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Author Topic: Current Buffer Unity Gain Opamp  (Read 4394 times)

Offline Blackfish_ArdRi

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Current Buffer Unity Gain Opamp
« on: January 26, 2013, 12:36:51 am »
Hello all!
First Let me say that I have very little experience in electronic circuit deign. My education did not include IC studies (unfortunately).
The problem:
I have a console with outputs of 150 ohms, they are intended to drive a 600 ohm load.
These outputs I would like to use as the monitor path going into a passive controller (attenuator and a/b/c switch, mute/mono) selecting between multiple monitors.
The controllers inputs are 10k ohm.
In order to use this passive device, I need to construct a device that raises the consoles output impedance into an acceptable range for the controller. I have been told this circuit is a unity-gain opamp current buffer. Correct me on the name, as that may help my research! :)
Can anyone point me to a circuit diagram or reference material for this type of device?
Thank you
James Mullen
NEP-Sweetwater

Offline radardoug

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Re: Current Buffer Unity Gain Opamp
« Reply #1 on: January 26, 2013, 03:45:32 pm »
You don't need to do anything, just connect them together. If the console outputs are transformer, you could terminate them by putting a 600 ohm resistor across each transformer output. Otherwise it will work fine. It is what is called a bridging connection.

Offline Blackfish_ArdRi

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Re: Current Buffer Unity Gain Opamp
« Reply #2 on: January 26, 2013, 04:35:53 pm »
thank you sir! I thought they would act as a bridging impedance, but they are transformer outputs, so the signal from the passive controller was lacking low frequency. I used a 600 ohm resistive load from a line isolating 1:1 transformer. worked. appreciate the help.

Offline John Roberts {JR}

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Re: Current Buffer Unity Gain Opamp
« Reply #3 on: January 29, 2013, 11:32:48 am »
Hello all!
First Let me say that I have very little experience in electronic circuit deign. My education did not include IC studies (unfortunately).
The problem:
I have a console with outputs of 150 ohms,
 
Generally active electronic outputs are very low source impedance with 150 ohm resistors added in series to provide a resistive source impedance, decouple capacitive loading (from cables etc), and balance + and - sources with similar impedance.
Quote
they are intended to drive a 600 ohm load.
Most professional gear is designed to drive as low as 600 ohms, not to actually always interface into 600 ohms. That is a mostly archaic left-over standard from the (not so) good old days.
Quote
These outputs I would like to use as the monitor path going into a passive controller (attenuator and a/b/c switch, mute/mono) selecting between multiple monitors.
The controllers inputs are 10k ohm.
Should be no problem to just connect as is.
Quote
In order to use this passive device, I need to construct a device that raises the consoles output impedance into an acceptable range for the controller. I have been told this circuit is a unity-gain opamp current buffer. Correct me on the name, as that may help my research! :)
Can anyone point me to a circuit diagram or reference material for this type of device?
Thank you
James Mullen
NEP-Sweetwater
That sounds backwards. A unity gain current buffer, would be used to drop the output impedance, but that does not seem like a problem.

What happens when you just hook it up?

JR
Circular Science     http://RESOTUNE.COM

"tune it or don't play it..."

Offline Blackfish_ArdRi

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Re: Current Buffer Unity Gain Opamp
« Reply #4 on: February 04, 2013, 04:29:02 pm »
well that was the strange part Mr. Roberts!
the console is a PM2000, transformer balanced outputs.
When I initially connected my Matrix outs to the monitor controller, my audio was skewed in frequency response. I "could" hear a full sine sweep, but from 60ish-800ish Hz was masked and comby and approx. 6-10 db (splc) lower than any tones above 800/1k. Tones in the upper mid-range octaves had a slight oscillation and distortion to them, and the tones above 6k were distorted and a good 6 DB (splC) higher than the 1k reference (1k sine at -18dbFS peak).
I inserted a line iso transformer from AudioMan and BOOM good sounding tunes (and tones).
I confirmed that I had not ingested any owsley or rohypnol in the past two days and looked at the circuit again. It should have worked just fine without the redundant transformer stage. 150 Ohm/600 ohm into 10kohm bridging impedance.
So I unplugged the isolators and...............
everything sounds just fine. PM2000 is a little mid forward, but otherwise solved.
still slightly confused on what I was screwing up and have been unable to recreate the problem, but am wondering if it has something to do with the stepped attenuator operating on unbalanced inputs and outputs and my powered speakers/amplifiers input impedance?.... where did I put my blotter...........

thanks for taking the time to help all!

Offline mpdonahue

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Re: Current Buffer Unity Gain Opamp
« Reply #5 on: February 08, 2013, 11:21:41 pm »
This may be a simple observation, but I suspect that the source of your problems is in how you wire the cable from the x-former balanced output of the console to the unbalanced input of the attenuator.
Put simply, you need to ground the unused pin on the output of the x-former when connecting it to unbalanced equipment. The symptoms you describe are exactly what you would get from a floating output of a transformer.
All the best,
-mark

Offline Blackfish_ArdRi

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Re: Current Buffer Unity Gain Opamp
« Reply #6 on: February 09, 2013, 10:53:00 pm »
thank you Mark!
Appreciate the insight and time.