Fried My Focusrite - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 5 Old 02-27-17, 02:15 PM Thread Starter
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Fried My Focusrite

I was hoping I wouldn't have to embarrass myself, but I don't want to fry my audio interface a second time.

Jumping into the Way-Back Machine and dialing in the day when I first started dabbling in REW:
I see myself getting ready for my first frequency response measurement. I'd done my research, had my REW Help handy, and started making connections. Microphone connected to Input 1, and AVR connected to Output 1 (left). Check. Loopback cable connected from Input 2 to Output 2 (right). Check. 48V phantom power on. SIZZLE.

That setup fried at least one channel (probably the loopback channel, I can't remember), which I think has something to do with the fact the 2i2 does NOT have separate phantom power for each channel. Looking back, I'd venture a guess that I used the wrong type of cable - possibly an XLR to TRS.

Would a TRS-TRS or TS-TS cable circumvent that problem?

Incidentally (my word of the week), I found an article which states:
"...you can have the delay through the measurement system automatically removed by connecting a loopback cable. With the 2i2, this should be a 6.3mm TRS-TRS loopback cable, as you cannot turn off phantom power individually for the two inputs. The photograph below shows the cabling."
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Last edited by Lumen; 02-28-17 at 08:00 AM. Reason: Changed Thread Title from "Fry my..." to "Fried my...". Original sounded accusatory.
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post #2 of 5 Old 02-27-17, 03:15 PM
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Re: Fry My Focusrite

The manual for the 1st generation model says that Phantom is only present on the XLR contacts of the combination jack. IMO it would be best just to turn Phantom power off until the loopback measurement is completed.

Regards,
Wayne



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post #3 of 5 Old 02-28-17, 07:18 AM Thread Starter
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Re: Fry My Focusrite

Quote:
Wayne A. Pflughaupt wrote: View Post
IMO it would be best just to turn Phantom power off until the loopback measurement is completed.
I would tend to agree, except I'm pretty sure that the loopback connection has to stay in place during timing measurements, which require phantom power for the mic to be on.

Quote:
Wayne A. Pflughaupt wrote: View Post
The manual for the 1st generation model says that Phantom is only present on the XLR contacts of the combination jack.
Good catch! I think I see what I did now, and I'd like to understand why. I treated the unit as plug-n-play, when in actuality I should have studied it. The manual is well-written, and contains all the info needed to avoid my mishap, but not explicitly - there are no measurement-related instructions, so REW connection details must be inferred.

After reading through it again(?), I found this instruction:
The front panel input sockets are Neutrik Combo, which accept either an XLR male connector ... or a ¼” (6.35 mm) jack plug. Note the Scarlett 2i2 has no “Mic/line” switch – the Focusrite preamplifier stage is automatically configured for a microphone when you plug an XLR into the input, and for a line or instrument when you connect a jack plug. Set the LINE/INST switch next to the socket to INST if you are connecting a musical instrument (a guitar in the example) via an ordinary 2-pole (TS) guitar jack, or to LINE if you are connecting a line level source such as the balanced output of a stage piano via a 3-pole (TRS) jack. Note the Combo connector accepts both TRS and TS types of jack plug.

So I must have plugged in an XLR to TRS (or XLR to TS) cable with the XLR end at the input. That would have triggered the 2i2 to activate phantom power to the mic. Regardless of TRS or TS termination at the other end of the cable, 48V was applied to the output. What little circuit theory I understood is now rusted over, and without a schematic I know even less. But I'd be willing to bet that the incorrect cable is the smoking gun.

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post #4 of 5 Old 02-28-17, 07:48 AM
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Re: Fried My Focusrite

Quote:
Lumen wrote: View Post
But I'd be willing to bet that the incorrect cable is the smoking gun.
Most likely. It’s been a long time since the subject has come up, but we used to recommend not using the mic input for a loopback sound card calibration. I know nothing of timing measurements though, so maybe that’s changed...

Regards,
Wayne



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post #5 of 5 Old 03-01-17, 12:05 PM
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Re: Fry My Focusrite

Quote:
Lumen wrote: View Post
I would tend to agree, except I'm pretty sure that the loopback connection has to stay in place during timing measurements, which require phantom power for the mic to be on.


Good catch! I think I see what I did now, and I'd like to understand why. I treated the unit as plug-n-play, when in actuality I should have studied it. The manual is well-written, and contains all the info needed to avoid my mishap, but not explicitly - there are no measurement-related instructions, so REW connection details must be inferred.

After reading through it again(?), I found this instruction:
The front panel input sockets are Neutrik Combo, which accept either an XLR male connector ... or a ¼” (6.35 mm) jack plug. Note the Scarlett 2i2 has no “Mic/line” switch – the Focusrite preamplifier stage is automatically configured for a microphone when you plug an XLR into the input, and for a line or instrument when you connect a jack plug. Set the LINE/INST switch next to the socket to INST if you are connecting a musical instrument (a guitar in the example) via an ordinary 2-pole (TS) guitar jack, or to LINE if you are connecting a line level source such as the balanced output of a stage piano via a 3-pole (TRS) jack. Note the Combo connector accepts both TRS and TS types of jack plug.

So I must have plugged in an XLR to TRS (or XLR to TS) cable with the XLR end at the input. That would have triggered the 2i2 to activate phantom power to the mic. Regardless of TRS or TS termination at the other end of the cable, 48V was applied to the output. What little circuit theory I understood is now rusted over, and without a schematic I know even less. But I'd be willing to bet that the incorrect cable is the smoking gun.
This thread covers some detail about the phantom power circuit.
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