FAQ - Headroom

Do I really need to read all of this?
Headroom vs Battery Voltage?
What happens when you clip?
How about "rail to rail" electronics?
How much voltage is needed not to clip?
Waveform of Slapped E
Slapped A
Slapped D
Slapped G
Popped A
Popped D
Popped G

Do I really need to read all of this?: Not really - this is fairly geeky stuff.
The punch line is you only need 1 battery for most pickups.
2 batteries are useful for really hot pickups when combined with a lot of tone control push.

But a longer technical discussion follows if you're interested.

Headroom vs Battery Voltage?: Headroom is the amount of extra voltage you have above the max excursions of the voltage passing through a preamp.
If you use an 18 volt system (2 batteries) then your power supply voltage will double for the preamp.
Having extra headroom never hurts but it is normally not required for our preamps.

What happens when you clip?: If an electronics system needs a larger voltage than a power supply can deliver then it will clip the signal.
In electronics we think of signals in 2 different ways, the time and frequency domains.

In the time domain you think of the waveform in Y (voltage signal level) vs. X (time).
A oscilloscope waveform as will be shown below is a classical time domain tool.

In the frequency domain you think of the waveforms in Y vs. Frequency.
A spectrum analyzer is a classical frequency domain tool.

To understand many problems it is much easier to think in one or the other domain.

Understand the audible effects of clipping is easier in the frequency domain.
In the frequency domain clipping will be represented by a series of even and odd high frequency harmonics and this sounds both "bright" and "harsh".

Some of the issues around the details of the clipping process are easier to understand in the time domain.
When the signal must get clipped ideally the signal goes flat at the max/min supply voltage then recovers quickly when the signal excursion reduces which is how our circuits behave.
Some other electronic designs exhibit 2 common faults:
1) The signal can get stuck at the rail for an extended time. If this time runs into the milliseconds then it is audible.
2) The signal output level can actually invert. As the signal level slews to the other extreme it will be very audible.

The Audere preamp when clipped goes flat against the rail then recovers very quickly but you will clip the input of your power amplifier when plugged into a passive bass input (as we recommend) long before you clip the Audere preamp.

How about "Rail to Rail" electronics?: Many current op amps feature "Rail to Rail" output voltage swings. This design approach will allow a system to be constructed with smaller power supply voltages compared to the signal levels. However, I do not currently use this style of electronics. Sorry this is going to get very technical but I will keep it simple as is possible. If you look carefully at the design of a rail to rail system then as the signal level is moving toward a rail you have to transition from a P type semiconductor to a N type semiconductor when the signal level is not close to the 0 voltage level. P type and N type transistors almost never have the same performance characteristic. I considered using several "rail to rail" parts but I can measure (and often even see in the waveform) the extra distortion this creates at a lower voltage level than the clipping point of more conventional designs.

How much voltage is needed not to clip?: This depends on 3 factors:
1) How large is the input? Scroll down as I have included oscilloscope pictures of a PJ bass being slapped and popped really abusively (let's just say you probably would not let me touch your bass if you saw me do this...) in Mid Z-Mode. This was a 4 string bass and slapping a 5 string can also be louder on the Low B. You have to watch the level in Low Z-Mode as this is adjustable and you can set it really loud (not recommended). But in Mid Z-Mode a 5 string slapped would in all cases be less than +/-0.5 volts peak - in Low Z-Mode I would set it for less than 0.7 volts peak.
2) How much gain is taken by the electronics? The Audere preamp has a gain of 1.0 assuming Mid Z-Mode and the tone controls are in the center detent. So the only gain in the tone controls - maxing the treble and bass gets you 15 dB or the output is 2.5 times the input. So the output signal level is running 0.7 x 2.5 or 1.75 volts peak or 3.5 volts peak to peak.
3) How much overhead is required? Overhead is required with any good electronics design for several reasons - for example, you do not want to hear a pop as the electrics explode because you connected to battery backward for a second as you were installing it. Our overhead level is about +/- 1 volt all together.
Putting this all together taking a 9 volt battery - age it to 7.2 volts remove 2 volts for the overhead then subtract 3.5 for the signal and you still get 1.7 volts of headroom. So in this case clipping or other forms of distortion would not be possible. If you run some really hot pickups like Dark Stars - you can get close to clipping; I tested a Dark Star and there was not a lot of extra signal range.

Waveform of Slapped E:

Slapped A:

Slapped D:

Slapped G:

Popped A:

Popped D:

Popped G: