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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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Old 5th March 2011, 06:26 AM   #11
Mooly is online now Mooly  United Kingdom
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Originally Posted by linuxworks View Post
yes, it does get simpler.

you switch the gain as you switch inputs. you have ONE gain stage, one attenuator stage and you vary the atten based on which input you have selected.

to create an op-amp gain stage for each input is extreme overkill for this problem.
Hi linuxworks
I don't think you understand the idea in post #9

Just one "gain" stage is used for as many inputs as required. I used this very technique here, post#2
My MOSFET amplifier designed for music.

The switching here was electronic (FET's) but a single mechanical switch "at the input side" is fine. R1 can be scaled for each input such that any particular input can have either true gain for a low output source, or attenuation for a high output source. In the preamp here the second opamp simply maintains correct phase.
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Old 5th March 2011, 10:46 AM   #12
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So basically we can have a digital circuit (with a PIC controller?) which measures the levels and remembers (in flash memory?) the setting for each input and uses a digital volume control chip to set things, or we have a set of resistors into an op-amp?
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Old 5th March 2011, 10:54 AM   #13
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So basically we can have a digital circuit (with a PIC controller?) which measures the levels and remembers (in flash memory?) the setting for each input and uses a digital volume control chip to set things, or we have a set of resistors into an op-amp?
I guess so

Or if you really really wanted...
something ancient like the old NE570/571 compander can be used as an AGC circuit. The data sheets give all the applications.
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Old 5th March 2011, 05:17 PM   #14
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Hi linuxworks
I don't think you understand the idea in post #9

Just one "gain" stage is used for as many inputs as required.
I got that part, but it was still using multiple attenuators and switching between them. my suggestion was to have one volume control element and vary *it* based on which input was selected.

if you like resistors, maybe we could split the diff (lol) and use a digital pot and have the cpu vary that on a per-input basis.
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Old 5th March 2011, 05:20 PM   #15
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I guess so

Or if you really really wanted...
something ancient like the old NE570/571 compander can be used as an AGC circuit. The data sheets give all the applications.
agc's have pump/breathe effects to them. there's no one time constant that works for all program material.

if you want a good compressor, get a pro audio one and be done with it years ago, there was a guy making a RNC (really nice compressor) for a semi reasonable price. it was a good performer and its effects were not too bad if you set them right (had a bunch of front panel knobs you can set).

but is an agc really what the OP wanted? if you could get a 'volume fixup' for each input, isn't that kind of the use-case, here?
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Old 5th March 2011, 05:44 PM   #16
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CPU's and digital pots... it's a lot of faffing about

"but is an agc really what the OP wanted?"

Have to see what the OP (I couldn't remember the name to spell either) comes back with.
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Old 5th March 2011, 05:58 PM   #17
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CPU's and digital pots... it's a lot of faffing about
I have to agree, Mooly. I'm a PIC enthusiast, and I have designed a few volume controllers using them with digipots &c. but a source selector and a bunch of presets, be they pots or discrete resistive dividers is a simple hardwired solution with no programming involved.

w

If you want remote though, the circuit is going to get complicated, willy-nilly, and a PIC, some relays and a digipot are probably the way to go.

Last edited by wakibaki; 5th March 2011 at 06:01 PM.
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Old 5th March 2011, 07:08 PM   #18
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I agree with Mooly... it's called a mixer. You connect a source to an input and if it is too loud, you move the associated slider towards you. If the source is not loud enough, you move the associated slider away from you. If slider goes to either end and the volume is still not within range, you turn a knob labelled gain or trim. Set and forget.

:)ensen.
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Old 5th March 2011, 09:02 PM   #19
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and what I'm suggesting is effectively the same thing; just not replicating the gain changing or attenuating element. you instantiate that *once* and you change its setting based on whatever rules you want, be it input or output or both or phase of moon

having many attenuators (the mixer model) seems overkill and if the attenuator at all matters in the audio path (it does) then having a good one times the number of inputs - that gets large and expensive.

talking to a digital pot is like 10 or 20 lines of code. there are examples online and if you only needed to change that value based on inputs, its still quite a simple program. the controllers today have EEPROM and so you can set values and things there.

having one good quality settable attenuator and a controller is the more flexible solution and will give the best audio quality. having to have several attens just seems too 'old school' (lol) to me.
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Old 5th March 2011, 11:04 PM   #20
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Yeah, but you could build this about as quick as I drew it, on a bit of perfboard.

Put a pot in instead of one of the dividers, turn it till you get a reasonable level, measure the resistances from the wiper to both ends. Solder in those resistor values, in one of the other positions, move the input signal onto that one. Do it again, etc.. You can even forget the opamps if you've got enough signal. I just used 2 because they're in the package, and I arranged them as inverting. If you leave them out, you won't have much to complain about in terms of signal quality.

var_attn_sw.jpg

Any errors? Objections? Miss anything? Anybody?

w
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