[50] Guitar Direct Box

Date: October 17th, 2007 | Comments : [11] | Categories: DIY.

Guitar Direct Box

I posted this design for a direct box in an online forum:

Download the schematic here

I simplified the design and converted it to run off a single 9v battery. This allows powering it with a typical pedal power supply or a battery, and switching the power on/off with the input jack as is done in pedals so you don’t need a switch… Want more headroom? Feed it 12v or even 18v.

I changed a bunch of resistors to 10k because it’s a common value and I have plenty of them around. I deleted the input pulldown resistor since this is a direct box and you are not going to switch it… the input impedance is 1M, which is plenty for guitar and typical of the tube amps that they are usually plugged into. You can parallel R3 with a 10 to 47pF ceramic if RF becomes a problem but likely won’t be needed at these gains in a metal box.

I’m not that familiar with the op2134 but if it’s a fet then it will be fine. The input impedance of a 5532 is too low for this application. The TL072 will be almost as quiet at these impedances. Put in a socket and try different chips.

Without some frequency shaping with an EQ, guitars will typically sound a little lifeless and thin when going direct.

I deleted a bunch of other components for simplicity’s sake, and changed other minor things besides converting to a single 9v supply. I also added the unbalanced output.

The “GND” point should be isolated from the chassis but Vee can be connected to chassis.

I have a box almost exactly like this that I’ve used for years. It can be built on the AMZ Multi-Purpose Opamp PCB.

 

11 Responses to “Guitar Direct Box”

[610] Will Says: 6:39 pm, February 10th, 2009

Hey,

I can’t access the schematic. Could you update the link, or send it to me?

Thanks@

Will


[622] admin Says: 6:25 pm, February 28th, 2009

The link has been updated and the schematic should be available again.

-Jack


[743] Scott Morrison Says: 12:38 pm, July 11th, 2009

Hello Jack,

Very nice design! I have a couple of questions. I’ve not actually heard the unit, but was wondering if there’s any audible noise associated with the [J-FET] TL072 in this particular circuit. I’ve used this IC in “line level” applications with good results but with lower level signals I often needed to go to something quieter such as the [bi-polar] NE5532.

Also, is the [unlikely] presence of 48V phantom power a concern? I’m designing a special application “sub-mixer” with a couple of low impedance balanced line outputs to feed a house PA system. Like yours, my signal ground is midway between V and gnd so I too need to cap couple to the outside world. But it occurred to me that my rig could very well find itself connected to a console input with 48VDC phantom power present. Yow! This would therefore require two additional considerations. (1) The voltage rating of the coupling cap would need to be increased, and (2) the polarity would need to be at both ends of the cap (non-polar, or 2 electrolytics back-to-back) to allow for a positive voltage of 4.5V on one side and 48V on the other. I’m trying to be careful here because it would be too easy for this to happen, resulting in damaged coupling caps, op amps, and who knows what else. Your circuit has a 10K to ground, so that might shunt the phantom power and provide protection.

Thanks much,
Scott Morrison


[751] doro Says: 12:55 pm, July 29th, 2009

hi jack,

can you post a pcb layout for the schem?

can’t piece together with the schem since im not that really familiar with schems.

thanks in advance.

doro


[752] admin Says: 1:49 am, July 30th, 2009

The direct box can be built using the Multi-Purpose Opamp pc board. The link has been added to the original post.

-Jack


[2192] Robin Feathers Says: 9:49 am, April 29th, 2011

Great circuit Jack, Thanks!

Is it possible to harvest the phantom power, regulate it down and use it to power this circuit?


[3841] richard44 Says: 8:40 am, April 22nd, 2013

Hi there. I have a few questions on your guitar direct box schematic. Is it correct that this box has two outputs: a 1/4″ jack (typical guitar unbalanced out), as well as an XLR balanced output, where Out on the schematic would go to XLR Pin2, -Out would go to XLR Pin3, and XLR pin1 would go to ground? If that is correct, can you use both the 1/4″ output and the XLR output at the same time without a signal drop or other noticable audio degradation? Thanks so much in advance for your attention.


[3861] richard44 Says: 5:25 am, April 29th, 2013

One other question if anyone’s out there. . . Does the op amp require a Vref bias voltage (4.5 volts)? From the schematic it does not seem that there is a Bias voltage, but many other circuits I have seen require such. Thanks.


[3862] admin Says: 12:45 pm, April 29th, 2013

GND is the bias connection. Vcc goes to pin 8 of the opamp and Vee goes to pin 4.

This circuit can be built on the Opamp Multi pc board that I have for sale – the link is at the bottom of the original post.

-Jack


[726574] Timbo Says: 9:26 pm, August 26th, 2015

So about the phantom power running through… Would that mess it up or is it protected?


[729347] admin Says: 9:35 am, August 31st, 2015

I would turn off the 48v phantom power, though it should not hurt the circuit.

regards, Jack




 

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