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Old 22nd April 2008, 02:31 PM   #1
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Default mic preamp automatic gain

Hi all,

I was wondering what is the best method of having an automatic gain in a hi-fi mic-preamp for a music instrument. I have been looking for plastic resistive material motorized pots and struggling to find a suitable part. Alps and other companies produce motorized pots but with carbon resistive material, e.g. Alpha Taiwan. a) life span is short <100000 cycles, b) no dc current through for the servo, c) wiper is noisy at high gains, >40 dB... among other problems. In addition, Alps dual units aren't truly dual, they have a common pin (1 and (1) ) so the servo option is made difficult even if they had resistive material ... other manufacturers like State Electronics in USA, do produce in small quantities plastic resistive pots but very expensive $140 usd each in a hundred units. I have been looking at faders, I must mention that I have space limitations too, the dual plastic resistive material ones come with a travel of 100 mm, (overall size: 157x13x39 mm)... and the motor current demand is from 800 mA to 1A at 10V dc (quite a lot I would say). The other option could be relays but I am dubious about the number of gain steps, some hi-fi mixers manufacturers say that even 0.5 dB steps are noticeable along with the noise introduced when changing gain (do i need to include a mute option?)

Does anyone know how the automatic gain control is done in commercial hi-fi mic-preamps? Thanks

Sergio
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Old 23rd April 2008, 06:30 AM   #2
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I'm not really sure what you're trying to do?, or what you mean by a "commercial hi-fi mic-preamp" - Hi-Fi's don't usually have mic preamps anyway, and if they did they wouldn't have AGC on them.

Where AGC is used, is on cheap taperecorders, to set the recording level, it's not done via a mechanical pot, but electronically, commonly using an FET.

However, for a start tell us what you're trying to do, apart from taperecorders audio AGC would be exceedingly rare.
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Old 23rd April 2008, 11:50 AM   #3
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Quote:
Originally posted by Nigel Goodwin
I'm not really sure what you're trying to do?, or what you mean by a "commercial hi-fi mic-preamp" - Hi-Fi's don't usually have mic preamps anyway, and if they did they wouldn't have AGC on them.

Where AGC is used, is on cheap taperecorders, to set the recording level, it's not done via a mechanical pot, but electronically, commonly using an FET.

However, for a start tell us what you're trying to do, apart from taperecorders audio AGC would be exceedingly rare.

Dear Nigel,

Hope you are fine,

I thought the definition of "hi-fi" is generally used for electronic equipment to reproduce or record high-fidelity sound; sorry if I misunderstood the term. What I have implemented is a very low-noise mic-preamp which would be used in a state-of-the-art electronic music instrument. The mic-preamp has been designed used a balanced transistor array input which requires a gain control. At the moment I have to do it manually using a pot. However, we would like to make the gain control automatic as well. I can't use FETS to do so since they will introduce distortion.

I hope this can help, thanks for replying my previous post,


kind regards,

Sergio
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Old 23rd April 2008, 07:46 PM   #4
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Like I said before audio AGC is generally only useful for cheap recorders - you wouldn't normally have any use for it on any professional music equipment - it's much too limiting.

But what you need for it to work for a recorder is fast attack and slow decay, and you're not going to get fast attack with a motor driven pot. If you can give us any clue as to why you feel you need it?, we might be able to make suggestions?.

Hi-Fi refers to high quality home playback equipment, never to professional or music equipment.
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Old 23rd April 2008, 09:22 PM   #5
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Hi Nigel,

thanks for your answer,

I do not need automatic gain control, I just need the user to be able to remotely control the gain of the amplifier while the instrument is being played. Some manufacturers of professional equipment like Aphex (e.g. 1788A-RC) suggest that relays are used to remotely control the gain with very small steps 1/4 dB (space and power is a constraint for me as to use e.g. 8 relays to have 256 gain levels) some others use faders from Alps or Penny + Giles to do so, e.g. Mackie, Yamaha, etc. I have been thinking of using LDRs as well but they aren't RoHs compliant at the moment and it would be a big risk to include them in my design. Typical AGC, with CMOS or FET switches like Maxim digipots will increase the noise of the amplifier and introduce non-linearities and distortion. They are O.K. for consumable audio amps but not as to be included in the path of the audio signal of a mic-preamp with noise levels less than 1.5nV/(Sqrt(Hz)) in the audio band. Hence my previous posts, thanks again for your interest, looking forward to hear some suggestions,

Sergio
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Old 24th April 2008, 11:24 AM   #6
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OK, so it's not AGC you want, just remote control of the gain - really the easiest way to not detract from any quality issues is to use a motorised pot - the manufacturers of which you already know.

Any electronic method is going to have some slight effect on the sound quality, although by controlling the gain at a suitable signal level point it's probably not significant?.
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Old 10th May 2008, 05:29 PM   #7
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Especially when using the attenuator in conjunction with tube amplification, you might want to take a look at US patent 5208548. Held essentially by Mesa/Boogie and used extensively in their Triaxis guitar preamp, it explains into sufficient detail a way of attenuating without affecting Tone. Just my 2c.
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