need automatic gain preamp - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: March 18, 2012, 12:02:00 pm
content: I'm looking to buy an automatic gain preamp, this to go between a mixer and a Pc being used for recording the audio. I can't really see much for sale and there are a few projects around maybe but not quite what I want. I did find quite a basic circuit using a 741. So I'm looking to buy a basic auto gain preamp or make one if simple enough. This is not for me but an organisation I volunteer with so needs to be reliable

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 18, 2012, 01:34:03 pm
content: Digitize at 24 bits and do it in software.

author: Simon
author_rank: Unknown
timestamp: March 18, 2012, 01:36:17 pm
content: Quote from: NiHaoMike on March 18, 2012, 01:34:03 pmthat don't help when I have some silly old geezer that can't read a volume meter and I'm no good at software for this sort of thing, it must be possible with analogue

author: T4P
author_rank: Super Contributor
timestamp: March 18, 2012, 02:11:54 pm
content: What i am about to say might not work , just a idea .If you are trying to increase the gain by the signal level ,buffer the input , split the input into 2 opamps : inverting and noninverting .Inverting outputs to the NFB of the non-inverting opamp , sounds convincing enough ? Just have to tailor the gain of the inverting opamp using a trimpot .I'm not sure if it works entirely or not , as i do not how to actually limit the gain of the inverting opamp

author: Simon
author_rank: Unknown
timestamp: March 18, 2012, 02:19:56 pm
content: no I just need a preset signal level to go into the computer so that people doing the recording don't need to fiddle with the computer volume levels as they can only just handle making a recording on a PC and one in particular does not know what a volume meter is for

author: sonicj
author_rank: Frequent Contributor
timestamp: March 18, 2012, 03:40:23 pm
content: what you're looking for is called a limiter/expander. they start around $150 and go up from there. you can pick up a used behringer MDX4600 for about $50usd.-sj

author: Simon
author_rank: Unknown
timestamp: March 18, 2012, 03:55:35 pm
content: so that is why auto gain preamp don't get me any results ?. Why so expensive ? I mean it is just a rectified output fed back to a mosfet that acts as a variable resistor in the gain loop. I'll have a look around. if anyone knows of a diagram / project instructions I am happy to make them one

author: sonicj
author_rank: Frequent Contributor
timestamp: March 18, 2012, 04:12:53 pm
content: automatic gain control should land you plenty of results. (AGC)agc from on semi:NE570

author: Simon
author_rank: Unknown
timestamp: March 18, 2012, 04:41:44 pm
content: looks like a winner

author: Simon
author_rank: Unknown
timestamp: March 19, 2012, 07:11:06 am
content: I found this:http://www.electronicecircuits.com/electronic-circuits/automatic-gain-control-pre-amplifier-circuit-diagramis it any good ?

author: Psi
author_rank: Super Contributor
timestamp: March 19, 2012, 07:22:54 am
content: Just be aware if you use a limiter/expander it will change how music sounds. (if you're recording music).Depending on how much is applied, and the specific song, it can take all the enjoyment out of a track or make it sound even better than the original.That's why some songs sound better on the radio than when played from the proper CD. (The radio stations apply some limiting to the tracks)

author: Simon
author_rank: Unknown
timestamp: March 19, 2012, 07:39:40 am
content: Yea this is for voice only so no problem there

author: Simon
author_rank: Unknown
timestamp: March 21, 2012, 06:14:47 pm
content: can people have a look at this for me. It looks a bit dodgy to me, it is basically applying a DC offset to the output to alter the volume ?http://www.electronicecircuits.com/electronic-circuits/automatic-gain-control-pre-amplifier-circuit-diagram

author: david77
author_rank: Frequent Contributor
timestamp: March 21, 2012, 08:52:59 pm
content: I haven't looked at the circuit you linked to in detail but it looks about right to me, I'm no expert, though.The FET Q1 acts a variable resistance. The output of the 741 is rectified using the diodes, this rectified signal depends on the output of the opamp and in turn is used to control the resistance of Q1 which alters the gain of the op. C3 slows down the loop to keep Q1 from going low R too fast, for example in short pauses and quiet passages of the audio signal.I think that should be about right, comments and corrections to my explanation are welcome.Do not expect too much from this circuit performance-wise. I'd expect it to introduce a bit of noise and distortion. A circuit like this may well be used for a cb-radio station mike or a simple PA system, I don't know what quality your old geezers expect.Have a look at this:http://sound.westhost.com/project67.htm

author: Simon
author_rank: Unknown
timestamp: March 22, 2012, 07:11:05 am
content: well from what I see it is like a summing amp and is adding a DC voltage to the audio input but then I'm no op-amp configuration expert. The feedback loop seems totally unaffected. I'll take a look at the circuit you linke to and then maybe look at the chip that I was linked to further back as that may give best results and I don't mind spending a few pounds.Quality wise I'm not after super HiFi quality but need it to be decent. we have just abandoned cassettes for memory sticks and MP3 and the end users are really happy with the quality. I don't want to trash that for the same of a nicety although I have had a few complaints back of varying volumes between sections of the overall recording (80 minutes long)

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 22, 2012, 08:55:05 am
content: Please reconsider doing it in software. Also, if you are going to compress to mp3 format, shoot for input of -3db max to give the encoder/decoder a bit of head room. Depends on the which encoder/decoder exactly how much you'd need but -3db is usually a good amount since it's not too far away from any 0db content you might have and you'll avoid any clipping problems.Speaking of clipping, make sure you test the input of the pc that it does not clip your signal especially since you are going to amplify it. Some cheap sound devices, though less common nowadays, could max-out the signal in the mixer before it hits the ADC.HTH.Also, did you know that the MP3 format has an adjustable gain feature built into the playback of the file? The audio is sliced up and encoded in frames of a certain length and each frame has it's own playback gain field that can be adjusted with software. No need to re-encode, just tweak little numbers in the mp3 file and there is plenty of software that can do it in varying degrees of easiness.

author: sonicj
author_rank: Frequent Contributor
timestamp: March 22, 2012, 11:05:25 am
content: Quote from: Ed.Kloonk on March 22, 2012, 08:55:05 amthats the problem... the operator does not understand concepts like clipping or signal to noise. the fools on tha mic needs to be clamped prior to the ADC.-sj

author: caroper
author_rank: Regular Contributor
timestamp: March 22, 2012, 11:40:12 am
content: Take a look at Audacityhttp://audacity.sourceforge.net/about/featuresamong its other features it list this as one of the effects filters:Adjust volume with Compressor, Amplify, Normalize and Fade In/Out effects.I have used it personally, so this is not a blind reference.I used it to mix a CD of recordings from several live sources and it Normalized them all perfectly.It is open source so it will cost nothing to experiment and could save you $$$ on hardware.

author: sonicj
author_rank: Frequent Contributor
timestamp: March 22, 2012, 12:12:08 pm
content: Quote from: caroper on March 22, 2012, 11:40:12 amfrom the audacity website:"If clipping is detected when you are recording, you should stop, lower the volume of your input source, and start recording again from the start. If the output meter clips then you need to make some or all of your tracks quieter using the track gain control.If the level of the input (recording) source is too high, you can try to lower the input level using the Mixer Toolbar, but if this doesn't work, you should try to lower the volume of the external input source (e.g., your microphone, cassette player, or record player)."Audacity does not do anything in real time.-sj

author: Simon
author_rank: Unknown
timestamp: March 22, 2012, 12:41:13 pm
content: I need this to be as transparent as possible to the person using the computer.I can't write software and a micro design is a bit beyond me atm.We are actually using audacity now so that may be an option but i need to have a look into it to try and keep it easy as there is still an option on screwing it up during recording. the idea is to make it totally fool proof and not require the volume levels to be changed all the time to stop clipping ect. there are different people doing the recordings each weeks and many different readers so a lot of variation in there.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 23, 2012, 07:33:05 am
content: Quote from: Simon on March 22, 2012, 12:41:13 pmFair enough. Don't want to be bashing my opinion, just thought a different approach might be beneficial. No problem.There are also some very nice portable digital recorders around due to the popularity of podcasters everywhere and those trade shows that podcasters have to cover. I know a few high end recorders can do auto-gain on the fly, maybe you could hunt around and find a cheap one that has auto-gain built in.

author: Simon
author_rank: Unknown
timestamp: March 23, 2012, 07:35:24 am
content: are you suggesting i use the guts of an existing device to build what I need ? or that we switch to using one. We have to keep the current system and use the computer so that we can edit the audio quickly.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 23, 2012, 07:40:45 am
content: You might be lucky to find one that has audio in and out and while passing through the auto gain. But I'm pretty sure that type of functionality is done digitally in the upper extreme price bracket of those recorders. Don't know if you'd use the guts because those things are tiny and are little computers inside. I don't think you'd be able to cannablise.

author: Simon
author_rank: Unknown
timestamp: March 23, 2012, 07:53:40 am
content: well we have been told of a professional piece of kit called a podblaster that does what we want and more. I think the organization I volunteer with are considering getting one now.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: March 23, 2012, 09:01:02 am
content: Well there you go!Can we look forward to a podcast from you in the near future?

author: Simon
author_rank: Unknown
timestamp: March 23, 2012, 07:52:23 pm
content: well I'm tryiong to go that way too but having some problems because i cannot find a reliable free service that will host our audio files for us. I did try youtube but got fed up with all the work it was. I'm happy to post up some MP3's here though