impedance matching devices

Jerome Hawkes · From: Fayetteville, North Carolina, USA

basic question here. i have read how important it is to have your pickups impedance matched with the pre-amp and i know thats one reason people use devices like the Black Box with Vari-Z load matching.

since i have never used one and have started doing some line direct recording, i'm wondering just what you are listening for when you do this? is this something where you just use your ears to listen to certain fundamentals or what? In other words, how do i know if i'm actually matching things up or doing more damage. also, Is this similar to/ or used as a DI interface?
thanks
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'65 Sho-Bud D-10 Permanent • '54 Fender Dual-8 • Clinesmith T-8 • '38 Ric Bakelite • '92 Emmons D-10 Legrande II

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

There's no reason to 'match' impedances between a pickup and a voltage amplifier section. The notion of 'impedance matching' is for power transfer - under certain idealized assumptions, maximum power transfer occurs when the source and load impedances are complex conjugates of each other. For a purely resistive load, that means the resistances are equal. Perfect impedance matching is never practically achieved, but closer is better from the stand point of maximum power transfer. Practically speaking, if you're matched more closely across the program frequency band, you get better power transfer. This has nothing to do with preamp buffers or anything like hat.

In voltage amplifiers like a preamp amplifying a voltage source like a pickup, the issue of impedance relations is that the source and input impedances act as a voltage divider, and at any given frequency, the signal level 'seen' by the preamp is attenuated by the ratio of the source impedance divided by the total impedance (source + input impedance). This impedance ratio tends to vary by frequency - my experience is that it usually affects higher frequencies more than lower, and the result of a relatively low input impedance is a rolloff of high frequencies into the preamp. So increasing the input impedance of the preamp reduces this effect and tends to increase the brilliance (and to some extent, overall level) of the signal seen by the preamp. That's the basic principle behind a high (and sometimes variable) impedance buffer or DI. The function of a maximum signal-voltage coupling device is to present a high input impedance to the source and low output impedance to the next stage.

I personally use a variable impedance matching device such as Brad Sarno's Freeloader. His Revelation Preamp has the same type of control in tube form, and some of his Black Boxes have it (my older one doesn't). They sound great, but I confess that I really like the Freeloader - it's small, inexpensive, hooks to the leg of my steel, sounds great, to my tastes, and a 9V batter lasts a long time. There are times when I want maximum brilliance, and for that I crank up the input impedance. But sometimes I find that lowering the input impedance works better for certain things. Once I set my amp EQ for a gig or session, I rarely need to touch it. The variable-Z is a surprisingly versatile and useful control.

My take.

Craig Baker · From: Eatonton, Georgia, USA - R.I.P.

Hi Jerome,
At the risk of answering questions that no one is asking. . . I'll jump in. In practical terms, it's impossible to match a pickup's. I say this because a pickup's impedance is constantly changing in relation to the note being played. Now, here's the good news. There is no need to "match" a pickup's impedance, since we are only concerned with transferring voltage from the guitar to the amplifier. No current is involved, therefore no power is transferred. Though it's not necessary or possible to "match" the impedance, there are a couple of things to keep in mind. The last thing you want the pickup to see is the capacitance of a guitar cord, OR too much of a resistive load "swamping" the signal.

A resistive overload might come from the wrong pot in a volume pedal, 50,000 ohms instead of 500,000 ohms for example. While this would be very unlikely, capacitance loading (great tone's worst enemy) happens every day and comes from even a short length of the very best guitar cords. This is what really kills the overtones that make a guitar sound so pleasing. Usually, when someone test-drives a buffer, they don't ever want to play without one. When you buy a steel guitar, you pay a lot for beautiful tone. By investing in a Black Box, Freeloader, Matchbox or Li'l Izzy you'll finally get your money's worth and hear all of that tone you paid for.

Best regards,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024
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Tom Gorr · From: Three Hills, Alberta

Lane Gray · From: Topeka, KS

What Dave and Craig said.
The only place where I have to match loads is my CB radio (or I guess speakers to amps). There, when you have a mismatch, some power gets reflected back to the output, which has to absorb it. Since pickups current is really low (approximately the cube root of diddly squat), a pickup can eat its own output.
Now as to your question on where to set the vari-Z: since a mismatch only effects tone, you turn it to where it sounds good.
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2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects

Jerome Hawkes · From: Fayetteville, North Carolina, USA

thanks guys for the info - although you've blown my brain up a bit Dave, haha - just trying to get my head around it. so there isn't a "matched" value you are trying to achieve, the Vari-Z is used for whatever sound you want.

i was thinking it had something to do with fixing the loss from running a steel thru a volume pedal - seems i've read the newer pedals have these buffers built in??
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'65 Sho-Bud D-10 Permanent • '54 Fender Dual-8 • Clinesmith T-8 • '38 Ric Bakelite • '92 Emmons D-10 Legrande II

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

One issue with a passive volume pedal is that the effective input impedance goes down as you lower the volume - and thus you lose brilliance. Now, that doesn't necessarily bother me - in fact, I often find it useful to soften the sound for backup playing. But I really like the Variable-Z approach - it puts that control at the tip of my fingers also.

Another issue with a passive volume pedal is that even the full-on impedance is lower than, let's say, the 1MOhm input impedance of the typical tube amp, let's say like an old Fender from the mid-late 50s onward. Most pedal steel volume pedals like old Sho Buds (what I prefer), passive Goodrich, and so on, use around 500KOhm pots. Guitar volume pedals like Ernie Ball use around 250K, which is fine for a Strat or Tele, but I find a bit low for pedal steel unbuffered (to my tastes, anyway).

Another point - most Peavey steel guitar amps have a significantly lower input impedance than those old tube amps - there was a thread a number of years ago that more or less got most of those numbers collected in one place. They were typically in the 300-500KOhm range, but note that the second (lower sensitivity) input on any of these amps (old tube Fenders included) are even lower, and sometimes it's useful to use the lower-sensitivity inputs.

A good buffer levels the playing field by presenting a consistent input impedance to the pickup, and presenting a low output impedance to the next stage, whatever that is - volume pedal, effects, amp preamp stage, whatever. With a buffer, even my old EB pedals (with the side jacks instead of the stupid jacks out front) that I used for guitar for years are now usable. Beyond this, being able to control that input impedance with a single knob attached to my steel is gravy, but damned useful.

Brad Sarno · From: St. Louis, MO USA

I think the term "matching" is a bit misleading because it can sound like it's dangerous to not do it. The basic principle is simply that if we place an electronic "buffer" near the pickup of a guitar...

[and by "buffer" I mean a device that electronically drives the signal but without gain or any change in loudness - unity gain]

... that buffered signal is then happy to drive long cable runs or thru numerous switches and jacks or things that can sometimes degrade the signal or suck high frequencies away.

In the case of these various buffers we use with pedal steel guitar, the idea of "matching" basically suggests that the input to the device is "high impedance" so that the pickup gives its full response, and the output of the device is "low impedance" which means it can drive miles of cable without losing tone. That is the "matching" factor.

Now in the case of the FreeLoader or Black Box or Telonics volume pedal, there is the additional control called "Vari-Z" or variable impedance. This is NOT a critical thing with regards to the matching. It's simply a certain type of tone control that is very effective with pedal steel pickups. This variable input impedance takes control over the treble peak in the pickup and lets one dial up a sharp and bright tone or mellow it out a bit for a fuller, darker sound.

But the short answer is that any of these buffers will do the "matching" thing. It's not something to sweat over, it's just a matter of taste. Some people prefer the tone of certain buffers. Some people prefer the sound of no buffer. The only rule is how it sounds to you. There's no electrical harm that can be done by not "matching" with a buffer.

B

Jerome Hawkes · From: Fayetteville, North Carolina, USA

thanks again for clarifying - got it now.

what brought this question on was trying to do some line direct recording. my steels, the old ones. all have volume controls on the steel. i will usually not run these wide open as i fell it muddies up the sound adding too much bass, so i'll back them off to around 7. then i run to a std goodrich volume pedal - of course, we never run these wide open but at 1/2 to 3/4 volume. so i am essentially running thru 2 volume pots that are attenuating the volume. on an amp, if its enough power, its no big deal, i don't play loud. but i've noticed going direct that i am really pushing the mic pre's up pretty high. my recorded tone is clean, but anemic.
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'65 Sho-Bud D-10 Permanent • '54 Fender Dual-8 • Clinesmith T-8 • '38 Ric Bakelite • '92 Emmons D-10 Legrande II

Jonathan Cullifer · From: Gallatin, TN

A little off topic, but even when driving speakers, impedance is not matched. It's usually more important to control the speaker than to maximize power transfer. The ratio between the output impedance of the amp and the speaker impedance is the damping factor (the merits of which are a whole different discussion).

For most things in a signal chain are lower impedance to higher impedance to maximize voltage transfer. See http://en.wikipedia.org/wiki/Impedance_bridging, it explains impedance bridging pretty well.

Jim Palenscar · From: Oceanside, Calif, USA

I'm a bit confused and would like some clarification of my previously held ideas. Ohm's law states that current is equal to voltage/resistance. If anything varies- so do the others- so isn't current involved in that the resistance is changing as a note is picked? And isn't the reason that a passive pot pedal changes tone throughout the sweep is that current is changing (along w resistance, etc) and that is why buffer amps and pedals like the Telonics minimize that effect due to minimizing the voltage/resistance/current change?

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

The simple form of Ohm's Law, Voltage = Current x Resistance, is true only for DC signals - a constant voltage/current. Audio signals vary with time, and can be represented as sums of sinusoidal signals of various frequencies, amplitudes, and relative phase relationships - the voltage and current for each of these frequencies is related through the complex impedance, which is generally different for each frequency. One needs to not only consider the amplitude relationship between voltage and current waveforms, but the phase relationship - it is much more complicated.

For example, if you add two DC signals of exactly the same amplitude, you always get double the signal. If you add two sinusoids of exactly the same amplitude and frequency but 180 degrees out of phase, you get a zero signal - they cancel out. Only if they're in phase does the sum amplitude double. Even passive elements like capacitance and inductance in a circuit change the phase relationships between current and voltage, and these must be considered in determining how signals pass through such a circuit.

The reason the tone changes as one sweeps through the range of a volume pedal's resistance is that the change in the load's input impedance affects different source voltage frequencies differently. The voltage source (the output of one or more pickups) has a complex impedance which includes (at least) resistance of the coil and associated wiring, inductance of the coil, and inter-winding capacitance, and these reactances (capacitive and inductive) can be quite large. As the load impedance changes, the relationship between the source and load can change in potentially complex ways.

I can't speak to the Telonics pedal because I don't know how it's designed. But a buffer simply presents a high input impedance to the source at all frequencies of interest, and a low output impedance to the next stage. A general rule of thumb is that if the input impedance of a load is at least 10x the output impedance of the source at all frequencies of interest, then the voltage will effectively transfer from source to load quite uniformly at all frequencies of interest. As the load impedance is reduced from this, then source frequencies with high source impedance will be cut more than frequencies with lower source impedance. Coils like in a pickup tend to have a resonant peak at midrange frequencies, and then at a certain frequency the inductive reactance starts to monotonically go up as frequency is increased. So in general, complex impedance just takes off at high frequencies. Thus high frequencies tend to get cut more than lower ones as the load impedance is lowered. What's happening is that the high frequency signal content (voltage) is getting dropped more across the source impedance because the ratio between the source impedance and total impedance (source + load) is higher at high frequencies.

Remember - the 'signal' being fed into an amp is a voltage level representing an audio waveform. The goal of pre-processing is to affect this voltage signal. The amp then preamplifies that input voltage enough to drive power tubes or power transistors enough to create a large current through the speaker load.

My take.

Bill Duncan · From: Lenoir, North Carolina, USA

So, is this why my old MSA D10 Classic sounds so good?

I thought it was my pickin'.
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Jim Palenscar · From: Oceanside, Calif, USA

Nice explanation- thanks Dave.

Erv Niehaus · From: Litchfield, MN, USA

I play with a Goodrich Match Box if for no other reason than to have a volume and tone control at my finger tips.

Georg Sørtun · From: Mandal, Agder, Norway

Having the volume and tone near by on my Goodrich Match Box, is one reason I always use one.
In addition: I prefer a 50K VP over 500K or active VPs, and also prefer the tone and low noise-floor I get when bypassing all input/eq stages in my amp - an NV112.

All in all, the "matching and level-balancing" I use is what I have found to make all my PSGs sound their best, although different. So, once the units are chained up I can focus entirely on playing these "things". Smile

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