Radial Engineering - SGI (Studio Guitar Interface)  [traced]

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Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 01 Jun 2015, 16:05

Hey fellow FSB'ers. It's been a while since I've contributed but I just bought one of these Units for recording some guitar parts. This thing is really useful for when you need to isolate the amp far away, the SGI makes it a piece of cake. I took the boards out of the enclosures and took some photos. The receive module is very simple, I'll post pictures of that PCB and schematic later. I'm working on the Transmit module, which is where most of the magic happens. Both units are transformer isolated to reduce hum, the ratio is 1:1. They're made by Allied and model number is AEE19-3460 as you'll see in the photos.

I'm having some issues tracing through the schematic, I believe I've got the power and input sections correct, but the IC section doesn't look like it connects to anything. I've been using a flashlight and my multimeter to check continuity and find all the traces but so far I have been unsuccessful in finding how the input and IC sections connect.

Anyway, here are the photos, and my first draft of the schematic. Forgive me if the components look all wonky right now, I haven't had time to go back through the schematic and make it look neater and more legible.

*** This is a positive tip DC Supply ***
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 02 Jun 2015, 01:58

Update

I looked over the schematic and PCB again tonight, and I've made some corrections to the schematic. Pin 5 on the IC should have been going to the 1/2 supply voltage, labeled Vb in the new schematic, not Vr.

Vr is actually the ground pin connection from the regulator. I also added a Vr pin to the ground path on the regulator to show that it's connected to the rest of the components along that trace. I'm still not understanding exactly what the IC section is doing. It looks almost like a current mirror? In which case I would assume the IC is buffering some part of the supply, but It's not jumping out at me. Probably because of the way i've drawn the schematic out.

Anyway, here is the updated version of the schematic.
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Re: Radial Engineering - SGI (Studio Guitar Interface)  [traced]

Postby MullisMan » 03 Jun 2015, 13:21

Update #2

I kept looking at how the input section connected to the JFET 2N4393 (the symbol for this NPN Jfet was corrected in this update as well). I realized that the pinout that I had used was incorrect and changed the gate and drain connections accordingly. This looks much better. I also connected the input jack terminals to ground, I had them all connected but never sent it to a ground node.

I'm convinced more and more that the IC section of the circuit is working as a current mirror with the PNP and NPN transistors for the Va and Vr part of the supply. I could still be wrong, it's been way too long since I worked with those circuits in college. I should look into some example circuits again.

If a mod would like to delete the previous versions of the schematic by all means please do.
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby soulsonic » 03 Jun 2015, 18:25

Variable input Z is a 1M pot in series with a 22k resistor? Exactly the same values I use! :shock:
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 03 Jun 2015, 18:58

soulsonic wrote:Variable input Z is a 1M pot in series with a 22k resistor? Exactly the same values I use! :shock:


Yep, although in the unit they're using the dual gang pot, but the other half of it is just grounded. There were some empty spots on the PCB for other resistors and caps so maybe it was from an earlier rev of the circuit? Anyway, I just made it a single gang pot since that's all that's being used.

Do you wire yours with the same rotation?
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby soulsonic » 03 Jun 2015, 19:02

Yes I do, and I usually use audio taper as well, because you don't hear enough of a change during the rotation when you use linear.

But alot of other things about this schematic make no sense to me - like everything around the 4558 and the two larger transistors. I don't see their purpose in the circuit at all.. :|
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 03 Jun 2015, 19:58

soulsonic wrote:Yes I do, and I usually use audio taper as well, because you don't hear enough of a change during the rotation when you use linear.

But alot of other things about this schematic make no sense to me - like everything around the 4558 and the two larger transistors. I don't see their purpose in the circuit at all.. :|


Me either, and I can't find a trace connecting the 4558 to the input section at all. I kept checking to see if maybe the output was being used or something but I can't find anything. I don't think they'd go to all the trouble of putting that many dummy components in the circuit but I'm at a loss. Could it possibly be regulating the supply even further ?
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby nickthemen » 04 Jun 2015, 09:27

Looks like power sag from the roland blues cube?
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 04 Jun 2015, 17:19

nickthemen wrote:Looks like power sag from the roland blues cube?


Hmm it does look kind of similar except for the IC connections. :hmmm:
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby coldcraft » 05 Jun 2015, 04:34

that JFET/BJT buffer is probably arranged more like the first example in this diagram,

Image

The funny opamp thing with the complimentary BJTs is a virtual ground for a split rail power. The box is powered by 15V and regulated to 12V, from which R8 and R6 generate +/- 6V where "GND" is buffered by Q3 and Q4. IC1B is a servo that gaurantees that GND and VB are the same voltage. By doing this, the gate of the JFET is DC coupled to the input as it is referenced to ground. VR in your diagram is -6V and VA is +6V. VB is GND.

I hope that adds a little more clarity.
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 09 Jun 2015, 18:30

coldcraft wrote:that JFET/BJT buffer is probably arranged more like the first example in this diagram,

[ Image ]

The funny opamp thing with the complimentary BJTs is a virtual ground for a split rail power. The box is powered by 15V and regulated to 12V, from which R8 and R6 generate +/- 6V where "GND" is buffered by Q3 and Q4. IC1B is a servo that gaurantees that GND and VB are the same voltage. By doing this, the gate of the JFET is DC coupled to the input as it is referenced to ground. VR in your diagram is -6V and VA is +6V. VB is GND.

I hope that adds a little more clarity.


Thanks Coldcraft! I was having some trouble finding pinouts that made sense from datasheets on the web, especially for the 2N4393.
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 09 Jun 2015, 19:35

Now i'm tasked with finding a transformer with comparable qualities to the original Allied Components model. From the datasheet here: http://www.alliedcomponents.com/tf_pdf/AEE19-3460.pdf, it looks like the transformer has a ratio of 1:1. The DC resistance is a little harder to match up on Mouser, as the primary has a resistance of 820ohms, and the secondary has a resistance of 1.08Kohms.

The datasheet lists this as a high frequency transformer, but doesn't give a frequency range. Does anybody have an idea of what should be used? Guitar has an 80hz - 1200hz range, is this what I should aim for?

So close! :horsey:
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 19 Jun 2015, 13:37

UPDATE

After I realized that the pinouts I was using for the JFET and BJT in the input section were incorrect and per Coldcrafts correction, I decided that I'd rather do this whole thing with OpAmps. So I left the power section the same but I added another op amp for the input section. The opamp buffer splits the input to a transformer isolated XLR output and a 1/4" output for direct recording to an audio interface. I have the 10megohm input resistor so that with the variable input impedance control we can get somewhere from 510K to 1.3 Megohms of impedance. Will that introduce a lot of noise since they're paralleled?

I also am wondering if my connections will work for the biasing of the op amps. I have the input impedance pot tied to ground or Vb if I understood Coldcrafts explanation correctly. Does this mean that the 10Megohm resistor is also at the same potential since it's tied to Vb?

I'm just wondering if I should connect the 2&3 lugs of the pot to Vb as well. Anyway, below is the schematic. Does this look correct to you Coldcraft?

StudioGuitarDI.png


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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby Njebamin » 02 May 2016, 18:50

I know this is a real old thread, but did anyone ever confirm this schem? And what about the receive box?
I'd be really keen to build a couple of these for live use!
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Re: Radial Engineering - SGI (Studio Guitar Interface)

Postby MullisMan » 30 Aug 2018, 15:46

Sorry to Necro a thread. I never did verify this exact schematic, but If you reconfigure the BYOC Amp Selector project to only be single channel with a transformer to isolate and then connect it to an XLR, you get the same result. Then build a receive module (i'll attach the schematic here) and you're good to go.
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