My DI+Reamp Schematic (please have a look)

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xeawr

Well-known member
Joined
Dec 14, 2016
Messages
166
Location
Germany / Austria
Dear GroupDIY fellas,

I have a quick request. Could you smart people out there be so kind as to have a quick glance at my schematic and tell me if it makes sense to you? Here are my issues:

a) I'm not sure about the H-pad, in the ReAmp signal path . Is it really necessary? Did I even get the correct values for the H-pad; is this a reasonable attenuation factor?

b) Doesn't the transformer do all the impedance matching, or do I really  need a resistor in parallel with the transformer?
Maybe I do, since
1. Jensen included an impedance matching pot in their design (notes) - see  attachment. For simplicity's sake I don't want such a potentiometer. However
2. the popular LINE2AMP also shows a resistor in parallel with the  transformer.

c) Will this schematic (work well  ::)). I know there are some other schematics around, I studied them carefully and tried to understand them. Mine varies in a number of points:
1. No Z pot for more simplicity (unless you convince me it is ABSOLUTELY necessary :D )
2. XLR input/output is split into TRS input for line level (for reamping) and XLR mic level output (for DI purposes).

d) Do I (really) need a pad in the unbalanced part of the schematic? I think basically all guitars have a  volume control, so I'd rather turn that down (to not "overdrive" the x-former), instead of engaging a pad at the DI box. Makes sense?

e In the data sheet (attached) it says "Do not pass DC through the windings." Would I pass DC through the windings if I  engaged the 48V at the XLR output? If yes - how to block? (caps?)

f) Any further, hints, insights, caveats?

I can't believe I made such a simple thing so complicated,  ;D but there is simply so much I don't know, hence the myriad of questions...
Thank you SO MUCH (!) for your help. I can't wait to build this thing so I'm really really curious to hear what you will say!

 

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attached is conceptually what has worked well,
though different transformers..
do suggest an output control for re-amping.
 

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A few things jump out.

1. I don't think your ground lift is going to do what you want it to - I think its just a drawing error - but right now, you're not grounding pin 1 of the XLR  to the Hi-Z input shield - you are stilting pin 1 above the HiZ shield by 100Ω in parallel with a 2.1uf cap (or maybe that is C2_100nF?)...  I think what you mean to do is have pin 1 terminated to the shield/chassis ground/power supply ground in one switch position and then lifted above that but the RC network in the other position - as it is in the Jensen drawing... that said, i understand switching between ground and ground thru the low value cap, but i don't understand the RC network...

2. the reamp path is a bit weird. in general a reamp box is to go from line level (+4dBm) to instrument level (-20dBm) and the impedance pot is there to mimic the high source impedance a guitar might present.

so first and foremost there is the gain issue - you have a 15:1 pad for the line in which looks like 602Ω (its a bit low for todays standards, but acceptable so long as the source can drive it) - assuming +4dBm in, were at -26dBm after the pad - which would be fine - except you have over 20db of gain thru the transformer - so your running a bit hot - but the bigger issue i believe tho is the source impedance driving the Reamp is really high thru the step up action of the transformer... You might actually be better served running thru the transformer the other way for the reamp...

I would definitely use the level control for the reamp send to the guitar - it will save you a lot of fiddling - as for the impedance pot - i have made reamp boxes with them and without - it just depends - some pedals don't like being fed from a low source Z - a fuzz face is the perfect example....

3. you should work it so the ground lift works with the reamp circuit as well...

4. one of the nice things about this transformer is all the shielding - be sure to take advantage of that - I would tie at least A3 and the transformer chassis to the cleaner ground - so the mixer...

 
Thanks for your info! I appreciate it very much. I redesign my schematic and then be back with another couple of questions... :)

I'm a bit busy at the moment, lots of projects, lot's of work, so it might take a while... (unfortunately) :(

By the way, I attached a picture from the west host DI-Box schematic, this is where I got stole the idea of the 10R+100nF network from. (bottom right)
p35-f2.gif

Anybody can enlighten me, what these parts are actually doing there?

Can anyone help me with this:
e) In the data sheet (attached) it says "Do not pass DC through the windings." Would I pass DC through the windings if I  engaged the 48V at the XLR output? If yes - how to block? (caps?)
I meant: Do I "pass DC through the windings", if I have +48v on pin 2+3 of the XLR plug and ground on pin 1 and feed it directly into the transformer?

Thank you so much for your help!

X
 

Attachments

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Couple of things. First I think your ground/chassis symbol is in the wrong place. If you have a metal enclosure then you need that to be connected to pin 1 of the XLR at all times so you need to move the earth symbol to the other side of the switch. Second, it your pahantom power is supplied via a pair of 6K8 resistors then the voltage at the opposite ends of the transformer will be the same so there will be no current flowing through the windings.

Cheers

Ian
 

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