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Old 25th October 2013, 11:04 PM   #11
jimminy is offline jimminy  United States
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I like the idea of your #3 better than buying an off-the-shelf solution, way more fun, but I'm not sure what that circuit looks like. I tried a search of "multi-input preamp" but didn't find anything appropriate on diyaudio so far. Do you have an example or link, or a better search term I could use to search it?

I'll keep looking in the meantime. Thanks!
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Old 26th October 2013, 12:20 AM   #12
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Here's a couple of examples. The first is adapted from an old Radio Electronics article, "Op Amp Audio Applications", and the second is from "An Applications Guide for Op Amps," National Semiconductor (now Texas Instruments) Application Note 20.
The first circuit I chose because it illustrates fairly obviously the circuit function.
The second is perhaps a little more technical, but shows that there's almost always more than one way to put components together for a desired effect. V1, V2, & V3 are the signal voltage sources. This circuit also includes the summing formula for determining Vout, -R4(V1/R1 + V2/R2 + V3/R3). With just one source supplying a signal, the formula simplifies to Vout = -R4(Vn/Rn). So Rn can be determined for each Vn input to provide a consistent Vout.
Search terms "summing" and/or "mixer" and "circuit" should return lots of results here and on the web.
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File Type: png mix1.png (4.9 KB, 56 views)
File Type: png mix2.png (3.4 KB, 55 views)
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Last edited by sofaspud; 26th October 2013 at 12:25 AM. Reason: word redundancy
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Old 26th October 2013, 11:19 PM   #13
Burki is offline Burki  Germany
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I think it is always a problem with too much gain of the power amp. It is always, believe me. Best
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Old 27th October 2013, 03:08 PM   #14
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Quote:
Originally Posted by Burki View Post
I think it is always a problem with too much gain of the power amp. It is always, believe me. Best
I've been thinking about this post, and in doing so have made a 180 to conclude it is mostly correct.
The OP says,
Quote:
Originally Posted by jimminy View Post
What I meant to say is that the goal is the set the volume control on the power amp once, stick it somewhere out of sight, and control volume using the volume controls on the individual devices.

Each device seems to have fairly different gain levels coming out of their headphone jacks and I would like to be able to compensate for that to even them out a bit.
IOW, the OP wants to vary the gain level coming out to the power amp from various devices, but also wants the power amp output to be evened out among them!
My solutions list is reduced to #1 and #2 only.
I may be slow, but I did get there...
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Old 27th October 2013, 03:56 PM   #15
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Why does everybody try to sell a mixer circuit? No one said that more than one source is played at a time.

A simple Opamp buffer stage, switching inputs and gain setting resistors appropriately via rotary or solid state switch.

Rundmaus
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Old 27th October 2013, 03:59 PM   #16
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Or, even simpler:

Operate the Opamp at fixed gain, high enough to get the desired volume from the weakest source. Reduce input signal of the other sources with voltage dividers before the input selector.
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Old 27th October 2013, 07:49 PM   #17
jimminy is offline jimminy  United States
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There's a good point there I didn't probably make clear. That is, there would indeed be only one source at a time. I assumed the summing part of the circuit is to isolate the channels rather than to act as a mixer for the various channels.
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Old 27th October 2013, 08:06 PM   #18
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Quote:
Originally Posted by sofaspud View Post
I may be slow, but I did get there...
And it ain't so bad when looking back to see the pack.
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Old 27th October 2013, 09:59 PM   #19
jimminy is offline jimminy  United States
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I think #3 still sounds closer to what I'm after. I don't need to even anything out dynamically after the devices such as with a compressor, and don't need anything to adjust itself automatically. Each device can just be leveled relative to the others one time to have their various gain differences evened out. Once that happens, the preamp should be able to just do what it does.
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Old 27th October 2013, 10:21 PM   #20
Burki is offline Burki  Germany
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Hello again, what I wanted to say is, that high gain amplifiers mostly do not need much voltage to be very loud. If there are more than one sources it can occur, that they are sometimes really loud and others are weak. If you want to control the volume of each unit by itself and not use a pre or passive solution that fits for all units at full speed, why not see every unit as a separate problem. I would not use resistors or so, even buffered. See what every unit has as output voltage and think of placing a transformer between the source an amp as reducer for example. Besides you will not have problems with impedance matching and galvanic separation. I hope you understand. Best
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