Double Bass QBG1 Pro Preamp

[synaesthesia]

Anyone got a report on this item?
Link Removed

 

[drurb]

Looks like a really nice unit. I'd like to see specs on the high-pass filter, specifically, the range of the cutoff frequencies achieved by rotating the knob and the slope of the filter.

 

[Jaromir]

It looks like it has all the features I like: the phase switch, HPF, pre/post switch and even impedance switch (1 or 10 Mohms).
Now the question is what is the basic sound of this unit, that's the most important thing...

 

[Francois Blais]

They say their M0070 preamp has the same functionality. (it's their pre-production model)
The M0070 specsheet says:
Rumble cut range (‐3 dB) 10 Hz (clockwise) to 37 Hz (counter clockwise)

The slope is nothing like Fdeck's HPFpre though, with -3dB.
Francis's unit has -12dB/octave, and its range is 35-140 Hz.

 

[Ric Vice]

Anyone got a report on this item?
Link Removed

The site lists two versions of the pickup one for a Realist Style Pickup and one for an Underwood Style Pickup. That's kind of interesting since the Realist has a very hot signal vs the Underwood and the timbre of those pickups is quite different. It's not setup for blending two different sources together. I like the adjustable input gain, that's something that the Headway EDB-1 dosen't offer.

Ric

 

[drurb]

They say their M0070 preamp has the same functionality. (it's their pre-production model)
The M0070 specsheet says:
Rumble cut range (‐3 dB) 10 Hz (clockwise) to 37 Hz (counter clockwise)

The slope is nothing like Fdeck's HPFpre though, with -3dB.
Francis's unit has -12dB/octave, and its range is 35-140 Hz.

I'm sure familiar with fdeck's unit as he and I collaborated early on concerning the choice of cutoff range and slope. The spec of -3 dB on the M0070 is the "3-dB-down corner frequency" and not the slope. The "corner" or "cutoff" frequencies are often those at which the response falls by 3 dB. So, in English )), that spec tells you that you can vary the cutoff frequency (3-dB-down) point from 10 Hz to 37 Hz. Unfortunately, it says nothing about the slope.

I use fdecks's HPF strictly to get rid of infrasonics so an upper cutoff of 37 Hz would work fine for me. Many players, however, use the fdeck HPF to cut some of the "boom" in difficult situations and that requires being able to set the cutoff substantially higher.

I see no practical use for HPF cutoffs below 20-25 Hz. Here's why. Consider how a HPF works. We'll assume that the slope is -12 dB/oct. If you set the corner frequency at 10 Hz, then the response will be down 3 dB at 10 Hz and down 15 dB at 5 Hz (12 more dB an octave below the cutoff). This means that anything between about 10 Hz and higher is untouched. In practice then, you haven't affected the really pesky infrasonics (around 20 Hz and below) much at all. Now, consider setting the corner at 35 Hz (fdeck's lowest point). You'd be 3 dB down at 35 Hz (already below the fundamental of the E), 15 dB down at 17.5 Hz, 27 dB down at 8.75 Hz, and so on. Now, you're talkin! That kind of setting will zap those infrasonics. fdeck's unit allows the choice to set the cutoff higher to tame potential infrasonics and boominess. IMO, a far better choice of parameter values.

 

[synaesthesia]

reply from Microlynx
"There are two poles, one at 16 Hz fixed and one that is variable using the
High Pass control. The combined 3 dB point can be varied between 20 Hz and
40 Hz. Below 20 Hz the roll-off is 12 dB/octave.

The high pass itself doesn't have a bandwidth but the overall response (with
tone controls flat) rolls off above 20 kHz at 12 dB/octave."

 

[durtler]

IMO it's best to separate the two functions. The high pass is just there to attenuate the sub-sonic frequencies. By that I mean the frequencies that can't be genrated by the strings of your instrument. I've spent a fair bit of time looking at the the voltages generated by various pickups and the act of pulling on the strings prior to release can create quite a high voltage. The spectral content of this is in the range of 5 to 20 Hz. Many heads have quite a bit of gain in this range and this creates the opportunity for vicious feedback.
The reason I made my high pass variable was because there are people out there with B strings. Below the B string frequency (about 31 Hz) the gain should roll off quickly, and in the range between the B string and E string (about 41 Hz) the rolloff is a bit less quick.
If you want to change the sound, use the tone control. The QBG1 (and M0070) have a shelving type bass tone control that can be used to cut back boominess (or boost a weak bottom end). It starts to kick in below about 110 Hz, which is about the G string.
There are many opinions on this so I welcome your feedback (pun intended).

 

[drurb]

reply from Microlynx
"There are two poles, one at 16 Hz fixed and one that is variable using the
High Pass control. The combined 3 dB point can be varied between 20 Hz and
40 Hz. Below 20 Hz the roll-off is 12 dB/octave.

The high pass itself doesn't have a bandwidth but the overall response (with
tone controls flat) rolls off above 20 kHz at 12 dB/octave."

What you're describing sounds like a cascade of 1) a -6 dB/oct HPF @ 16 Hz with 2) a -6 dB/oct HPF with variable cutoff. Bill Durtler and I have exchanged quite a number of emails in the past about design. Hi Bill! Glad you found the "string-pull" test useful.

The cutoff range of 20-40 Hz makes far more sense than the original report of 10-37 Hz.

By the way, those frequencies are "infrasonic," meaning below the frequency of sound, as opposed to "sub-sonic," which means below the velocity of sound.

 

[durtler]

Drurb, you are correct to describe it as a cascade. There are two other HP poles in the input side of the transfer function. One is inside the preamp at about 8 Hz. As you point out it doesn't do much. The other is formed by the capacitance of the piezo transducer and the input impedance of the preamp. This is the real reason I put a switchable impedance on the preamp. Most of the time the 1 megohm is best but some pickups have low capacitance and will lose bottom end at that setting.

Mea culpa on the terminology...