Passive DIs, Impedance, Pads, ReAmping, and Questions

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heatwalk

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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.
 

abbey road d enfer

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heatwalk said:
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.
I think the problem is that you think ReAmp is a mirror image of DI. No. Because in both cases, one wants to attenuate the signal.
Actually many use the step-down connection for both DI and ReAmping. I both cases, the 20dB attenuation is rather correct.
But a more appropriate solution for ReAmping is to use a 10k:10k xfmr, followed by an LR circuit that simulates the output impedance of a guitar and a volume pot.
 

heatwalk

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Ah maybe I confused you, or maybe I'm misinterpreting you? My noting the level increase you cited is because I know it's inevitable sending signal through it backwards, I'm trying to work under the constraints of what transformer I have available to me. I know if you open a dedicated reamp box and DI, it won't be a mirror. But in my situation I'm wondering if you can somewhat remediate the lack of two devices with some kind of resistive fix.

By "both cases", do you mean the 20dB L-pad described? Since the transformer I have at my disposal is what I have described, -20dB will get me back to the level I'm sending into the transformer when reamping (or -40dB total when acting as a DI), so if I have that in place, is a further gain reduction better included on the balanced side, or adding onto the L-pad? 

The LR circuit is pretty intriguing, I haven't heard of that before other than when I briefly saw the DIYre PPE which seemed like if I was building a dedicated unit would be the way to go. How common is an inductive component under the hood in reamp boxes, or is it more ideal scenario? It seems they touched on lack of inductive components as a pitfall of something like the radial.
 

abbey road d enfer

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heatwalk said:
Ah maybe I confused you, or maybe I'm misinterpreting you? My noting the level increase you cited is because I know it's inevitable sending signal through it backwards, I'm trying to work under the constraints of what transformer I have available to me. I know if you open a dedicated reamp box and DI, it won't be a mirror. But in my situation I'm wondering if you can somewhat remediate the lack of two devices with some kind of resistive fix.
OK, I mentioned that there is a possibility to use the same xfmr in step-down mode for both DI AND reamping. then it becomes just a matter of connectors. You need a 1/4" inch and a female XLR at the input, and a 1/4" jack and a male XLR at the output.

By "both cases", do you mean the 20dB L-pad described?
Have I mentioned an L-pad? I don't think so, and I don't think it's relevant, if the xfmr is used in both cases as a step-down.

The LR circuit is pretty intriguing, I haven't heard of that before other than when I briefly saw the DIYre PPE which seemed like if I was building a dedicated unit would be the way to go. How common is an inductive component under the hood in reamp boxes, or is it more ideal scenario? It seems they touched on lack of inductive components as a pitfall of something like the radial.
From memory, I can't cite any particular brand, but I know there are commercial units that inlude that. The easiest is to use a humpbucker without its magnet.
 

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heatwalk

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Oh ok, step down providing the desired gain reduction for either purpose. My bad. Hmmmm... meaning I would have to wire a switch on the 50k side that changes connection of the leads for the two different purposes right? Because the two sides of the transformer would change their connection to be unbalanced or balanced?

DI - 50k side the taps are + and Ground, 50/500 the taps are + and - of balanced signal
ReAmp - 50k side the taps are + and -, 50/500 side the taps are + and ground.

So then its step down as you described and you get the desired gain reduction, but am I dealing with an impedance issue now in that the reflected impedance when reamping in this manner would be too high? 10:1 step down means my line or mic out at say 150ohms is looking at something of the order of 100M from a 1M guitar amp input? Is this where the inductor then comes into play? Because it then adds to the source impedance into the amp?
 

abbey road d enfer

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heatwalk said:
Oh ok, step down providing the desired gain reduction for either purpose. My bad. Hmmmm... meaning I would have to wire a switch though that changes connection of the leads for the two different purposes right? Because the two sides of the transformer would change their connection to be unbalanced or balanced?

DI - 50k side the taps are + and Ground, 50/500 the taps are + and - of balanced signal
If the jack is not grounded, you can have both connections.

ReAmp - 50k side the taps are + and -, 50/500 side the taps are + and ground.
That is correct.

So then its step down as you described and you get the desired gain reduction, but am I dealing with an impedance issue now in that the reflected impedance when reamping in this manner would be too high? 10:1 step down means my line or mic out at say 150ohms is looking at something of the order of 100M from a 1M guitar amp input?
Losses in the xfmr limit the actual value of the reflected impedance, and anyway, it is of no consequence, since the source is of very low impedance compared to it.

Is this where the inductor then comes into play?
the inductor is there to make sure the amp or pedal sees a credible impedance.

See attached schemo.
 

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heatwalk

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Took a day to try and to absorb this.

abbey road d enfer said:
If the jack is not grounded, you can have both connections.

Ok I think I'm following along, but might be a bit confused still and getting the physics wrong regarding the ground lift implementation you illustrated so apologies if I'm getting some stuff wrong still, but I think this is what I'm gathering -

Groundlift no longer lifts the shield of the balanced cable from chassis as you might find on a DI, but disconnects the jack from ground to allow for operation as both a reamp or DI instead. Looking at input side - Instead of floating the shield of XLR, groundlift disconnects the jack from ground, allowing you to send a balanced signal for reamping purposes whereas if it is grounded, 1 and 3 tie together to ground along with the shield of 1/4", allowing for DI purposes, unbal to bal. If it is lifted, what happens if something sends signal via the 1/4" in? The shield of the cable is connected to the primary, but without it chassis grounded, what is produced on the other side?

And then, looking at output side, essentially the same thing in reverse? Grounding allows you to ReAmp as the shield of 1/4" connects to ground but ties shield and - of XLR together, while lifted allows you to DI as you have the balanced signal but prevents you from ReAmping?

abbey road d enfer said:
Losses in the xfmr limit the actual value of the reflected impedance, and anyway, it is of no consequence, since the source is of very low impedance compared to it.

Got it. I read a transformer acting as a step up has a limit of about 25dB gain but wasn't aware of impedance reflection limitations. Is there a standard rule of operation for this regarding reflected impedance, or as it relates to loss?

Thanks for the help. It's starting to make a bit more sense. Now I understand about the inductor, since the source impedance is so low in this configuration the inductor brings it back up. So ideal/optimal reamp solution actual simulates a guitar output as shown in schematic by implementing actual guitar components and wiring so an amp sees what is expected of a traditional guitar, complete with volume pot and inductance. And the reflected source impedance is negligible compared to that of the humbucker and volume pot.

Touching back on my initial idea, does it work from purely a signal conversion point of view? I attached a drawing of what I was initially thinking. Recognizing now that while it doesn't simulate the output of a guitar and might not be the proper solution, lacking the elements you discussed, for simply wanting to run a line out into an amp or pedal does it work since in this manner, I think the amp would be seeing a credible impedance regardless of inductor (or lack thereof)? My initial intent was using it from more of a "plug and play" standpoint with what I had at my disposal and I don't have a spare humbucker laying around right now. In use, ideally I would prefer doing volume work via automation or fader instead of a pot on a pedal, for while I suppose in reamping it is already tracked, my thought being the box is more likely to be next to the amp and not in a control room. Downsides, beyond whats already discussed, would be stepping up noise, reducing my dynamic range, etc.

If I placed the ReAmp connection on the primary as suggested and added a series resistor or series resistor + pot instead of the inductor + pot on the secondary, would that work? It wouldn't give the LR characteristic of a guitar output, but impedance wise? Just thinking with what I have laying around.
 

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gridcurrent

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Another view.

Cuniberti's original "Reamp" used an O-10 Ouncer wired 500:30k feeding a 50k potentiometer.
If you are planning on driving stomp boxes and related guitar effects, then a high source Z from the reamp box is important.
Credit owed to Timothy who posts here.

However, if your plans include simply feeding an amplifier from say your Studer A827 or Prism converter, a step-down transformer is vastly superior.
I have built dozens of boxes in the past 20 years, with transformer ratios ranging from 5:1 to 14:1.
Client's reactions have varied from "happy" to "delirious".

A passive di box normally has a "pin 1 lift" on the XLR output.
In contrast, a reamp type box should have a "pin 1 lift" on the XLR input.

In my opinion, better to build dedicated boxes.





 

heatwalk

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gridcurrent said:
Cuniberti's original "Reamp" used an O-10 Ouncer wired 500:30k feeding a 50k potentiometer.
Well this sent me on a little trip and came across the schematic. I'd only seen the jensen application design before. His original design then is the only one I see using a step up transformer, with the potentiometer afterwards wired like a guitar pot. Maybe the lack of variable impedance function post "level" as seen in the jensen is because the step-up already bumps up the reflected impedance so there isn't a need? What is the point of the RC across pins 2 and 3?

gridcurrent said:
If you are planning on driving stomp boxes and related guitar effects, then a high source Z from the reamp box is important. However, if your plans include simply feeding an amplifier from say your Studer A827 or Prism converter, a step-down transformer is vastly superior.

What is the reason behind amp vs pedals being different? I figured that high source Z from reamp is important regardless, but for scenario 2, is that just a matter of maintaining fidelity which is more difficult with stepup, so some kind of buildout resistive network post transformer becomes more essential?


It seems like there are a handful of solutions here that all borrow elements from one another. The Jensen uses a 1:1 transformer with volume and "impedance" pots, the OG design (thanks gridcurrent for pointing me in that direction) is a step up plus level pot, some use a variation of a stepdown and pot or stepdown and LR like the NYDave design or what abbey road suggested. Some people report the impedance pot doesn't do much for them... Options options, seems like no singular answer.

The Cuniberti and Jensen employ pads pre transformer as I was thinking. But if a level control is important on the secondary, makes me think if I can use a dual deck 50k or 100k pot, wiring one for variable L-pad a la the wolfbox, and the other deck for level control in reamp mode.


gridcurrent said:
In my opinion, better to build dedicated boxes.

But this is where my fun is! Solving a problem whilist giving myself a headache trying to do so under nonideal limitations
 

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abbey road d enfer

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heatwalk said:
What is the reason behind amp vs pedals being different?
many pedals have a low-ish and non-linear input impedance (Brian May's rangemaster), that reacts very differently with a low-Z source tha with an actual guitar. Much less a concern whn driving an amp.

The Cuniberti and Jensen employ pads pre transformer as I was thinking.
The input pad is not necessary with the Jensen since it can handle line level, but the other uses a step-up xfmr that is supposed to handle mic level, hence the need for an input pad.
The Cuniberti has a natural  high impedance output; in addition it's inductive, so it does not need the addition of an inductor.

But if a level control is important on the secondary,
Some kind of level control is advisable, at least when experimenting. If your set-up does not change significantly, you can make it fixed.
 

heatwalk

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Natural high impedance out with high inductance from being a stepup whereas the Jensen is a 1:1 so wouldn't have as many windings? Don't see induction measurements on many datasheets for transformers
 

abbey road d enfer

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Don't see induction measurements on many datasheets for transformers
The problem is that it's a can of worms. Inductance varies with frequency and level, so most manufacturers publish a nominal impedance value, which is not something that is intrinsic to the transformer. It's the recommanded impedance of the circuit that drives or load the xfmr. But actually, it's more complicated than that.
On the primary side, the nominal impedance is the maximum source impedance of the driver for correct LF response and THD. All transformers benefit from being driven by the lowest possible impedance.
On the secondary side, it's even more ambiguous; it's often the maximum impedance that loads enough the secondary for taming the inevitable HF resonance.
But in fact most mfgrs chose to present the secondary impedance as a number that coincides with the voltage ratio. According to this, a xfmr with the same number of terms on both the pri and the sec is a 1:1, which translates as a 10k:10k, or 600:600, although the second figure does not really coincides with the optimum loading.
Transformers are complicated simple things.
 
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