Update: reStomp re-ampers now available for sale in the shop.
I had an enquiry about re-amping and decided to look into the idea more closely. Traditionally, when recording a guitar (particularly an electric guitar), it is usual to experiment with the combination of guitar, pickups, effects and amplifier to get the required sound and record using a microphone adjacent to the speaker. The guitar sound is recorded as the performer intended leaving only EQ and reverb (and maybe other spectral effects) to be adjusted at the mixdown stage.
Re-amping is an idea that advocates recording the clean guitar sound (via a DI) as well as the sound the guitarist wanted which allows you to revisit the whole effects chain and amplifier/speaker combination at the mixdown stage.
The problem we are looking to solve here is that the balanced line output from the recording system is not suitable for connection back to the effects chain/amplifier input. The re-amping box therefore takes the balanced line signal and converts it to an unbalanced signal that mimics the output of a guitar. At the same time, the transformer isolates studio ground from the effects/amplifier. The problem is not a new one so we can follow established principles for impedance matching and isolation and offer the following schematic:-
R1 thru R5 form a balanced T-attenuator with a gain of -12dB. R6 and C1, along with the primary of the transformer form a Zobel network. R6 and C1 were chosen according to a comment from a Jensen Transformers engineer who suggested that the values should be found empirically with minimum capacitance and maximum resistance to give minimal ringing on the corners of a square wave. These values need to be adjusted for the particular make and model of transformer used. VR1 is a conductive plastic potentiometer from Omeg. Note that Omeg uses the “opposite” standard to far-east manufacturers, so the 47k LOG pot will be marked 47kB. Normally, the B-suffix would indicate a linear taper.
C2 and R7 go some way to isolate the re-amper circuit from “dirty” ground. SW1 isolates the unit (and the amp) from studio ground if necessary.
Which leads us to choice of transformer. There are many transformers we could use from Sowter, Lundahl, Jensen, etc, but they are all very expensive (considering it’s a guitar amp we’re feeding). We have therefore opted for a good value transformer from OEP – the A262A3E (this seems to be identical to the Vigortronix VTX 101-003 – so that might be worth trying if you cant get an A262A3E).
Here are a few photographs of the prototype: –
The design works very well and we now offer the re-stomp for sale in the shop.