I had an enquiry about using the new unbalanced piezo preamp to buffer a guitar lead and I thought I would do a write-up on the low-down of buffering and bypassing.
The basics of impedance and the effect of long guitar cables.
When we talk about impedance, we are talking about resistance to the flow of current. In a d.c. circuit we call this resistance – but music is an alternating (a.c.) signal and for a.c. we call resistance impedance. The important thing is that the amount of impedance is dependent upon the frequency of the signal.
If we measure the d.c. resistance of a guitar lead with a multimeter, the resistance will be practically zero (no more than a few Ohms for the longest of leads). If we measure the impedance of the guitar lead at low frequencies (from say 20Hz to 2kHz) the impedance will be low and the signal will be passed with very little attenuation (i.e. without reduction in the amplitude or level of the voltage). However – as a result of the guitar lead capacitance – above 2kHz or so the impedance of the cable rises and higher frequencies are attenuated. This has a similar effect as turning the tone control down on your guitar – high frequencies are lost and the sound initially becomes less bright and sparkly and progressively gets duller and muddier as the length of the cable increases.
Every guitarist should be familiar with the idea that long guitar cables sound less bright than short ones – no matter what make or model of cable.
Power transfer and voltage transfer
If we are sending a high-current signal down a lead (such as connecting a loudspeaker to our power amp) or we are sending a signal over a long distance (such as a telephone line) we are concerned with transferring maximum power. The circuit impedance at the driving end of the cable is called the source impedance and the circuit impedance at the receiving end is called the load impedance. We can show with some simple arithmetic that maximum power is transferred when the source impedance equals the load impedance. However, with a small signal (such as the output from a guitar pickup) over a short distance we are usually concerned with maintaining the voltage of the signal.
Low impedance source – high impedance load
In order to ensure we don’t lose voltage as we transmit and process our guitar signal we want to have a low(er) source impedance and a high(er) load impedance. This is because some small amount of current will flow in the circuit and a high impedance source can not deliver very much current – whereas a low impedance load will demand significant current. If the source cannot deliver the current demanded by the load, the voltage (at the load resistor) will drop noticeably – i.e. the signal will be attenuated.
We can compensate for the reduced signal by turning up the gain in the following stage, but amplifiers act on signal and noise, so it is better not to lose signal in the first place. Also, signal lost in a long guitar lead is frequency-dependent so turning up amplifier gain will make a muddy signal louder but will not restore missing high frequencies.
A buffer is a device which takes a high impedance source and converts it to a low-impedance source. Usually buffers have unity gain – which means that the voltage at the output is the same as the voltage at the input.
The important thing about buffers is that they stop the guitar-lead from filtering out the high frequency content of the signal.
Here is (nearly) the simplest possible buffer circuit for guitars (this type of circuit is called a JFET source-follower):
Every guitar player should build this design and hear the difference it makes to their tone. Even with a short lead you will likely notice that your guitar sounds bright and sparkly. Without this circuit in place your guitar will at best sound comparatively mellow and at worse (with a long guitar lead) sound positively dull. Of course some guitar amps will have circuitry to compensate for a dull signal coming in by having a “bright” switch – but having a bright switch is not the same as not losing your highs in the first place.
There are two places you can put this buffer; ideally, you buffer the signal before the guitar lead – and this is what happens in e.g. active bass guitars which incorporate electronics and a battery. However, if you have a passive guitar and you don’t want to make it active, you have two choices. Firstly, use a pedal board in front of the amplifier and use a shortish guitar lead. The lead from the pedal board to the amp can be longer. Secondly, you can put the buffer at the end of a long guitar lead – but this is the less satisfactory option.
Buffering – Executive Summary
- Long guitar leads cause loss of high-frequency tone from the signal.
- Ideally, the guitar pickup should be buffered before driving a long lead.
- If the guitar is not active, buffer at the earliest opportunity (before effects pedals).
- Wireless guitar transmitters buffer the signal as part of their design.
The effects chain and amplifier effects loop
In the first part, we considered only the case where a guitar is connected directly to an amp. If we want to add in effects pedals we have two options – we can either place the effects before the amplifier or we can place the effects in the amplifier’s effects loop (if it has one). If you aren’t aware, the effects loop is the two jacks on the back of the amp labelled effects send and effects return. The effects send is the signal coming out of the preamp and the effects return is the input to the power amp (or to a later section of the preamp). If you do not use the effects loop then the send is connected to the return internally in the amp.
The advantage of using the effects loop is that the guitar plugs directly into the amp and the amp acts as a buffer for the guitar signal. Of course, if the amp does not have a high (enough) input impedance then we still might lose some high-frequencies with a long cable.
The disadvantage of using the effects loop is that the effects pedals get the signal after the guitar preamp (which may contain the main overdrive stage for the amp) so some of the effects may be in the wrong place in the chain.
Bypassing is sending the guitar signal straight past an effects pedal circuit when the effect is not in use. The idea of true bypassing comes about because back-in-the-day a lot of effects pedals had a switching arrangement where even when the effect was out of circuit (bypassed) it still had an influence on the signal. This situation arises because you need a three-pole switch to do true bypass and light an LED and back-in-the-day three-pole switches were more expensive and less reliable than two pole switches.
With a three pole switch, in the bypass position (as shown above) the effects circuitry is completely disconnected from the guitar signal.
With a two pole switch, in the bypass position (as shown above) the input signal is always connected to the effects circuitry. This is an additional load on the signal which causes “tone suckage” (a bad thing).
One way to get around needing a three-pole switch is to implement a 2-pole latching relay operated by a microcontroller. A separate output on the microcontroller operates the LED. This system has the advantage that almost any type of switch (momentary or latching switch action) can be used.
There are other ways to do bypass such as the BOSS/Ibanez method which uses JFETs to switch the signal. Also, there are methods to do true bypass and LED using a 2-pole mechanical switch, such as RG Keen’s millennium bypass circuit.
Bypassing – What is the take away?
- Without true bypass an effects pedal may cause tone suckage even when it is bypassed.
- Arguments about 3-pole mechanical foot-switches being less reliable than 2-pole switches may still be true.
So what is the best arrangement for me?
If you watch YouTube channels like Premier Guitar’s Rig Rundown series you will quickly realise that every touring guitar player and their techs have their own idea of the right way to put a system together. Some systems are very simple and straightforward and some are mind-bogglingly complex.
The best advice we can give is the following. Whatever your setup:
- Make sure that you preserve signal integrity as much as possible.
- Better to keep a good original signal than to compensate for tone suckage.
- Better to use tone controls to cut unwanted brightness than to try to restore brightness that has been lost.
Finally, there isn’t any special sauce which is the only true way to solve problems of tone suckage due to long leads and poor bypassing. Buffering addresses the long guitar lead problem and there are many perfectly good buffering circuits out there. True bypass simply takes away the problem of tone suckage in poorly designed effects.
See here for our article on building your own buffer.