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 Post subject: Measuring IM on a fisher X-100-C
PostPosted: Jul Thu 12, 2018 7:48 pm 
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Joined: Oct Thu 27, 2016 6:25 pm
Posts: 24
I was wanting to measure intermodulation distortion on my fisher. Manual calls for IM analyzer. Can I use an HP 333a distortion analyzer?? Just wondering because IM uses two signals I believe to be from push pull circuit. Do I have that correct?? Thanks


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 Post subject: Re: Measuring IM on a fisher X-100-C
PostPosted: Jul Thu 12, 2018 8:06 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 24042
Location: Annapolis, MD
first, the definition of intermodulation distortion: It's a fancy way of saying that a non-linear circuit will cause mixing of 2 pure signals to give additional frequencies. This is what a mixer does for us in a typical superhet. In a high-performance amplifier, however, we don't see it a a good thing....;)
Except for the fact that push-pull amps can be non-linear, they are not relevant to the measurement.

Basics here:
https://en.wikipedia.org/wiki/Intermodulation

to measure IM distortion, you need an instrument that accurately measures the new frequencies that are created.

EDIT: It looks like the 333A is a basic harmonic distortion analyzer:
http://web.mit.edu/6.101/www/reference/ ... gilent.pdf

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 Post subject: Re: Measuring IM on a fisher X-100-C
PostPosted: Jul Fri 13, 2018 2:21 am 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 7485
Location: Ohio 45177
I think you are right, as I looked up the info after I noticed the adjustment for IMD on my X 100 B. I think they apply something like 10 khz and 11 khz signals to the input and see what resultant artifacts show up in the output spectrum. The usual analyzer for THD applies maybe a 1 khz signal and looks at the harmonics of that frequency in the output. So for IMD you would need to apply the two frequencies of equal amplitude from generators that are known to have much lower distortion products themselves, than the amplifier. Needless to say, I have not accomplished that check myself but have been utilizing the original factory tubes for what that is worth.


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 Post subject: Re: Measuring IM on a fisher X-100-C
PostPosted: Jul Fri 13, 2018 8:32 pm 
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Joined: Dec Fri 08, 2017 5:40 pm
Posts: 163
Location: 44141
Just Google "Crown IMA". The manual will probably give you more info than you want.

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 Post subject: Re: Measuring IM on a fisher X-100-C
PostPosted: Jul Sun 15, 2018 10:59 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10448
Location: Powell River BC Canada
A Heathkit IM 5248 will measure IM, SMPTE method. 60 and 7000 Hz. (Internal)



Society of Motion Picture and Television Engineers.

They cared because if the optical track recording / playback wasn't
working right the sound was as bad as a 16 mm projector that has been
dropped a time too many. (I managed to drop one down a flight of stairs.)


Some amplifiers we serviced had adjustable screen voltage on the
output tubes that needed adjustment for min IM whenever replaced.

Attachment:
IM   worth a thosand words.jpg
IM worth a thosand words.jpg [ 126.25 KiB | Viewed 314 times ]

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Steve Dow
ve7aso@rac.ca


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