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Old 7th June 2013, 01:55 PM   #1
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Default Low frequency modulation distortion in foil resistors

Did anybody notice the comments in the latest Audio Precision newsletter about metal foil resistors having low frequency modulation distortion? They suggested that conventional metal films are better choices for audio. IMO, that should raise some high end eyebrows.

Now, my question is what's the test setup to reveal this, particularly if one doesn't have the $$ for an AP setup?

AP Powerpoint article
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Old 7th June 2013, 03:32 PM   #2
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Example(s) of "metal foil" resistor(s)?

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Old 7th June 2013, 03:43 PM   #3
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Quote:
Originally Posted by bear View Post
Example(s) of "metal foil" resistor(s)?
(SALES PAGE)

screengrab below
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File Type: png distro_screengrab.png (12.9 KB, 158 views)
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Old 7th June 2013, 04:39 PM   #4
jcx is offline jcx  United States
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it appears that Hofer is saying that some bulk metal foil don't perform proportionately as much better than some thin film just considering TCR

a guess would be that the extreme low TCR are obtained in part by balancing mechanical strain and TC of substrate and foil - so there is a more complicated thermo-mechanical effect that doesn't work to full benefit above fractional Hz and below kHz

but the foil still has as low TCR as any thin film as far as I know – so it shouldn't actually be worse than thin film – just not as much better as might be expected by the DC TCR ratios

it would be useful to know which “some bulk metal foil” are that show this – Z foil? - differ between chips, leaded, “naked”, fully encapsulated? as well as the numbers, plots
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Old 7th June 2013, 04:53 PM   #5
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Quote:
Originally Posted by Conrad Hoffman View Post
Did anybody notice the comments in the latest Audio Precision newsletter about metal foil resistors having low frequency modulation distortion? They suggested that conventional metal films are better choices for audio. IMO, that should raise some high end eyebrows.

Now, my question is what's the test setup to reveal this, particularly if one doesn't have the $$ for an AP setup?

AP Powerpoint article
You don't need an AP just ten of the same resistors and any analyzer. even a PC. This was covered in the Blowtorch thread a few years back.
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Old 7th June 2013, 05:11 PM   #6
qusp is offline qusp  Australia
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jcx, even the naked parts are not naked as you probably know, they have clear conformal coating.

one would hope they are meaning Zfoil, Sfoil is less and less of the vpg and TC catalog these days anyway
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Old 7th June 2013, 05:11 PM   #7
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Simon,

Do you have any information as to the veracity of the claim made?


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Old 7th June 2013, 05:15 PM   #8
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Quote:
Originally Posted by transistormarkj View Post
(SALES PAGE)

screengrab below

specific examples... as in brand, types, styles, power ratings...

of course I'd also like to know at what % of rated power these problems showed up (vs. metal film for example).
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Old 7th June 2013, 05:45 PM   #9
qusp is offline qusp  Australia
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there is only a couple brands. Vishay Precision Group aka VPG is the main one, texas components are an offshoot of VPG and manufacture small run (as low as single quantities) custom value foil parts where you can choose the exact resistance, TC, absolute tolerance as well as matching in pairs, quads etc. for a price ...

the best known part here on DIYA is the TX2575 'naked resistor'

below is also some of my ASMP naked zfoil SMD resistors.

and some TX220Z power resistors

the 'naked' parts are only available from texas components, not vishay. they came about due to audiophiles there is no objective reasoning and they arent naked anyway. it was thought that no dielectric may 'sound' more transparent ... I get them for the excellent tolerance, ability to buy matched, odd values to 0.0005% and because they are sexy

I also use thin film a lot; in fact those are pretty much all I use these days, those 2. lately i'm loving the vishay thin film soic8 resistor networks for input stages.

I believe there may be some other brands, but i'm not sure what they are.

most parts will be cheaper to buy from texas components than the VPG equivalent at digikey, who seem to have some OTT prices on some of the parts, like more than double the price
Attached Images
File Type: jpg TX2575.jpg (199.6 KB, 140 views)
File Type: jpg Zfoil SMD ASMP identical lorez.jpg (135.9 KB, 136 views)
File Type: jpg Zfoil SMD ASMP identical 2 smaller.jpg (201.5 KB, 135 views)
File Type: jpg TX220Z 400R web.jpg (292.3 KB, 132 views)

Last edited by qusp; 7th June 2013 at 06:01 PM.
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Old 7th June 2013, 06:15 PM   #10
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If the foils had a high TC, which they typically don't, I could see a LF distortion issue, but it's not clear that's what they're talking about. Or, do they mean plain old IM distortion, but at low frequencies rather than the usual standards? Maybe I need to shoot 'em an email as I'm quite curious about this.
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