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Old 25th June 2006, 09:34 PM   #21
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I think you meant "extensively". The rest of us that responded negatively about carbon comps were voicing our preferences and also raising some concerns about the pitfalls of using them. I learned something new - I hadn't considered the moisture absorbtion problem at all. Considering that some of the carbon comps I have in my junk box have the carbon element (porous) exposed at the ends, I shouldn't really have been surprised.

There are 4 issues with NOS carbon comps - noise, microphonics (2 sides of the same coin), moisture absorbtion, and drift - both thermal drift and storage drift. I'd think you'd want to be real careful about any nos carbon comp resistors used, especially if the seller is asking a premium. If I were going to use them, I'd limit my consumption to some brand known to be decently constructed, like Allen-Bradley. (if anyone else knows a decent brand, sing out). It's a good idea also to generously derate the resistor wattage to minimize thermal drift. I use a factor of two wattage derating for resistors - with CCs, you might want to go a bit further. Using something like a Riken carbon film for voicing may be a decent compromise, though I haven't tried them myself.

I'd also like to point out that film resistors have their issues too, including end cap integrity, end cap and lead materials, and (some) self inductance.
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Old 25th June 2006, 10:51 PM   #22
rdf is offline rdf  Canada
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Hi lndm! I also don't use them - fresh from the baker's oven or not - because I don't like what they do to the sound. For me it's like Russian KY40-9 PIO caps, everyone talks them up but I always end up pulling them out.
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Old 25th June 2006, 11:12 PM   #23
lndm is offline lndm  Australia
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I believe I'm in the minority, and I can make sense of the dilemma.

I try where possible to replace carbons I've used in prototypes with metal films/oxides, but they occasionally disappoint me. I'd rather use them if possible but sometimes it doesn't work out.

Maybe I'm sensitive to something, I do like a warmer sound. Over time I've developed the opinion that carbon resistors 'don't bite', and I can count on their 'support' if I don't use too many of them.

What do you make of the thought that some metal film resistors cause glare and a metallic sound. I don't think this describes what I hear but it's the closest I've read about from others.
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Old 25th June 2006, 11:36 PM   #24
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I don't have any specific opinion about the sound of any resistor type, except that perceived differences of that sort are really going to be dependent on circuit type and topology, other component choices (output transformer, for example), equipment lineup, and of course, individual hearing differences. In the end, you go with what works for you. The choice should be an informed one, so one is not terribly shocked and surprised by other problems along the way.

My personal view on this matter is that if I want to inject some 2nd harmonic character into the signal, I'd use an open loop single ended topology. I have a bunch of threads in the analogue and solid state discussing some experiments I've done with JFET-based open loop preamp circuits. I'm currently using a JFET SRPP phono preamp of my own design, though it's going to get the heave-ho sooner or later (probably later, as I've been sidetracked with some tube designs) because of issues with output impedance and RIAA accuracy. I used all metal film reistors in these designs.
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Old 26th June 2006, 01:28 AM   #25
rdf is offline rdf  Canada
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Maybe it's the type of carbon (or application and circuit as wrenchone suggest) but I've always found them too 'textrured' up top for a lack of better word. Not bright for sure, just something happening with sibalance I didn't like. I haven't seen anything in umpteen distortion spectral analysis so can't really explain why.

Re: metal film, no real opinion. Most in the bin are 20-year old Philips. I usually prototype with generic Chinese metal oxides, because they're available and cheap, then replace them with Mills because I have a bunch and they're non-magnetic.
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Old 26th June 2006, 02:21 AM   #26
poobah is offline poobah  United States
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rdf,

Do you know about "voltage coefficient"? It's not your imagination, cc's sound different. The even order distortion can even be measured!

Yea!


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Old 26th June 2006, 03:49 AM   #27
rdf is offline rdf  Canada
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Not applied to resistors, thanks for the head's up! Near the first link on Google:

http://www.geofex.com/Article_Folder...carboncomp.htm

It's a possibility though I can't recall a circuit in which they swung much voltage. Maybe their effect didn't appear in measurements for being swamped by the tube's characteristic. It would also be unpredicatble and depend on circuit location. The Geofex article isn't exactly right, two series stages with comparable content can partially cancel the 2nd harmonic at the expense of more 3rd and up. Following a tube stage with a carbon comp could theoretically result in less distorion up to the point of overload. Less sweet, more edge, whatever that translates to in guitar terms. Yepes vs. Fast Eddy?
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Old 26th June 2006, 04:07 AM   #28
poobah is offline poobah  United States
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Weird huh? That's what lndm is talking about in plate loads where you do get some serious swing.

CC's are for guitar amps... spread the word.



Forum Advisory: No fools or charlatans were harmed in the making of this post.
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Old 26th June 2006, 06:38 AM   #29
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Default carbon comp resistors

I've found that there are tremendous differences in the quality of carbon comp resistors - some brands from the 1950s and 60s drift terribly. The late A-B types seem the most stable (of course they are the newest).

If used in places where their noise and drift is not a problem, carbon comps are not always bad, sonically. They have to be used with caution. Using larger than needed sizes (i.e. 1 or 2W) usually results in lower noise and better stability. However, I generally default to using metal oxide flame proof types.

Another flaw with carbon comp resistors is that their resistance goes down at high frequencies. This manifiests itself for high (50K and higher) values and the resistance begins to drop above about 100KHz or so by as much as a factor of 2!. This won't be a problem for most audio designs, but affects wide-band and RF circuits. Carbon film and metal film types don't show this effect.

- John
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