CMOY Headphone amp mod questions

[jasonsmusicgear]

I am planning to build some CMOY headphone amps for my band and possibly use them as monitors.  My current mixer only has 2 auxes and we have 5 people in the band.  So my idea is to put an extra input and output in the amp so that each person can put their own instrument into the amp and therefore have more control of their individual monitor mix.  

Here's the layout I found at http://tangentsoft.net/audio/cmoy-tutorial/


and here's the layout with my proposed changes


I would like the guitar signal to pass through unaffected, but I want the individual user to be able to adjust the volume for both "inputs" separately.

Here's my questions:
1. Will this work the way I have drawn it?
2. What value pot should I use for the guitar input?

Thanks
Jason

[jasonsmusicgear]

bump

[PerroGrande]

Hi Jason,

Several comments:

The CMOY amp is stereo.  You have added an input (guitar) that only goes to one channel.   The guitar/instrument input will be heard only in one ear.  The other input on your drawing still shows up as a stereo input.  I assume you're going to use this for the monitor signal -- but is your monitor signal stereo?  If not, you can certainly just connect it to both channels and use a mono jack.

Adding a volume control on the guitar input without a buffer stage will present a variable impedance to the guitar and could conceivably suck tone.  Also, mixing signals by simply tying the two inputs together isn't really considered best practice.  Consider reconfiguring the CMOY to operate as an inverting configuration which would allow you to run it as a summing amplifier (i.e. mixer).  I'd still recommend a buffer on the guitar input prior to any volume control, just to make sure you're presenting a very high impedance to the guitar (no tone sucking!).

[jasonsmusicgear]

Thanks PerroGrande you have been an immense help!  Of course your help brings up a few more questions, sorry. 

1. My monitor signal is mono like you suggested.  If I plug that signal into the stereo input jack will I only hear the monitor signal in one ear?

2. Will the GGG discrete buffer project work for this purpose (found at http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=75&Itemid=26)  I have the parts for this project so it would be easy.

3. If I use 1 - 9 volt battery to power the CMOY, can I also connect the buffer circuit to the same power supply without any consequences.  (The idea would be to turn on both circuits with the same switch)

4. If I use 2 - 9 volt batteries to power the CMOY how should I modify the buffer circuit to handle the extra voltage.

4. I have no idea how to reconfigure the CMOY as an inverting configuration, could you suggest some websites or schematics that would explain this to me.

Thanks again

Jason

[PerroGrande]

Hi Jason,

Some answers:

1) Yes -- you'll end up with the input in only one ear plugging a mono plug into a stereo jack.  Just replace the stereo jack with a mono one -- wire the tip to both channels of the cmoy amplifier.

2) The GGG JFET buffer will work well for this.

3) You can run both circuits off the same 9v supply without issue.  The buffer requires very low amounts of current and will not significantly contribute to decrease battery life.  The two circuits can share the same ground and the same Vcc, so one switch is all that is necessary (or use the stereo jack trick).

4) When you speak of running the CMOY on 2 9V batteries, I assume you are running it as a split supply (+9v, Ground, -9v).  You can run the buffer from the +9v and Ground without modification.  When done this way, the buffer circuit sees the same 9v difference between the positive supply and ground as it does in the single battery configuration.  Signal ground is the same for both portions of the circuit, so there is no issue there.

5) I'm posting this from work -- when I get home, I'll sketch up a sample of how CMOY could be flipped into an inverting (i.e. summing) configuration.  However, if you look at the various op amp tutorials that are available around the 'net, you can compare the difference between the inverting and non-inverting configurations.  The good news is that the modifications are usually simple.

I built several similar projects for myself and various friends to use on-stage.  For me, I got sick and tired of playing shows where I couldn't hear myself in spite of repeated pleas to the sound guy.  For that, I added a headphone amp/in-ear monitor at the end of the effects chain.  I can control exactly how much of "me" I want to hear in my in-ear headphones.  I did another one for a bass player, but he wanted to hear his instrument before any effects, processing, etc.  For this, I built a combination headphone amp/DI box that he can attach to his guitar strap.  Works like a champ.  For the bass model, I tailored the frequency response (at the player's request) to roll-off some highs, so he wasn't hearing as much finger noise, etc.

[PerroGrande]

Sorry -- things this evening took longer than expected... 

Several circuits to discuss that may assist in your project.

First is a version using two batteries to produce a +/- 9v split supply. 



The "ground" is virtual and is taken from the junction point of the two batteries connected in series (as shown in the lower right hand corner).

[PerroGrande]

Second option involves running the circuit from a single battery, but providing the full 9V supply to the buffer circuit.

I'm not a big fan of running op-amps with a single supply like this one, but it does simplify a few things (like power switching).



In this case, vref (which is half the supply voltage in this case) must be produced somehow.  There are a lot of ways to do it -- I merely showed a simple voltage divider for the point of completeness.

[PerroGrande]

The normal disclaimers apply...  I have not shown finishing touches such as power supply filtering, caps to shunt RF noise, nor have I shown various power switching options. 

A couple of other notes.   

Both circuits are mono and should be fairly simple to build.  I would be tempted to test it in mono mode with the output wired to a stereo jack (just connect the output to both left and right channel) and see how it works.   If you find that you need a separate channel for each ear (i.e. the cmoy couldn't produce enough current when the headphones were connected in parallel), just split the signal for the line and the guitar right after it comes off the potentiometer and build a second op-amp circuit. (in other words, you don't need two buffers).

[jasonsmusicgear]

It sounds like you would prefer the first schematic with 2 batteries so that is what I will go with.  Here's a few more questions:

1. I ordered the OPA2134 so I changed the pinout on your schematic (Red numbers were the ones changed).  Does this look right?

2. You changed a couple of values from GGG's buffer schematic, I wanted to make sure this was on purpose.  GGG's schem has C1 at 0.1uf and C2 at 10uf.



3. When you said that it needs some finishing touches such as power supply filtering and power switching options, would the power section from the schematic below work just as well for this?



Thanks Again

Jason

[PerroGrande]

Hi Jason,

You can use the original values from the GGG circuit for the caps.  I think I just grabbed the wrong value when I was drawing up the schem. 

Your 2134 pinout mod looks correct. 

With the 2-battery configuration, you don't need the voltage divider from the CMOY circuit. You can just take your reference ground from the junction point of the two batteries.  The filter caps are okay, although I doubt that you would need ones that large.  They won't hurt anything, but you could probably get away with a smaller value cap if space/size is an issue.  I would also switch both the positive and negative sides using a double-pole switch. 

Yes -- I do prefer to work with a true bi-polar supply for an op-amp if at all possible.  The circuits are simpler, as there is no need to worry about creating Vref, etc.  Plus, most op-amps are designed for this type of setup, and really do perform best.  In many cases, you also end up with more headroom.  On the other hand, the downside is that you have the weight and expense of two batteries. Powering such effects from wall warts can also be a bit of a headache.  (Of course, you could provide the pedal/unit with AC, and have the power supply circuitry internal, but this adds complexity back in to the situation.)  As with most things, it becomes a trade off.  In the DIY world, we often have the luxury of creating products solely for our own use, so we don't have to answer to cost-accountants when we want things like bi-polar supplies and trim pots...

[the_random_hero]

^MAX1044 charge pump to create +9 and -9? How much current does OPA2134 draw?

[PerroGrande]

The 2134 draws 4-5ma (per amplifier) quiescent, but can source 40ma per amplifier into  a short.  Because this particular application is going to be driving headphones, I'd be a little concerned about the current when running from the 1044 charge pump.

[the_random_hero]

Isn't there a MAX660 that works basically the same way and is good for around 100mA? I think I remember seeing it when I was looking at power supply options for running two NE5532's.

[jasonsmusicgear]

Please check my mods again to the power section.  I think I got rid of the voltage divider but I wasn't really sure what that was.  I also extended the red line over so that the ground would be from the junction of the two batteries, and I added another switch like you said.  Thanks for all your help.

[PerroGrande]

Yep -- looks good!

[jasonsmusicgear]

Thanks so much PerroGrande, I'll let you know how they turn out.

[jasonsmusicgear]

I started building this over the weekend and realized that I forgot to ask about a guitar out.  I modified the schematic to include a guitar out (the part in red).  Please let me know if this will work correctly.  I need the same signal going out that comes in.



Thanks

[PerroGrande]

You can put it there (in red), or you can put it in parallel with the original input. 

[jasonsmusicgear]

Here's my finished CMOY headphone amp.  Thanks so much PerroGrande for all your help.  This amp works exactly like I wanted it to, your schematic was perfect.

One of the 1/4 inch inputs is for a monitor in and the other is for guitar in.  The guitar input uses the GGG buffer circuit.  There is also a guitar out on the other side so I can route the guitar signal back to the board.


Now I just need to find a good belt clip so I can wear this during performances.  Does anyone know of a good source for a sturdy belt clip?

Jason

[PerroGrande]

Awesome!  Glad to be of assistance.  Your build looks good, too.

I got some belt/pocket clips from Allied Electronics a while back for a similar project.  They are originally for a Serpac enclosure (I think).  However, they aren't quite deep enough to be a really solid belt clip -- would classify them as more of a "pocket clip". 

So -- if anyone has a source of good belt clips, I'd be interested too.  I actually considered modifying a cell phone clip at one point, but I couldn't find one cheap enough...