Why the preference for the 1-Megohm input Z?

[earthtonesaudio]

So I've been reading an old R. Penfold book, "Audio Amplifier Projects," and the one for the guitar preamp (TL071 clean opamp) has its input Z set at around 56k. 

But, most stompbox lore says a 1M input impedance (like a tube amp) is ideal for guitar. 

I know this has something to do with pickup output impedance, so obviously it varies from guitar to guitar.  But I don't quite get it.  Obviously effects with lower input impedances exist (Fuzz Face comes to mind) and work with some limitations, but what's so special about 1M?

[R.G.]

I've typed it in several times before. I guess once more won't hurt. Maybe the other pedal sales businesses can use the information too. It's not lore. It's not a preference. It's a design compromise based on conflicting electronic requirements.

You have to understand what a guitar looks like electrically. A pickup is a coil of wire around magnetic materials. That's an inductor. A typical pickup will have 4K to 10K of resistance in the many turns of fine wire, and two to six henries of inductance. Whatever signal the strings make in the coils appears in series with the resistance and inductance electrically. So the source impedance of the guitar rises with increasing frequency. At something like 7kHz, the self capacitance of pickups shut down further signal.

At 7kHz, a 4H inductor has an impedance of Xl = 2*pi*F*L = 6.28*7000*4 = 175k. To have any chance at reproducing both the bass and treble ends of the signal relatively equally, the input impedance the guitar sees must be more than ten times the guitar's source impedance. On the face of it, it needs 1.75M; more would be better. Zachary Vex is a tub thumper for even more, 5-10M. These higher values do - under the right circumstances - produce sparklier treble.

One conflicting factor is noise. All resistors make noise as a result of being at temperatures above absolute zero. The thermal noise of a resistor is proportional to its absolute temperature and its resistance. Bigger resistance, bigger noise.

A second conflicting factor is capacitor pulldown. The point in having many of those 1Ms is to pull the capacitor leakage below insignificance to prevent popping. The smaller the resistor the better.

It's a compromise. I think the deciding factor is that guitars sound like we expect them to when they drive the 1M the guys used in the "golden age". That value was set then because of cable and capacitive requirements in tube amps in general, not just guitar amp. It carried over.

If you lower the input impedance of an effect or an amp, it *will* bleed off some treble. The Fuzz Face does this naturally, and it helps in keeping it from being too shrill - see the "technology of..." articles if you haven't already. That may be good, bad, or indifferent, depending on your taste.

[soulsonic]

Bootstrap circuits are an interesting way to get really high impedance without having resort to using big noisy resistors. I plan on experimenting with those kind of circuits soon - ones with lots of feedback to control the gain.

[grolschie]

Thanks for that great insight R.G. That post really needs it's own page somewhere. 

[ClinchFX]

R.G., can you comment on the following please?  I respect your expertise and am just interested in an educated critique of my thoughts.

Many circuits I've seen have a 1M input resistor to ground followed by a series capacitor and then another resistor connected to bias voltage, and then the input of the Op-Amp.  The input impedance will then be the parallel combination of the 1M input resistor, the bias voltage resistor and the input impedance of the Op-Amp.  Obviously, if the Op-Amp is FET input, its input impedance will be irrelevant.  If the resistor to V bias is 1M, then the input impedance will be 500K at best.  It appears that most folks in the effects business look only at the input resistor and ignore any other components that contribute to input impedance.  I've seen very few schematics where the true input impedance is more than 500K - often less, particularly when a 741 is used.

Way back during my technician-in-training days (1967-1970), one of the subjects was transmission line theory, where we were taught that load impedance should be equal to source impedance.  In the years since, and in further recent studies, this rule has held up well.  As I see it, a pickup has very non-linear frequency response, and a load mismatch helps compensate for this response.  Wouldn't it be technically more correct to design for a matching impedance and use filtering to shape the response?  One of the potential issues I can see with high load (pedal input) impedance is that the effect of cable capacitance between the guitar and pedal will be more significant than it would with a load impedance that matches pickup impedance.

I look forward to hearing your comments.

Peter.

[gez]

So I've been reading an old R. Penfold book, "Audio Amplifier Projects," and the one for the guitar preamp (TL071 clean opamp) has its input Z set at around 56k. 

Most Penfold projects for guitar have only moderate input impedance.  A lot of his earlier projects weren't true bypass; they either used simple switching where the signal is always connected to the input (always loaded, even when bypassed), or the effect was always meant to be on.  In the case of filters, a switch would sweep it so high that it sounds like bypass.  Either way, he wouldn't have noticed a drop in volume between "bypass" and effect.  Liikewise, any loss of high end would be compensated for elsewhere in the chain.   In short, he probably didn't know it was a problem.

[R.G.]

Many circuits I've seen have a 1M input resistor to ground followed by a series capacitor and then another resistor connected to bias voltage, and then the input of the Op-Amp.  The input impedance will then be the parallel combination of the 1M input resistor, the bias voltage resistor and the input impedance of the Op-Amp.  Obviously, if the Op-Amp is FET input, its input impedance will be irrelevant.  If the resistor to V bias is 1M, then the input impedance will be 500K at best.  It appears that most folks in the effects business look only at the input resistor and ignore any other components that contribute to input impedance.  I've seen very few schematics where the true input impedance is more than 500K - often less, particularly when a 741 is used.

You are absolutely correct.

Another common mistake is putting a 1M pulldown resistor on the input or output jack itself. A little thought shows that with a few pedals you can be below 100K with the parallel combination even when every effect is bypassed.

Way back during my technician-in-training days (1967-1970), one of the subjects was transmission line theory, where we were taught that load impedance should be equal to source impedance.  In the years since, and in further recent studies, this rule has held up well.  As I see it, a pickup has very non-linear frequency response, and a load mismatch helps compensate for this response.  Wouldn't it be technically more correct to design for a matching impedance and use filtering to shape the response?  One of the potential issues I can see with high load (pedal input) impedance is that the effect of cable capacitance between the guitar and pedal will be more significant than it would with a load impedance that matches pickup impedance.

Matched source and load impedances are the way to get maximum power transfer. For RF transmission lines, that is the ideal case, as matched impedances also keep the transmission line from having standing waves and reflections.

In general, that's not what we want for audio signals. We want maximum voltage transfer, not power transfer. For maximum voltage transfer, the load impedance should be at least ten times the source impedance. If we used current, not voltage amplifiers, we would want input impedances less than one-tenths of the source impedance to suck in every last bit of current the source could put out. But for the voltage amplifiers we use, the input impedance acts like a voltage divider. A matched input impedance immediately loses half the input signal to loading just by ohm's law.

Another tough issue is what impedance would you match? A single coil is very different from a humbucker, and they both vary with volume and tone control settings. And then there's that inductive nature of the pickup impedance. To match the impedance of the pickup, you'd have to make your inputs also be inductive, rising from maybe 5-10K at DC to 100K-200K at 7kHz just at a guess. That's a tough job; we're lucky we don't have to do that.

In short, he probably didn't know it was a problem.

There's a lot of that going around... 8-)

[Processaurus]

Interesting discussion, apologies if anyone's mentioned it here already, but an often overlooked factor in guitar impedance/loading of the pickups is the volume pot on the guitar, which electronically looks exactly the same as a resistor to ground to the pickups, identical to the impedance setting resistors of concern in the circuit.  That's why 5M input preamp/boosters like the SHO and mosfet boost seem a little silly, as the impedance of the volume control is probably 1/10 that (500K).  You'd have to take the pot out of the guitar (or put the high impedance preamp in the guitar, before the volume pot) to get true high impedance.

[earthtonesaudio]

Thanks R.G. and everyone for thoughtful answers.  Maybe this should go in the F.A.Q. if it's not already.

Interesting discussion, apologies if anyone's mentioned it here already, but an often overlooked factor in guitar impedance/loading of the pickups is the volume pot on the guitar, which electronically looks exactly the same as a resistor to ground to the pickups, identical to the impedance setting resistors of concern in the circuit.  That's why 5M input preamp/boosters like the SHO and mosfet boost seem a little silly, as the impedance of the volume control is probably 1/10 that (500K).  You'd have to take the pot out of the guitar (or put the high impedance preamp in the guitar, before the volume pot) to get true high impedance.

...Perhaps those of us with volume knobs in our guitars are not hearing the full potential of those high Z input boosters!  But I can't justify amputating the volume pot from any of my guitars.  I like fuzz faces!

[soulsonic]

Oh boy, I'm seeing a really good argument from buffered bypass systems!

Think about it: you have a nice hi-Z buffer at the beginning of the chain that doesn't require any extra pull-down junk on the input because it's hardwired and doesn't need a bypass switch. Following that is the usual signal chain, but instead of everything having to use all these big resistors try attempt and keep a high impedance despite all the parallel loading, we can just be smart and use smaller cap pull-downs, etc, etc... and be able to realize a much cleaner, more managed, signal chain.

[ClinchFX]

Thanks R.G. and everyone else for a sensible and informative discussion.  When I raised these questions last year on another forum, I was flamed by a member who uses aggression to compensate for his lack of knowledge

Soulsonic, I'm beginning to see a lot of sense in buffered systems.

This is a pleasant forum and I'm proud to be a member.  There is a wealth of real knowledge here, and if I don't post here very often, it's because I rarely have anything to add.

Peter.

[JimRayden]

Oh boy, I'm seeing a really good argument from buffered bypass systems!

Think about it: you have a nice hi-Z buffer at the beginning of the chain that doesn't require any extra pull-down junk on the input because it's hardwired and doesn't need a bypass switch. Following that is the usual signal chain, but instead of everything having to use all these big resistors try attempt and keep a high impedance despite all the parallel loading, we can just be smart and use smaller cap pull-downs, etc, etc... and be able to realize a much cleaner, more managed, signal chain.

And since this buffer would be in the beginning of every pedal chain you use anyway, it would be smart to build it into a guitar, which will consequently get rid of cable capacitance problem also. Which in turn brings us to active pickups...

---------
Jimbo

[soulsonic]

Yeah, I'm really starting to think that putting a simple good buffer at the beginning of the chain makes alot of sense. People complain that buffers break the interaction between the guitar's volume knob and the input of the effects and that this screws up some of the good mojo you can get from rolling back the volume with certain pedals - BUT, I believe that you can probably solve this problem by using a simple passive Volume pedal after the buffer. Haven't tried it yet, but I bet it would do the trick. If the buffer was mounted in a guitar, you could just put the guitar's volume control after the buffer.

BTW, great looking pedals, Peter! Where do you find those beautiful fancy footswitches?

[ClinchFX]

Yeah, I'm really starting to think that putting a simple good buffer at the beginning of the chain makes alot of sense. People complain that buffers break the interaction between the guitar's volume knob and the input of the effects and that this screws up some of the good mojo you can get from rolling back the volume with certain pedals - BUT, I believe that you can probably solve this problem by using a simple passive Volume pedal after the buffer. Haven't tried it yet, but I bet it would do the trick. If the buffer was mounted in a guitar, you could just put the guitar's volume control after the buffer.

BTW, great looking pedals, Peter! Where do you find those beautiful fancy footswitches?

Thanks for the compliment.  I think I somewhat over-engineered the Blue and Mosfet Classic box.   I have the metal laser cut and do all the folding and punching myself in a press I made using a car steering rack for mechanical advantage.  The pedals really are my own designs - not versions of other established designs with changed component values, although the EP-PRE is pretty much the input preamp from the tape echo, with a switched capacitor power supply.

Ahh, the switches - what a marathon.  I have a supplier here in Aus who is agent for a Taiwan switch manufacturer.  The switch was in the catalogue, but I doubt that they had ever manufactured it in the "stomp" plunger configuration.  Switches in the first batch I received were mechanically unsatisfactory, so I re-designed the plunger and toggle pin and requested that they manufacture them to my spec.  After months of back-and-forth with drawings and samples, I finally had a reliable switch.  BTW, I specified gold contacts because it's been my experience that the majority of failures in other switches are contact failures because, in a pedal, there's not enough current for "contact wetting" in other types of contact material.  The down side is that, with gold contacts, maximum switching VA is very limited.  So far, I haven't had any failures with new switches and no reported failures from customers.  The switches don't come with plastic washers.  I buy the black nylon washers separately.

I think pedals like the Fuzz Face make use of the varying impedance of pickups that R.G talked about.  Rather than a volume pedal, I think it would be necessary to use pickup simulation as suggested by Jack Orman to get close to an unbuffered sound.

Peter.

[ashcat_lt]

The following is quoted from another forum, where I  post a lot more than this one.  The original thread was re: DI boxes and how/why they don't cause loading issues.  I never really got a satisfactory answer on that, but I think the graphs here fit well with the topic of this thread.

ash - this is both interesting and easy to test using my 5spice model of a PAF pickup with 500k pots:



C1 represents the capacitance of about a 10' cord and the input of the amp, while R3 is the amps input impedance, nominally 1000k for starters.

The graphs show the output frequency response as the amp input impedance R3 is reduced, first from 1000k to 100k:



and then from 100k down to 10k:



It would seem, that with these values, the first graph is showing mainly a damping of the resonant peak, then as input impedance is reduced further, it is cutting deeper into the treble as the resistance load interacts with the pup inductance, acting as a low pass filter.

John
 

[JimRayden]

great graphs, thanks.

---------
Jimbo

[ClinchFX]

The following is quoted from another forum, where I  post a lot more than this one.  The original thread was re: DI boxes and how/why they don't cause loading issues.  I never really got a satisfactory answer on that, but I think the graphs here fit well with the topic of this thread.

Yeah ash, you didn't really get any definitive answers, and I'm not sure I can do any better.  Assuming that the pickup model is correct for a PAF, a 200K load seems to reduce the resonant peak by only about 3db, just enough for the ear to discern.

I expect that the pickup model doesn't take into account the fact that the response of a guitar is bass heavy.  As I see it, (and this is only my take - I hope someone else will join in) the heavier strings that produce the lower notes will create much more pickup output than the lighter strings that make the higher notes.  As an electronics person, I admit that it's all too easy to concentrate on the electronic principles of the pickup model and forget about the guitar.

The transformer in a passive DI box converts impedance, voltage and current, which are all inter-related.  A pickup, being high impedance, generates some voltage and very little current.  The transformer in the DI box converts high impedance in the primary winding to low impedance in the secondary.  The relationships of impedance, voltage and current dictate that the secondary voltage will be lower than the primary voltage and the secondary current will be higher than the primary current.  The result is that the primary of the transformer in the DI can present a high impedance to the guitar while the secondary of the transformer can present a low impedance to the input of a mixing console etc.  The transformer primary is in parallel with any pedal or amp that the guitar is feeding, and will lower the impedance of the total load on the pickup.  I doubt, however, that many DIs live up to the claim of 1M or higher input impedance, unless the console has a fairly high input impedance.

For a year or two, I was resident sound guy in a club.  During that time, I dealt with only one guitar player who used a DI straight into the console.  I didn't take a lot of notice, because he insisted on setting up the channel EQ himself, but this band sounded fantastic.

Peter.

[brett]

Nice graphs Ashcat.
Given that the peak is in a fairly obnoxious place in musical terms, the "flat" and nearly "flat" curves at 100k and 200k look better to me than the peaky graphs for higher Zin. 
Is it a coincidence that several of Penfold's designs have input impedance of 220k in the case of resistors or 250k in the case of pots?  I doubt it.
Why did Jimi use those curly leads?  Because they roll off more highs?
What is the "right" balance between lows and highs, anyway? 
Every amp has a unique frequency response, and tone stacks aren't designed to be anywhere near "flat".

[frank_p]

Brett, sorry, I feel "relatively" mean tonight,
(I really mean relatively because I understand how someone think he must be "right" in that way of thingking).

Jimmy was more interested in playing, I think.
And Mao is a dictator (this, I don't think, it's a reality).
Sorry again, but please change your signature,
Unless you endorse his way of life (not his idea of a valuable world, which I would in "theory" agree to some point).

This is a friendly post.
And a way to avoid more troublesome response.
What he preached was not what happened...
Hope that some young Chineses don't read your endorsements. 
Yours.
F.H.P. 

[Ben N]

I believe that you can probably solve this problem by using a simple passive Volume pedal after the buffer. Haven't tried it yet, but I bet it would do the trick. If the buffer was mounted in a guitar, you could just put the guitar's volume control after the buffer.

Which brings us to: http://dozin.com/jers/guitars/tiger/schematic.html