Simplest headphone amp circuit (something like you'd get in a walkman or MP3 player)? - diyAudio
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Old 12th March 2014, 07:22 PM   #1
777funk is offline 777funk  United States
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Default Simplest headphone amp circuit (something like you'd get in a walkman or MP3 player)?

I always see headphone amps needing a transistor output. Would an MP3 player have this? I'd like to build a utilitarian headphone amp (mixer actually) to listen to my monitoring while recording and a second input for the mix that I'm playing along with.
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Old 12th March 2014, 07:23 PM   #2
paulb is offline paulb  Canada
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Probably an opamp.
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Old 12th March 2014, 07:28 PM   #3
Mooly is offline Mooly  United Kingdom
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Any portable device that drives headphones will have an amp of some sort. As power consumption is all important, this could even be a Class D (switching) amp.

For something easy to build and that sounds great, why not try a TDA2822M chip. It doesn't come any easier than this and it will work on a wide range of supply voltage.
TDA2822M
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Old 12th March 2014, 07:38 PM   #4
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Default Nice Mossfet sweetness

I have built a few of these and they sound nice. Getting expensive for the
MPF-102 though.

JFET iPod preamplifier
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Old 12th March 2014, 07:40 PM   #5
777funk is offline 777funk  United States
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Wow! Class D even. Never would have thought it but that makes sense with power consumption.

You are speaking my language Mooly. Thanks for the recommendation there.
EDIT: just saw your post sfltrack. I like the idea of JFETs (even simpler). I've got a bunch of JFETs (mostly J201 or similar) but I'm guessing they're too small to do what you have there. I don't know too much about hte MPF's.

This would be off topic (but same project), I want to take signal off of my mic preamp. I'm guessing I'd want to pull this signal before the output transformer or phase splitter (I want unbalanced, correct)?

second question:
Would I need to put a high impedance input buffer device (say an FET) before the TDA2822M chip so I wouldn't notice any signal loss due to loading the signal from my preamp? I'd hate to loose any sound quality due to feeding the monitor amp.

last question:
to mix 2 signals without bleed, I'd guess I'll need an opamp (or FET amp) channel per signal fed with maybe a pot right after this stage? Sorry if this is a simple question. I can picture the idea but have never seen it in a schematic.
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Old 12th March 2014, 07:48 PM   #6
Mooly is offline Mooly  United Kingdom
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Yes, unbalanced for these circuits. Whether you need a buffer or not depends on the preamp that would drive the headphone amp. If you used a 47k volume control on the headphone amp then any preamp should drive that kind of load many times over so I wouldn't foresee any problem there.

To mix two signals with no interaction really requires an opamp run in inverting mode.

Post #12
Buffering two signals together
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Old 12th March 2014, 07:59 PM   #7
777funk is offline 777funk  United States
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Thanks Mooly, I saw your attachment in that post (and attached it here for simplicity) I can remove it if need be.

So both inputs go into the inverting side of an opamp via a resistor (R1 and R2). Wouldn't there be some crosstalk of the signals or interaction/loading between signal source 1 and 2? I guess if I use 1M resistors the input impedance would be high but then I'd have to use a huge feedback resistor to get gain and maybe noisy not sure there.

EDIT: sorry for the many questions. Mooly, you mentioned two signals having no interaction require an opamp, would an FET per input channel not provide the required isolation?
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Last edited by 777funk; 12th March 2014 at 08:08 PM.
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Old 12th March 2014, 08:05 PM   #8
Mooly is offline Mooly  United Kingdom
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There is no interaction what so ever and that is because of the way the inverting opamp configuration works. It makes the perfect mixer.

Basic theory... the opamp output will do whatever is needed to keep the difference between the two inputs at zero volts.

So signal wise there is nothing to measure or see at the inverting input and so no interaction or modulation of one signal interfering with the other. The output from the opamp is the perfect combination of the two applied signals.

If you just mixed resistively then you would get interaction.
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Old 12th March 2014, 08:16 PM   #9
Mooly is offline Mooly  United Kingdom
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I see you've edited the post

1meg is to high as you say, noise and hf roll of could be a problem.

The secret of the inverting stage having no interaction between inputs (and you can add as many inputs as you want to the mixer) is that the voltage on the opamp inverting input really is virtually (I say virtually to keep the theorists happy) zero all the time... that takes some getting used to and figuring out but its true. Scope it, measure it with a DVM if you apply DC voltages to the inputs... you won't measure anything on that point in the circuit but the output will be a true mix of the applied signals... however many inputs you add.

Use the rule I posted on how it works and put some numbers in and you will see how it works (remember to take the inversion into account in the calculation)

That's me done for today
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Old 12th March 2014, 08:30 PM   #10
777funk is offline 777funk  United States
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Thanks for your insight. Wow, I'll have to test this out on a breadboard.

I had always heard opamp gain was R3/R1 and not vice versa. That almost struck me as an error in that diagram but I'm assuming that's not the case for some reason. No need to answer that Mooly but if anyone knows the why here.
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