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Measurement & test amplifier
Measurement & test amplifier
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Old 10th August 2018, 12:20 PM   #1
mbrennwa is offline mbrennwa  Switzerland
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Default Measurement & test amplifier

I am looking for a new measurement amplifier for my bench (mostly for loudspeaker testing). So far, I have used an old integrated HiFi amplifier. This amp is a bit impractical as a bench instrument, and it seems to be dying.

What I need / want:
  • Accurate and repeatable gain setting. A switched volume control would be good (for example -20 dB / 0 dB / +20 dB / +40 dB gain settings).
  • Strong enough to drive loudspeakers hard, both at low and high impedance (for distortion testing)
  • Protection against the usual mishaps would be nice (short circuits etc.)
  • Low noise and distortion.
  • Anything else I am missing?

The whole thing should be a (semi-) finished unit. I want to save my time building stuff for the stuff I want to build, not for the stuff I need to test my stuff (I hope you get my point). A kit with assembled PCBs and machined chassis parts would be ok, too (but no heavy chassis work please).

Do you have any suggestions or ideas where to look?
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Old 10th August 2018, 12:33 PM   #2
Mooly is offline Mooly  United Kingdom
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Measurement & test amplifier
What about an LM3886 chip amp or an LM1875. Both are 4 ohms capable. Any chassis work is restricted to drilling a few holes in a suitable box for sockets and so on.

2 x LM1875 based kit Stereo Amplifier | eBay
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Old 10th August 2018, 02:42 PM   #3
jackinnj is offline jackinnj  United States
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Measurement & test amplifier
Quote:
Originally Posted by Mooly View Post
What about an LM3886 chip amp or an LM1875. Both are 4 ohms capable.
I use an LM4780 for testing -- instead of a big heatsink use a computer heatsink with fan. Thermal impedance is quite low and the whole setup quite compact:
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Old 10th August 2018, 02:46 PM   #4
Mooly is offline Mooly  United Kingdom
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Measurement & test amplifier
Looks good
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Old 10th August 2018, 04:39 PM   #5
mbrennwa is offline mbrennwa  Switzerland
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I have zero experience with chip amps. Do they have built-in protection (shorted output, DC output)? What are the "stronger" chips that will work well with high output currents (low impedance speakers)?
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Old 10th August 2018, 06:19 PM   #6
Mooly is offline Mooly  United Kingdom
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Measurement & test amplifier
All the popular chip amps have protection to varying degrees, for example the LM1875 data sheet says 'Protected against AC and DC shorts to ground'.

The LM3886 says 'The performance of the LM3886, utilizing its Self Peak Instantaneous
Temperature (°Ke) (SPiKe™) protection circuitry,
puts it in a class above discrete and hybrid amplifiers
by providing an inherently, dynamically protected Safe Operating
Area (SOA). SPiKe protection means that these
parts are completely safeguarded at the output against overvoltage,
undervoltage, overloads, including shorts to the
supplies, thermal runaway, and instantaneous temperature
peaks.'

Have a look at Tom Christian's threads (and his own site) on the LM3886 to see just what is possible. Even a basic LM3886 application has pretty impressive performance:

LM3886 Done Right
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