Preventing Tone Loss:
Impedance, Capacitance,
Resonant Frequencies and You

November 16, 2011

Note: this article is addressing the influence of impedance on passive magnetic pickups as found on the majority of electric guitars.

The signal coming out of your electric guitar is a puny thing. About 200 millivolts (on average) with just a bare trickle of current. It's the amplifier's job to take this tiny signal and turn it into a loud, vibrant tonal entity.

But first the signal has to reach the amp.

Without hitting you with a lot of math and a bunch of confusing graphs, let's just talk about the concepts behind preserving the sound of your guitar plugged straight into your amp with a single cable and why there are so many things that can (and do) go wrong.

There are four main reasons your tone changes as you increase the total length of your cable and add effects in front of your amp:

• Cumulative Capacitance
• Resonant Frequency Changes
• Improper Impedance Loading
• Noise

Cumulative Capacitance

As the total length of cable increases, so does the amount of capacitance added to your signal. The average instrument cable has a capacitance of about 38pf per foot. With a 20 foot cable that adds up to 760pf. That capacitance in parallel with the output impedance of your pickup will usually be above the highest frequency produced by your amp so it's fine. But as the cable length increases the cumulative capacitance builds to the point where it's not only noticeable but objectionable – like turning down the tone knob on your guitar. This is part, but only part, of the "blanket on the amp" so many players are familiar with.

Resonant Frequency Changes

What makes a Strat sound like a Strat? The wood? Yes, in part, but then Strats and Teles would sound more alike than they do. The pickups play a huge role, and while people commonly talk about different pickup impedances it's actually the pickup inductance that plays the biggest role in the tone of a certain pickup.

One of the biggest reasons for this is different pickup inductances result in different pickup resonant frequencies. A Strat may have a big bump around 5KHz, a Tele may have a different sized hump around 1KHz, and a Les Paul may have a smaller hump around 800Hz. These resonant frequency humps are not static – they are affected by the load placed on the pickup. So when you put 500K pots in a Les Paul instead of the stock 300K pots, a lot of the difference you are hearing is due to the resonant frequency moving up and becoming more prominent as the load is decreased (no math, I promised, so for now let's just say higher impedance loads actually load down pickups less – more is less, in other words).

Remember this – impedance loads affect the resonant frequency, both which frequency is prominent and how prominent it is. Cable capacitance affects this, too.

If the cable is too long or the guitar is plugged into a device with too low of an input impedance (all too often, both), the resonant frequency moves down and becomes less prominent. This means that the basic characteristics of your pickups, the sound that made you choose Lollars over Duncans over DiMarzios or whatever, are changed or even eliminated. This is a major source of bland tone.

Improper Impedance Loading

So how do you know if a device has too low an input impedance? Some devices list it, others don't. The great majority of amplifiers have a 1M (1 million ohm) input impedance, which is ideal for almost all passive pickups. A lot of pedals also have a 1M impedance, though 470K or so is also common (just under half the ideal 1M). A 470K input impedance changes the tone subtly, making the guitar a little less bright. And that's OK here and there, but you have to make informed decisions about these things – whether that 470K is "OK" depends on cable length and other factors. Many volume pedals have a 250K input impedance, which is just barely passable (and they really darken the tone when not on full volume). Some old fuzzes and most wahs have input impedances of 100K to 47K, which is just way way too low (but the loss of high end helps mask how noisy the old fuzzes and most wahs are).

Once the pickup hits too low an input impedance, the highs go away, both because of an impedance mismatch and because the resonant frequency moves way down and diminishes in impact. Now you have dark bland tone – even worse than regular bland tone.

A note on coloration: while the science behind what I'm saying is absolute, the effect all this has on tone is subjective. One player's "too dark" is another player's "warm". At Psionic Audio we try to make any tonal changes controlled and intentional so you can dial in a specific sound then keep that sound consistently. There are better ways in our opinion to make a tone “warm” without losing the resonant peak response of the pickups.

Another note on that resonant peak response: this is where the vocal quality of guitar is. This is the area where exciting things happen when amplified, and (when not improperly loaded down) this happens in the range of about 800Hz-5KHz – exactly where our ears are most sensitive to subtle differences.

Noise

We've talked about improper loading reducing treble but it's also important to note that it reduces overall level as well, even though it's most noticeable in the higher frequencies. And as high impedance signals (such as comes from your guitar) get weaker they become more and more susceptible to picking up noise.

So with improper input impedances we have improper loading, and we have a lot of cable. The highs are rolled off, the response and character of the pickups are diminished, we've lost overall level, and we've added noise as we go.

This brings us to the signal to noise ratio. This is very important to understand – thankfully it's easy to get (hey, we're all guitarists).

On the one side we have the level of the signal – the good information we want to send from the guitar to the amp.

On the other side is the level of the noise – the crap behind the good information.

With the scenario above, the overall signal is decreasing in level while the overall amount of noise is increasing. We want a lot more signal and a lot less noise – this is called a good signal to noise ratio.

So what do we do when our signal finally reaches the amp? Well, it's a little bit dark, so we turn up the Treble knob. Hey, now I can hear treble frequencies from the guitar, but I can also hear the treble frequencies from the noise more. Rats. I had to turn up the Gain a little bit because the guitar wasn't pushing the amp enough – darn, that made the noise louder, too.

People will sell you a lot of things to try to get rid of the noise or EQs to make your guitar sound pretty again, but really they are just turd polishers. You can gate noise or you can prevent noise. You can turn up the highs (and the fssshhhh part of the noise along with it) or you can keep the highs intact all along.

But whichever way you go, noise and high end are only part of the puzzle. No pedal or effect can recreate the resonant peak of the pickups once it has been lost. When it's gone, it's gone, and you can have bright bland or dark bland or noisy bland or gated bland, but whatever way you slice it, you have bland.

So, What to Do?

What is the solution, and what precisely is Psionic Audio trying to sell me?

There are a few different options, each of which helps prevent, or entirely prevents, the problems described above.

First, use good cables. You don't have to spend a ton on cables to have good quality. We use Mogami cables with Neutrik plugs, but Canare, Switchcraft, Lava Cable, George L, and Evidence, among others, all are great choices too. While you cannot prevent cable capacitance from building up and effecting your tone just by choice of cable alone, these low to moderate capacitance cables do reduce the total amount of possible capacitance, and they ensure good quality connections using good quality plugs. Note: every time you connect a plug to a jack you risk losing some signal and increasing noise. There's a reason high end products use Switchcraft, Neutrik, and Amphenol connectors – you should, too, and you should keep them clean.

Second, be aware of the input impedances of your effects, from your guitar's volume pot to your pedals all the way to the amp. Any good tech can modify the input impedance of most circuits to maintain proper loading.

Third - hey, what if the pedal is "true bypass"?

While we'll discuss "true" and other bypass methods in an upcoming article, for now let's just say that it is both good and bad. It's good because most effect circuits used to be tied to the input jack at all times, which caused terrible loading problems making all the symptoms we've discussed even worse. But it's not perfect.

First, when the true bypass pedal is off, cable capacitance is in play and the resonant peak of your pickups is changed.

But then when you turn the true bypass pedal on, most of the time the output of the pedal is buffered, so cable capacitance and resonant peak changes aren't a problem at that point.

Sounds great, but the problem is you set the amp for a certain sound when the pedal is off. Turn up the highs and turn up the gain to compensate for all the tonal loss encountered. But then when you turn on the true bypass pedal, the overall sound gets brighter and louder, so now your amp is too bright and too loud. And if the pedal in question is an overdrive pedal, hey, your amp is brighter when the overdrive pedal is on, and you can better hear the noise floor of the overdrive pedal. That's backwards from what most players want.

So true bypass, by itself, isn't the solution.

We recommend using a very high quality buffer in your signal path. No, not one in a Boss or Ibanez pedal – those are terrible and grainy sounding. We'll be doing a full article on buffers soon, but for now let's say the ideal buffer is one that adds no noise and doesn't change the sound or feel of the guitar but magically makes the output very low impedance.

Once the signal is very low impedance then cable capacitance is no longer an issue. Whatever the resonant peak characteristics of your pickups are when they go into the buffer are what they remain no matter how long the cable becomes after that point. Post buffer you no longer lose signal and the signal is less susceptible to picking up noise.

Perfect, right? Well, almost.

With the caveat that no buffer is "perfect" (ours included) it is possible to get a buffer that is "practically perfect" (well above and below the frequency range of the amplifier). But very good quality buffers are relatively expensive. As we now get into an area where we're talking about “the competition” we won't name names here, but there are very good sounding buffers available today, and we are confident that our Psionic Audio buffer (which is built into every production pedal we offer) more than holds its own against anything else on the market.

So yeah, that's our big sales pitch - if you're serious about your tone and you use pedals, you should buy a buffer. We make a good one, but even if you don't buy ours, buy one.