I’ve got a spare CAPI CA-11 transformer meant for passive DI applications and wanted to build a box that operates as both a DI and ReAmp after watching a video by the guys at Electrical Audio (https://www.youtube.com/watch?v=7ryJanXTMjI) where there are male and female XLR in parallel.
Before jumping into building I did a little research which brought up the issues surrounding doing this, thus why dedicated reamp boxes exist. Since running it backwards it is now a step up transformer, you get a gain boost, the potential for transformer saturation (but hey maybe you want that), and issues with impedance matching. The easy fix for level bump and transformer saturation is to just be level conscious in your DAW or on a fader (as suggested by EA) but the impedance matching still won’t be ideal. But that being said, looking at a dedicated reamp box like the Radial ProRMP, the ~1:1 impedance ratio suggests to me that the impedance issue still exists even if there is no gain increase, so what’s up with that? A resistor on the unbalanced side to get it up to the ballpark of guitar pickup impedance range? There is a level control on it, presumably a pot on the unbalanced side which can further increase the impedance the amp sees?
Moving on…
The CAPI transformer has taps at 50k:500 and 50k:50 impedance ratios, or 10:1 and 31.6:1 turns ratios, -20dB and -30dB level loss. Run as a DI, assuming a 1k mic pre, that gives you a reflected impedance of either 100k or 1M for your instrument to look at, respectively. Flipping it around for reamping purposes, assuming a 1M input impedance of a guitar amp, the reflected impedance is 1k for the 50k:50 ratio, 10k for 50k:500, and level change now an increase instead of loss. Thus, I wanted to know if simply putting in a U-pad pre-transformer would alleviate my problems, on both the gain and impedance front. I guess challenges arise since there are two taps, so I could either design a pad for each of these taps, pick one, or meet in the middle.
I guess before discussing solutions, I’d like to clarify - if using a mic instead of line level, is any of this necessary? No pad and impedance is fine as the box is operating in the opposite of its intended fashion - mic to unbalanced line? While a 10k reflected impedance seems fairly high for a mic, at 1k reflected impedance I’m ok, no?
On to line level, as I mentioned it is a little difficult with two taps a factor of 10 apart regarding reflected impedances. Looking at the 50k:500 tap, at a reflected 10k input impedance when using a 1M amp, that’s in the ballpark of a lower end of being bridging input, so is there really that much of a concern, impedance wise? The 1k reflection by the 50k:50 is pretty low though for line level, so I understand that that would need to be addressed. But, sigh, this brings up another issue of the variance in source impedance. 600 vs 100 ohms is a big difference once you start multiplying and reflecting. So would a U-Pad also fix this problem through the setting of the shunt resistor?
The other issue is accounting for level change. If dealing with a 20-30dB step up, then the answer is build a pad that attenuates that much. But, Line to Mic level pads are usually 40-50dB loss, so would I be designing with this standard in mind or trying to cancel out the 20 to 30dB step up?
Finally, is the unbalanced pad still fine for reamping? It would still provide the level change I know, but if there isn't a balanced pad, while the transformer would get hit harder the impedance could works out in my favor, and with the balanced pad in place then transformer doesn't saturate and the impedance is still ok, correct? I'd be using an L-pad at 10k,1k.
All this being said, if the guys at EA are fine using this build on their records then I’d lean that its fine for me even if it some technical disadvantages exist, but in the name of learning and trying to combat the potential pitfalls of sending signal opposite how it was intended, I’d like to at least discuss and consider a solution. Any and all thoughts would be appreciated.
Before jumping into building I did a little research which brought up the issues surrounding doing this, thus why dedicated reamp boxes exist. Since running it backwards it is now a step up transformer, you get a gain boost, the potential for transformer saturation (but hey maybe you want that), and issues with impedance matching. The easy fix for level bump and transformer saturation is to just be level conscious in your DAW or on a fader (as suggested by EA) but the impedance matching still won’t be ideal. But that being said, looking at a dedicated reamp box like the Radial ProRMP, the ~1:1 impedance ratio suggests to me that the impedance issue still exists even if there is no gain increase, so what’s up with that? A resistor on the unbalanced side to get it up to the ballpark of guitar pickup impedance range? There is a level control on it, presumably a pot on the unbalanced side which can further increase the impedance the amp sees?
Moving on…
The CAPI transformer has taps at 50k:500 and 50k:50 impedance ratios, or 10:1 and 31.6:1 turns ratios, -20dB and -30dB level loss. Run as a DI, assuming a 1k mic pre, that gives you a reflected impedance of either 100k or 1M for your instrument to look at, respectively. Flipping it around for reamping purposes, assuming a 1M input impedance of a guitar amp, the reflected impedance is 1k for the 50k:50 ratio, 10k for 50k:500, and level change now an increase instead of loss. Thus, I wanted to know if simply putting in a U-pad pre-transformer would alleviate my problems, on both the gain and impedance front. I guess challenges arise since there are two taps, so I could either design a pad for each of these taps, pick one, or meet in the middle.
I guess before discussing solutions, I’d like to clarify - if using a mic instead of line level, is any of this necessary? No pad and impedance is fine as the box is operating in the opposite of its intended fashion - mic to unbalanced line? While a 10k reflected impedance seems fairly high for a mic, at 1k reflected impedance I’m ok, no?
On to line level, as I mentioned it is a little difficult with two taps a factor of 10 apart regarding reflected impedances. Looking at the 50k:500 tap, at a reflected 10k input impedance when using a 1M amp, that’s in the ballpark of a lower end of being bridging input, so is there really that much of a concern, impedance wise? The 1k reflection by the 50k:50 is pretty low though for line level, so I understand that that would need to be addressed. But, sigh, this brings up another issue of the variance in source impedance. 600 vs 100 ohms is a big difference once you start multiplying and reflecting. So would a U-Pad also fix this problem through the setting of the shunt resistor?
The other issue is accounting for level change. If dealing with a 20-30dB step up, then the answer is build a pad that attenuates that much. But, Line to Mic level pads are usually 40-50dB loss, so would I be designing with this standard in mind or trying to cancel out the 20 to 30dB step up?
Finally, is the unbalanced pad still fine for reamping? It would still provide the level change I know, but if there isn't a balanced pad, while the transformer would get hit harder the impedance could works out in my favor, and with the balanced pad in place then transformer doesn't saturate and the impedance is still ok, correct? I'd be using an L-pad at 10k,1k.
All this being said, if the guys at EA are fine using this build on their records then I’d lean that its fine for me even if it some technical disadvantages exist, but in the name of learning and trying to combat the potential pitfalls of sending signal opposite how it was intended, I’d like to at least discuss and consider a solution. Any and all thoughts would be appreciated.