Building a clean preamp/DI box with XLR out

[Hardtailed]

Here's what I'm looking for: I really dig Metallica's clean tone which is usually a JC120 head straight to the board (using the line out). My current setup doesn't give me a real clean tone (Marshall 50W, if I set it up for a nice clean tone, the drive s*cks). It's nice for the semi-clean stuff, but sometimes I need a real sparkly tone with no hint of overdrive.

So I thought about doing the A/B thing but I don't want to carry a second amp around and I'd rather leave my PodXT at home (it doesn't quite give the sparkle I'm looking for anyway, the top end sounds too restricted).

So I'm looking for a SansAmp kind of thing, all analog signal path, XLR out (fed back through the monitors, so no need for a 2nd amp). Except without the overdrive/modelling part.

Given the following schematic of a JC120: http://www.jnd.com/kdawg/my_jc120.pdf

What part would I remove, what part would I keep? I would like to have chorus and reverb built-in if possible (although a reverb tank is of course out of the question, so I guess I can scrap this one), but the distortion channel is of course useless. You know, just everything in one box. I'd like to have the A/B switching built-in too, but that I can figure out. Having stereo outputs with a mono option would be killer.

Also, since this would be sent to the board's preamp, I guess I don't need to run it on 15V. It could be easily modified to run on a battery or a 9V adapter by adjusting the circuit to provide less gain. As I understand, even the highest output microphones usually don't go above +/- 3V. So I can easily provide a strong enough signal out of a 9V supply that can be further amplified by the board's preamps.

The balanced output puzzles me a bit too. Do I have to use a transformer? Can this be done with opamps?

Basically, what I'm looking at is a stereo active DI box with some tone shaping and chorus based on a JC120 preamp.

[Joecool85]

I'm looking for something similar...have you checked the cheap DI boxes on musiciansfriend?

[Hardtailed]

I'm looking for something similar...have you checked the cheap DI boxes on musiciansfriend?

You mean the cheap Behringer ones? The SansAmp clones?

I don't know... I don't like giving my money to this company. I've heard they are very noisy, and since it's all SMD crap, it can be a PITA to upgrades opamps and caps. Plus I'd have to use a separate A/B box (although I do own a Boss LS2...). Plus, they are more overdrive oriented as I understand.

Looking at the JC120 schematic, I think I can easily replicate the tone stack, which I guess is a big part of the sound. Maybe I'll just do something simple, opamp based, with a similar tonestack. The chorus part I'm having a hard time understanding. I'm looking at various chorus schematics right now (CE1, CE2...), maybe I'll implement one of those instead. Or maybe I can just snatch a cheap one on the used market and fit the board in my enclosure.

The problem is, since the output would be XLR, I can't put a pedal after it... unless I put some loop... hmmmmmmmmmm.
Nah, I'm sticking with my all-in-one-box concept 

[d95err]

Putting all this in a single box would be a very complex circuit. I'd suggest building a series of boxes each with a single task instead. You'd need:

* An A/B box (a simple passive A/B would work fine. Three jacks a DPDT switch and a box)
* A clean preamp and tonestack (Checkout the Runoffgroove Tone Mender)
* A Chorus (The JC120 uses an analog MN3007 delay chip, so perhaps the Small Clone chorus would be similar)
* A DI box (Take a Mouser TM018 10k:10k trafo and put a jack on either side)

That's it.

To get anything near the JC120 sound though, it needs to be stereo. The JC120 uses a very simple stereo trick with the chorus. One channel has the delay (chorus signal) only, the other has only the dry signal. So preferably, you should have two outputs from the chorus, one dry and one wet. They you'd need a stereo DI box.

[Hardtailed]

Yeah, but the main goal is practicality. I'm not a big pedalboard fan  , I want as little stuff as I can get buy with. I'll just make the box bigger if need be.

I've already begun drawing a schematic. I'm not very experienced in circuit design so I'll post it here later, hoping some people can help me. It's just a typical pedal input section (inspired from the Marshall Guv'nor I'm cloning at the moment) with not much gain, followed by the JC120's tone stack (will experiment with capacitors' value though).

For the chorus, I may just get the Small Clone or CE-2 PCB from tonepad and stick it in the box with a switch to bypass it. That's gonna be a lot of knob, but I like big pedals
If I could clone just the chorus part of the JC120 that would be great though.
I didn't know the JC120's chorus work that way. So basically, all I have to do is send the chorus output to a second XLR output (but I have to provide mono ability).

I guess I'll go with a transfo for the balanced out since this will be connected to my Marshall on the bypassed output, don't want ground loops.

I got a cool idea though: I could power it from phantom power! Extracing the DC is not very hard, just need to build a voltage divider then to not blow my IC to smoke. I'll read up a bit more first.

[d95err]

Just make sure to build it in separate units and test each one separately. Then put them all in one box if you want to.

For the chorus, if you want mono just mix the dry and wet signals together. There are more advanced ways to get a nice stereo chorus from a single chip. The tonepad Small Clone board has a small extra stereo mod board you could try.

Phantom power would be cool. The higher voltage you can run the preamp on, the cleaner it will be.

[Hardtailed]

For the chorus, if you want mono just mix the dry and wet signals together. There are more advanced ways to get a nice stereo chorus from a single chip. The tonepad Small Clone board has a small extra stereo mod board you could try.

Do you know any ressource on the net for chorus circuits? I'm having a hard time finding much info about how they work. I understand that those BBD chips are the key for an analog chorus, but have no clue how to use them. I want something fairly simple, just for additionnal sweetening of the clean tone.

[d95err]

Do you know any ressource on the net for chorus circuits? I'm having a hard time finding much info about how they work. I understand that those BBD chips are the key for an analog chorus, but have no clue how to use them. I want something fairly simple, just for additionnal sweetening of the clean tone.

The base of a chorus is to modulate the pitch of the signal and mix it with the dry signal. This is achieved by using a short delay (typically a few milliseconds), and modulating the delay time.

An analog chorus is rather complex. The tricky part is to use pre- and post-delay filters to only allow certain frequencies into the delay, and to avoid noice from the very low quality delay signal you get from BBDs.

It's probably best to use an existing design, rather than trying to design your own. Since Roland and BOSS are essentially the same company, perhaps the BOSS CE-2 chorus available at Tonepad would be closest to the JC120 chorus. If you want something simpler than a chorus, then perhaps try a phaser. You could use the same stereo trick, separating the wet and dry signals.

The simplest solution of all would be to buy a Berhinger chorus stompbox, take it out of the cheap plastic box and stick it into your design (although that's "cheating")... 

[Ardric]

One thing that sets the JC-120's chorus apart, AFAIK, is the use of an envelope detector to vary the LFO rate.  I think it's the Q25/Q26 stuff on the above linked schematic.