How to figure out input impedance of a buffered circuit

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How to figure out input impedance of a buffered circuit

Postby JakeAC5253 » 15 Oct 2011, 04:11

How do you go about figuring out the total input impedance of a circuit which uses a buffered bypass? I have read that the input impedance of a buffered circuit is very high, but I am a bit confused having never been formally taught the characteristics of buffer amplifiers as a switching mechanism, and the answer may be entirely too simple. I am only concerned with the input impedance of the effected circuit when the pedal is in activated mode. I will attach a schematic so that you can have a direct reference, but this isn't the exact circuit that I am working with, parts in the input section are different.

Here is my confusion. There is a pulldown resistor before the buffer switch, and I assume that this is so that the full switched impedance does not over-load (not overload) the guitar which is driving it. I have downloaded the datasheet for the specific part, which according to the schematic is a 2SC732TM-GR, and I don't see anything quoted as input impedance, so it must be circuit dependent. How do I figure this out?

BOSS SD-1 Schematic.JPG
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Re: How to figure out input impedance of a buffered circuit

Postby mictester » 15 Oct 2011, 09:43

In this case, the base bias resistor largely determines the input impedance, so in this case it will be the 470k (plus the 10k series input resistor). The series capacitor will also add to the series resistance, but varies with frequency (look up the formula for capacitive reactance - which is the AC analogue of resistance). The series capacitor is normally chosen to have an insignificantly small reactance at the range of frequencies you want to pass, so can usually be ignored (except in a Rangemaster, where the slope of the treble boost is determined by the reactance of that capacitor).

The audio routing in the circuit you attach is achieved by the FETs - these are configured to behave as switches, and have no effect on the input impedance because they are separated from the outside world by the buffer transistors. As an aside - the 470k (nominal) input impedance on this pedal won't affect the tone of any guitar you connect to it (don't believe the "true bypass" hype!). In the bypassed mode, the pedal will actually help with the guitar frequency response by converting the high input impedance of the guitar to the low output impedance of the output buffer - the low output impedance will drive long cables without treble loss!
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 11:03

True bypass is not just "hype"
Here, the "bypassed" signal goes trough at least 2 transistors and one fet, 3 active components for a single pedal in "bypass mode".
with 5 pedals, it means at least 15 active components and a bunch of resistors and caps, it can't obviously be "transparent", considering the low quality of the transistors and caps.
Added to that, 470K is not ideal for a guitar signal if your cable is more then 10 feet long.
An all true bypass signal chain is silly, yes, but more than one buffer in the chain is sily too.
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Re: How to figure out input impedance of a buffered circuit

Postby mictester » 15 Oct 2011, 11:17

kleuck wrote:True bypass is not just "hype"
Here, the "bypassed" signal goes trough at least 2 transistors and one fet, 3 active components for a single pedal in "bypass mode".
with 5 pedals, it means at least 15 active components and a bunch of resistors and caps, it can't obviously be "transparent", considering the low quality of the transistors and caps.
Added to that, 470K is not ideal for a guitar signal if your cable is more then 10 feet long.
An all true bypass signal chain is silly, yes, but more than one buffer in the chain is sily too.


Just for the sake of amusement, get yourself (say) a dozen Boss pedals, a mechanical A/B switch (and a friend to operate it) and your favourite guitar amplifier. Put all the pedals in series, bypassed, for one path, and the guitar direct to the A/B switch for the other. If you don't know if you're listening to direct or through the pedals, you won't be able to tell the difference. Really - you won't - unless you're one of those golden-eared types who claim that leads are "directional" and that oxygen in cables "degrades" the audio passing through them!
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Re: How to figure out input impedance of a buffered circuit

Postby FiveseveN » 15 Oct 2011, 11:31

considering the low quality of the transistors and caps

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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 11:53

mictester wrote:
kleuck wrote:True bypass is not just "hype"
Here, the "bypassed" signal goes trough at least 2 transistors and one fet, 3 active components for a single pedal in "bypass mode".
with 5 pedals, it means at least 15 active components and a bunch of resistors and caps, it can't obviously be "transparent", considering the low quality of the transistors and caps.
Added to that, 470K is not ideal for a guitar signal if your cable is more then 10 feet long.
An all true bypass signal chain is silly, yes, but more than one buffer in the chain is sily too.


Just for the sake of amusement, get yourself (say) a dozen Boss pedals, a mechanical A/B switch (and a friend to operate it) and your favourite guitar amplifier. Put all the pedals in series, bypassed, for one path, and the guitar direct to the A/B switch for the other. If you don't know if you're listening to direct or through the pedals, you won't be able to tell the difference. Really - you won't - unless you're one of those golden-eared types who claim that leads are "directional" and that oxygen in cables "degrades" the audio passing through them!

Sorry, i did exactly that (5 pedals) and the difference was obvious, though i'm not at all one of the people you're talking about haha ! (Cheap patches, good 5 meters cable from guitar to pedals/switch and an average 1,5 meter cable between pedals/switch and amp)
I compared too a few cables, with and without a buffer, even a crappy old 12 meter one etc.
I don't talk when i do not have tested myself.
FiveseveN wrote:
considering the low quality of the transistors and caps

Are Asian electrons inferior to American electrons?

Buffer are often built with generic "allrounder" transistors not at all intended for low noise (at least they should be ALL fets)
And with 5 pedals, you got 10 buffers, for me it's 9 useless ones, and ten times too much noise.
Caps in the path are often not that good, and of low value, it's good to limit lows in ONE circuit, when you do that in 5 pedals, you just get mud, even in bypass mode.
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Re: How to figure out input impedance of a buffered circuit

Postby tschrama » 15 Oct 2011, 13:10

kleuck wrote:...Added to that, 470K is not ideal for a guitar signal if your cable is more then 10 feet long....



if doubt that cable length determines the optimal input R . Could you explain?
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Re: How to figure out input impedance of a buffered circuit

Postby mictester » 15 Oct 2011, 13:21

tschrama wrote:
kleuck wrote:...Added to that, 470K is not ideal for a guitar signal if your cable is more then 10 feet long....



if doubt that cable length determines the optimal input R . Could you explain?


The (spurious) theory goes that the cable capacitance will reduce the treble content of the signal. It's nonsense - do the mathematics. The capacitance of a couple of metres of cable matters at VHF (100 MHz and upwards) - it's insignificant at audio frequencies. It's akin to the idiots who claim "skin effect" in speaker cables...
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 13:25

Well, it's not cable length, but cable capacitance actually.
The cable capacitance tunes the resonance peak of your PU an lowers it too.
So with a long cable (assuming it's a good one, capacitance will be about 70-100 pf/meter) you want a higher impedance from you first active item, with a shorter cable, it can become a little harsh.
Depending on the cable, my homemade LP Jr sounds really differently -cause i use high value pots too.
Docs are here http://buildyourguitar.com/resources/lemme/ and here http://terrydownsmusic.com/technotes/gu ... cables.htm

mictester wrote:
tschrama wrote:
kleuck wrote:...Added to that, 470K is not ideal for a guitar signal if your cable is more then 10 feet long....



if doubt that cable length determines the optimal input R . Could you explain?


The (spurious) theory goes that the cable capacitance will reduce the treble content of the signal. It's nonsense - do the mathematics. The capacitance of a couple of metres of cable matters at VHF (100 MHz and upwards) - it's insignificant at audio frequencies. It's akin to the idiots who claim "skin effect" in speaker cables...

YOU should do the maths : a pickup is not a resistor, but mostly an inductance.
And considering the eddy current, the Pu/cable cap filter can be a 18 db/oct one
Alternatively, you could just TEST some 3 meters, 6 meters and 9 meters with identical components and with your ears, would be interesting.
I did.
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 13:39

Here, some tests i made on a french forum, pics, capacitance and samples : http://www.guitariste.com/forums/access ... 1,500.html
There's a particularly long and crappy cable, alone and with a 3Mohms input buffer.
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 14:49

Tried to translate:

Unknown 80's hi-end cable (solderless) Length : 5m30 Cap : 490pf Metric cap : 90pf/m

Image
Image
Cordial122 Length : 6m15 Cap : 570pf Metric cap : 90pf/m
(Cordial says : 70pf/m)


Image
Image

Stagg n°1 Length : 3m Capa : 365pf Metric cap : 122pf/m

ImageImage

Stagg n°2 Length : 5m95 Capa : 925pf Metric cap : 155pf/m

ImageImage

Unkown cable Length : 2m95 Capa : 356pf Metric cap : 120pf/m

ImageImage

Old and crappy cable Length 14m Capa 4n37 ! Metric cap 312pf/m (!!!=60 meters for a Cordil 122)

ImageImage

Sommer Spirit : 5m90 Cap : 447 pf >> Metric cap 75pf/m (sommer says : 78pf/m)

Hi-end vintage cable

Cordial122

Stagg 6m

Crappy old cable

The 14 m crappy cable, proceeded by my 3Mohms buffer & the Vintage unknown cable (the good one)HDG

Vintage Hi-End>>Buffer>>Stagg

Bridge, then center, P90s.
Amp impedance : 500K ; Buffer : 3Mohms

If you don't hear the difference in brightness between my vintage Hi-End cable without buffer, and before a 3Mohms buffer...
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Re: How to figure out input impedance of a buffered circuit

Postby mictester » 15 Oct 2011, 15:19

A quick check here a while ago, when a well-known guitarist claimed that he could hear the difference between cables:- three cables, all slightly different materials, but crucially the same length (2.5m) and with the same connectors on them.

The first measured at 92 pF/m, the second at 98 pF/m and the third at 118 pF/m. Measured losses at 20 kHz (considered to be the upper limit of hearing for most of us), using a signal generator (600 ohm O/P Z) and a FET preamp stage at the other end with an input impedance of 450 K - in all cases the signal in was identical to the signal out - there was no measurable loss. Next test was an actual Di Marzio PAF pickup, with a 1 mH choke inductively coupled to it, fed from the signal generator. The level was set on the generator to deliver 40 mV at the pickup terminals at 15 kHz. Measured results into the same preamplifier were identical to within 1 mV....

You may have some resonant effect with your pickups and your guitar lead, but it's going to be ultrasonic. If it isn't, you should consider putting an active preamplifier at the pickups in the guitar body, or you should consider that the design on the guitar electronics is sub-standard and should be returned to the manufacturer for correction as it clearly not suitable for most use.

Incidentally, the guitarist who claimed to be able to hear the differences between cables was tested in the studio - he was asked to "audition" some "high-end" cables for a manufacturer... At NO TIME, in ANY of the tests could he correctly identify a cable by "sound" alone. He was tested with his own guitars on his own rig...
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 15:30

Are you deaf ?
Just listen to the samples, i recorded the differences, and they are not ultrasonic.
The signal generator is just a silly idea, unless you find one with a 2 to 6 H inductance with an alnico construction on the ouptut, just like an actual Pick-Up, to simulate the eddy currents too.
Not to say that a 2m50 cable (if good) is too short to hear or measure anything indeed.
It's just like pretending to test the effects of the cable capacitance on passive guitar rigs, by using a CD, a Hi-Fi amp and a 50 cm cable : just useless.

Read the chart : http://buildyourguitar.com/resources/lemme/table.htm
And the sims : http://terrydownsmusic.com/technotes/gu ... cables.htm

If you can't hear.

Almost all guitarist know that you can make a guitar brighter just by replacing the volume pot with another with higher value and vice-versa -and i did a couple times.
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Re: How to figure out input impedance of a buffered circuit

Postby mictester » 15 Oct 2011, 18:34

:roll: You have no idea whatsoever, have you?

Here's a clue: What is the resonant frequency of your 2H with 250 pF? It's 22.3 kHz.

My guitar amp (like most) has no significant response above 4 kHz, so no, - I'm not deaf - there's just nothing to hear!

Still - the whole thing is moot - all the professionals I deal with tend to radio-link their guitars to their pedal boards to give them greater freedom on stage. There are one or two die-hards who persist in using cables, but they are very much in the minority. Nowadays I have quite a lot of business building high input impedance mixer stages for fitting into guitars to drive radio transmitters. I also include a compressor in the path (and a corresponding expander in the receiver) to enhance the signal to noise ratio. There's also pre- and de- emphasis to further help with the radio link noise issue. The guitars are usually powered by rechargeable batteries, and I've been working on an inductive charging system to allow them to charge when they're sitting in their stands or in their flight cases.
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 19:14

Ho, yes, you choose just the lower scenario hum ?
Do the maths : it will be about 7 Khz :)
And with my amps, it's obviously audible, strat/fender users won't disagree.
And by the way, my amps still delivers after 4 Khz (and it's not always a good thing)
But if you don't want to hear the samples, read the charts or sims (not mine BTW) what can i do ?
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 15 Oct 2011, 21:45

BTW, when you do the maths for the average HFE of the 2SCXX, you find an input impedance of about 400Kohms, more probably UNDER 400 Kohms : lame
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Re: How to figure out input impedance of a buffered circuit

Postby mictester » 16 Oct 2011, 09:34

An announcement:

I now obviously know nothing at all about electronics, guitars, effects, amplification, radio, music or psychoacoustics. My customers also have no idea what either they or I am doing with music electronics despite their spending tens of thousands of pounds with me each year. The Japanese electronics manufacturers who hired me in the 1970s and 1980s to design products for them were also misguided, and the fact that many of these products are still in production and sell well is just due to the pretty colours of the boxes or the stupidity of the market.

"kleuck" knows everything, so any questions on any of these topics should be directed to him.

Bye!
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 16 Oct 2011, 09:44

You obviously don't know what is an RLC resonant circuit or how to calculate it's cutoff, nor can you use the "Play" button in your browser.
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Re: How to figure out input impedance of a buffered circuit

Postby kleuck » 16 Oct 2011, 10:28

...Nor calculate the actual impedance of a BJT-based buffer, a clue : between 380 & 420K for the DS-1 circuit.
But here, non lows loss, and the transistors are good low-noise-ones i admit.
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Re: How to figure out input impedance of a buffered circuit

Postby GuitarlCarl » 17 Oct 2011, 01:28

I've used enough crappy cables in my time to know most of the time if its messing with my sound it AIN"T THE CABLE! It's the crappy solder jobs on the leads... repair and play on... I think I've got pretty good ears and I can't hear a big difference between 90 pf and 150 pf caps as far as what frequencies are coming thru. Shit those are the sizes you use to ALLOW treble bleed at lower volume, but if they were to allow enough to make a difference in TONE then they'd be used for the main capacitors in yer guitar. Buffer vs true bypass.... Who gives a crap? Wah pedals were made for saxophones, someone did what they shouldn't have, and used one on a guitar. Your LP jr sounds better with one kind of cable? Good use those. True bypass is all hype? No, but it ain't a BIG ASS DEAL either! Blackmore uses a tape player for a preamp, should I? Page mics dinky amps at a distance in a big empty room, everyone OUT I'm fixing to play, duh... Whatever you like play it. Two brains arguing about sound? Sounds like crap to me. I love you guys, but you're both fucked up.

Jake, fuck the math. Breadboard it and use your ears. Mictesters math is probably great, and Klueck is counting decimals... Seriously? Do the shit yer way and use YOUR ears. Guitar PLAYERS don't give a crap about the math.
I want it to sound like bees buzzing around in a 55 gallon drum...
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