What exactly does a Treble Bleed Cap do...?

[RandomHero]

Ok, I know what it does in theory, but I want to know because my guitar Jamie is due for a rewiring. The part I'm talking about is the .001uf capacitor commonly found on Teles between the hot and centre lug on the volume control.

I've read it in a couple contexts. One said that the purpose of the cap is to retain treble frequencies as the volume is rolled off, as many players experience a muddiness caused by (a phenomenon I don't understand) when they lower their volume. This makes sense as it would always allow that tiny bit of treble straight from the pups to the output.

Another place said it's a futile attempt to overcome treble loss through long patch runs due to capacitance in longer cables... also that it destroys the linearity of the volume control and wrecks the proper function of the tone control. The two downfalls stated in this excerpt (a book by Ritchie Fliegler) also make sense.

What's the low-down? My guitar is pretty trebly (mainly because my tone control is currently non-functioning,) but even by nature the pups have bite. I want it to stay that way if I drop the volume, but not at those costs! Anyone shed some light on the subject?

[UnklMickey]

hmmm sounds like the perfect test-bed for some experimentation?

you say your tone control is currently non-functioning.

if it is both:

a -- still in place.

and

b -- not currently being used for some other task.



i would suggest the following set-up:

connect the .001 cap in series with the tone pot.

place that series network in the place where the treble bleed cap would normally go.

at maximum resistance of the tone pot, you can hear the same sound you have now.

at minimum resistance, you'll hear what the treble bleed cap would sound like.

too much effect from the treble bleed cap? dial in some resistance. sound just right now? tape the tone pot so it won't move. take the guitar apart and measure the resistance of the pot between the 2 terminals you had in the circuit. this is the value of the resistor you'll use in series with the treble bleed cap.


sound like a plan?

[JohnH]

I think both those contexts are correct, and differ in opinion as to whether the idea works or not. My belief is that it is indeed the capacitance of the cable that causes a loss of treble as you roll down the volume. The treble bleed compensates and improves this, but does not fully avoid the issue while still being worth doing if you find that this treble loss is an issue.

Cables have a capacitance of about 30-40pF per foot,, so 300pF to 1000pF for typical cables, depending on length. 1000pF = 0.001microF. These cause an interaction with the pups, dependent on the pup resistance. At full volume, the cable sees a relatively medium/low impedance from the guitar, since it is directly connected to the coils. At reduced volume however, the signal is fed through parts of the volume pot, giving a relatively higher impedance. The guitar impedance, in combination with the cable capacitance, creates a low pass filter, and the cutoff frequency is lower with higher guitar impedance. hence at lower volume, you get a greater treble cut effect due to the cable than at high volume. The pup inductance is also a key part of all this.

The extra cap feeds a bit more treble through to compensate, and its size is of the order of the cable capacitance. The best value is chosen by ear though, and it depends also on how long the cable should be. Too much treble bleed, and the sound gets brighter as you turn it down, great if that was what you wanted. It has no effect at full volume.

I now prefer to address treble loss of cables with an active buffer stage, which fixes the issue at all volumes and gives extra clarity.

Finally, here is a link to a great article that covers pup impedance, inductance and capacitance.

www.buildyourguitar.com/resources/lemme/index.htm

cheers

John

[RandomHero]

Active buffer stage? Like an onboard preamp? Explain please.

Is there a simple active buffer I could use in order to let my controls act as they do now and experience the benefits you described?

Unk- My tone and volume pots are both either fried or corroded beyond repair. I loaned them (equipped in a different guitar) to someone else and got them back in this condition. The switch is also giving me some noisy spots, hence the total rewire. I'm pulling out the stops too, this is going to be done clean, properly, with heat-shrink tubing and everything. I want this to be the last time I have to do this.

[JohnH]

My favorite buffer circuit is as on the 'LP Maximiser', schematics board. The schematic is there, a picture of the circuit board and info on wiring it in.

John

[UnklMickey]

admin said:

... I want this to be the last time I have to do this.

nice in theory, but wait till some new idea comes along....

[ChrisK]

The Lemme link is great. I had an email thread going w/ him in early summer of 2004 regarding his PU tester. While I could have saved some shipping charges since I was going to Germany in July of that year, I decided not to buy it (~~$900) and bought an LCR meter instead.

Analog Devices (www.analog.com) have some (relatively) new ICs that measures impedance directly (AD5933 and AD5934). These return the complex impedance (not just Ohms, but a real and imaginary data word at each freq. point) and the demo board and software looks like a $100 way of doing what Lemme is doing.


As the volume is rolled down, the effective output impedance (well, resistance for a pot) does increase, reducing the cable capacitance driving ability. It's complex (as in AC circuit analysis) and quite hard to explain simply. Also, as the volume is reduced, our ears hear differently (why we have loudness controls on stereos and not just volume).

If anyone is interested in modeling PUs and circuit components, Linear Technology has a free download of switcherCAD from their site. While setup for switching power supply design (with their ICs naturally), the fundamental "engine" inside is pSpice. While incurably churlish in interface, it is "learnable" and powerful.

The Lemme link is a good place to start for building PU analysis models.

While most folk yearn for simple answers, the analysis process and hence the "seeing" of the interaction and response of even the "simple" components within a passive electric guitar is complex and often counterintuitive.

For example, the addition of a capacitor across a PU does attenuate the high frequencies above the cutoff frequency. It also often creates a frequency response peak at the cutoff frequency. In essence, it emphasizes certain frequencies that may make a PU sound brighter. A 0.001uF cap across a 3 Henry PU creates a peak at about 3 kHz. Even the 0.022uF tone cap when the tone pot is at "0" creates a smaller peak at about 600 Hz with the same PU.

This is the basis for the new "Grease Bucket" tone control on the Fender Highway1 models. It rolls off the high without creating this peak (and introduces an even more complex analysis since it has a pot, a resistor, and two capacitors).


So yes, the (anti) treble bleed capacitor will help preserve the high frequencies. This is truly low voltage and high impedance, so see tag line below.

[Mini-Strat_Maine]

chrisk said:

This is truly low voltage and high impedance, so see tag line below.

Yeah, but for much experimentation, outside test clips or an LP-type (or anything with access to the pots via the back) would be very handy. All that unstringing/restringing could discourage all but the most dedicated experimenter.
I haven't added anything to it for awhile now, but I made a Micro$oft Works database with cap values and other info for treble bleed ( or "retainer") circuits that I found on various Web sites. The cap recommendations run from 0.0001 µF to 0.0027 µF, sometimes with a resistor (47K to 330K, but mostly around 100K to 150K). Almost 70% of those that advocate a resistor along with the cap (11 of 16) say the two should be in parallel.
Many of them suggest experimenting to get the sound you want.
All this LCRZ stuff makes me want to catch a nap.

[ChrisK]

I have a couple of variable air gap capacitors salvaged from old AM radios.

Each goes from about 30 to 1,400 pF and is ideal for finding "tuning" values along with some series pots. For larger values, I pad one with some of the 1% film capacitor values that I have thru several decades (yes, there really are 1% caps).

[mlrpa]

I use a .001 cap in my Pro540R. I use 2 stacked hums, and a Kramer Quad in the bridge. The .001 adds a bit of brightness at half vol. And I love it. I can actually get single coil sounds out of the stacked hums, without any rewiring. Try it, you'll love it!

[RandomHero]

I finally addressed the issue with the buffer from JohnH's LP Maximizer. I have to say that I'm impressed.

My uncle is an electronics technician with a flair for home project and helping his nephew. I thought my job on the buffer circuit construction was a bit shady, so we got out his breadboard kit, which had a requency generator. I had him make it generate an AC current of about a volt with a frequency of 220; the A on a guitar, and wired it into the oscilliscope and the amp. A pure, unwavering sine wave. We hooked the buffer onto the breadboard and ran the frequency through it. The exact same result on the amp and the meter.

After installing the buffer, I found my harmonics jumping out more, everything has more punch, and the highs are -so- crisp I find myself using my tone control. (Not common for me.) The next guitar I get is going to include one of those magical little buffers, too.

[JohnH]

RH - thats great to hear. Im glad that worked for you!

John

[ChrisK]

Yes,

Getting rid of the unwanted high cut tone control (cable) may indeed lead one to use the variable tone control.

This may explain the entire genre' of music created thru 25 foot (and longer) coil cords.

Hopefully, the success of Marshall (EL-34's) wasn't due entirely to overcoming cable'ific attenuation.


Apparently I've discovered how to use the smilies.

[JohnH]

Having found all that extra brightness - sometimes you may want the smooth cool sound of a 20' curly cord. The buffer, by taking away the effect of the cable capacitance, is giving you back those highs. But it is also taking away a part of what normally contributes to the pickup sound, and what distinguishes different pickups and coil combos from each other.

So my current approach is to look for ways to add a very small cap in a controllable way. Maybe 1nF or 1.5nF. This gets connected hot to ground, before the volume control - which is then followed by the buffer. That way it does not cause treble bleed at low volume. Ideally, it would be engaged as a push pull on the tone pot, or it could be a separate switch. or the tone control becomes a rotary switch with a range of cap options. Either way, switched out - I get maximum punch and brightness, switched in I get the cool liquid tones that I expect from humbuckers and series combos. This feature is as least as useful as a new wiring or pup combo, and is a very minor addition to a design.

John

[Runewalker]

JH: "My belief is that it is indeed the capacitance of the cable that causes a loss of treble as you roll down the volume. The treble bleed compensates and improves this, but does not fully avoid the issue while still being worth doing if you find that this treble loss is an issue.
...The extra cap feeds a bit more treble through to compensate, and its size is of the order of the cable capacitance...I now prefer to address treble loss of cables with an active buffer stage, which fixes the issue at all volumes and gives extra clarity. "

JH, are you suggesting that the buffer supplants the benefit of the tone bleed cap, or in some installations the tone bleed cap/resistor combination? What would happen with only the buffer circuit and no tone bleed compensator? And is the tone bleed redundant or worse with the buffer circuit?

RW

[JohnH]

In general, I think that with the buffer, you dont need bleed caps beacuse the buffer stops the source ofthe problem (the cable) from interacting with the volume control pot. Thats how I have it on my Strat.

However, on the LPmax I do have them as part of a slightly unusual dual volume control circuit. It just sounded better to me that way in this design.

It really depends on each case, and is worth testing to hear the effects, for each guitar.

John