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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Oct 2018, 11:31 pm   #1
1100 man
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Default Auto level control for Minimod.

I've recently built a Minimod AM modulator and found it to be a great little thing, but I would like to be able to use it from a variety of audio sources without having to adjust the level to maintain constant modulation.
The sources will be headphone output jacks and so the volume control on the device will affect the volume.

Is there a relatively simple way of achieving some sort of auto level control? I've looked at the SL6270 but that seems designed for microphone level signals.
What I need is a chip that I can feed headphone level audio in to and get a similar but fairly constant level out!

Any suggestions gratefully received!

Many thanks
Nick
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Old 13th Oct 2018, 12:57 am   #2
boxdoctor
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Default Re: Auto level control for Minimod.

I would suggest that you look at the BA 3308.https://www.cricklewoodelectronics.c...keep_https=yes
Easy to use, i.e. does not "take off" too easily, works well at line level, cheap, and being S.I.L., easy to use with veroboard. I've built quite a few items with this chip incorporated, and found them really versatile and un-fussy to use. Stereo, too. Tony.
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Old 13th Oct 2018, 11:37 am   #3
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Default Re: Auto level control for Minimod.

Just checked the above link, invalid. Sorry. Try this https://www.cricklewoodelectronics.com/BA3308.html
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Old 13th Oct 2018, 12:12 pm   #4
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Default Re: Auto level control for Minimod.

Ebay sellers sell them too.
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Old 13th Oct 2018, 11:53 pm   #5
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Default Re: Auto level control for Minimod.

Many thanks Tony, that looks to be just the thing. I've just read the data sheet for the surface mount type and it looks quite sensible.
It's a relief to know it's still available in a SIL package.
I notice the Vcc pin is connected via a 220R resistor to the power rail with a cap to ground. Is this to do with the 'pop' muting at power on? Have you done this on your ones?

I don't doubt I will source them from Ebay, although I am now a bit wary of buying semiconductors this way. I had issues with an unusual dual transistor that turned out to have completely different internals than the type number printed on it!

Thanks for the help,
All the best
Nick
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Old 14th Oct 2018, 8:56 am   #6
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Default Re: Auto level control for Minimod.

Hi Nick,
I am very wary of some of the vendors on Ebay.
Old saying: "All that glitters is not gold." - Long O.T. story.
Re the 220R res and associated cap. I think it is probably an anti-thump measure as you say, and I have never incorporated it myself.
Dont forget that any form of A.G.C. in an analogue audio circuit may cause "breathing" effects unless adjusted carefully. I would be inclined to have an input gain pot. to avoid excessive breathing effects, which, to me, sound unpleasant.
You may have to play around with the feedback circuit to get the ideal overall gain you require. Tony
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