Author Topic: Guitar output impedance?  (Read 2208 times)

Ruptor

Re: Guitar output impedance?
« Reply #20 on: January 10, 2018, 10:28:09 AM »
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?

Ruptor

Re: Guitar output impedance?
« Reply #21 on: January 10, 2018, 10:30:40 AM »

anotherjim

Re: Guitar output impedance?
« Reply #22 on: January 10, 2018, 12:02:41 PM »
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.
"So lets stay within the limit of sureness: lets consider the fuses you have available and lets see what you have to do to light them up..." Farfisa Partner 15 drum machine manual.

thermionix

Re: Guitar output impedance?
« Reply #23 on: January 10, 2018, 12:10:51 PM »
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.
FDH

Ruptor

Re: Guitar output impedance?
« Reply #24 on: January 10, 2018, 01:45:41 PM »
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?

thermionix

Re: Guitar output impedance?
« Reply #25 on: January 10, 2018, 03:52:45 PM »
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.

Ruptor

Re: Guitar output impedance?
« Reply #26 on: January 11, 2018, 04:01:58 AM »
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:

thermionix

Re: Guitar output impedance?
« Reply #27 on: January 11, 2018, 02:18:15 PM »
I wonder what it sounds like without the tone controls attached?

A little bit more high end.

Quote
it 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.

anotherjim

Re: Guitar output impedance?
« Reply #28 on: January 11, 2018, 03:38:27 PM »
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.


"So lets stay within the limit of sureness: lets consider the fuses you have available and lets see what you have to do to light them up..." Farfisa Partner 15 drum machine manual.

Rob Strand

Re: Guitar output impedance?
« Reply #29 on: January 11, 2018, 04:20:15 PM »
Quote
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
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.
The mind often distorts without gain.

Ruptor

Re: Guitar output impedance?
« Reply #30 on: January 13, 2018, 04:24:13 AM »
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.
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:

anotherjim

Re: Guitar output impedance?
« Reply #31 on: January 13, 2018, 07:01:32 AM »
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.



"So lets stay within the limit of sureness: lets consider the fuses you have available and lets see what you have to do to light them up..." Farfisa Partner 15 drum machine manual.

PRR

Re: Guitar output impedance?
« Reply #32 on: January 13, 2018, 05:37:47 PM »
> 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 lists reactances for their pups; typical of many.

At the pup you can plot curves like this.


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.

Ruptor

Re: Guitar output impedance?
« Reply #33 on: January 16, 2018, 06:44:21 AM »
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. :)

thermionix

Re: Guitar output impedance?
« Reply #34 on: January 16, 2018, 02:18:38 PM »
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.
FDH

ashcat_lt

Re: Guitar output impedance?
« Reply #35 on: January 16, 2018, 03:26:13 PM »
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.

Ruptor

Re: Guitar output impedance?
« Reply #36 on: January 18, 2018, 10:21:47 AM »
I guess I haven't noticed any problems with hum because my guitar has Humbuckers not single ended pickups. :icon_rolleyes: