guitar reamping box

[Joe Kramer]

just ordered a 10k:10k transformer from edcor like he just said he purchased above... and you said i can wire it exactly the same as the jensen transformer schematic?

Yes, same as the Jensen schematic, with a few notes.  The Edcor xfrmr does not have external mu-shield or an internal Faraday shield, so the wires designated "WHI" and "BLK" on the schemo can be disregarded.  However, use a metal case to minimize hum.  The schemo shows an XLR connector for the input, but I used a 1/4" TRS jack instead for easier connection to unbalanced equipment.  If you use a TRS input jack, TIP is RED (hot), RING is BRN (cold), and SLEEVE is ground.  Both input and output jacks should be isolated from the case, and all grounds should be tied to a single point inside the case.  That's about it.

BTW, depending on how inventive you are with the recording process, reamping can be applied to anything--bass, drums and drum machines, keyboards, vocals--you name it.

Have fun!   

[TimWaldvogel]

Hey I am using a metal casing.
. But what do you mean by isolating the jacks? I usually use open mono nuetrik jacks not the enclosed ones. And I already have a female xlr from radioshack. I was gonna use a on off type swith for breaking The ground

[Joe Kramer]

But what do you mean by isolating the jacks?

A jack that's "isolated from the case" does not make the usual connection between the jack's ground sleeve and the case.  To do this, you need either a jack with a plastic bushing, or some kind of plastic washer or other to insulate the bushing from the case.  As you can see in the schemo, the output jack "floats" on the xfrmr secondary and isn't connected to the case ground at all.  The input jack also floats, though with the Ground Lift switch in the "preferred" position (lifted), the RC network directly grounds radio frequencies only.  For that to function properly, you must not ground the body of your XLR connector directly to the case.  So, using the Edcor xfrmr, your only ground should come from PIN 3 of the XLR, through the RC network/LIFT switch, and should tie to a ground lug inside the case.

[TimWaldvogel]

Ok so the ground from te xlr and the ground/sleeve of the 1/4" connect to the transformer an do not make contact to the case? Or do they connect at a central ground point to the case

[TimWaldvogel]

I will use a plastic 1/4"  jack. I am just curious if the grounds make contact with the case whatsoever

[PRR]

> how a 1:1 transformer ...converts anydamnthing?

It breaks the ground.

In a large studio the source (tape deck, DAW, etc) is in the control room on one power circuit and the amplifier is out in the studio on another circuit, or a different point on a long power circuit. Guitar amp inputs are unbalanced, the ground is a signal reference. That works fine for a short cable from a guitar hanging on a strap, but can make trouble when the source has its own ground. microVolts of stray voltage make large buzz.

[Joe Kramer]

I am just curious if the grounds make contact with the case whatsoever

The answer to your question is no, the grounds of the jacks make no contact with the case whatsoever.  If you connected the jack grounds, you would bypass, and therefore defeat the purpose of, the transformer.  That is one of the main and most useful functions of transformers, to "transform" voltage/current from one part of a circuit to another without making any physical contact whatsoever.  Among other things, (as PRR said) this sort of isolation helps reduce noise (by breaking ground loops).  

[TimWaldvogel]

An one more question. Do you use a different ratio or different transformer to make a direct box out of a edcor transformer. Looking to make a direct box too with the edcor equivelant of the jensen di?

[Joe Kramer]

I've never made a xfrmr direct box, so you're on your own with that.  The Jensen site has lots of good schematics with part numbers, so it's just a matter of figuring out which Edcor part is roughly the same.

Happy hunting!   

[DougH]

Forgive my dumb question, but what it the point of "re-amping" anyway? I just read a little blurb about it in the latest issue of Guitar Player magazine, but the concept still doesn't make any sense to me.  

It supposedly allows you to "re-amplify" a safety track so you can reuse the performance later. I did a session where the producer split my signal out with a direct box for a clean safety track (as well as micing my amp). Then on one track he reused that and ran it through his onboard plugin fx in his DAW s/w and consequently ruined the guitar sound for me, and used that instead of the mic'ed track, grrrrr...   But you could run the safety track back into a guitar amp for re-recording of the performance. The re-amp box converts from the lo-Z of the mixing board output to a typical hi-Z guitar signal. I agree with Ashcat in that it is probably unnecessary for plugging directly into a guitar amp. With ~1M input impedance, who cares how low the impedance is of the input signal. If you wanted to run through a Rangemaster or Fuzz Face first, which requires a hi Z sig to sound right, it might be necessary. But I suspect a re-amp box doesn't really do all the "pickup simulation" stuff you would need in that case anyway. I believe it just serves as an impedance converter. So IMO it is probably kind of unnecessary. But when I home record I don't record safety tracks, etc. Part of the performance, for me, is the sound. You either catch it and get what you want or you don't- without a net IMO...

I suspect producers like gizmos like this more than performers do. Esp if it allows the producer to go back and ruin the track after the fact.

[Processaurus]

I've been using a simple 10K volume pot in a box for reamping, and it has worked great.  Just ran a guitar cable out of one of the outputs of my interface.  It attenuates line level down to guitar level so it doesn't blast the effects with too hot of a signal.  It just loses one of the balanced lines.  I really see no need for a transformer, for a home recordist, unless you really want the balanced line because you're running it through a long snake and are getting noise.  For ground loops, you could just put a ground lift switch in, and isolate one of the jacks.  A cheap transformer would likely screw up the sound way more than wiring it straight through unbalanced.  Actually, rather than a pot, an attenuator made with a rotary switch with a voltage divider made of a ladder of metal film resistors would be more hi-fi, and make settings more repeatable.  Kind of like you see on nicer mic pre-amps.

Many engineers don't have the working knowledge of electronics we do, and falsely assume there is something gained by converting a low impedance source to a hi impedance one, to run it into a hi impedance input, and "match" them.  A hi impedance input will happily accept a hi or low impedance source with no issues.  The only issues with audio signal impedance is running a hi impedance source (probably guitar) into a low impedance input (say, a 10K line input on a mixer, or passive DI), where you get tone suck from the pickups being loaded.

This is ignoring the rare fuzz face type effects that load the pickups, and a low impedance source (like a line in, or buffered guitar) will squash them.  For reamping with those, you'd want to add a series resistance, or go further and make a pickup simulator like the thing at AMZ. 

[TimWaldvogel]

Well a couple of things here. I could totally sell these reamping boxes to the local studios. And I don't thing the pot is horible. Just not as good as it can be. And from I here edcor transformers rock. Illegal be making more of these I'm the future I have a feeling

[PRR]

> For ground loops, you could just put a ground lift switch in, and isolate one of the jacks.

Since the destination is unbalanced, lifting the ground is equivalent to breaking the circuit. No sound.

The pot is usually necessary: most sources are stronger than any guitar, and you need the equivalent of the guitarist's arm-strength and axe-pot to set up the amplifier's tone and overload.

Metal-film switch attenuator seems like lily-gild to me, but if that's what the Buyer wants to see....

Given the low level and narrow bandwidth at guitar jack, if the transformer is well-suited to the pot (nominal impedance = or > than pot) the transformer is probably not hindering "the sound". Again, whatever the Buyer wants to see.....

[TimWaldvogel]

Technically it's not full breaking the ground just keeping the ground from transfering from transformer tothe 1/4" Jack right? 

[Processaurus]

> For ground loops, you could just put a ground lift switch in, and isolate one of the jacks.

Since the destination is unbalanced, lifting the ground is equivalent to breaking the circuit. No sound.

If things are buzzing, it is because the ground circuit is still being made through the 3rd prong on the amp and the 3rd prong on the mixer going back to the breaker eventually.  The ground loop is cut.  One could put in the cap and series resistor between in and out ground as per Jenson's schematic in the ground lift position, if there where issues just opening the ground connection.

Though the "ground lift" (it's more like chassis ground lift, as opposed to cutting ground between input and output) on Jenson's Reamp schematic is honestly kind of strange and superstitious, because all it does is disconnect a DC connection from the metal box and the shield on the XLR cable.  The box can be seen as merely a continuation of the XLR cable's shield.  The switch isn't useful, unless the bare metal box is touching something else metal, like an amp chassis or pedal.  Some rubber feet would do the same thing, or a sticker.  Or moving it.

It's a good point that guitars aren't very wide bandwidth.  The transformer thing makes it more universally useful in a real recording studio with XLR snakes going into another room.  I thought it was important to let home recordists know that they need nothing except an inline pot or attenator to get good (ie, identical sounding to an expensive reamp box) results plugging their soundcard into their amp.  They could even make a special cable with a two resistor voltage divider soldered right into the amp side plug.  No ground loop worries if the amp is plugged into the same outlet as the PC.

Metal-film switch attenuator seems like lily-gild to me, but if that's what the Buyer wants to see....

The real value of the stepped attenuator over a pot is merely that the settings are limited, and repeatable.  The metal film part is the lily gilding!  I have some things to do this evening, but I'll try to put together a schematic for an alternative, xfmr-less reamp box sometime soon.

[Processaurus]

> For ground loops, you could just put a ground lift switch in, and isolate one of the jacks.

Since the destination is unbalanced, lifting the ground is equivalent to breaking the circuit. No sound.

If things are buzzing, it is because the ground circuit is still being made through the 3rd prong on the amp and the 3rd prong on the mixer going back to the breaker eventually.  The ground loop is cut.  One could put in the cap and series resistor between in and out ground as per Jenson's schematic in the ground lift position, if there where issues just opening the ground connection.

Ah, I thought of a problem, even though you would definitely get sound, by cutting the shield at one end and relying on the building's wiring to make the ground connection, it might be noisy, because the building's grounds being used to connect the PC and the amp will probably have all kinds of appliances leaking power onto it and causing a changing voltage potential between the two pieces of equipment's ground reference.  The 1:1 isolation transformer is a good idea if the amp can't conveniently be plugged into the same outlet as the pc.  But if things can share an outlet the xfmr isn't necessary.

[Processaurus]

From AMZ's article about the Jenson design, the Reamp patent is eerily similar:


Yeah, the ground switch thing as drawn in Jenson's schematic is an error, the 1/4" output jack should not be isolated from the chassis ground.  Like a DI, the chassis needs to be permanently connected to either the input or output ground, and not left floating, just like you wouldn't isolate a pedal's hammond chassis from the jacks.  The ground lift should break the connection between in and out's ground, not in's and the chassis's ground with the out left permanently floating.  So either don't use an isolated jack, or draw a wire from the white wire over to the sleeve connection on the 1/4" out.

  However, use a metal case to minimize hum. 

Unfortunately the case won't effectively shield electromagnetic interference from power transformers, wall warts, etc. (a concern because it would often naturally be stacked on top of an amp) unless it is thick steel or other ferrous metal, aluminum doesn't block it at all.  A forumite, Mike, turned me on to this stuff for magnetic shielding, it works well:

http://www.goldmine-elec-products.com/prodinfo.asp?number=G16600A

[Joe Kramer]

the ground switch thing as drawn in Jenson's schematic is an error

By "error" I think you mean to say the Jensen schematic is not drawn exactly as in the patent you posted? At any rate, the Jensen schematic can and does work perfectly exactly as drawn.  After using this circuit, the only mod I might suggest is to raise the value of the Impedance control (to 100K or so?) because it has only slight effect using the value shown.

[Processaurus]

the ground switch thing as drawn in Jenson's schematic is an error

By "error" I think you mean to say the Jensen schematic is not drawn exactly as in the patent you posted? At any rate, the Jensen schematic can and does work perfectly exactly as drawn.  After using this circuit, the only mod I might suggest is to raise the value of the Impedance control (to 100K or so?) because it has only slight effect using the value shown.

Nope!  It's wrong as in they missed drawing a line.  App notes can be wrong.  If it doesn't seem fishy just from the electronics you know, reference any DI schematic (as these are just a backwards DI, with a volume knob), like this one from jenson.  Note the 1/4" input is connected to the chassis. Yours isn't grossly noisy to the point of being obviously malfunctioning because the chassis is still AC connected to the XLR through a cap an resistor, but you may have wondered why the switch doesn't do anything useful.  It is permanently lifted.  In the case of when you might like to connect it, like when reamping to just a pedal with no 3rd prong power, you wouldn't be able to, and the pedal's ground would be floating in a DC way.

[Joe Kramer]

Nope!  It's wrong as in they missed drawing a line.  App notes can be wrong.  If it doesn't seem fishy just from the electronics you know, reference any DI schematic (as these are just a backwards DI, with a volume knob), like this one from jenson.  Note the 1/4" input is connected to the chassis. Yours isn't grossly noisy to the point of being obviously malfunctioning because the chassis is still AC connected to the XLR through a cap an resistor, but you may have wondered why the switch doesn't do anything useful.  It is permanently lifted.  In the case of when you might like to connect it, like when reamping to just a pedal with no 3rd prong power, you wouldn't be able to, and the pedal's ground would be floating in a DC way.

Okay, I see what you mean by the Jensen DI schemo.  And come to think of it, after having built this, http://www.jensen-transformers.com/as/as002.pdf, I did find that the output jack sleeve needed grounding to the case.  IOW, same error.  And all this time, I believed their apps were as exemplary as their xfrmrs.   

As for the reamp, I've never had a problem, though I tend to use the same set-up in the same way every time I use it.  I guess I should check it with some other sources/destinations before I send other builders off on futile missions.  Sorry for the mis-guidance on my part.  Thanks for the eye-opener.