Passive DI / Reamp Box design queries

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crduff

Member
Joined
Nov 2, 2022
Messages
7
Location
Glasgow, Scotland
Hey, this is my first post on the forum — looking for a little guidance with Passive DI / Reamp Box design. I see there are a few threads on passive DIs already, but thought it may be helpful to combine my questions in the same place.

I've looked over the Jensen schematics — 'typical' (AS007) and 'full-featured' (AS066) DI boxes, and the reamp box (AS092) — and have a couple of queries to improve my understanding and inform design considerations. Basing this off the AS066 design, I don't need the -10dB pad or high-cut at the input, so I will omit these going forward. Anyway, I have a few questions if anyone is able to chime in:

1) What is the purpose of C1 and C2 on AS066, is this to block unwanted DC components across the input, or something else? Are these strictly necessary?
2) On AS007 there is a 6.8kΩ resistor across the input, is this to ensure the transformer always sees an impedance > 6.8kΩ? If so, on AS066, when the pad is not engaged, is there anything lacking with this resistor not being present?
3) AS006 has an optional AC coupling block — is this worth considering for connecting synths which may exhibit some DC components, or is this over-engineering?

Regarding using this as a reamp box, which I appreciate is not simply the reverse of a DI and there will be tradeoffs taking this approach:
For now I only foresee using this to cancel interference over long runs to amps/getting more appropriate signal levels to use effects pedals as outboard. Considering this, I don't need the output level pot (as AS092) as I'll be controlling the input level from the desk, and I will omit the impedance pot since this would change the operating point of the voltage divider in the pad. As such, my queries relate to impedance matching and the pad operation.

4) Given the variance with guitar/pedal/amp impedance to begin with, is there any significant issue with using this circuit in reverse as a reamp box since we should be in a (roughly) appropriate range for the output impedance at > 100kΩ?
5) Since I'll be varying input level at the desk, I expect the pad will remain disengaged when using the box in reamp 'mode'. However it may be useful to have the pad engaged to allow greater level control given the signal level is stepping up. Considering, is it practical to replace the pad for a symmetrical T-pad (of course assuming no impedance issues when the pad is not engaged, as noted in question 2)?

I haven't studied electronics for a few years, so I'm very out of practice! Hopefully someone can provide some wisdom in any case. Would be happy to discuss and draw up a schematic once the I have a better grasp on the above.

Cheers,
Cam
 
Hey, this is my first post on the forum — looking for a little guidance with Passive DI / Reamp Box design. I see there are a few threads on passive DIs already, but thought it may be helpful to combine my questions in the same place.

I've looked over the Jensen schematics — 'typical' (AS007) and 'full-featured' (AS066) DI boxes, and the reamp box (AS092) — and have a couple of queries to improve my understanding and inform design considerations. Basing this off the AS066 design, I don't need the -10dB pad or high-cut at the input, so I will omit these going forward. Anyway, I have a few questions if anyone is able to chime in:

1) What is the purpose of C1 and C2 on AS066, is this to block unwanted DC components across the input, or something else? Are these strictly necessary?
Yes, it's not unheard of for there to be a little DC on an output. In this case you would be running DC through the primary of the transformer which, even if it's not enough current to damage the transformer, it will cause significant distortion.
2) On AS007 there is a 6.8kΩ resistor across the input, is this to ensure the transformer always sees an impedance > 6.8kΩ? If so, on AS066, when the pad is not engaged, is there anything lacking with this resistor not being present?
No idea. I guess if you don't use a cap, it's to limit current to an acceptable level.
3) AS006 has an optional AC coupling block — is this worth considering for connecting synths which may exhibit some DC components, or is this over-engineering?
Using a transformer for a DI these days is over-engineering. You can easily make a pedal with a low power op amp (1mA quiescent current) that has very high input impedance and an impedance balanced output. Run it on two 9V batteries and you have nearly perfect DI. Of course nobody sells a device like that because that would be too simple, two 9V is odd, it has no pzazz, etc. For some reason people are transfixed with using transformers for DIs and I really don't know why. Once upon a time they were a good solution if you didn't want to need power at the end of a long cable and you have a desk with lots of mic inputs to utilize. But nowadays that's not really the most common recording scenario.
Regarding using this as a reamp box, which I appreciate is not simply the reverse of a DI and there will be tradeoffs taking this approach:
The aforementioned battery operated single op amp, ultra-high-Z, impedance balanced output would be near perfect for re-amping whereas the circuits you're looking at would definitely not. For re-amping you want a very very high input impedance like 10M (just enough to keep the op amp input from drifting) and a through jack so that the input impedance is defined by the amp it's connected to. Record dry signal and then reverse I/O to re-amp.
 
Hey, this is my first post on the forum — looking for a little guidance with Passive DI / Reamp Box design. I see there are a few threads on passive DIs already, but thought it may be helpful to combine my questions in the same place.

I've looked over the Jensen schematics — 'typical' (AS007) and 'full-featured' (AS066) DI boxes, and the reamp box (AS092) — and have a couple of queries to improve my understanding and inform design considerations. Basing this off the AS066 design, I don't need the -10dB pad or high-cut at the input, so I will omit these going forward. Anyway, I have a few questions if anyone is able to chime in:

1) What is the purpose of C1 and C2 on AS066, is this to block unwanted DC components across the input, or something else? Are these strictly necessary?
2) On AS007 there is a 6.8kΩ resistor across the input, is this to ensure the transformer always sees an impedance > 6.8kΩ? If so, on AS066, when the pad is not engaged, is there anything lacking with this resistor not being present?
3) AS006 has an optional AC coupling block — is this worth considering for connecting synths which may exhibit some DC components, or is this over-engineering?

Regarding using this as a reamp box, which I appreciate is not simply the reverse of a DI and there will be tradeoffs taking this approach:
For now I only foresee using this to cancel interference over long runs to amps/getting more appropriate signal levels to use effects pedals as outboard. Considering this, I don't need the output level pot (as AS092) as I'll be controlling the input level from the desk, and I will omit the impedance pot since this would change the operating point of the voltage divider in the pad. As such, my queries relate to impedance matching and the pad operation.

4) Given the variance with guitar/pedal/amp impedance to begin with, is there any significant issue with using this circuit in reverse as a reamp box since we should be in a (roughly) appropriate range for the output impedance at > 100kΩ?
5) Since I'll be varying input level at the desk, I expect the pad will remain disengaged when using the box in reamp 'mode'. However it may be useful to have the pad engaged to allow greater level control given the signal level is stepping up. Considering, is it practical to replace the pad for a symmetrical T-pad (of course assuming no impedance issues when the pad is not engaged, as noted in question 2)?

I haven't studied electronics for a few years, so I'm very out of practice! Hopefully someone can provide some wisdom in any case. Would be happy to discuss and draw up a schematic once the I have a better grasp on the above.

Cheers,
Cam
Capacitors C1 and C2 are coupling capacitors, C1 is an electrolytic and its paralleled by a poly cap (C2) this paralleling might be beneficial at high frequencies. These caps avoid any DC from going into the transformer, also, they avoid any DC from flowing through the switches (particularly S1b) which could create audible pops when toggling it.
 
For some reason people are transfixed with using transformers for DIs and I really don't know why.
The traditional reason is galvanic isolation. If you have an amp with a 60V chassis, you won't have happy times strapping that to 0V with a cable shield. Of course, you shouldn't have crappy amps like that, but sometimes people bring in really broken stuff and you have to deal with it.
 
Yes, it's not unheard of for there to be a little DC on an output. In this case you would be running DC through the primary of the transformer which, even if it's not enough current to damage the transformer, it will cause significant distortion.

No idea. I guess if you don't use a cap, it's to limit current to an acceptable level.

Using a transformer for a DI these days is over-engineering. You can easily make a pedal with a low power op amp (1mA quiescent current) that has very high input impedance and an impedance balanced output. Run it on two 9V batteries and you have nearly perfect DI. Of course nobody sells a device like that because that would be too simple, two 9V is odd, it has no pzazz, etc. For some reason people are transfixed with using transformers for DIs and I really don't know why. Once upon a time they were a good solution if you didn't want to need power at the end of a long cable and you have a desk with lots of mic inputs to utilize. But nowadays that's not really the most common recording scenario.

The aforementioned battery operated single op amp, ultra-high-Z, impedance balanced output would be near perfect for re-amping whereas the circuits you're looking at would definitely not. For re-amping you want a very very high input impedance like 10M (just enough to keep the op amp input from drifting) and a through jack so that the input impedance is defined by the amp it's connected to. Record dry signal and then reverse I/O to re-amp.
Thanks for the response, makes sense re: the coupling caps over the input. Honestly hadn't really considered going the op-amp route, was quite attracted to the concept of the DI not requiring a power supply although one that would integrate into a pedal chain is a neat solution. Will explore this a bit further.

Any thoughts on the pad design — is a T pad the way to go?
 
Capacitors C1 and C2 are coupling capacitors, C1 is an electrolytic and its paralleled by a poly cap (C2) this paralleling might be beneficial at high frequencies. These caps avoid any DC from going into the transformer, also, they avoid any DC from flowing through the switches (particularly S1b) which could create audible pops when toggling it.
Very thorough explanation here, thanks a lot.
 

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