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Old 13th February 2004, 10:41 PM   #1
maylar is offline maylar  United States
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Default DC controlled gain circuit

I'm looking for a circuit that's basically a remote volume control. Don't wanna send audio through long wires to a pot, so it's gotta be DC level controlled gain. I'd like at least 20 dB of control and DON'T want digital solutions. Any ideas?

Thanks.
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Old 14th February 2004, 02:03 AM   #2
Eva is offline Eva  Spain
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Switched-resistor-network volume control 'digital' IC's found in consumer equipment tend to be a very crappy low cost thing but there are also high quality ICs with outstanding specs

If you don't want to use switched networks, they you could use VCA, but you would need a DC control value from the microcontroller [from a DAC or from a integrated PWM signal]

As a starting point, you may try National Semiconductor LM13700 IC, it contains 2 current controlled amplifiers. It has no wonderful specs but it's cheap and I've seen it used quite often for VCA and signal limiting in PA equipment and as a part of remote level control for active subwoofers
[As an alternative, there are also $30 multiplier ICs with outstanding specs]

And if you don't like solid state volume control at all, you could use motorized potentiometers, they are a simple yet elegant solution
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Old 14th February 2004, 11:56 AM   #3
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A simple, and rather functional solution would be to have a pot control current through a nunber of LED:s, and then let each led shine on an LDR that serves as one leg of a voltage divider. The tracking might not be wonderful, but that can be compensated using trim pots as the other leg of the voltage divider.

Rune
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Old 14th February 2004, 02:29 PM   #4
maylar is offline maylar  United States
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There's gotta be an analog VCA schematic out there somewhere... maybe an adaptation of AGC with a wide dynamic range?

I've considered using a JFET as a voltage controlled resistor in the feedback loop of a simple opamp, but I question the linearity of such a circuit.

As for a motorized pot, I have one and might actually use it if it comes to that.
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Old 14th February 2004, 03:10 PM   #5
Did it Himself
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Nice VCAs click on the Products button then choose That Corporation for their range
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Old 14th February 2004, 06:36 PM   #6
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Hi!

I am using the LM13700 in a limiter for my subwoofer.
It works and os easy to use, but S/N-ratio is not convincing.
No issues with hum (here this circuit is as good as normal
OP amps), but white noise and poor 1/f-corner...
THD..., not good but acceptable.

I would prefer a switched resistor network from Analog Devices or similar. But then you would need some logic control signals.
If you do not like digital signal conditioning, please note that these
switched networks do not digitally scramble and mix up the signal.
Also the controll signal can be generated without microcontrolers
or other clocked stuff.
(Well I do not generally reject digital signal processing, but it is
not easy to make it really good, my DIY know how gives definitely better results in analogue solutions ..... some professionals
should think about this too.....)

If you only need it for one chanel (subwoofers or similar),
you could also set up it with a photoresistor and LED (glued together and somehow encapsulated to get independend of external light...).
This would probably be the best low cost solution.

Cheers
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Old 15th February 2004, 03:35 PM   #7
maylar is offline maylar  United States
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richie thanks for the link to That Corp - it's what I was looking for. But at $9.00 each US, it's a bit exotic.

I have no aversion to digital controls, except for the extra complexity of clock and counters required. And for a remote volume control it requires somehow saving the setting when power is removed. Ugh.

I used LDR's long ago in studio audio gear. Their drawbacks are their size and reliability (incandescant light). Although I can't find them listed anywhere and I don't wanna make my own.
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Old 15th February 2004, 07:04 PM   #8
Steven is offline Steven  Netherlands
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Quote:
Originally posted by maylar

I used LDR's long ago in studio audio gear. Their drawbacks are their size and reliability (incandescant light). Although I can't find them listed anywhere and I don't wanna make my own.
No need for incandescant lights. Use an LED. You can buy ready-made LED/LDR combinations. See e.g. http://optoelectronics.perkinelmer.c...oductID=VTL5C4

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