FREQBOOST: An Adaptation of C. Anderton's Frequency Boost
Optimized to add a 400 Hz "Sting" to the SRB-EQ
last update: June 12, 2024
Copyright 2024 by H. Gragger. All Rights Reserved. All information provided herein is destined for educational and D.I.Y. purposes only. Commercial re-sale, distribution or usage of artwork without explicit written permission of the author is strictly prohibited. The original units  with their associated  trade-names are subject to the copyright of the individual copyright or trademark owner. The Author is by no means affiliated with any of those companies. References to trade names are made for educational purposes only. By reading the information provided here you agree to the Terms of Use.
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Index

Treading On The Stinger
Selecting A Suitable Schematic
Design Criteria And Remarks
Verdict
Sound Samples
Reference
Update History

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Treading On The Stinger

In an earlier article the Author has described a wrapper around the on-board EQ section of a Music Man Stingray Bass (hereafter: SRB), which is an indispensable prerequisite to make the EQ functioning without being directly connected to a low impedance pickup, for the purpose of lending Stingray flavour to a generic bass by means of a stomp-box.

A list of criteria had been compiled in an attempt to describe what constitutes the SRB's tone.
After having listened to a lot of recordings, the Author felt that beyond all the criteria already found, there was something else lacking. From a frequency perspective, the SRB heard on said recordings has something that resides between an "o" and an "a" vowel, which  cannot be attributed solely to the pre-amp electronics.

While different maker's pickups in general sound different on their own, individual coils or rather windings will yet sound different depending on the mounting location (relative to the length of the vibrating string) and different if combined too.

Combined windings tend to cancel out or enhance certain frequency bands, so it were not further surprising if the SRB's pickup, which is a parallel combination and resides in a (back then) rather unconventional position on the instrument, would exhibit a peculiar boost or deficit somewhere in its spectrum.

Accordingly, simple R-L-C pickup simulations cannot possibly account for any of those effects, so all of those models used hitherto for evaluating the combination of SRB pickup and its preamplifier are necessarily ignorant of them.


Playing with the 10 band graphic EQ on the amp quickly revealed that the frequency of interest resides around 400Hz, which indeed lends a certain sting to the bass guitar. A small amount of boost there gives it a lot of that quality, a certain aggressiveness. It reminds of a dog baring its teeth and growling. This growl seems to an extent be prevalent on basses with J-Bass type pickups, and not surprising, the (early) SRB has its coils just about in the very region.

An according filter was built and  housed in the very SRB-EQ stomp box, which at the flipping of a toggle switch evokes this   quality at a whim.

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Selecting A Suitable Schematic

Looking for a suitable schematic, the purple peaker by Dan Armstrong came to mind. However this seemed a bit over the top for the purpose because it was a high Q circuit and rather inflexible. The preferred choice was an adjustable one, since too much of a boost here will definitely sound obnoxious. It is better to rather feel something without being able to put the finger on it, and then feel something is missing when the effect is turned off. Also, maybe a slightly smaller Q resp. broader bandwidth filter would be less obvious. High Q filters are to be applied with caution.

Looking through the archives brought Craig Anderton's Frequency Booster (hereafter: C.A.) to the daylight. Unfortunately, this was rather a circuit sketch than a complete unit, since it needed to be driven by a buffer. A later published, only slightly more complex version that fixes this flaw, had appeared in a book[1] under project #10.

A fellow tinkerer, Vivek Mehta has commented on this on diystompboxes[2]. He identified it as being actually a Sallen&Key filter, which has adjustable Q (by circuit gain) and is adjustable in the boost amplitude. He has rearranged the circuit slightly to achieve a zero gain condition on the pot's minimum, whereas Anderton's version starts from 3 dB.  Perfect.

Note: In retrospect, for the purpose finally used, the original version would have been fine, because the boost section is permanently wired to a value greater than 3 dB. However, in view of providing for potentially making  it a generic control, Vivek's version was preferred. Sonically, there is no difference.

This circuit had been destined by C.A. to be a self-sufficient stomp-box, but several things had to be adapted for the particular application.

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Design Criteria And Remarks

The following are random thoughts in no particular order:
  • A low-pass filter was included on the front stage to kill all RF intrusion. It is puzzling why this is still neglected.
  • Note the clever biasing. The booster stage needs to be driven by a low impedance section. Both stages are DC coupled, thus all capacities and the need for further biasing are avoided.
  • The filter stage according to Vivek Mehta has the frequency forming network rearranged to meet the 0 dB boost condition. Provision has been made on the PCB for paralleling caps for fine-tuning the resonance, but 47 nF worked out of the box.
  • The achievable Q depends on the matching of the caps, however no higher Q's are envisioned.
  • A small ceramic tailoring cap has been added to guarantee stability. It may be redundant.
  • A build-out resistor has been added to help tame switching pops, if this is the output stage. It also serves as one half of the output attenuator.
  • With increasing boost factors, the overall loudness increase is noticeable upon switching to bypass, but this lies in the nature of things.
  • For the add-on version, no volume control is necessary, for its gain is not far from unity and the SRB-EQ has a volume control. For a stand-alone box, a volume pot may be added according to C.A.'s original drawing.
  • Note the low impedance split-rail network.
  • Note the crafty bypass switching. The circuit gets its DC reference from the preceding stage. When the ground leg resistor is tied to ground via a capacitor (the usual method), the circuit works. Alternatively, saving a capacitor, it can be tied to the reference voltage. When this connection is broken (e.g. by a switch as shown) then the ground leg resistor is floating and the op-amp reverts to a +1 follower. This is not a true bypass circuit, but this is not necessary since there are essentially two buffers in series. Switching is clickless.
  • If a stand-alone stomp-box version were envisioned, a true bypass variant would probably be preferred.
  • There is one snag. The gain of IC1B is two when enabled, while the follower's gain is one. But there is an easy solution to that. A 1:1 output divider tied to the "bypass" switch (via a cap of course) knocks the voltage down, whenever there is a low impedance in view - such as UB/2. This is a low impedance network - at least AC wise. In a stand-alone version, if a true bypass were used, the effect would be permanently jumpered "on". In this case, the associated SPST switch can be eliminated and with it the output divider.
  • If somebody wanted to incorporate this buffer plus peak filter into a complete (re-) design together with the SRB-EQ, it can make the discrete buffer in front of  SRB-EQ version 1.1 redundant; yet no harm happens if it remains there (such as in the Author's prototype). Note however, if you put the two designs together in one melting pot in order to create a single PCB,  there should be provision for proper power supply filtering and all that, as shown in SRB-EQ V1.1. Note also that one of the resulting series capacitors (the bigger one) should be eliminated.
  • For the finished prototype, the FREQBOOST landed behind the SRB-EQ. No big difference was apparent if it was in front.
  • The Author felt, that this boosting function needed no performance control, so the trim pot used for the evaluation phase remained there, as this was destined to remain a set-and-forget adjustment. A boost of 7 dB (approximately 3 o'clock on the pot) worked well.
  • A single pole toggle switch utilizes C.A.'s "bypass" circuitry for de/activation. The Author does not recommend to hard-wire this filter permanently on, since some bass may come along that already incorporated this sonic quality and may not benefit from an additional accentuation.
  • Since the SRB is a one-trick pony, there is only one sound. More flexible basses (like the Author's L-2000) may provide surprisingly different tones. Some of its pickup combinations seem to completely swallow that boosted range - which is in order with respect to diversity. Anything beyond would make all tones sounding equal.
  • The SRB-EQ's "flat" setting (on the Author's unit) is: bass and treble 11 o´clock, volume 1 o´clock.
  • The Author looked at the later SRB's 3 control EQ (known als 3EQ). Its mid control acts upon a different range, so this is no replacement for the unit suggested here. Bass and treble control work differently too. They do not duplicate the 2EQ. Still cudos to the designer(s) who again managed to cram a huge amount of control into a space that small...
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Verdict

The original goal of the SRB-EQ was to add Stingray flavour to a different bass, meaning its heavy boost of low and high frequencies. In this light, adding a full-blown mid-control to a tonally well balanced bass seemed counterproductive. Still the colouration introduced by emphasizing a mid frequency band to a fixed value (which, if it is there at all, is intrinsic to the SRB due to its static configuration and thus not variable) seemed alluring.

Logically, some time was invested into the simulation of the late Stingray's 3EQ, which turned out well spent. However, the 3EQ is sufficiently different in its response by means of mid and high centre frequencies, filter bandwidth and low end, that going the 3EQ way appeared to steer away from the original concept. Which does not mean that it is a bad EQ by any means. It's different.

The path chosen, using the 2EQ (commonly deemed the Stingray tone) embodied in the SRB-EQ followed by a fixed (yet defeatable) boost at 400 Hz seemed like a perfect match. As the situation demands, this effect should be turned on and off, because it will not always augment tone. But as with everything, the beauty is in the ear of the beholder...

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Sound Samples 
The subsequent recordings have been done using the following setup and no further processing:
  • L-2000 modified, pickups as specified, treble down to 1/3. EB cobalt flat strings.
  • SRB-EQ bass and treble 11 o´clock, volume 1 o´clock.
  • FREQBOOST (internal) boost pot @ 3 o´clock
  • Warwick Pro-Tube IV, direct out
  • Recording device: Focusrite PC interface into DAW (all recordings untreated)

Note: "inside pair" means _N║B_ and "outside" means N_ ║  _B. Those options are not available on a stock L-2000.
"Native" in the current context means stock B║B.
First half of recording respectively: L-2000 through SRB-EQ (flat), FREQBOOST off. Second half of recording: FREQBOOST on. Listen to both in a sequence, then wind back to the start and notice that there is "something" missing.
Note the change in loudness. Nothing else was changed.

Comparison parallel inside pair ( L-2000 modified) w/ SRB-EQ flat and boost off vs. boost on
Note how the 400 Hz boost is rather swamped by the bass content. 


Comparison parallel native bridge ( L-2000 stock) w/ SRB-EQ flat and boost off vs. boost on
Note the typical J-Bass style bridge pickup "bark" and how the boost emphasizes this. Bass is naturally reduced.
This is a situation where one might consider not using the effect because it may be too much.
 

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Reference

 [1] Craig Anderton: Do-It-Yourself Projects For Guitarists (#10, p. 34f);
     https://kupdf.net/download/diy-projects-for-guitarists-craig-anderton_5b0c82aee2b6f5ad7608d1cd_pdf
[2] diystompboxes.com (Vivek Mehta):  LTSPICE analysis of Craig Anderton Frequency Booster (Project #10);
      https://www.diystompboxes.com/smfforum/index.php?topic=129972.0
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Update History
  • June 12, 2024: first release
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