analog Devices Design Guide.http://www.analog.com/static/imported-files/design_handbooks/405029458307440275176Section_III.pdf

author: Simon
author_rank: Unknown
timestamp: June 05, 2012, 07:16:07 am
content: well it don't hurt to reinvent the wheel now and then

author: Rerouter
author_rank: Super Contributor
timestamp: June 05, 2012, 07:34:56 am
content: a reinvention of the wheel was the only reason the gear was madestill that is more simple than i thought it would be :/ may look into including one in a design or 2 down the track.

author: codeboy2k
author_rank: Super Contributor
timestamp: June 05, 2012, 08:31:55 am
content: Quote from: Rerouter on June 05, 2012, 07:34:56 amOne obvious use would be on the output of an ARB/function generator. Just set the desired RMS output level and it will be what you set.You'll have to make sure you have the needed bandwidth, but this could be useful on some of the cheaper ARBs that fall off in level after 20Mhz or so.