Resistor Article for Audio

[DonH50]

Vishay is one of the most-respected companies in precision resistors. They recently announced a new line of metal foil resistors combining low noise and excellent thermal stability. Since they have produced resistors with those traits for many, many years it is nice to see them continue to advance the state of the art.

Here is a recent article talking about resistors and their impact on the signal chain, and why low noise and thermal stability matter to designers (and to listeners): http://www.eetimes.com/design/audio...plifier-and-other-high-end-audio-applications

Enjoy! - Don

 

[muralman1]

I was looking forward to reading all about resistor design, but only got a Linux come on.

 

[Steve williams]

Works fine at my end

 

[DonH50]

I think the article is also on the Vishay web site -- http://www.vishay.com/ You'll probably need a PDF reader.

 

[microstrip]

Don,
Vishay manufactures excellent high quality resistors, but the article does not demonstrate their impact in sound quality in audio. Although the authors prudently do not make any firm statement about audio, just some vague general considerations, the quoted thermal example and picture are misleading and do not apply to typical applications, such as preamplifiers. Preamplifers are usually operated in class A and signals have very low power, these thermal variations do not happen in a typical application.

However, some known brands, such as conrad johnson, use their expensive bulk foil resistors with great success. I remember that I upgraded myself my cj Premier seven long time ago and the upgrade was only changing good quality resistors in the power supply for Vishay's bulk foil. And the sound difference was easily noticed. Once again, our ears can notice effects that the usual classical measurements do not see.

A very interesting theory was presented by Gérard Perrot (founder of Lavardin) about the audio thermal memory distortion in semiconductors in 90's. See:
http://peufeu.free.fr/audio/memory/

 

[DonH50]

I'll look at the article, thanks.

I have not run the numbers myself for audio, but have seen thermal distortion from resistors in simple op-amp circuits when you look far enough down (in my case of a few years ago, 16 bits). If you have say a gain of 4 (12 dB) in an opamp, the output resistor sees 4x the voltage swing that the input resistor sees. On average, this just affects the gain, but for slowly varying signals you can see the nonlinearity caused by the gain changing with resistor (and opamp) thermals since the output R changes more than the input R, inducing thermal-related signal nonlinearity. This happens in low-level signals -- it's a problem of a nonlinear resistor ratio due to thermal tails.

For most of my work, the resistor thermal time constants are in the mud, so I choose low-Tc resistors primarily to reduce gain and offset errors. However, in some situations where the DAC switches and dwells, resistor (and opamp, and even wiring) thermal tails are a real problem. I verified the Rs were part of the problem with a simple divider to take out the opamp.

Without knowing the actual signal levels and resistor's (and opamp's) thermal characteristics (tempcos and time constants) I cannot say whether it matters or not for audio. Not saying you are wrong, just that I don't know, and when I have some spare time may try to dig deeper. My past experience may have misled me.

It is interesting to note that years ago those little metal resistors everyone wanted to throw into circuits for their better matching and lower tempco were actually noisier and had higher self-capacitance and hysteresis than standard carbon film. One of those trades one has to really look closely at to ensure the right film is used...

Thanks - Don

p.s. In present day, semiconductor thermals and self-heating are a HUGE problem! Smaller devices means greater thermal density, faster thermal time constants, and much greater nonlinearity due to thermal effects.

 

[microstrip]

Don,
As you wisely say, we have not information enough to know if there will be any significant variation in gain due to thermal effects.

My main point is that the quoted paper uses a non audio condition to make conclusions about audio - if you tap an amplifier violently with an heavy hammer and hear something in the speakers can you immediately conclude that micro phonics plays an important aspect in amplifier performance?

Figure 4 is misleading - it was taken from the excellent old classical ref[4] but omits the caption words "scale exaggerated" and the error values 150 and 600 micro-volt at half and full scale.

 

[DonH50]

Got it, good points. We all have to beware the driver's ed effect... - Don

 

[garylkoh]

I was looking forward to reading all about resistor design, but only got a Linux come on.

That's the problem with a lot of websites these days - advertising.

You have to either wait for the Linux ad to end, or click the X to get rid of the ad.

The place to get more information on the Vishay foil resistors is from their Precision Group website - http://www.vishaypg.com/foil-resistors/

The Vishay Z-foil resistors are some of the best sounding resistors on the planet. Unfortunately, they are incredibly difficult to find retail, and some of the more expensive ones can cost $25 each in 1,000 minimums. Nevertheless, they are not a silver bullet.

There are some other excellent sounding resistors for small signal audio - including http://www.prpinc.com/

But for large signals - in crossovers particularly, common bifilar wirewound resistors sound fantastic. Much better than expensive film resistors that have such a great reputation and following.

 

[garylkoh]

It is interesting to note that years ago those little metal resistors everyone wanted to throw into circuits for their better matching and lower tempco were actually noisier and had higher self-capacitance and hysteresis than standard carbon film. One of those trades one has to really look closely at to ensure the right film is used...

A little knowledge is a dangerous thing!! Somehow, audiophiles have come to associate the TO-220 package and the Vishay, Caddock names with "good" and "expensive" in resistors. But I have not found anywhere in the signal path of an audio circuit where a thin film power resistor would bring significant benefit.

However, in one of my entry models, I have a thick film resistor in a shunt. It's just speculation, but may be the noisy resistor in that particular spot of the crossover there acts like dither to make the treble far smoother.

I have been asked many times why I use "cheap" resistors in my crossovers. In a loudspeaker crossover where high power and heating are a big factor, wirewound is often a far better solution than film resistors, and the very, very expensive Z-foil to be better, but not much better than an excellent bifilar-wound resistor.

 

[microstrip]

I have been asked many times why I use "cheap" resistors in my crossovers. In a loudspeaker crossover where high power and heating are a big factor, wirewound is often a far better solution than film resistors, and the very, very expensive Z-foil to be better, but not much better than an excellent bifilar-wound resistor.

You are not alone. We learn a lot looking to internal pictures in reviews and fliers.
I was astonished to discover that one of the most expensive amplifiers in the world, the Wavac SH833 only uses cheap RS aluminum encased wirewound resistors in the amplifier board!
Even Focal uses many of them in their top model, the Grande Utopia EM.

 

[DonH50]

A good wire wound resistor has about the lowest noise and highest power dissipation rating that can be had. The tempco can be high, depending upon the wire type, but there are ways around that in both the design and construction of the resistors (and things that use them), and for low-value resistors in a crossover I can hardly imagine tempco is a big deal (basically the same tempco as the speaker voice coil, for that matter). They have been panned in audio for their supposed high capacitance and inductive effects; true in the RF world, but I suspect that does not matter at audio frequencies. I hadn't really thought about film resistors in crossovers, for example; that strikes me as poor matching to the application, but what do I know?

Too many audiophiles read a little bit, assume they know it all, apply parameters totally out of context, and go for "the look" instead of what really matters, IMO.

Actually, that's not just audiophiles... - Don

 

[garylkoh]

Thank you, Don. Hopefully everybody who reads this thread no longer asks "Whaatt?? You are not using Vishay resistors in your $30,000 xxxxx amplifier??"

Unfortunately, it's not only audiophiles - even some designers are guilty of that and follow the flavor of the day because it will sell product when reviewers mention the wonderful quality of components applied.

 

[DonH50]

You could always throw in a few and ground them to provide something for the reviewers to see. IIRC, a certain preamp in the 80's had a group of metal film Rs hooked to nothing; their sole purpose was for reviewers/buyers to see them and feel they were getting what they wanted.

 

[garylkoh]

Haha! That's a great idea. I'll just put it down to "internal cosmetics".