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Suggestions for amp w/ phone output

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Quercus

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Hello all,

I'm working on an amplifier based on the TDA7294. It has a built-in mixer with three inputs (based on the CircuitLib article here: http://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer). I also need the amp to have a phone jack. At the moment, I have two thoughts on how to accomplish the headphone output - see images attached (don't worry about missing resistors, caps and the like... this is just a concept diagram).

Option #1 - Use a passive voltage divider on the output of the main amp

Option #2 - Build two parallel amplifiers, one based on the TDA7294 for the speakers, and a second amp based on the LM386 - an independent switch in the phone jack will activate the mute function of the main amp when phones are plugged in.

I'm leaning towards Option #2 for several reasons: 1) I'm skeptical about relying on the internal ring switch of the phone jack for audio fidelity, 2) this allows for independent volume control of the two outputs, 3) (and most scientifically), well... it seems a better way.

Note - the selection of the LM386 over something like the LM4910 is due to power requirements. My amp already has two sets of power rails - a +/- 36V for the main amp, and a +/- 12V for the op amps. I can use the 12V rails for the LM386, the LM4910 would require more work to provide a 6V power supply.

I'd appreciate your thoughts or comments or options I've not considered.

Thanks.
upload_2016-6-12_9-53-20.jpeg

upload_2016-6-12_9-53-43.jpeg
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It really makes very little difference, historically headphone sockets have almost always been fed via resistive attenuators from the main outputs (even in highly respected HiFi amps). It's also usual NOT to rely on the switching on the socket, but instead to have a speaker switch to turn the speakers off if you want them off (quite often done as part of the speaker switching arrangement for one, two, or zero pairs of speakers).

If you have some specific reason for wanting independent volume controls?, then add an LM386 or similar to feed the headphones, but again you don't usually want the main speakers muting automatically when you do so. However, it's unlikely to provide any better quality using a separate little amp.

You're not very explicit about exactly what you're trying to do?, but normally the mixer would include a headphone amplifier (for PFL purposes, as well as monitoring the main outputs), and the power amplifier would be a completely separate unit. Even in combined mixer/amplifiers this is still effectively how it's done - just in one box. But again, there's no muting of one from the other
 

audioguru

Well-Known Member
Most Helpful Member
Your opamps are missing the EXTREMELY important resistors that set their gain. You probably don't need the input opamps, instead feed the signal directly into the volume controls.
A cheap little LM386 amplifier is designed to drive a toy's loudspeaker fairly far from your ears. Its noise (hiss) is too loud to be a headphones amplifier with the headphones very close to your ears. Instead use resistor attenuators from the main amplifier. Most headphones amplifiers use a low noise audio opamp selected to provide enough output current like an OPA2134 dual. Look at the Cmoy headphones amplifier and its design notes in Google.
 

Quercus

Member
Thanks for the feedback. Let me clarify what this project is - this is an amp for a set of computer speakers. I'm using "real" monitors instead of a set of self powered speakers, so I need the amp. I have two computers, plus an aux input, and I want to be able to select any one, each, all. The amp will sit right next to me on my desk. I want to use the amp as my headphone source simply because it's easier to have the phones plugged in on the desk and have a volume knob there to turn instead of using the computer's volume control. This is why the mixer is built in and the headphones should switch the speakers off.

The appeal of dual volume controls, one for speaker and one for headphones is simply so that I don't have to adjust the volume as I switch back-and-forth between speakers and phones. But all things considered this is a small point and one that I'm flexible on.

I looked at the Cmoy headphone amp as well as a few comments that the sound quality from this was quite variable and the frequency response not very smooth, so I had discarded it as an option. I'm willing to reconsider if people have had good experience with it.

If I go with the the option 1 - passive attenuators on the amp output - I would want the main audio to by-pass the ring switch in the headphone jack, so I'd need another method of muting the speakers when the phones are plugged in. Using a relay seems clunky - is there a solid-state option that would work?

Thanks.
 

audioguru

Well-Known Member
Most Helpful Member
When the Cmoy headphones amplifier uses an OPA2134 dual audio opamp then its distortion is approaching 0.00008%, its noise is almost nothing and its frequency response is from as low as you want (you pick its output capacitor value) to 250kHz.
It will work fine from 12V as its power supply voltage.
 

AnalogKid

Well-Known Member
Most Helpful Member
I've been down this road. For me, the extra effort for independently adjustable headphone volume (and a lower output impedance that made for less volume variation among different phones) was absolutely worth it.

ak
 

spec

Well-Known Member
Most Helpful Member
Off the top of my head, I don't think there is any reason reason why you can't have a passive circuit with an independent volume control for the phones. Hope this hasn't been suggested already.

spec
 
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