Download Carbon Comp Distorti..

Monday, March 18, 2002 10:18:21 AM
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R.G.
3/13/2002 10:01 PM
Carbon Comp Distortion Calculation
This is going into the next update of the Tube Amp FAQ along with about a zillion other things, but I
thought you'd like to see it.
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FAQ: ... I would like to hear opinions concerning the use of carbon composition resistors.
Obviously, the vintage amps we all love had them, and they certainly have their share of mojo, but they
used them in the 60s because that's what was available.
Now there are lots of options. It's my understanding that they drift a lot, compared to their stated values.
What's the story on using carbon comps to replace otherwise operating resistors in a modern amp, or in
rebuilding a vintage amp to play through where reliability and low-noise operation are as important as the
tone?
If anyone can shed some light on the pros and cons of the carbon comps, I will appreciate it. Is this purely
a "mojo" thing, or is there some science involved?
What about metal film resistors? Used in almost every "modern" hi spec amp today including Soldano and
so on....
R.G.: Carbon comps have excess noise, high drift, high pulse power, and high variability. They also have
a high voltage coefficient of resistance (as well as a high thermal coefficient of resistance - temperature
drift).
That means that the resistance actually varies with the voltage across the resistor. That induces a certain
amount of second harmonic distortion only in the case where the signal across the resistor is large - in the
50V and up range to be really audible.
In only those conditions, you can hear a certain amount of "sweetening".
You also hear the excess noise and drift. Us them sparingly only where your personal ears tell you that
they're useful. For instance, use metal film for low noise in input stages, and carbon film in PI and output
stages.
From an electrical circuit point of view, especially as regards noise, all resistors connected to a tube that
handles signal are in the signal path. They don't have to be in series between tubes or used as a plate
resistor to be in the signal path from the standpoint of noise contribution.
Only resistors with both large voltages across them -and- large signals in those resistors can offer the
carbon comp sweetening distortion. That leaves out grid resistors of all kinds (low voltages across them),
all bypassed resistors (no AC signal across them), and first-stage preamp plate resistors (signal is too
small).
Since the distortion is small, it's going to be most noticeable in the plate(s) of the phase inverter and the
tube just before the plate inverter. Earlier stages are too early - not enough signal - and too sensitive to
noise because of the later amp gain.
I'm always amused at people who advertise putting carbon comp resistors in their 9V powered effects to
give them some kind of magical vintage sound. Urban legend is tough to kill, though.
In my own rough and ready estimation, you have to have at least 50V of DC and about a 50V or more
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signal on a resistor to get the sweetening distortion. Obviously, 50V is not a hard and fast dividing line,
just where I judged it to be significant. Here's some web references and an example calculation
==============================================
From www.metechinc.com:
The Voltage Coefficient of Resistance-VCR-describes the reversible change in resistance due to
increased voltage gradients. ... CAUTION: VCR (like TCR) is not a linear property. VCR's can increase
significantly when measured at higher voltages. Testing in your application is always prudent.
(translation: the very proportionality of resistance itself varies with the applied voltage; worse, the amount
of variation gets larger at larger voltages too.)
From http://www.texascomponents.com/7%20reasons/Reason%207.pdf
... Reason 7. Non-Measurable Voltage Coefficient As mentioned earlier in our section on resistor noise,
resistors can change value due to applied voltage. The term used to describe the rate of change of
resistance with changing voltage is known as voltage coefficient.
Resistors of different constructions have noticeably different voltage coefficients. In the extreme case the
effect in a carbon composition resistor is so noticeable that the resistance value varies greatly as a
function of the applied voltage.
From http://www.riedon.com/handbook_default.htm:
Voltage coefficient is the change in resistance with applied voltage. It is associated with carbon
composition and carbon film resistors, and is a function of the resistor's value and its composition.
A typical carbon comp resistor voltage coefficient can be seen at http://www.irctt.com/pdf_files/IBT.pdf which shows carbon comp at 0.005%/volt for that company's products. Another was 0.008%/V
From http://www.irctt.com/product_selector/irc/frames3.cfm you can find carbon film at 0.001%/V.
So - for a 50V swing, the change in resistance will be .25% in CC, 0.05% in CF, compared to
unmeasurable in metal film, wirewound and bulk metal foil. That is, with 50V across it, the small signal
resistance is 0.25% lower than it is with 0V across it. But what if we're feeding the resistor a DC current
with enough AC imposed onto it to make the *signal* swing 50V? The high end of the signal will be 0.25%
smaller than the low end of the signal.
That's pure second harmonic, by the way. You probably can't hear it as other than a sweetening or
fullness.
I'd have to do some more calculation to relate the change in resistance to straight % distortion, but you get
the idea. At 9V, even carbon comp sees only 0.045% change, pretty neglible. At 100V, you get 0.5%
nonlinearity, and at 200V, 1% nonlinearity.
I guessed at an arbitrary point below which I think the voltage coefficient distortion would be negligible.
The exact cutoff point is arguable, but I think we'd all agree that there is some range below where the
distortion is not noticeable, and above which it has an audible effect.
The numbers are for IRC CC's. They'd be a lot worse from other makers, maybe.
From:
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Wild Bill ([email protected])
3/14/2002 2:15 AM
Re: Carbon Comp Distortion Calculation
At last! A truly scientific basis for discussion abut carbon comp mojo!
As for .25% giving an audible "sweetening", I'm afraid that my fingers just don't have much sweetening at
all! :)
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---Wild Bill
From:
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Subject:
R.G.
3/14/2002 4:16 AM
Re: Carbon Comp Distortion Calculation
I was truly relieved when my thumbnailing turned out to produce results that correlated with available info,
and also gave a good way to predict where carbon mojo could and couldn't be had.
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Gino ([email protected])
3/14/2002 3:04 PM
Re: Carbon Comp Distortion Calculation
... in other words, changing out those NOISY carbon comps as plate resistors on the first and second
stages of old FENDERS isn't going to do a dang thing to the tone -- esp in the first stages, whree a 'trickle'
of noise would sound like a waterfall after being amplified thousands of times over througout the circuit....
VERY cool article.....
From:
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Subject:
R.G.
3/14/2002 3:57 PM
Re: Carbon Comp Distortion Calculation
You certainly ought to get the most bang for the buck latest in the amp. PI plate resistors should be good.
This is really good news for people making their own amps or modding. You make the front end quiet with
metal film and get tube squish from CC's in the later stages where the noise doesn't hurt you so much.
R.G.
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Mike Conner ([email protected])
3/15/2002 4:03 PM
Re: Carbon Comp Distortion Calculation
Before changing those noisy plate resistors, check their value. If they're off spec (didn't they have like
20% tolerance or something) or have drifted, it might be possible that the off spec value is contributing to
the uniqueness or tone of an amp.
Replacing a drifted 150K or 80K plate resistor with a 100K might have some effect, I guess (maybe not?).
Just replace the drifted one with as close a value as possible if you don't want to change the amp.
Mike
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Anon
3/14/2002 10:52 PM
Re: Carbon Comp Distortion Calculation
Monday, March 18, 2002 10:18:21 AM
What about the AC effects? How do various resistor topologies behave to "guitar frequency" signals in a
high impedance circuit?
From:
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Subject:
R.G.
3/14/2002 11:30 PM
Re: Carbon Comp Distortion Calculation
What about the AC effects? How do various resistor topologies behave to "guitar frequency"
signals in a high impedance circuit?
There's always more research to be done - you might have a real winner of a topic to dig into there ... I'm
still drinking my victory beer from digging this one out 8-)
My personal guess is that for the 40Hz to 7kHz of signal that comes out of a typical guitar pickup, you'd
have a huge difficulty measuring any frequency effect on any of the common resistor technologies.
There are really only three possibilities for frequency effects. Those are
(1) spiral cut carbon film
(2) spiral cut metal film
(3) wirewound
In all these cases, the effect is from inductance of the conductor and distributed capacitance. However,
the inductance of a spiral cut resistor is easy to measure. Just hang a high quality cap across it to make a
parallel RLC circuit and find out where it resonates. The inductive effects can be calculated in a
straightforward manner from that.
I would bet money that you won't see audibly significant effects below 20kHz, and probably not below
100kHz. The inductive effect gets worse with higher resistor values which need more turns of resistive
material to get a long enough conductor string. Carbon and metal film resistors are used in noncritical
radio circuits, and they couldn't be if the inductances were very high.
Carbon comp will be a relative winner there, though. Because of the highly distributed nature of the
granular contacts in the carbon, there are many paths, and CC's are relatively resistive to high RF ranges
- out where we can't hear the difference 8-).
R.G.
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Alexander ([email protected])
3/15/2002 12:46 AM
Certain Mfr.'s are gonna wanna kill you...
Thanks, R.G !
Science triumphs over 'mojo'/urban legend again.
I'm sure half the builders here have an amp that contains metal film resistors and metallized
polypropelyne caps and they sound GREAT. But there's always that nagging doubt...bottom line: Trust
your Ears ! If it sounds good, it IS good.
We all oughta chip in and publish this post in Vintage Guitar & Guitar Player magazine to help straighten
out the superstitious masses !
Damn, I just found an old electronics warehous that has THOUSANDS of old .5 & 1 watt CC resistors !
Thanks again for the continued enlightenment.
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Alexander
Retrodyne Amplification
From:
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Subject:
b
3/15/2002 5:10 AM
Re: Certain Mfr.'s are gonna wanna kill you...
Where is a good online source that sells CC resistors? Mouser?
From:
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Anon
3/15/2002 2:43 PM
Re: Carbon Comp Distortion Calculation
Thanks for bringing up this subject, RG. It's long overdue for serious study.
Eddy currents are generated when current flows in a conductor. No one has ever checked out the Eddy
current effect on resistors. Another science project!
Many metal film & Carbon film resistors are made by depositing the material on the outside of a ceramic
cylinder. We know that as frequency rises, the signal moves to the outside of a conductor. But at WHAT
frequency this begins to occurr, and the effects of phase shift & harmonics, is anyone's guess.
In an AC circuit, a resistor is not a resistor. It's an impedance.
Anon
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kg ([email protected])
3/16/2002 9:32 PM
quick'n'dirty skin effect...
well, there's no ONE frequency at which skin effect starts.
it's ALWAYS there in varying degrees, since every conductor creates an electromagnetic field when
current flows. the greater the number of electromagnetic flux lines surrounding the conductor, the greater
the inductance of the conductor. this is exactly how chokes work.
because the center of the conductor has more lines of flux surrounding it, the center has more inductance.
therefore, at higher frequencies, the impedance at the center of the conductor is increased, which pushes
the AC signal to the edges.
the skin effect is therefore a decrease in effective conductor cross section, and correspondingly an
increase in effective wire resistance. this increases power loss in the conductor and can lower the Q of an
LCR resonant circuit.
this is not to be understood as a simple increase of impedance due to self-inductance (which would be
largely reactive, and not very resistive), but rather an increase in power loss and dissipation due to higher
effective resistance of the conductor itself.
now here's the key: the skin effect is only an issue if the increase in effective wire impedance has some
negative effect on the circuit... i.e. it is a wideband circuit which has frequency components spread out
such that some experience the lower impedance, and others the higher impedance, and this is
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undesirable. in other words, the skin effect becomes an issue when the CHANGE in impedance becomes
too large for the circuit in question.
ironically, the THINNER the conductor is, the less skin effect is an issue, since the CHANGE in
impedance will not be so drastic... the thinner conductor has a higher impedance (well, higher DCR really)
to begin with, so the change in impedance does not alter the circuit operation that much.
in the same way, tubular conductors are used to minimize the change in impedance due to skin effect,
since they have no center which would be a low impedance at LF and a higher impedance at HF. their
impedance therefore remains more stable across an extended frequency range than a solid conductor's
would.
foil or strap conductors are preferred in this respect as well, since their edges will not be encircled by so
many EM flux lines, and will thus carry more of the current.
the extreme is the litz wire, which consists of multiple conductors insulated from one another and woven
together in such a way that, over the length of the conductor, each strand has the same "distance" from
the center, and thus will be encircled by the same # of lines of flux. constructing the wire in such a way
makes sure that each strand will carry the same current.
furthermore, because each individual strand is of a much smaller diameter than the effective aggregate
diameter, the frequency at which the skin effect starts to become an issue is pushed outside of the
passband of interest. it is used extensively in RF coils to preserve high circuit Q, but is a very expensive
construction technique.
IMHO, skin effect is REALLY HONESTLY not an issue worth obsessing over at audio frequencies.
ken
From:
Anon
Date:
3/16/2002 11:32 PM
Subject:
Good stuff, KG!
Good stuff, KG. I still find myself pondering over phase relationships in a complex wave form. (Current
leading/lagging voltage, etc.)
Knowing that resistors, capacitors and inductors all contain an L/C/R component can be mind boggling.
This is where comparing the sonic difference of resistors has brought us.
I can't wait to see RG's final report on the matter. I used to think it was all "hogwash" until I started
experimenting with solid wire & CC resistors in tube & SS audio circuits. Using this technology, were
really making instruments, not "perfect" amps.
Anon
From:
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Subject:
Doug B. ([email protected])
3/15/2002 10:46 PM
Re: Carbon Comp Distortion Calculation
R.G. Would the wattage of the resistor have any effect on this?
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- Doug B.
From:
Date:
Subject:
R.G.
3/18/2002 4:05 AM
Re: Carbon Comp Distortion Calculation
Probably. Now that I think about it, it's got to be the per-unit voltage stress across carbon granules that
makes for the variation in resistance. With a higher wattage resistor, there's a lot more resistive filling, so
the per-unit voltage stress is probably smaller.
It's a theory, anyway. I bet if I looked back at the AB and IRC data sheets I could find out.
R.G.
From:
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Subject:
Mark Abbott ([email protected])
3/16/2002 7:58 AM
Re: Best places for Carbon comp resistors.
Dear RG
As always, great work!
I suppose I'm asking for the short answer here, where would you use them?
I gather you would replace the 82K, and 100K resistors in P.I. circuit, but would you replace any other
resistors in the P.I. circuit?
Would you replace the bias resistors with carbon comp resistors, when the amp is being pushed I seem to
recall that you have about 96VAC across them.
Again thanks for your help.
Yours Sincerely
Mark Abbott.
P.S. I'm intending to do the change to resistors in my Boogie Mark 1/S.O.B. I own, and for the record the
original Boogies came out with Carbon Comp resistors.
From:
Date:
Subject:
R.G.
3/18/2002 4:13 AM
Re: Best places for Carbon comp resistors.
I suppose I'm asking for the short answer here, where would you use them?
The short answer has to be "wherever the signal swings are biggest. The plate resistors for the PI are the
prime candidates. The stage just before the PI is the next best.
Grid resistors of all stripes are least likely, because the actual resistance there matters almost not at all.
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Case in point - the grid bias resistors for the output stage. Yep, there's the same voltage there as on the
PI plate resistors. However, the actual resistor value could change around a lot and not affect the signal.
On an AC basis, this is in parallel with the PI plate resistor and rp, so the value of the bias resistor can't
change the value much - it's got to be much larger than the plate resistor for signal transfer reasons.
Likewise, cathode resistors shouldn't matter, as there's low voltage across them (in preamps, anyway)
and they're usually bypassed. No big signal swings.
Cathode followers also should not exhibit the CC distortion. There can be a big signal swing, but the
impedance of the cathode is so low that the actual resistor value doesn't affect the signal much. The
distortion effect is swamped.
Hey. I didn't know I knew this much about this 8-)
R.G.
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