I feel I owe an explanation and apology to clear this up.
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I initially sourced a dozen pairs of the IR 240/9240 parts from a Chinese vendor.
This was three years ago when I was planning a balanced F5T build. Other things came up and I needed a small amp quickly, so I quickly built a F5T, V1 using these parts.
I encountered a massive mismatch on negative supply one channel of the build when I deployed it, so I sourced the Vishay IRF240 and IR 9140 parts locally.
In the mismatched channel, I simply replaced the IRFP240 (where the problem was) and the problem disappeared. No further action was needed.
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I later built a bunch of the F5/BJT circuits proposed by juma, but tried a bunch of probably wrong mods. He provided patient tutelage to help me get it up and running, and it sounded great when it was done exactly as per his recommendations. It was using the same old IR parts but had only one pair per channel.
Both amplifiers had H2 of under -80dB and only small ripples, or nothing after. Sounded absolutely fantastic, all of them.
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Unfortunately I loaned this second amp out and when it came back one channel had a badly distorted sound, traced back to a blown output device. I took the opportunity to change both of them, this time using the Vishay IRFP240 and the IR IRFP9140. The two channels sounded a bit different to each other, so I decided to change the intact channel as well, with the mixed-manufacturer set. Since I had done this before I had no reason to worry.
That is when I seemed to find the audible performance degraded badly, and I began to read as much as I could. This post led me to believe that I should use parts from the same manufacturer: http://www.diyaudio.com/forums/solid-state/249866-matching-pairing-mosfets-2.html#post4137651
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Obviously this is a slightly simplistic understanding on my part and since Mr. Pass has weighed in, this matter should be closed with the understanding that the devices are fine as long as they meet the target specifications. For better or worse, I already have the Vishay IRFP9140 on the way from Digikey (no Asian sellers this time) and they will be here in about five weeks.
I apologise for any confusion caused and if anyone was offended by my posts.
Cheers!
------------------
I initially sourced a dozen pairs of the IR 240/9240 parts from a Chinese vendor.
This was three years ago when I was planning a balanced F5T build. Other things came up and I needed a small amp quickly, so I quickly built a F5T, V1 using these parts.
I encountered a massive mismatch on negative supply one channel of the build when I deployed it, so I sourced the Vishay IRF240 and IR 9140 parts locally.
In the mismatched channel, I simply replaced the IRFP240 (where the problem was) and the problem disappeared. No further action was needed.
------------------
I later built a bunch of the F5/BJT circuits proposed by juma, but tried a bunch of probably wrong mods. He provided patient tutelage to help me get it up and running, and it sounded great when it was done exactly as per his recommendations. It was using the same old IR parts but had only one pair per channel.
Both amplifiers had H2 of under -80dB and only small ripples, or nothing after. Sounded absolutely fantastic, all of them.
--------------------
Unfortunately I loaned this second amp out and when it came back one channel had a badly distorted sound, traced back to a blown output device. I took the opportunity to change both of them, this time using the Vishay IRFP240 and the IR IRFP9140. The two channels sounded a bit different to each other, so I decided to change the intact channel as well, with the mixed-manufacturer set. Since I had done this before I had no reason to worry.
That is when I seemed to find the audible performance degraded badly, and I began to read as much as I could. This post led me to believe that I should use parts from the same manufacturer: http://www.diyaudio.com/forums/solid-state/249866-matching-pairing-mosfets-2.html#post4137651
-----------------
Obviously this is a slightly simplistic understanding on my part and since Mr. Pass has weighed in, this matter should be closed with the understanding that the devices are fine as long as they meet the target specifications. For better or worse, I already have the Vishay IRFP9140 on the way from Digikey (no Asian sellers this time) and they will be here in about five weeks.
I apologise for any confusion caused and if anyone was offended by my posts.
Cheers!
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Nelson said that Vishay took over Harris, so they are likely very similar, and may well be the same part. Mouser stocks Vishay.
IR is not Harris.
IR parts has the frequency dependent gain problem.
Cheers,
Patrick
In a prior post in this thread the IRFP240 and IRFP9240 were called out as suitable replacements.
Mouser has both in stock:
IRFP240 as PN 844-IRFP240PBF
IRFP9240 as PN 844-IRFP9240PBF
The Vishay IR P channel parts will work, but the Harris version does have a
lower distortion figure.
Interestingly, the gain shelving on the IR P channel parts does give some
negative phase second harmonic in the midrange/top on the F5
lower distortion figure.
Interestingly, the gain shelving on the IR P channel parts does give some
negative phase second harmonic in the midrange/top on the F5
For the record, IR transferred not only the license etc. but, the facility and equipment to Vishay. 15 or more years ago. On Semi bought Fairchild. I'm sure you can search on Harris 
