Treble Bleed Explained

With factory-standard guitar wiring, you may have noticed that as you turn down your guitar’s volume pot the output signal progressively loses treble. This effect, denoted 'Treble Bleed',  is due to a Passive Analog Low-Pass Filter that is created by the combination of the increasing resistance of the volume pot (on the hot side) and the capacitance of the guitar lead. This is shown in the circuit below (the tone circuit has been removed for clarity).

In the circuit, the capacitance of the cable is marked C. As you can see, as the volume pot is turned down, the pot acts as a voltage divider with resistance to ground being gradually reduced and the resistance leading to the output (the 'Hot' Resistance) being gradually increased. It is this resistance leading to the output, in conjunction with the cable capacitance that forms the passive analog low-pass filter.

With the volume control up full, the resistance portion of the filter circuit (the 'Hot Resistance') is very small (effectively zero) and the cut-off frequency for the low pass filter is very high indeed - way above the audible frequency range.  When the volume is lowered, however, this resistance is increased. The result is that the cut-off frequency dips down into the audible range - cutting audible higher frequencies from the guitar’s output signal.

Crunching the Numbers

Remember that the cutoff frequncy of a passive analog low-pass filter is calculated as follows:

fcut-off (Hz) = 1/(2πRC)

The capacitance of typical guitar cable is approximately 100pF per metre, and the frequency range of human hearing is typically 20Hz – 20KHz (ignoring guitar amp related deafness). For augments sake, let’s assume a 10-metre cable - that’s an approximate cable capacitance of C = 1000pF across the guitar output. So running the numbers to find the initial resistance at which we hear an effect:

20000Hz = 1/(2πR0.0000000001F)

solving for R: 

R = 79577.4715 Ohms or 79.5 KOhms.

So given that we can start to hear an audible effect with a 'hot' resistance at the pot of 79K, if we are using 500K linear taper volume pots this begins to have an effect quite early on in the volume reduction (basically just under to 90% mark). If we are using 250K pots, we start to hear it a little later (around the 60% mark). 

We'll talk about the various solutions to this problem (if indeed you are a guitarist who wants to solve this problem - and many don't) in a future post.

12 String 335 - Polishing and Shielding

Great progress today - i *finally* got the polishing finished on my 335 12 string, and I also managed to shield the pickup cavities ready to receive their glorious GFS Surf 90s!

I started the polishing process after having wet-sanding with 1500 grit paper and applying a final coat of tru-oil thinned to 50% with turps. Polishing proceeded in 4 stages, with the first stage being a wet-sand with 2000 grit wet-and-dry using boiled linseed oil as the lubricant. I wet-sanded until the surface felt smooth and sanding marks covered the majority of the surface. The surface wasn't as dull as it probably should have been, but this wasn't unexpected given that I didn't leave all my wet-sanding until the last coat of Tru-oil was applied.

After the 2000 grade wet-sand, I then applied sucessive Stewmac polishing compounds - starting with Medium, then moving to Fine and Swirl Remover to remove the sanding scratches and bring out the shine.

I applied each successive compound using my electric drill and some small (3cm) foam polishing pads I purchased from Aussie site polish-up.com.au.  The shine that resulted was surprising, with the Tru-oil displaying a satisfyingly reflective gloss finish, and a deep translucency that unfortunately shows up every little mistake I made when applying each layer of oil. The flame maple has really popped, and the 3d or "chatoyance" effect as you move around the guitar is fantastic. In short, I'm very happy with how the finish has turned out!

With polishing complete, it was time to turn my attention to preparing the guitar for wiring - notably, shielding the pickup cavities with some dual-sided conductive copper foil. This stuff is great - it comes with stickum on the back and simply lays down like tape (if a little harder to handle). Each strip lays over the strip next to it and the whole thing forms a wonderfully conductive sheet (resistance is effectively negligable).

"chatoyance"

You can even solder onto it! I was able to easily solder wire from the neck cavity to the bridge cavity without any trouble at all. When the bridge is finally installed, i will be able to solder the bridge ground wire to the shielding around the bridge pickup,  and then run another wire from the shielding out to the wiring harness. Exciting times ahead!

12 String 335 - Vintage 50s Wiring Harness

I got the wiring harness for the 335 wired up today based on the "Vintage 50s Wiring" scheme used by Gibson in the (unsurprisingly) 50s and 60s. This wiring scheme attaches the tone capacitor (and therefore tone pot) to the output of the volume pot rather than the input as is the case for so-called "Gibson Modern Wiring" that is used in all 335, Les Pauls and SGs today.

I'm hoping that this wiring scheme, in combination with my GFS Surf-90 pickups, will lend the guitar a very 50s jangly sound.

Everything went together fairly easily using 500K CTS pots and 0.022 uF orange caps from Stewmac. Because my push-pull CTS pots wont fit through the f-hole in the 335 body, my planned series/parallel and coil tap modifications had to be scrapped for this build. Hopefully in a later build I will be able to explore the plethora of wiring modification possibilities.

At present I satisfied myself with a simple modification to the 50s Vintage wiring scheme to allow the volume pots to operate independently. The problem with both 50s Vintage and Modern wiring is that when both pickups are selected, turning one one of the pickup volume pots to zero grounds out the entire signal from the 3-way switch (including any signal generated from the other pickup). A simple modification swaps the volume pot input and output lugs, placing the pickup input wire onto the pot's swing arm.  When the pot is turned to zero, only the hot wire from the pickup in question is grounded rather than the whole circuit, allowing the other pickup to continue to supply signal to the 3-way switch as normal.

A second simple modification I am considering is the addition of a "treble-bleed" circuit to each of the volume pots so that the high-end jangle is not muddied as the volume is turned down. This simple circuit involves a resistor and capacitor wired in parallel across the volume pot "hot path" to allow high signal frequencies to bypass the increasing resistance as the volume is turned down.

As the pot is turned and the resistance increases, the resulting signal, while attentuated for the majority of the frequency range, is not reduced for the higher frequencies. These higher frequencies stay in the signal and the resulting output retains the high jangly tones even as the volume is rolled off.