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Topic: Sarno back box specs |
Stuart Tindall
From:
England, UK
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Posted 16 Dec 2023 4:46 pm |
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I have an old Sarno Black Box with a 12au7 tube.
I recently decided to use it again as a buffer immediately after my guitar.
I checked it over and found I was getting a reading of 32kohm to 1meg impedance with the vari z control on the input jack as it states on the specs, but I get a reading of 1 meg on the output jack, which to my mind is way too high.
Normally a reading of 100k would seem right to allow the signal to flow better and give the buffering effect.
Does anyone else have an older SGBB and what should the output impedance reading be.
I fear I may have a problem!
Stuart |
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Tucker Jackson
From:
Portland, Oregon, USA
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Posted 17 Dec 2023 6:28 pm
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Hi, Stuart. I think your device is working properly. The BB Quick Start Guide says this:
"Output Impedance - Approx 1K Ohms" |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Stuart Tindall
From:
England, UK
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Posted 18 Dec 2023 2:31 pm
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Tucker, as I suspected there is an issue, as output impedance should be low compared to input.
You quoted 1k ohm output, mine is 1meg ohm, which is higher than input. This will not buffer the signal, but I'm not sure what might be the cause of this.
It would be good if somebody could explain whats going on here, because I'm sure this is not as it should be. |
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Dave Stagner
From:
Minnesota, USA
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Posted 18 Dec 2023 2:42 pm
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First, check to be sure you’re measuring it correctly! I’m sure Brad Sarno will step in with technical details here, but normally output impedance doesn’t jump like that.
_________________
I don’t believe in pixie dust, but I believe in magic.
1967 ZB D-10
1990 OMI Dobro
Recording King lap steel with Certano benders |
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Fred
From:
Amesbury, MA
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Posted 19 Dec 2023 1:10 pm
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| If you're just measuring across the output jack with an ohm meter you're not measuring the output impedance. I don't know the circuit but Brad Sarno does. Emailing him through his website would be a good place to start. |
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Bruce Derr
From:
Lee, New Hampshire, USA
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Posted 19 Dec 2023 9:25 pm
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Impedance is for AC (usually measured at 1 kHz for audio circuits) and is not the same as DC resistance. The Black Box output circuit is probably configured as a cathode follower, the output of which is characteristically low impedance. There must be a coupling capacitor in the output circuit. The capacitor will pass a 1 kHz signal unimpeded but will block DC voltage. It would look like an open circuit to your meter, which is measuring DC resistance, not impedance. You're meter is probably measuring a terminating resistor from output to ground.
The Black Box's input impedance, on the other hand, is set by a pot and resistor connected across the input. Since those are purely resistive, the DC resistance and the impedance at 1 kHz are essentially the same, so your meter can measure it. That's not the case with the output impedance. |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 20 Dec 2023 10:09 am
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Fred and Bruce have it right.
You can't measure the output Z that way. The output Z is probably about 500 Ohms or so, less than the stated 1K. But measuring with a ohm-meter won't let you see the AC impedance. It only shows the 1Meg drain resistor which is why you get that reading.
Best,
Brad |
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