On this page I plan to provide some useful information that hopefully won't be too technical. My objective is to add something every week or so.  Much of the information is somewhat subjective and I welcome your feedback, whether you agree or disagree. 

How do I open a QBG preamp to change the battery?

Why do I need a preamp?

What does the High Pass control do?

What should I look for in a preamp?

I have an electric bass.  Are your preamps useful for me?

Do your preamps work with active pickups?

Why are some of your boxes made of wood?

How does the QBG1 compare to other preamps?

What is a dBu?


Help! I have a QBG1 or QBG2 and my battery has died. How do I open the thing?

Simply unscrew the black nuts from the two quarter-inch phone jacks and slide the top cover back. I wanted to make sure I can open the preamp without needing any tools, yet have a solid design that can stand a bit of abuse.

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My amp has high input impedance. Why do I need a preamp?

There are two answers to this question.

First, to get great sound from your transducer, a good quality preamp isn't a luxury, it's a necessity. One of the main functions of the preamp is to buffer the transducer by providing a high impedance resistive load as close to the transducer as possible. This maximizes the frequency response of the transducer and minimizes the likelihood of hum and noise pickup.

Even though your amp has a high input impedance (say 10 megohm) the capacitance of a long cable will provide a substantial capacitive loading which will affect the frequency response of your system. The low output impedance of the preamp can easily drive this capacitance and maintain the frequency response. The low impedance also makes the setup much less sensitive to noise pickup.

Second, there is the matter of convenience. Personally, I like to have control of my sound within easy reach. To avoid feedback, my amp is some distance away. So if I didn't have my preamp and wanted to change my volume or adjust my tone, I would have to reach way over or even get someone to quickly hold my bass while I fiddle with my amp.

With my preamp, all I have to do is reach down to my tailpiece, so I can make adjustments virtually on the fly.

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What does the High Pass control do? I don't hear any difference when I adjust it

The High Pass filter is not a tone control. It is intended to reduce the level of any frequencies below those generated by the strings of your instrument. These frequencies are typically not audible but can really stress your amplifier or speaker. I've made it adjustable so that the lucky folks with a low-B (or C extension) can get the full tonal range of their instrument while the rest of us can optimize our setup for an E string.

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What should I look for in a preamp?

That is a matter of opinion. For what it's worth, here is what I think:

There are also a few nice features that can improve your sound even more, such as:

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Do your preamps work with electric basses?

The short answer is: they work great with electric basses. While this was not originally my target market, I've sold quite a few of my preamps to electric bass players who like the small size, excellent performance and extensive features, all at a great price. The DI alone is worth the money. The high input impedance isn't required for electric bass but it doesn't hurt, either. My one caveat is that the enclosure isn't designed to be stepped on, so be careful.

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Do your preamps work with active pickups?

The M0070 and QBG1 should work with all passive pickups, including piezos and magnetic types. This covers the vast majority of transducers in use. They will not work with active pickups or pickups that require a bias voltage. The one example we have encountered where our preamps won't work is the Schertler STAT-B series. These pickups require a bias voltage and have a low output impedance. We have found a solution for this which involves making a minor modification to the circuit, so if you have an active pickup like the Schertler, please contact us before you buy one. We would be happy to make this modification at a reasonable cost.

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Why are your QBG1D preamps in a wooden enclosure?

Wooden enclosures offer several significant advantages.

Of course, wood isn't as strong as metal and so our QBG1D enclosures are a bit fragile. When mounted behind the tailpiece they are well protected and should give years of service.

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How does the QBG1 compare with other leading preamps?

Our goal was to make a preamp that was as at least as good as anything we could find on the market. The table below shows the published specifications for some other fine preamps compared to the QBG1. If you know of a preamp we should be including in this comparison, please send me a link and I'll do my best.

Disclaimer:  Please keep in mind that everybody (us included) specifies their products in a way that makes theirs look best. This means it may not always be possible to make apples-to-apples comparisons. Use these specs as a guide if you wish but it's up to you to do your homework.

ModelQBG1Pro-EQ Platinum BassPre-A 111Pure XLR
THD+N @ 1 kHz0.03%0.05%0.05%Not Spec’ed
Residual Noise10 microvolts27 microvolts35 microvoltsNot Spec’ed
Max Input Level+13 dBu+10 dBu-7 dBuNot Spec’ed
Max Output Level+13 dBu+10 dBu+15 dBuNot Spec’ed
Input Impedance10 M or 1 M10 M0.01 M1 M
Size - cm
Not Spec’ed12x12x4
Not Spec’ed
Weight - grams
135 -incl. battery
Not Spec’ed
Gain (main out)-3 to +20 dB *Not Spec’ed? to +41 dBNot Spec’ed
Features and Controls
Clipping indicator?YYNN
Low Battery indicator?YYN/AN
Phase switch?YYNY
Adjustable low pass?YYNN
Impedance switch?YNNN
Pre/Post EQ switch?YYNN
Ground Lift?YYYN
Phantom Power with Ground Lift?YYNot Spec’edN
XLR Direct Out?YYYY
Phantom Power in?YYYY
Enclosure materialReal woodSteelCast aluminumAluminum?
Fits on instrument?YNNMaybe
List price (US$)$189$210Approx.$450$210

* The gain of early versions of the M0070 is adjustable between -6 and +6 dB.

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What is a dBu?

The short answer is that it's a logarithmic unit of signal strength. It stands for decibel-unloaded.
0 dBu = 0.775 Vrms. The derivation of this value is a bit technical.

The original use of the dB (decibel) is to provide a convenient way to quantify the ratio of two power levels. Mathematically, this is described as dB = 10*log(P1/P2), where P1 and P2 are two arbitrary powers. This use can be used to define the gain of an amplifier. For example, an amplifier with 20 dB gain would have an output power that is 100 times greater than its input power, since 10*log(100) equals 10*2 equals 20.

This relationship only holds true, however, if the amplifier has a defined input and output impedance. Most audio amplifiers have a high input impedance and low output impedance. In such cases the definition of a dB is stretched a bit to dB = 20*log(V1/V2). In the example amplifier with 20 dB gain, the output voltage would be 10 times greater than the input voltage. This voltage definition is valid whether or not the amplifier has a defined input and output impedance.

A common reference impedance level for audio circuits is 600 ohms. In a matched 600 ohm audio circuit, both the source impedance and the load impedance are 600 ohms. The relationship between power and voltage for a resistor is P = V^2/R where V is the rms voltage and R is the resistance. If the source level in a matched 600 ohm circuit is set to provide a power of 1 milliwatt (mW) to the load, the AC voltage across the load will be 0.775 Vrms. This voltage level is used to define the reference voltage level if the "load" is not 600 ohms but a high impedance. When the "load" is a high impedance the circuit is said to be unloaded, hence the dBu.

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