Reamp Box design with transformer bypass and stepped attenuation

[Processaurus]

There was a question recently about reamp boxes, here is a design with a couple minor bells and whistles that could be helpful for someone gearing up to do some serious reamping.  It uses a ~10K 12 step attenuator for the output to the amp, rather than a volume knob, the idea being that often with recording it is better to trade options for being able to get reproducible results, especially if you need to go back and punch something in later.  Also you can use metal film resistors and have a higher quality attenuation stage, as pots are usually carbon comp.

One question that gets brought up with reamping is whether you even need a special box to put in between the line output of the computer/interface/tape machine and an amp or pedal.  Sometimes you don't, and can save your time.  The specialized reamping box comes into play when you have problems. 

- One problem is groundloops from connecting grounded recording equipment to a grounded amp, especially if they are plugged into different outlets.  Some computers make a lot of weird robot chirps that show up as noise on the ground connecting the equipment.  Here the isolation transformer comes in handy, and is sometimes the only solution that works 100%.  Also a transformer performs noise rejection by turning a balanced signal into an unbalanced signal, which helps noise for long cable runs, like through a snake.  Anything over 30 ft or so and it seems like you start getting noticeable noise with guitar cables.  On this design there is a switch to take the transformer out of the circuit, in case you don't need it, as I'm a believer there's no reason to add unnecessary electronics when you have something that sounds good.  It uses a DPDT switch to "true bypass" the transformer, so there isn't any loading on the input except the 10K attenuator.

- Another problem is of matching levels, as line level is a bit louder than guitar.  With digital playback you want a healthy signal coming out of your converters so you aren't losing resolution, so having an analog attenuation stage (volume knob) is great because you can get that without slamming the input of the amp or effect, which is built for quieter, guitar signals.

- The other problem the special reamp box addresses is simply one of connectors, this one has XLR, male and female, as well as 1/4" TRS wired in parallel for the input, because you never know.  The XLR male could be used as a thru, or one could use the box backwards, as a DI, in a pinch (as long as the attenuator was all the way up).


Schematic. Based on a Jenson app note. I used Jenson's values for the resistor (51 ohms) and cap (.01uF) in the ground lift switch.  I wasn't interested in the impedance knob, but one could add it if you needed to address the special case of using fuzz faces and similar vintage fuzzes that rely on the high output impedance of a guitar.  A lot of recording engineers think you need to make your signal high impedance to hook it up to a high impedance input, but they are wrong .  A regular high impedance input, like pedals and amps have, is perfectly happy being driven directly from a low impedance output, and will be quieter than if you add unnecessary series resistance (the Jenson "impedance" knob).

Front. 1/4" jack goes to the amp or pedal being used to do the reamping. The toggle switch is for bypassing the 1:1 isolation transformer, and connects the 1/4" jack to the XLR hot signal. The knob is a 12 position switch that does attenuation, in 3dB steps(and then 5db steps towards the bottom of the range)

Parallel balanced ins/outs. One could use the XLR out to split off to another piece of gear with XLR ins. The red switch is ground lift.  Note the chassis is grounded to the amp side, so don't let it touch grounded metal from the recorder/computer side or it will complete the ground circuit you were trying to lift.  Some rubber feet would be a good precaution.

[defaced]

Nice work, thanks for sharing!  This is something that's been "on the list" to build for a while.  What transformer did you use?  I know there's alot of opinions on the topic, so I'm curious one where you ended up and your impressions of it. 

[Paul Marossy]

Nice! 

[Processaurus]

Nice work, thanks for sharing!  This is something that's been "on the list" to build for a while.  What transformer did you use?  I know there's alot of opinions on the topic, so I'm curious one where you ended up and your impressions of it. 

Thanks, yeah, it is a great recording accessory, I definitely recommend something like this or the unaltered Jenson design for home recording setups where you want to use your pedals, amps, or spaces for rerecording.  The transformers were pretty random, I made two boxes, one with a big heavy military surplus line isolation transformer and one with a smaller cheapie I had lying around.  Both were 1:1 turns ratio, and 600 ohms per winding.  I used a scope to help make a simple Zobel network to minimize ringing with the smaller, cheaper transformer.

The Jenson transformer is undoubtedly a quality part, but there are alternatives out there.  I was checking ebay a while ago and there were plenty of surplus military transformers from communications equipment or what have you, that are also high quality, have the specs and pinout printed on them, have shielded cans, and can be had for $10-$20.

[defaced]

Excellent info!  Thank you for sharing.  I'll keep my eyes peeled for some surplus parts.

[ghosttone]

This is very cool.  Any idea how I would wire up a fixed attenuation stage in place of the stepped rotary switch?  I believe you can use a couple of resistors in a T formation but I have no idea of the values.  Say for 6 dB of attenuation?  I just finished soldering a reamp based on the jensen design and it works great, but the signal seems too hot.  I am always at the bottom of the volume pot.  Having some extra fixed attenuation would be great (especially since my version includes a bypass switch for the volume pot.)  I am ok at soldering but I have zero electronics theory, so any help would be much appreciated.

[slacker]

If you look at Ben' schematic the string of resistors adds up to 10k Ohms and the -6dB tap is 5k down from the top, so half way. So you want two equal value resistors for -6dB, if you want this as extra attenuation before the pot you can just use the pot as the bottom resistor and put a resistor the same value as the pot between the top of the pot and the output of the circuit. This in effect gives you a pot that can only be turned half way up.

[slacker]

Note the chassis is grounded to the amp side, so don't let it touch grounded metal from the recorder/computer side or it will complete the ground circuit you were trying to lift.  Some rubber feet would be a good precaution.

I've never built anything using XLR connectors so I'm probably missing something but on the photo it looks like the shells of the XLRs would connect to the chassis, are they isolated in some way?

[ghosttone]

Thanks, slacker.  I ended up using a 1.5 K as the "terminating resistor" as shown here: 

http://www.diyrecordingequipment.com/wp-content/uploads/2012/02/block-diagram.png

Gives me a more usable level going into a guitar amp.  I will see how it works with stomp boxes but so far so good!

[Processaurus]

Note the chassis is grounded to the amp side, so don't let it touch grounded metal from the recorder/computer side or it will complete the ground circuit you were trying to lift.  Some rubber feet would be a good precaution.

I've never built anything using XLR connectors so I'm probably missing something but on the photo it looks like the shells of the XLRs would connect to the chassis, are they isolated in some way?

Good eye, yes, they are connected to the chassis.  Mic cables are supposed to be manufactured without the shells connected to the pin 1 ground (which connects to the cable shield), and it is up to the piece of gear whether it connects pin 1 and chassis ground, but I notice sometimes they are soldered together for some unkown reason in the plug with a little wire.  Especially el cheapo cables (though it makes no sense because it is an extra step in manufacturing??).  That little wire will make the ground lift not work on any equipment with metal XLR jacks.  If you see that in a cable you have open, you can just snip the wire, with confidence.  You can check you mic cables if they are wired that way without taking them apart by beeping the pin 1 connection and plug shell with your multimeter.

It may be best to preemptively use plastic XLR jacks, here, however, if you want the reamp box's ground lift to be compatible with even miswired XLR cables.  The plastic XLR jacks will have a lug for the plug's shell connection, you would connect that to pin 1 and then connect the two of them to the ground lift switch, as shown on the schematic.  In this case, with your grounding, you are treating the shell of the XLR plug like a continuation of the cable shield, rather than a continuation of the equipment's chassis.

[slacker]

Cheers for the explanation, makes perfect sense.
Funny you mention the el cheapo cables. Before I asked the question I tested the only cable I had to hand to see if the shells were connected or not, you can guess the rest.

[wavley]

Nice work, thanks for sharing!  This is something that's been "on the list" to build for a while.  What transformer did you use?  I know there's alot of opinions on the topic, so I'm curious one where you ended up and your impressions of it. 

Thanks, yeah, it is a great recording accessory, I definitely recommend something like this or the unaltered Jenson design for home recording setups where you want to use your pedals, amps, or spaces for rerecording.  The transformers were pretty random, I made two boxes, one with a big heavy military surplus line isolation transformer and one with a smaller cheapie I had lying around.  Both were 1:1 turns ratio, and 600 ohms per winding.  I used a scope to help make a simple Zobel network to minimize ringing with the smaller, cheaper transformer.

The Jenson transformer is undoubtedly a quality part, but there are alternatives out there.  I was checking ebay a while ago and there were plenty of surplus military transformers from communications equipment or what have you, that are also high quality, have the specs and pinout printed on them, have shielded cans, and can be had for $10-$20.

Peterson Goodwyn uses an Edcor transformer in his project http://www.diyrecordingequipment.com/line2amp-manual/  They run right around $7 and I've had great results using them in a box inspired my R.G.'s Hum Free and as output transformers for a multi out mod of an Altec mixer.  I must confess that I've never built a proper re-amping box because I have an old DOD DI that I use in reverse that works great for this.  I keep meaning to build one with a few more bells and whistles, but haven't gotten around to it yet.