It seems to me that choosing the input impedance of a DIY box is a bit of a minefield. I measured the resistance of my Epiphone SG dual Humbucker pick up guitar and it goes from 3.7 K to 470 K depending on the switch, tone and volume control settings. :icon_confused: Hoping for some inspiration I looked at the input circuit of my early 70's Orange Matamp OR100 but notice there was a modification on the site that said later models had 220 K resistors added. This mod on the later unit changed the input resistance from 1.068 M down to 182 K :icon_confused: so no enlightenment from that. :icon_rolleyes:
I can see that you want a high input impedance like 1 M ideally but in the real world there can be a significant distance from the guitar to the pedal to allow movement on the stage like 10 or 20 foot cable so I wonder if Orange added the resistor to combat noise pick up. Over 200 K would suit my needs and simplify the circuit I want to build since it wouldn't require an extra input buffer and is better than existing Orange Amps being sold. Any thoughts on whether > 200K would be too low or not?
Quote from: Ruptor on December 20, 2017, 12:46:30 PM
It seems to me that choosing the input impedance of a DIY box is a bit of a minefield.
Yes. And that is a bit of an understatement. :icon_lol:
QuoteI measured the resistance of my Epiphone SG dual Humbucker pick up guitar and it goes from 3.7 K to 470 K depending on the switch, tone and volume control settings.
That's because you're not measuring the pickup itself, but one or more pickups in various combinations and possibly splits, depending on what the switching does, as well as the tone and/or volume controls. And because resistance is not what you need to measure.
Humbucker pickups by themselves mostly measure 4K and up, not far from your 3.7K minimum. That goes through the switches, then the volume pot. When volume is max, the volume pot is in parallel with the pickup, and so you have the volume pot end-to-end resistance (250K to 2M, generally) parallel to the 4K-ish bucker.
But what you need to know is not DC resistance, but what the AC impedance is over the usable frequency range, up to 5K to 10K Hz. Those pickups are actually high-value inductors, usually 2H to 4H for 'buckers, and so their impedance rises linearly with frequency, and is in the range of 100K to 400K at the high end of audio. Amps and pedals will subtract treble more than bass if they're less than about 10x the AC impedance at treble, so you really need on the order of 1M and up in whatever the pickup is driving. In fact, one old trick to fake a pickup is to use a biggish inductor, or a transformer primary of a few Henries to get the same kind of rolloff.
Quote
Any thoughts on whether > 200K would be too low or not?
200K is too low unless you like treble rolloff.
You don't have to match the impedance.
The load affects the high frequency response.
Maybe look here, figs 14 and 15
http://buildyourguitar.com/resources/lemme/
(As written "external" load means external to the pickup not external to the guitar."
At the end of the day use 470k to 1MEG.
Adding resistor will not "combat noise pick up".
Put 220K across your guitar and play. Is the tone OK? Top-end all there? ALL your guitars? Now, and the one you buy next month? Then go for it. Quite a few amps are in this zone. Maybe to cut some harshness (but different cut for different guitars).
The use of 20 30 40 foot cables clobbers highs more than a few hundred K of loading. But guitar-amplifier interfacing has always been a "good enough" bodge, and a 50 foot cable is so cool.
Thanks for your replies and thanks to Rob for the link about pickups where the guy has an interesting method of investigating the frequency response of pickups using an external magnetic driver. I looked at some pedals and some are as low as 80K on the input then I tried putting resistors across the guitar lead and watched on the oscilloscope. 47 K knock the signal right down to roughly 25% and 820 K took it to 80% but it says nothing about the frequency content other than I can't see any change in wave shape. Going by what people say when they modify Orange amps from 180 K input load back to 1 M like the original 70's amps the amplifier gain is the only change and there is no mention of any change in frequency content perhaps because it is so insignificant. I guess with a distortion pedal it doesn't matter what it does to the signal so long as it sounds like what you want. :icon_neutral:
When folk speak of low amplifier input impedance not having a noticeable cut of their highs, you have to ask what they are playing with.
They may always have a buffered pedal between guitar and amp. That stops the amp input impedance load for changing the guitar tone.
They may have active pickups or tone control. Ditto as above.
They may have low impedance pickups on a passive guitar. These can be tolerant of load down to 100k or less. Without doing any measurement, the value of volume control in the guitar is a guide. If it's 500k to 1M, then the amp impedance must be as high as is practical - usually 1M. If its 100k to 250k, then it probably can tolerate less that 1M input impedance.
There may be treble loss, but the players amp & cab design or eq choices won't show it up.
The player has severe hearing damage and wouldn't notice treble loss if it walked up and smacked them around the head.
Quote from: anotherjim on December 21, 2017, 12:07:36 PMThey may always have a buffered pedal between guitar and amp. That stops the amp input impedance load for changing the guitar tone. They may have active pickups or tone control. Ditto as above.
Are you saying Orange assume that everybody will be using active guitars or pedals allowing them to drop their amplifier input resistance?
More likely someone tested it with a Telecaster or Strat & didn't think to try a Les Paul.
QuoteI looked at some pedals and some are as low as 80K on the input then I tried putting resistors across the guitar lead and watched on the oscilloscope. 47 K knock the signal right down to roughly 25% and 820 K took it to 80% but it says nothing about the frequency content other than I can't see any change in wave shape.
Normally, I wouldn't expect that much attenuation, maybe a few dB. You might be seeing changes in peaks as opposed to the perceived level. The 470k DC resistance you measured before might also have something to do with it.
QuoteGoing by what people say when they modify Orange amps from 180 K input load back to 1 M like the original 70's amps the amplifier gain is the only change and there is no mention of any change in frequency content perhaps because it is so insignificant.
Normally it would just give you a little more highs. For most pickups 180k would have an audible roll-off. At about 330k it's hard to tell. And 1MEG is usually a good upper bound. If you consider the guitar has a 250k volume control and a 250k tone control in parallel with it, the pickup is seeing at least 125k, loads like 1MEG only change the total load by a small amount, like 10% - that's why you can't hear much change beyond 1MEG.
QuoteI guess with a distortion pedal it doesn't matter what it does to the signal so long as it sounds like what you want.
For effects like an echo, phaser or a chorus, you want the clean signal to come through so you don't want to load the pickup. But a distortion pedal you can do what you want.
> resistors across the guitar lead and watched on the oscilloscope
Your ears/brain are much smarter than a 'scope!
Bass is big. Treble is small. You can cut everything over 800Hz and hardly see a difference in the total. But your ears will feel like they are full of wool. Very dull sound. No sing no zing.
*Listen* with various loadings. As you come down from a Meg, the treble goes out first. Playing through a blanket. Because much guitar technique is about managing those higher partials (and because you need the scream to cut through a band), you really want to preserve these highs as much as possible.
Quote from: Rob Strand on December 21, 2017, 08:06:06 PM
The 470k DC resistance you measured before might also have something to do with it.
Yeah you shouldn't see a resistance anywhere near that across the output of any common, passive pickup guitar. Usually below 20k. You might have an open pickup, and you're reading a volume pot with nothing in parallel. It can still making sound, thin and weak, via a capacitive coupling in the coil.
If you have active electronics though, anything's possible.
My first thought, for a reading of 470k from the jack socket, is that you have a guitar with one pickup selected by a switch (or only one pickup fitted) and a 500k volume pot turned most of the way down.
Volume pot all the way down would be "zero" ohms across the output jack.
500k volume pot all the way up is 500k ohms in parallel with the dc resistance of the pickup, usually 6-16k or so. In a common gibson type guitar, one pickup on.
Quote from: thermionix on December 22, 2017, 05:55:02 AM
500k volume pot all the way up is 500k ohms in parallel with the dc resistance of the pickup, usually 6-16k or so.
But what actually minds is the AC resistance of the picup(s).. :icon_wink:
QuoteVolume pot all the way down would be "zero" ohms across the output jack.
No, not if its wired pickup to wiper of volume pot, which some guitars are - mostly 2 pickup/separate control guitars but some single pickup versions do the same, even though they could be wired wiper to jack. Think about it, if 2 volume pots are wired wiper to jack, then turning one down would turn the other down too. Although there is probably more treble loss with low volume control settings (due to the lower wiper resistance to ground loading the treble out), some manufacturers and players prefer the lack of interaction between controls by wiring it that way. With 2 pickup bass guitars, pick up to volume wiper is pretty much the standard way - Fender, Rickenbacker etc.
(http://www.1728.com/gibson.gif)
Now, the OP said it's an Epiphone humbucker SG, which almost certainly means 500k pots. The Epi standard (according to web searches) appears to be volume wiper towards the jack (via the selector). To read 470k at the jack would require no pickup at all selected, which isn't likely unless modified with additional switching that can do that. Either that 470k is an "anomalous" reading, or its wired pickup to wiper and the volume is turned down.
Well, let me re-phrase. In any factory-wired Gibson or Epiphone "standard" LP/SG/similar that I'm aware of, a 470k DC resistance reading across the output jack would indicate an open or disconnected pickup.
Wiring the pickup hot to the volume pot wiper is a noise problem, and fairly rarely done. But it's certainly possible.
Quoteit goes from 3.7 K to 470 K depending on the switch, tone and volume control settings
QuoteIn any factory-wired Gibson or Epiphone "standard" LP/SG/similar that I'm aware of, a 470k DC resistance reading across the output jack would indicate an open or disconnected pickup.
Maybe we need confirmation from the OP. From what I can see that's the case. However, across all Gibson guitars both connections are used.
QuoteWiring the pickup hot to the volume pot wiper is a noise problem, and fairly rarely done. But it's certainly possible.
It's pretty common for basses. Noise isn't an issue because as you tend to have one of the pickups cranked to full. You can also back one pickup-up off entirely without killing the signal (and there's no need for a pu selector.)
With a 3-way switch, in the middle position, the wiper to wiper connection causes the two volumes controls to affect each other.
My 70's Gibson SG was wired pickups to volume pot wipers. I changed it, didn't like the interaction thing and put it back.
Rickenbacker 4001/3 wire pup to wiper and have a 3 way selector.
Yes, doing it that way does mean that noise can increase when turning volumes down, since the cable to the amp gets a much higher source impedance and worse with 500k pots than 250k's. This point may be very pertinent to the OP if his pickups are wired to the wipers.
I don't know how the pickups are wired but they are not open circuit since the switch selects either one or both and you can hear the difference in sound from each of the pickups. The limit values are the lowest and highest resistance values I could measure by turning the pots to the extremes so I guess when the Pots are at maximum the one pickup is directly across the output since you say the pickups are 500K.
I guess your pickup hots are wired to the volume wipers. I would not expect that from a factory-wired Epiphone SG. Anything is possible, and it may have been changed by someone. I would recommend moving p/u hots to lug 3 (and output to wiper) so you have less hum and noise when you roll your volume off.
Here is the wiring of my guitar. Switch on the left, then volumes, then tone and output jack on the right.
Where is the picture?
(https://s14.postimg.org/6gsao6we5/SGpots.jpg)[/url
Oh! here it is. (https://postimg.org/image/6gsao6we5/)
Picture doesn't quite prove it - can't see the left side too well. But, if the blue screened cable is from a pickup then you DO have independent volume wiring since the hot wire goes to the control wiper.
All you need to do to make it standard is swap the wiring between wipers and ends of each volume pot. Leave the grounded end as it is.
But you don't have to look at the wires to know this. If, with both pickups selected, you can turn one of the volume controls all the way off without silencing the other pickup, then you have independent volume. Standard wiring lets either volume control all of the guitar output when both pickups are selected.
The other tell tale, which works even with single pickup guitars (yes, although independent working is irrelevant, the volume control can be wired that way), is that noise increases with anything less than full volume. With normal (pot wiper to output) wiring the noise increases with less than full and decreases again at or near zero.
All because of the way the source impedance to the amp input can increase and in turn increases the sensitivity of noise pickup. This is probably allowing noise in far of greater magnitude than the self noise of the components can produce. Better screening and grounding would help. Yes, that Epi' looks well screened compared to the average, but it could be better.
Yep, it sure looks like the pickup hots are going to the wipers. Also looks like factory original wiring. I am surprised. It works basically the same, but doesn't cut as much hum/noise when the volumes are rolled off. With the fact that most modern players will be plugging into high gains and whatnot, it's an odd choice.
It's also weird that they ran a strip of aluminum tape to connect the cover plate shield, when the rest of the control cavity isn't shielded, but doesn't really need to be since all the wiring is shielded. Nothing wrong with it, just another odd choice.
The red and blue wires coming from the left are from the pickups and are wired like your drawing back on post #14 although the tone caps look to be connected differently. My guitar is about 25 years old so I don't know how that fits with modern amps. I can't say I have noticed noise pickup but then I am using a 70's Orange Matamp OR100. I could swap the pickups to the pot ends but what effect would that have on the tone controls?
If you're not experiencing any noise problems, it might not be worth the trouble to change anything. The tone controls by themselves can be wired a couple different ways and still work and sound the same. They can be connected to the wipers of the volume pots, like in 1950s Les Pauls, or connected to Lug 3, which is much more common practice. Which way is preferable is just a matter of personal taste.
It is strange that the tone controls can't be taken completely out of circuit. I wonder what it sounds like without the tone controls attached? The whole point of valve amps is they have a wide band with incredible high frequency capability so it seems strange the guitar would cut off the high frequencies. :icon_rolleyes:
Quote from: Ruptor on January 11, 2018, 04:01:58 AM
I wonder what it sounds like without the tone controls attached?
A little bit more high end.
Quoteit seems strange the guitar would cut off the high frequencies.
Too much high end would sound harsh to most people. Guitar speakers are usually designed to roll off the highest frequencies.
Yep, electric guitar with full range reproduction isn't the most popular sound.
Some custom options include fitting a switch or pull pot to solo a pickup direct with no controls connected.
Passive wiring is a load of compromise which by good fortune and time honoured acceptance prove to be workable. Both pickups on means extra loading* working towards smearing & dulling the sound - but then this setting is rarely used for lead work - out of the 3 selector positions I do find it the least useful for foreground work.
*Extra load on each pickup is (possibly) both volume pots in parallel. Both tone cap/pots in parallel. The other pickup winding in parallel. This is why this selection never seems to be much louder than either pickup soloed.
QuoteThe whole point of valve amps is they have a wide band with incredible high frequency capability so it seems strange the guitar would cut off the high frequencies
A completely unloaded pickup has a high peak at high frequencies.
The loading tames it to sound more "musical", which usually means some amount of peaking.
For a hifi phono cartridge we choose the loading so it produces no peak or a small peak.
In the first case we are shaping the tone.
In the second we are reproducing what is already there.
Quote from: Rob Strand on January 11, 2018, 04:20:15 PMA completely unloaded pickup has a high peak at high frequencies.The loading tames it to sound more "musical", which usually means some amount of peaking.
Are you saying that the modern idea of using high input impedances like 1 M is wrong then? This is why I asked the original question because looking at different pedal and amplifiers they range from very low of a few K to 1 M. From what you say I guess having a low input impedance would be like levelling out or dowsing all the pickup responses to make them all sound the same. I can't see how loading a pickup would change its frequency output unless the load was phase shifting like capacitive or inductive. Dumping a resistor across a pickup should cut all frequencies not just high ones so I am more confused now. :icon_rolleyes:
Pickup impedance is not all pure resistance. It's a coil so has inductance. The impedance of an inductor rises with rising frequency.
So at higher frequencies, the source impedance of the pickup is higher.
That is why reduced load impedance cuts the high frequencies more than the lower.
Distortion effects generally sound more musical when the source signal has few upper harmonics. A low input impedance to the effect is a cheap & easy way to cut guitar harmonics. Such effects only sound as intended when fed directly from the guitar.
With a tone control capacitor present, combined with the inductance of the pickup, then not only is treble rolled off due to the capacitor, but a resonant peak is formed in the response curve about the roll-off frequency. You need both capacitance and inductance to get that peak which is an important part of guitar tone creation.
Reducing load impedance to the pickup (which you can vary with pickup to wiper wiring) both flattens and widens the peak.
It's common for the response peak to centre around 700Hz give or take.
Phase shift happens, but if it's stationary and soloed, you can't hear an effect from it. With more than one pickup active, the phase shifts are heard as nothing more significant than tonal changes due to the result of the mixing adding or cancelling some harmonics to different degrees.
> Dumping a resistor across a pickup should cut all frequencies not just high ones
As you speculate, a pickup is "all" reactance. 5 thousand turns of wire is a big inductance. Also some capacitance. Then we have to carry the signal off to the amp/pedal, more capacitance. OTOH we have volume/tone pots to break-up the reactances. It is not simple.
IronStone (http://ironstone-guitar-pickups.co.uk/guitar-impedance-matching-lcr/) lists reactances for their pups; typical of many.
At the pup you can plot curves like this.
(https://s18.postimg.org/8bh9rrnsl/pup-curves.gif) (https://postimg.org/image/8bh9rrnsl/)
But this neglects the non-negligible effect of axe pots, and guitar cord.
And it isn't like the pup signal is "flat". The magnetic pickup has a rising response. The steel string is heavy and a probably falling response. The pup pole is large and averages-away small (high-F) vibrations.
I have been looking for a Spice model for the Humbucker pickup and my guitar wiring but haven't found a sensible one yet. :icon_rolleyes: Why would they make the peak frequency at 4 KHz like in that picture since it adds colour to the signal? I would have thought they would want the actual string vibration not a synthesised or deformed version within the audio range. This is the circuit for each of the pickups on my guitar so it saves me drawing it.
http://guitarelectronics.com/1-humbucker-1-volume-1-tone-north-coil-humbucker-south-coil/
I can just copy it straight in to LTSpice and make my own model of my guitar. :)
Guitar pickups, effects, amplifiers, and speakers are all about coloring the signal. That's where "electric guitar tone" as we know it comes from. A flat-response system, if it could be built, would end up a louder version of the sound of your guitar unplugged. Not very interesting with a solid body.
There is more information than you could ever want about this stuff over at guitarnuts2.proboards.com
The fact of the matter is that no, we don't usually want to hear exactly what the string itself is doing, especially not way up high. Everything in the actual treble area is noise anyway.
I guess I haven't noticed any problems with hum because my guitar has Humbuckers not single ended pickups. :icon_rolleyes: