Pickup Tone and Impedance Loading

Brad Sarno · Post by **Brad Sarno** » 8 Oct 2005 7:59 am

Hey Dave, thanks for that correction on the LTD & Session 400 impedances. It's those darn transistor circuits threw me off when reading the input. There's a 1Meg to ground, but also a 470k to the B+ and I guess that's where we get 330k. It does look like with the way the input jacks are configured that the input #2 gives more like 44kOhms.

The Peavey inputs are cap blocked so it's not so simple to just read the load with a meter. You may just get the value of the volume pedal instead.

Also consider how these load values change with the volume pedal position. At full volume pedal, you actually get the lowest load value because the wiper at full lets the secondary load reflect entirely in parallel with the pedal load. With the wiper in a lower position, you have to incorporate the pot resistance in any given position other than full, thus the load value rises as the volume pedal goes down. It's more dramatic with the amps that have lower input Z's, like the Nashvilles. So that theory would say that the tonal response would actually get a big brighter as you back down the volume pedal.

I think I need one of those equalizers that'll make me sound like Buddy.

Brad

ed packard · Post by **ed packard** » 8 Oct 2005 8:09 am

Vern: I don't have to ask how you know that... looks like you have been there. My first pickups were made using a hand drill (motorized), clamped in a bench vise, speed controlled with a variac (soft start essential), and turns counted with one of those mechanical lever operated devices. Controling tension was fun.

This thread has been fun. Thanks Brad for starting it. Gave me something to do while sitting on my back porch here in the mountains of AZ and watching the leaves turn while sopping up sunshine. Typical rockin'chair, shotgun, hound dog, and jug place!

We have done everything except whip out the integral signs for a duel re the mag fields of the pickup...probably because we are partially differentiated...pun intended.

Dave M: Looks like you have collected the I/O info re a bunch of gear...care to compile and publish it on the Forum for general consumption and reference?

Jay: One further comment on the Ohm Meter thing...if there is a semiconductor junction in the loop (diode/transistor) your Ohm Meter may also give a false reading. You can tell this if you get different values by reversing the leads.

Post by **Dave Mudgett** » 8 Oct 2005 8:56 am

Brad - yes, the solid-state circuits are different. Transistors are not naturally high-impedance like a tube, so at AC, one needs to consider the overall transistor circuit. They're usually capacitively coupled, and if there are any nonlinear devices in the input circuit, Ed's caveat is important.

Ed, I guess I may have collected a bunch of manuals. Let me root around and see what I can come up with. But that list above covers a lot of the amps steel players actually use - Session/LTD 400, 500, NV 400 and offshoots. Just looked at specs for a NV 112:

High gain input: 220K
Low gain input (-10dB pad): 68K
Pre-EQ Patch return: 220K
Post-EQ Patch return: 1 M

I haven't looked, but I assume most of the modern Peavey steel amps are in the same ballpark. The white, brown, blackface, and silverface Fenders are generally 1M input 1, 136K input 2. Earlier tweed amps vary quite a bit. One needs to look at the circuit. The Fender Field Guide at http://www.ampwares.com/ffg/ has those for a wide variety of old Fender amps.

I think the point here is that, if someone isn't getting enough high-end, it may be worth finding the input impedance and consider using a buffer like the Black Box or some other kind of High-Zin / Low-Zout preamp or effects unit.[This message was edited by Dave Mudgett on 08 October 2005 at 09:57 AM.]

Bob Kagy · Post by **Bob Kagy** » 8 Oct 2005 9:41 am

Great thread.

In this context, I've got a setup that results in something I don't understand. Here's what it is:

I run a 99 Carter D10 w/GeoL TPPP p/u's straight into a SGBB, then to a Hilton VP, then to a Webb; all Geo L cables.

If I remove the SGBB, I get way, way fewer highs. With the SGBB in the chain I get worlds more.

If I understand the main points raised here, that shouldn't happen unless the output impedance of the SGBB is not what the Hilton VP is expecting. Also, the tone trim pot in the VP is unable to compensate even at its extreme ccw position.

Any comments anybody?

Thanks, BK

Michael Barone · Post by **Michael Barone** » 8 Oct 2005 10:00 am

I think the point here is that, if someone isn't getting enough high-end, it may be worth finding the input impedance and consider using a buffer like the Black Box or some other kind of High-Zin / Low-Zout preamp or effects unit.

I was attempting to offer a solution to this exact point with my earlier post, suggesting the use of impedance-matching resistor pads on both sides of an op-amp.

So the input chain would be: Pickup to "pi" pad to op-amp to "pi" pad to pre-amp.

A "Tee" configuration could be used instead of the "Pi".

The first pad would match the pickup to the op-amp. But all you might need here is just a coupling capacitor, since an op-amp can be made to show very high input impedance of 1M.

The second pad would match the op-amp output to the pre-amp. A relatively accurate calculation is important here to interface the low output impedance of an op-amp with an amp input impedance of 220K, for example, as Brad referenced originally.

The op-amp could provide gain to restore the pad loss.

Actually, the effects pedals mentioned perform some impedance matching close to this idea since most I think have op-amps doing the work with outboard parts, but a calculated pad could be made for an exact match. No?

I am not an engineer, just a tech. Just wondering if anyone else thinks like this.

Or am I wrong with this whole thing?

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

ed packard · Post by **ed packard** » 8 Oct 2005 11:22 am

MB: The output impedance of the pickup is n ot a constant value. It varies with the frequency it passes (vibrating string to coil output). At 0 Hz it would be the resistance of the winding(s). In the 3 to 6 KHz range (where the highs are lost)the Z is in the hundreds of K's hence the pot pedals tend to load the highs out. A single voltage follower op amp will do HiZ in/Loz out at a gain of one...no need to "impedance match", just not load, and be able to line drive.

Impedance matching is generally to achieve maximum power transfer and highest signal to noise as I recall. Our problem is more getting the most voltage out from a varying impedance source in the audio band, so lowZ out into HiZ in is the rule of the day...preserves signal level across the band. Re input Z, one meg is nice...ten meg is better. Re output Z, industry standards have been 600 ohms, 50 ohms, and now it looks like the 32 ohm (headphone) value is in.

Not too sure about the "pad(s)"issue but if you like the results, publish it. Most of us do not want what we get direct from the pickup (just want all we can get to start with)...then we turn around and tweak it to taste. What you don't have, you can't tweak.

Restoring lost signal level generally costs signal to noise ratio.

Jim Phelps · Post by **Jim Phelps** » 8 Oct 2005 12:57 pm

Forgive me if it's already been mentioned and I missed it, but it looks like these input-impedance measurements being discussed are regarding using pot pedals or Hilton pedals. What about Goodrich lightbeam pedals, done any measurements with that? I notice my old Goodrich saps a bit of the highs when playing through my old Gibson/Moog Lab Series L5 solid-state amp but not nearly as much as a pot pedal. Haven't done the a/b with the old Fender Bandmaster yet. I assume that any lightbeam pedal does buffer or isolate the pickup from the amps input circuit so there's no loading effects, but there is a distinct slight loss of highs with my Goodrich. I have no idea what the input impedance of a Lab Series is... maybe I can find it on the internet. I don't have a VTVM anymore...sold it, my oscilloscope, sig. gen., all of it, too much stuff to drag down here.[This message was edited by Jim Phelps on 08 October 2005 at 02:02 PM.]

Michael Barone · Post by **Michael Barone** » 8 Oct 2005 3:11 pm

Ed, thanks for your explanation. You have made the picture clear to me regarding the objective here, which involves a variable impedance source.

Upon retrospect, I probably should have concluded this after reading posts by Dave & Brad also.

Thanks to Brad for starting this great, educational thread.

Mike

ed packard · Post by **ed packard** » 8 Oct 2005 3:43 pm

MB: glad to be of assistance.
JP: I would not assume that ANY lightbeam pedal isolates with a high enough impedance...I compared the pickup output spectrum with both the Goodrich pot, and the Goodrich LDR, both without the output to amp connection. They both seemed to load about the same.

Not sure what the circuit is inside. They may try to restore some highs via the pots on the bottom. The output Z appears to be about 100 Ohms as I recall, so line driving S/B good.

My old tek 453 died a couple of years ago...high voltage diode problem in the display...have not found a replacement. The input stages still work as an amplifier so probes and chan 1 out can be used as a buffer for measurements. The computer pgm I have is both an FSA and an o'scope with swept freq in sine, ramp, and square. Much more convenient. Worst problem with the computer is the line in Z. Solution is to buffer with the voltage follower circuit.

The basic program is free (low resolution = 1/3 octave)...$99 for the souped up version.

Brad Sarno · Post by **Brad Sarno** » 10 Oct 2005 1:52 pm

Cool discovery. I just opened up my Rick Bakelite 6 lap steel and noticed that they have the volume control set up not as a volume control, but as a variable load. Only 2 of the pot's legs are used. That explains why I found myself using it as a tone control too. This guitar has a tone control as well. The "volume" control acts like a tone control until you get near silent and then it quickly quiets down to off. It really helps tailor the tone of that Rick pickup. It's cool to compare how the tone control has a totally different effect on tone than the "volume" control does.

Brad

Jim Peters · Post by **Jim Peters** » 10 Oct 2005 3:58 pm

I have a VTVM for sale if anyone is interested(under amps and accesories). JP

Brad Sarno · Post by **Brad Sarno** » 10 Oct 2005 4:45 pm

Oh Yea, Jim. I want it. I'll give you a call.

Brad

David L. Donald · Post by **David L. Donald** » 20 Oct 2005 10:39 pm

Great discusion.

Brad I just emailed you twice and it got returned.

Time for a BB in SE asia.
!

Dang that Revalation Preamp looks SOOO cool!
Both electronic design wise and the esthetics.
Love those knobs.

Pete Burak · Post by **Pete Burak** » 27 Dec 2005 11:58 am

"It's such a simple thing I think it's just a case of overlooking the obvious."

So did we ever find out what Jim is referring too?
Do tell!

Chris Erbacher · Post by **Chris Erbacher** » 27 Dec 2005 12:59 pm

i'm curious as to what jim is referring to. jim, we won't tell anybody if you tell us...

Donny Hinson · Post by **Donny Hinson** » 27 Dec 2005 1:08 pm

Before this thread dies, can anyone out there give me a good reason why every instrument amp made doesn't have an input impedance of at least 1 megohm? (It seems like such a small thing to ask for.)

Ken Prutzman · Post by **Ken Prutzman** » 27 Dec 2005 1:47 pm

Does the cords used make any difference?

Kenny

Pete Burak · Post by **Pete Burak** » 27 Dec 2005 4:42 pm

O.K. Let's see if we can finger this one out.

Jim writes:
"Although I haven't had the chance to examine a lot of new steels, my guess is you wouldn't find it there, so steels are probably OK as far as the thing I refer to."

So we know it doesn't have to do with steel tone, past vs. present.

Other clues?...

"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."

Are we talking electric guitars now?

"Another reason is I don't want to encourage people to make any kind of modification that they may botch up,..."

"That's because what I'm talking about is so simple anyone should be able to see it. The manufacturers surely know but don't care."

"It's such a simple thing I think it's just a case of overlooking the obvious."

"Everyone with a little electronic knowledge would know the electronic theory concerning this, but maybe no one is paying attention to some basic areas of application."

"Don't let all the knowledge of impedance, reactance, frequency, etc. cause you to overlook the simple things."

I dunno?... I remember a friend had an old Les Paul "Studio" with low impedance pickups that sounded pretty sweet... Were all guitars back then Low Impedance?

Don Benoit · Post by **Don Benoit** » 27 Dec 2005 5:04 pm

I'm surprised that no one has commented as to how the natural resonance of the guitar body itself affects the tone and overall frequency response of the whole setup. IMHO the mechanical resonance of the guitar will affects the vibrations of the strings and this will be transferred to the pickup. The more mass a guitar has, the lower the natural resonant frequency of the guitar. Could this be the reason why the old heavy push pull Emmons and Sho-Buds steel guitars have a better tone and growl than the new lightweight steel guitars?

------------------
http://www.steelguitar.ca

Chris Erbacher · Post by **Chris Erbacher** » 27 Dec 2005 10:22 pm

my guesses: plastics? all copper wires with good connectors? really nice and heavy metal bridges? dense wood for guitar bodies? loving the notes you are playing? to me these things are obvious, maybe i'm missing the really obvious??? (black sounds the best) any ideas fellers? i'm not sure if you guys have heard of steve kimock or not, but he is a really heavy tone freak guitarist (i've heard he used to play pedal steel,now he plays the non-pedal variety). he is one of the guys partly responsible for the knowledge that led up to the creation of two-rock amplifiers, really really sweet and pure sounding (they come with quite a bit of sticker shock too). according to him things like an all aluminum chassis will affect the tone (for the better). that is not so obvious, but i thought i would share it. if you are interested in what he has to say, look in last month's guitar player magazine for an interview, or go to his website to the gear section, pretty interesting read. maybe it is so obvious that it is understood...

Donny Hinson · Post by **Donny Hinson** » 28 Dec 2005 12:06 pm

Does the cords used make any difference?

Oh yes, especially with modern, high-impedance and humbucking pickups!

The standard rule is to keep cords as short as possible for the cleanest sound. Nothing robs your "sparkle" like 20 feet or more of coax cable, no matter what brand you're using.

Lonnie Portwood · Post by **Lonnie Portwood** » 28 Dec 2005 8:44 pm

You ask about "cords"? About two years ago I set up my Fulawka s-12 in the Fulawka room and was getting an awful muddy tone through my Fulawka Vol pedal to my dd-6 rev/delay. to my Sho-bud Christmas-tree amp. Ed Fulawka heard me and came to the bandstand, pulled out my George-L 3' cord and replaced it with his own cord which he makes up.My sound came alive. Ed then replaced the second George-L to the Boss pedal with another "marked" improvement. Then he replaced the third cord, which I think was a Fender, and I could not believe the overall improvement and the warm punchy sound that resulted. Now somebody "s'plain dat!!! Lonnie P.

Jim Phelps · Post by **Jim Phelps** » 28 Dec 2005 9:13 pm

Now somebody "s'plain dat!!!

Really simple. The majority of modern guitar cords are loaded with capacitance. It's like turning your tone control about 1/3 down, more with more cords. The cord from your guitar to your effects, a big tone-sucking capacitor; the cord connecting your effects, eq, etc., another cap and the cord going from the last effect into your amp, another one. Attempting to make the nice flexible cables we all like and making them cheap as possible is likely the cause of the really low-quality of cables. Higher quality coax usually isn't as flexible but has less capacitance and a clearer sound.

Post by **Dave Mudgett** » 29 Dec 2005 3:17 am

I'm surprised that no one has commented as to how the natural resonance of the guitar body itself affects the tone and overall frequency response of the whole setup.

Of course, we have, but not in Electronics:
http://steelguitarforum.com/Forum10/HTML/200939.html

Before this thread dies, can anyone out there give me a good reason why every instrument amp made doesn't have an input impedance of at least 1 megohm? (It seems like such a small thing to ask for.)

Tubes are naturally high-impedance, transistors are not. Op-amp feedback resistors complicate this some - there are design tradeoffs. Check one more advantage to tubes, IMO.

But let me say that for some things, a lower impedance may be just the ticket. For example, I like the way a jazz guitar sounds into a lower-impedance solid-state amp.

Personally, I like what Brad's now doing with a variable impedance control on the front end of his SG Black Box and the new Revelation preamp. To me, this is a different type of "Presence" control. A typical presence control varies the amount of feedback in the power amp section of a tube amp, which changes the linearity, gain, and bandwidth of the amp. This input impedance control is different, but I expect it will be useful. I guess I'm gonna have to have one now.

Ken Fox · Post by **Ken Fox** » 18 Apr 2006 3:18 pm

Just a note on the Nashville 400 input impedances. With the mod kit R3 and R10 are changed from 220K to 100K. That is at main input and the pre eq input.

I just noticed that today looking at the Nashville 400 mod kit instructions and schematic.

I have a 1983 LTD400 with the 1502-4 speaker. The amp sounds great. I am changing the Opamps to Burr Brown chips. I am reluctant to to change R3 and R10, as I think it may alter the frquency response of the amp too much. If the amp had a 1501-4SB I would do the complete mod, as more bass is needed.

Any thoughts apprectiated!