Looking for headphone amp/mixer ideas

[bassmannate]

Hey, guys. I've got a crazy idea for a headphone amp/mixer. I'm looking to mix my bass with mp3/smartphone for practice. I'd like to to have passthrough so I can go out to an amp/FOH console.

I'd also like to have some kind of option to receive a monitor feed from a FOH/monitor console. This is where it gets tricky. I know that Rolls makes a headphone amp that can receive a speaker level signal. Would it be possible to have a separate input that could do all 3? Line level from a console, speaker level from an amp and headphone level from a phone? Even with a selector switch, this would go a long way.

This all comes about because I recently started playing bass at church again after about a 5 year hiatus. During that time, I had worked as the tech director for a church and know what kind of a pain amps on stage can be in such a small setting. Having said that, I have a hard time with not being able to hear myself in a monitor mix and would prefer to be able mix myself in with a monitor feed of some sort.

I've done a little thinking already. For the bass input, I'm thinking a high impedance buffer into a lm386 would work well. The buffer would not only match impedance with the bass but allow me the ability to have a parallel passthrough to go to a direct box or something without worrying about loading. That's about as far as I've gotten though since I still don't completely understand how active mixers work. 

Is this all just a pipe dream? Am I trying to cram too much into a single box?

[PRR]

> a crazy idea

Sounds sane to me.

> Am I trying to cram too much into a single box?

Ever see a 128 input 16 sub-bus recording console with six separate monitor outputs?

It IS more tricky the more stuff you tie together, but is is all doable.

Personally, I like to draw it out. Crayon on paper, MS Paint.... the tool is not important, the idea is to get the plan onto your visual cortex which is smarter than words.

Without working-out your verbal description, I would *suspect* that Rolls or Bringer already make a box which does all you want (and probably more, just so it can be sold to more users). Certainly seems that Bringer's tiny band-mixer has all the inputs and outputs, smaller than a small pizza, $99 list, $69 dent-sale, $?? CraigList. We had a tiny Rolls mix/phone box covered with ins/outs. Question is whether it has the HIGH impedance input you need.

[Transmogrifox]

I agree with PRR this is not a crazy idea, and not too much to cram into a box.  The circuitry itself is really simple, and the space-hogs will be the pots (or sliders if you use those) themselves.  The rest is as simple as a moderate-complexity guitar FX stompbox.

You may also consider getting one of the little mixers like the Rolls and modify one of the inputs with a buffer for bass High Z, and maybe one of the inputs with an attenuator for powered speaker inputs.  I would expect for an amplified monitor channel that is rarely more than 30V peak (60V peak-peak) so a 10:1 attenuation would be in the right ballpark.  I recommend audio transformer isolation for speaker inputs because it's generally not a good idea to ground amplifier outputs.

Still the basic mixer idea is pretty simple, and the LM386 typical application circuit is fine for a headphone amp.

[jimilee]

I built a 2 channel mixer in to a headphone amp for a local player  so he could learn tunes from his MP3 or from his computer's sound card. I found everything at tagboard. He plugged his bass in 1 channel and audio in the second channel.

[bassmannate]

I built a 2 channel mixer in to a headphone amp for a local player  so he could learn tunes from his MP3 or from his computer's sound card. I found everything at tagboard. He plugged his bass in 1 channel and audio in the second channel.

Yeah, build sounds like what I'm wanting. Add a switch to attenuate for speaker levels and I'm good.

[jimilee]

I built a 2 channel mixer in to a headphone amp for a local player  so he could learn tunes from his MP3 or from his computer's sound card. I found everything at tagboard. He plugged his bass in 1 channel and audio in the second channel.

Yeah, build sounds like what I'm wanting. Add a switch to attenuate for speaker levels and I'm good.

With vero, all things are possible.

[Transmogrifox]

Just finished simulating something you might be interested in looking at:
http://cackleberrypines.net/transmogrifox/TransmogriNotes/stage_mixer/
Enjoy

[bassmannate]

Just as i was looking to start designing something. Going to have to use some ideas here. Trying to design something that will run on my board power supply.

[notnews32]

MAOPedals gets all the credit for this project coming to fruition, but check this out:
http://guitar-fx-layouts.42897.x6.nabble.com/JamMix-Headphones-Practice-Play-Along-Amp-td26681.html

"You can mix your guitar in with the audio at any level and pan it anywhere left to right. So you can mix your guitar opposite the main guitar and become the "2nd or 3rd guitarist" hearing both you and the original guitarist(s) in different parts of the headphones. Or you can mix your guitar right on top of the main guitar if you're practicing note for note. Still pretty cool.
Gain - from clean to heavy distortion.
Mix - adjusts where the guitar is mixed into the stereo field
Level - volume mix of the guitar"

You can use your guitar along with a diskman, or whatever...

[Transmogrifox]

I found the JamMix schematic. 


It wants 18V, but I don't see any reason you couldn't power it on 9V if you didn't mind losing the headroom.  It's worth a try.

FWIW the schematic I posted will work fine on 9V.  In fact, if you only intend to plug the output into headphones or a line out you don't even need the power amp section.  I designed that to double as a low-power practice amp going into a high-efficiency speaker (one may be amazed at how loud a speaker can be at 1 Watt).  Some spec 92 dB SPL per watt at 1 meter, which is more than enough for a bedroom practice amp.

The JamMix is stereo, so that's one advantage.  It has all the drive needed to be freakin' loud on decent headphones. 

Probably the main building block you can use from mine is the powered speaker isolated input so you can use it to mix in a powered monitor output.

[bassmannate]

I found the JamMix schematic. 


It wants 18V, but I don't see any reason you couldn't power it on 9V if you didn't mind losing the headroom.  It's worth a try.

FWIW the schematic I posted will work fine on 9V.  In fact, if you only intend to plug the output into headphones or a line out you don't even need the power amp section.  I designed that to double as a low-power practice amp going into a high-efficiency speaker (one may be amazed at how loud a speaker can be at 1 Watt).  Some spec 92 dB SPL per watt at 1 meter, which is more than enough for a bedroom practice amp.

The JamMix is stereo, so that's one advantage.  It has all the drive needed to be freakin' loud on decent headphones. 

Probably the main building block you can use from mine is the powered speaker isolated input so you can use it to mix in a powered monitor output.

Very nice! Yeah, I figured I could drop the power amp section since this is just to drive a pair of Sennheiser HD280s.

One question I have about the speaker input though. We keep talking about transformer isolated input but I keep seeing things like the Rolls headphone tap that just use a 150r to step down the signal. Is the transformer just for protection from a potentially bad amp or something? I think I'm missing something there.

Anyway, I need to start a block diagram to see what exactly I want and what it's going to take. Might even sum it to mono to keep it simple. Still trying to figure out how complex I want to make it.

[Transmogrifox]

The transformer serves  a few purposes:
1)  A potentially bad amp as you suggested.  Just a layer of safety.
2)  Ground differential noise isolation.  If the speaker output is referenced to ground (even through a capacitor) then there is a high probability the connection between the mixer and the monitor amp will be the cause of a nasty hum in the system that you can't get rid of.
3)  Speaker and cable transient protection:  Unplugging cables can cause speakers and/or long cables to have an inductive kickback (all stored energy in the system has to go somewhere).  Protecting from this can be done without a transformer, but the transformer does prevent this excess energy from being shot down the ground wire, possibly resulting in a loud pop.  Where does the system's stored energy normally go when a cable is unplugged?  Generally just oscillates between the speaker and cable until the speaker dissipates all the stored energy (which is no problem for the speaker).  It's best to keep this within the cable and speaker wiring and not provide other paths to other places for it.
4)  To protect non-isoloated amplifier outputs from being accidentally grounded.  Suppose an amplifier manufacturer allows one of the speaker outputs to be ground referenced or some installation technician decides one of the speaker legs needs to be grounded.  If you connect the non-grounded output to the ground on your unit, then you effectively short the amplifier output and you're at the mercy of the amp's overload protection circuitry to prevent damaging the amp.  Speaker outputs are meant to be isolated from ground, so it's a good idea to keep it that way.

I would say #2 is the main reason I would include transformer isolation in my own unit.  As a best practice I prefer to avoid situations where a speaker outputs could be grounded because this puts too much faith in whatever amplifier is being used.

A second thought is that by ground isolating the speaker input, you protect all of the equipment you're connecting to your box.  If your box serves as a path to convey damaging energy to (for example) a mixer board input, then maybe your little box survives ok, but you just blew a channel out of a $5000 mixer.  Somebody won't be happy.

[bassmannate]

Just a quick update on what I've been doing with this as well as my schematic WIP. I've got an instrument buffer and a stereo mixer based off a couple TL072s. Still working on figuring out the speaker and stereo inputs. Please excuse the messy wires. I'm in a bit of a sleep deprived state seeing as we just brought the new baby home yesterday. All the more reason to get this thing built so I can practice with the squirt in the room.


Forgot to put a value in for all the pots. Planning on making them A10k. Please advise on whether this is a good value or not. Haven't quite learned how to choose those values and I'm just going off what I've read. Also, could I just take my headphone output from the TL072s or do I need a separate amplification section? Going to have a master volume on this.

Haven't decided on whether I want balance controls or not. I don't want the thing to be too cluttered with knobs all over the place.

Last is a question regarding Eagle. The library for the TL072 shows supply pins but they're not showing up when I add them to the schematic. How do I get these?

Edit: Also been looking for a good schematic for a bipolar power supply. Looking to supply it with +9v to get +-9v. Looks like the specs for the LT1054 are a bit more favorable over the MAX1044? Any thoughts? I don't have any experience with using bipolar power yet other than using a resistor network to get 0, +4.5, +9.