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Old 27th March 2013, 02:20 PM   #1
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Default 2 preamps in the same poweramp

Hi, I recently built the Rod Elliot Guitar amp. It works fine now, no oscillation or anything bad. I was wondering if I could build a very simple hifi-preamp with an AUX-Input, with no tone controls or anything fancy and run it in the same poweramp as the guitar preamp. For playing backing tracks while I play the guitar. But how can I connect the 2 preamps ? One would apply voltage to the others output ? I guess that wont work well. Or does it ?
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Old 27th March 2013, 08:58 PM   #2
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You can't tie the main outputs of two input amp stages together, they would fight each other and run against the current limit of the IC's or transistors all the time, and overheat.
To have two inputs affecting one power output stage, you need a circuit called a "mixer". The most common mixers these days use two low level (preamp) input amps the outputs of them drive two resistors, say 1k or 10 k. The gain of the two input amps is variable so that the outputs reach some standard level. These resistors are tied together at the bottom where the mixing occurs, and because the levels are attenuated by the resistors, then another low level amp stage is necessary to boost the signal back up to the 1.6 Vac or so it takes to drive the power amplifier stage. The resistors keep the current at the mixing point from reflecting back to the output of the driving IC or transistor and overloading it.
See this thread for some mixer basics Basic Mixer Design: need input or look up the schematics of various commercial mixers on the PA thread and copy those. I've purchased a "disco" mixer which I use as the hub of my stereo system, mixing outputs of a turntable, CD player, and FM radio. It used NJM4558D op amps which I upgraded to ST33078 for less hiss. I've since purchased a 12 input PA type mixer for dynamic or condensor mikes, which uses NJM4560 op amps. These IC's have hefty output current capabilities to drive long cables to a remote power amp.
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Last edited by indianajo; 27th March 2013 at 09:03 PM.
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Old 27th March 2013, 10:12 PM   #3
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IndianaJo is right in that you need a mixer but I'm afraid I didn't follow what he was saying, so I apologise if I am repeating things.

The circuit you need is a standard summing circuit and it is simply both signals going to the -ve, inverting, input of an op amp. Each of the gains are the Rfb divided by either input Rs. What you get out is the sum of those two gains. So you can do it all on one op amp if needed, and have three pots (one on each of the inputs, to vary their mix, and one in the feedback loop (but put something in parallel with it) for overall gain, if you need that. You won't probably as your amp will have a volume control. Usually you would buffer the inputs and/or reverse the phase inversion, but it can actually be done on a single amp. Try it with a couple of PP3s and a breadboard.

Last edited by ChristianThomas; 27th March 2013 at 10:14 PM.
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Old 27th March 2013, 10:42 PM   #4
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Simple solution is use two 1k resistors to mix the signals together.
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
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Old 27th March 2013, 10:52 PM   #5
JMFahey is offline JMFahey  Argentina
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Easier: connect both preamp outputs to the Power amp input through a 4700 ohms 1/4W resistor each.

Even easier: connect the Guitar preamp to the Power Amp input through a 4K7 resistor.
Add 2 RCA jacks or a stereo jack and connect each "hot" pin to the power amp input, through a 10K resistor each.

That's what all practice amps have today, no preamp added, and called "CD input".
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Old 28th March 2013, 07:14 AM   #6
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Thats soundy fairly easy. But I need a preamp, my Mp3 Player, which I want to use, is simply not loud enough to hold against the guitar at a little higher volumes
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Old 28th March 2013, 07:37 AM   #7
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Originally Posted by JMFahey View Post

That's what all practice amps have today, no preamp added, and called "CD input".
Is it really? They are lazy sods, aren't they?

Still, no complaining that the op amp isn't any good.
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