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Old 28th April 2012, 12:26 AM   #1
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Default Amp testing equipment - should terminators be used

Hello, I'm new to amp testing and just purchased/awaiting a HP8903A Audio Analyzer and an Owon SDS6062 Oscilliscope. In the meantime I'm trying to get the appropriate cables and connections.

Question 1: Will I need to add a BNC terminator on the input side of the amp since the HP8903A output impedence is 600 ohms?

Question 2: Will I need a terminator on amp output side also? I plan on outputting the amp to the input side of HP8903A and teeing to SDS6062.

Question 3: If terminators are required what size (50, 75 or 600 ohm) will be needed? All cabling will be 75 ohm coax cable.

Thanks in advance for any advice!!!
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Old 28th April 2012, 12:49 AM   #2
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Location: Canandaigua, NY USA
No terminators are needed or should be used. All you need is a load for the amp. Nor does the impedance of the coax matter. This is audio, not RF!
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Old 28th April 2012, 01:33 AM   #3
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Good luck with the HP8903 -- Pete Millett has some software which extends the utility of the device. HP 8903 Software
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Old 28th April 2012, 01:49 AM   #4
tomchr is offline tomchr  Canada
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No terminators needed on the HP 8903.

The only time you will need terminators is for square wave testing. That's not supported on the 8903, though. I use an HP 3312A function generator which has 50 Ohm output. To avoid ringing on the edges of the square wave, the terminator is needed. What many people tend to forget is that to get clean edge rates on a square wave, proper termination is needed as the edges require fairly high bandwidth to reproduce properly.

Pete Millett's software require the National Instruments' USB-GPIB adapters, which are ridiculously expensive. For a lower cost alternative, look at the ones offered by Prologix. On my website I have a few scripts that use those adapters. I'm working to convert the scripts to Python so you don't have to have MATLAB or Octave running, but that's not likely to happen fast. The software doesn't extend the measurement capability, it just automates it. Which, granted, is handy for large sweeps of, say, THD vs frequency and power.

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Last edited by tomchr; 28th April 2012 at 01:52 AM.
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Old 29th April 2012, 07:33 PM   #5
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I also have an automation program on my site to use with the Prologix device. It's far from bug free, but doesn't require any other software. Will I get back to it in this lifetime? No idea, but it's worth a shot if you have a Prologix. I use it for all my testing.

On square wave testing, obviously not with the 8903, you can certainly apply very high bandwidth edges to the input if you use a proper terminated cable. IMO, how amplifiers respond to this varies quite a lot and may not be meaningful. It would be great if there were a rise and fall standard for square wave testing. IMO again, good designs should limit the incoming slew rate, leaving nothing to chance.
I may be barking up the wrong tree, but at least I'm barking!
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Old 30th April 2012, 05:43 PM   #6
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Thank you all for the quick response and I appreciate your time!!! Hopefully this thread will help out others in the future.
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