Pickup Tone and Impedance Loading

Brad Sarno · From: St. Louis, MO USA

Here's a topic that I hope some find useful. I've done a lot of experimentation with how the load that a pickup sees directly effects the tonal response of the pickup. It's a very simple LR filter network where the pickup is the L (inductor) and the load that it sees is the R (resistance).

If you use a normal pot pedal, then your pickup sees 500kOhms and also whatever follows the pot pedal. For example, if you use a pot pedal into an RV-3 which is 1Meg, you get 333kOhms of a load that the pickup sees. Anything after the RV-3 doesn't matter because it's buffered from the pickup by the active circuitry in the RV-3. If you use a pot pedal into an old LTD or Session 400, you get the same because those amps are also 1Meg (or 1000kOhms) at the input. If you use a Hilton pedal or Hilton Digital Sustain, your pickup sees 1Meg. That helps account for the high fidelity and extended frequency response you get in that situation. If you use a Black Box your pickup sees 450kOhms.

What you can find is that the tone of the pickup itself is very sensitive to the loading. Low impedance will give a darker mellower tone and higher impedance will give a clearer, brighter tone.

Many of the Peavey amps use a relatively low input impedance of 220kOhms. The Nashville 400, 1000, and 112 have this. If you use a pot pedal into the Nashville, you get a load of 153kOhms. That's quite low. If you plug into a Nashville using their 3-wire hookup, you get 220k, still kind of low. If you use a pot pedal into a typical delay or reverb pedal and then into the Nashville, you get 330kOhms.

There is a tremendous tonal difference between 153kOhms and 1Meg, and you'll experience more of the difference when using hot pickups. It's amazing how much tone shaping takes place right there, before you ever hit a preamp or equalizer. Sometimes if you've lost highs by having too low of a load, it's really hard to get them back with the preamp. And if you try, you may find that you just can't regain that sparkle, you just get hiss from the electronics.

I think I need to get out more....

Brad

Brad Sarno · From: St. Louis, MO USA

The calculations for the pot pedal loadings were at full pedal. The overall load values actually rise just a bit as the pedal comes down.

If you've ever heard the clarity gained by just adding an RV-3 or DD-3 pedal to a rig with a pot pedal into a Nashville amp, this helps explain the brightness gained. Your were feeding an amp input impedance of 220kOhms but by adding the pedal, you change that value to 1Meg.

Seriously, I'm gonna get out more.

Brad

Lee Baucum · From: McAllen, Texas (Extreme South) The Final Frontier

Brad - I have a little Guyatone Micro Delay stomp box that I use quite often. I've always thought my guitar sounded brighter when I used it, but I figured it was just my imagination. Thanks for sharing the info with us.

Lee, from South Texas

James Morehead · From: Prague, Oklahoma, USA - R.I.P.

Hey Brad, Before you leave to "get out more", what effect would a twin reverb have on this scenario, or would it all be the same as the Peaveys?

Buck Grantham R.I.P. · From: Denham Springs, LA. USA

Forget about getting out more and tell us more about this. It is very intresting stuff.

Michael Barone · From: Downingtown, Pennsylvania

Brad, I just wanted to know if you ever calculated a resistor pad, like a "pi" pad for impedance matching, using a pad before, and a pad after an op-amp. I was just wondering if an op-amp or 2 would affect the tone while making up for the attenuation in the pads, once the desired matching is achieved.

Just a though.

------------------
Mike Barone
Sho-Bud Pro-1 5&4 with RHL | Nashville 112
Assorted Guitars & Keyboards

Jon Light (deceased) · From: Saugerties, NY

Yeah, Brad. I'll go out and have fun. You stay right there and keep this stuff coming.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

What Brad is saying is absolutely correct. The input impedance of the device right after the pickup is a critical variable. Voltage transfer depends on the ratio of input impedance to pickup impedance, and note that this is a function of frequency. Voltage transfer approaches 1 as this ratio gets large, and drops as the ratio goes down (it's 50% when these two impedances are equal - one half of the voltage is dropped over the pickup, and one half over the input impedance).

Remember also that, with a lot of windings and a magnetic field in the pickup, inductive impedance of a pickup is large, so pickup impedance goes up with frequency. If this doesn't see a very high input impedance, high end is lost more severely than low end. Don't be mislead by the "20 KOhm resistance" of your pickup windings. In the high frequency range, the actual complex impedance may be many times higher than the D.C. resistance. The old 'rule-of-thumb' in tube amp design was to present a 1 MegOhm input impedance. Old Fender amps like the Twin Reverb have that value in the #1 input. Back in Physics and E.E. labs, old high-input-impedance vacuum-tube-volt-meters (VTVMs) were routinely preferred, over solid-state units, for measurements where it was important to minimize loading effects.

A good high-input-impedance preamp, effects unit, or compressor (not overused) buffers the pickup from the amp. A cheap preamp or other effects unit may actually degrade things if the input impedance is not high.

Jim Peters · From: St. Louis, Missouri, USA, R.I.P.

I always use my tube screamer right out of my guitar(6sting) to avoid the tone sucking that can occur. For steel I use 3 chords(NV112), and don't need the tubescreamer.There is a big difference going direct without the 3 chords.
Brad, you the man(how about posting a clip from Javier's CD)-great stuff! JP

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Brad, you are correct in all your figures. In my experiments it seemed like anything below 150K really sucked the highs out of the magnetic pickup. As you go above 150K, the sucking gets dramatically less. You have the input impediance, you quoted, on my products correct. The input impediance is very high for a reason. I didn't realize the Peavey 3 cord hookup impediance was so low. Brad, I have my digital pot working now.
I can change the taper with either the resistance or capacitance in the timing signal.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

While y'all are doing these "studies", maybe you will come up the reason why steels 40 years ago (when all we had was a pot pedal) had so much more highs than we hear nowadays? Nothing today even comes close to those searing highs on those old hit records. Just compare (for yourself) the "highs" on the records of 40 years ago when Tom Brumley, Buddy Emmons, or Ralph Mooney, were playing, to what you are hearing on recordings now.

Yes...maybe you can 'splain all that to the audience here.

(I already know.

)

Bob Snelgrove · From: san jose, ca

Donny,

Twin Reverbs?

bob

Brad Sarno · From: St. Louis, MO USA

Hey Donny, maybe because the pickups were wound so light, and there wasn't as much inductive reactance. Just a guess.

Brad

Brad Sarno · From: St. Louis, MO USA

Hey Keith, that digital pot sounds very interesting. Can you design any taper you'd like?

Also, I noticed what you said. When you get down around 150kOhms, a lot of tone gets lost. I notice that it really starts to clear up around 300k and by 450k it's pretty open. Up above 600k and on up it really sparkles. Also, the hotter the pickup, the higher the load impedance needs to be to get clear tone. Vintage, "underwound" pickups can get away with much lower impedances and still sound bright.

Brad

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

I suggest the following link to Prof. Steve Errede's Physics of Music/Musical Instruments course at U. of Illinois Urbana-Champaign:
http://online.physics.uiuc.edu/courses/phys498pom/498pom_labs.html

If you scroll down to the "Electric Guitar Pickup Measurements" link, you'll find useful frequency response curves for an assortment of Strat and P-90 pickups on page 8. The magnitude of the complex impedance near the resonant peak is near 1 MegOhm. I believe there are 3 basic effects going on here. The DC resistance is about 6-8 KOhm. As frequency increases past a few hundred Hz, inductive reactance increases, and this increase becomes very rapid as the resonant peak is approached, between 4 KHz and 10 KHz, depending on the pickup. Then, past the resonant peak, capactive reactance from inter-winding capacitance now shunts the signal, reducing impedance. The exact shape of this curve and the resonant frequency depend on the number of windings, magnet material and strength, geometry, etc. It's pretty hard to make simple generalizations about how changes will affect this curve. Even given a constant geometry and magnetic material, more windings raises inductance, capacitance and resistance, so it's not exactly obvious what the overall effect will be, but one thing is that it generally does lower the high end and broaden the resonant peak. A stronger magnet raises inductance, which sharpens the resonant peak. Bill Lawrence seems to be one of the few pickup makers to 'tailor' his pickups using this kind of technical approach, although there are other good-sounding pickups. The point is that I don't think it's possible to simply say "more windings always means less highs", but clearly, many modern pickups sound different than old pickups. Either way, it is clear that even a high-impedance amp loads down a pickup at some frequencies, and that is going to affect the amplified tone.

Tubes are 'naturally' high-impedance devices, while solid-state is 'naturally' a low- to medium-impedance device. But it is possible to make a high-impedance solid-state amp, so I don't think that's the complete answer - there's more to it than linear amplification.

My own experience is that solid-state gets harsher more easily when pushed in the high-end region. Tubes are more forgiving, hence there's a smaller penalty for pushing in the high-end. There is a tradeoff between clean and powerful low-end and sweetness in the top register. Another change from 40 years ago is speakers. Modern high-power speakers are more able to clinically reproduce with high fidelity. Even the high-headroom old Altec-Lansings and JBLs sound different than modern clean speakers. These speak to the nonlinear aspects of amplification. Distortion is not always evil, even for a PSG, IMO. The standard was "it should sound good", not "the specs should be so-and-so."

But part of the change may be changing tastes. It seems that nobody wants to hear it, but many consumers of popular music these days are turned off by the 'high whining' sounds of the high strings of a PSG. When playing around my part of the world, that kind of high-string playing is tolerated in very limited doses by fellow musicians and audience alike. That's true whether using a tube amp or a solid-state amp, although I get more complaints when pushing a solid-state amp like that. Most popular music these days is bottom-heavy, and this is often even exaagerated in live performance. Most people I know get annoyed at a piercing steel, or Telecaster for that matter.

[This message was edited by Dave Mudgett on 06 October 2005 at 07:59 AM.]

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Donnie, maybe you can tell me why there is so many more plastic parts on a big John Deere tractor in 2005, than there was in 1967? Maybe you can explain the cost difference to me also? 2005--$120,000.00, 1967--$12,000.00 Donnie, in the good old days pickups were wound with less ohms, and this created a bright sound. That has little to do with the CD's Nashville is producing nowdays. It has more to do with the producers, who don't want steel guitar cutting through. Donnie, I would suggest you buy some of Jake Hooker's and Bobby Flores' CD's. You will find these are better than the good old days.
Brad, I have been experimenting with Digital Pots. You can program them with a micro-controller, or you can simply alter the timing resistor, or timing capacitor. Here is the best invention I have come up with lately; Creating a pot wiper position that tracks a sensor voltage!

Brad Sarno · From: St. Louis, MO USA

Hey Dave, very cool stuff. Thanks for that. I think there's something to what you are describing about how steel tone has darkened over the years. I suspect a few reasons. One I think is that steel became "uncool" somewhere back a few years ago so they mellowed the tone to hide it in the mix a bit. I have a theory too that when lots of guys went solid state in the '70s and '80s, the highs were less pleasing and thus harder to make work in a mix. I had some real cool feedback from Russ Pahl regarding his steel tracks on Gretchen Wilson's big hit album. Forgive the shameless plug here. Russ brought a Black Box to those sessions part way thru the recording of the album. The engineers really liked how the tube enhanced highs and reduced harshness allowed them to mix the steel louder and brighter that what was typical. They said that generally steel gets mixed until its harshness starts to interfere with the vocals and then it gets backed down a bit. I found that interesting coming from some top contemporary country engineers. It's like they want more steel and brighter steel, but the tone that so many get just won't allow it to mix well. Also since people went solid state, they tried to tame the harshness of the transistory highs by overwinding the pickups. Then you found lots of steel guitars that didn't have that sweet vintage sparkle anymore.

Brad

Randy Beavers · From: Lebanon,TN 37090

Brad said "he really needs to get out more." I don't think he's going to be getting out much once people start to hear that new preamp. He'll be breathing clouds of solder smoke.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Once your pickup loses highs, there is no way to get them back, unless you re-create them artificially. Re-create them artificially by turning up the bright switch on an amp, or turning up the trebble on a amp. Artificially created highs are not the same as the highs created naturally by a guitar pickup. Now Donnie, which is best, artificial ice cream, or non-artificial ice cream? The "real deal" is better 99% of the time.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Donny, he's saying that if the pickup loses highs, it's hard to replace them:

Brad Sarno · From: St. Louis, MO USA

Donny, just to comment on a bright switch. In the typical amp situation, a bright switch, when off, is totally bypassed from the circuit. When it is "on" it works as a bypass to allow highs pass thru across the gain pot. I get that you are explaining how tube amp EQ's are generally passive and subtractive, but the bright switch in particular does not work by cutting highs. Now in an active circuit like in a modern Peavey, those EQ controls are active and actually are using amplifier and filter circuits to boost or cut particular frequencies. I think the point Keith was making was that once the overtones are lost, to try and regain them with an equalizer will not truly replace the lost sound. Not to mention the unpleasant artifacts of noise and phase distortion you "create" by heavy EQ use.

Brad

Jim Phelps · From: Mexico City, Mexico

I agree that if the pickup (and guitar, they work together you know) haven't got the tone, especially that treble sparkle, you're not going to get it back with the amp or EQ. Try and make a Les Paul sound like a Tele and find out.

There is another very BIG reason that todays instruments don't have the sparkle that the instruments in the "old days" had, that I discovered about 15 years ago. I'm still waiting for the experts to say anything about it, in all that time I still haven't seen a thing and it truly amazes me.

[This message was edited by Jim Phelps on 06 October 2005 at 03:50 PM.]

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