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The Sting Ray Bass-EQ
(SRB-EQ) A reliable wrapper around a (2-control pre-EB) MM Stingray Bass type pre-amp last update: June 10, 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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| MAIN PAGE>MUSIC STUFF>SRB-EQ Index  How It All Came AboutIdentifying
The Key Ingredients To Stingray ToneWhy A
Simple Outboard Amplifier Does Not SufficeFixing
The Mis-Match By Simulating The Presence Of A PickupExplaining
The CircuitPlaying
With The EQ Stage Component ValuesGain
Considerations, OPA Choice And Optional Post-GainThe Effect
Of Pickup Location And Promising Pickup CombinationsVerdictReferenceSound
Samples Random
Musings Update History
Back To Index How It All Came About
Back To Index Identifying The Key Ingredients To Stingray Tone It has been found that the SRB´s tone is
connected to several criteria:
The guy here plays a modded SRB that can be switched to passive. It still has that typical SRB tone. So much for that.
Warwick have nailed that part with their Corvette double buck, although they almost have gone over the top in the Author’s opinion.
A small boost in the 420 Hz region encourages this sound too. Astonishingly it seems rather nasal at first, which seems to contradict the SRB's otherwise gearing towards bass and treble. Of course it is just a generalization that the SRB is mid-deficient because of the clear favorization of treble and bass by the EQ's boost frequencies and not a proven rule. This hump here must stem from the way the pickup coils are spaced and magnetized.
Why A Simple Outboard Amplifier Does Not Suffice So the most
obvious approach would be to reproduce the SRB´s
electronic part and stuff it into a stomp box
enclosure, which others have done... However,
there is a snag with that. Others have stated[1], that this preamp
and the pickup itself are an inseparable unit whose
components hugely interact and thus cannot be viewed
individually.
There's
reason for that interaction. Unfortunately those
things cannot be measured readily on a real
instrument. They can however be simulated on the
PC. At least one merit for those things. The SRB Pickup On Its
Own (Unloaded) An unloaded pickup in general, meaning a pickup on its own, has a resonance determined by its parameters R,L,C. All three form a series resonant circuit that produces a hump in the frequency plot with a specific centre frequency and height (Q) that depends on the interplay of those parameters and a subsequent roll-off with a second-order steepness. Any external components present (as in all generic electric bass instruments) like wire capacity or voicing caps in combination with load resistance (like pots) modulate this towards smaller values. The interaction of those components is deliberately optimized to yield a pleasing tone, where the roll-off usually resides in the frequency range of interest. This is all well known and documented[2].The SRB's pickup is no exception to this, but its purported low inductance, low resistance, medium capacitance (unloaded) would result in a even higher resonance frequency, way beyond the frequency range of interest and way too high to play any role in sound shaping. Simulations (with the traded parameter values) have yielded a 12 kHz roll-off paired with an enormous Q (likely caused by the low DC resistance, meaning low damping). Such a pickup is tonally essentially "out of the way", which transfers tone-shaping practically to the electronics. The SRB Pre-amp On Its Own (no resonance) But this is academic, since we will never be able to listen to such a pickup on its own. However, the simulation reveals clearly that a buffer amp inserted between the SRB's pickup and the SRB's preamp defeats both of them. Only both together work as expected. The pre-amp seems to need a certain load to work properly. Otherwise it does something no doubt, but this is certainly not similar to the original. The
Author thus takes the liberty to postulate:
This all shows that it is surely not correct to assume that a stock SRB pre-amp can be stacked onto a generic bass in a stomp-box manner without additional trickery, particularly if the bass contains active electronics (i.e. a buffer inbetween). It remains to be
practically determined how well such a unit yields
subjectively useful results in spite of this apparent
mis-match. Somebody may still like it as is... Fixing The Mis-Match By Simulating The Presence Of A Pickup Conversely, disembowelling a generic bass instrument in order to retrofit it with a SRB-type pre-amp may not yield the desired result too, because it is not guaranteed that this pickup will harmonize with the pre-amp the way a genuine SRB pickup does. Besides, this approach is of course disregarding all other factors like pickup position and growl. Read more about this subject in chapter The Effect Of Pickup Location And Promising Pickup Combinations. To recapitulate, the electronics wants to “see” a series resonance circuit on its input in order to work properly. So what would be
more obvious than to use one of the circuits that have
recently become
en-vogue to simulate the presence of a pickup in
front of a fuzz-box? The Author has seriously
criticised this approach since it is
curing symptoms but not the cause[3].
For a brief
moment, it looked like advocating a simulator was akin
to falling for the very same trap again, but
simulations clearly revealed, that the pre-amp does
not work without a reactive load in front as
discussed in the previous chapter. Several
renderings of such a circuit fragment have been
suggested and have been incorporated into practical
designs (read more in[3]);
all of them use a
small transformer, a cap and a resistor to mimic a
series resonant circuit.
Indeed such a circuit snippet has been used with great success in the subsequent design. This way it probably cannot come any closer to reality. Back To Index Explaining The Circuit
The stomp box’s
input uses a buffer circuit to drive the transformer.
A j-fet was chosen for low current, but any variety
would serve technically. The transformer
is not used for voltage transforming purposes, but
rather as a coil.
It has been found that for this purpose the primary
and secondary sides can be stacked to increase its
inductance (read more in [3]).
Since there are coil taps readily available, two have
been wired to a jumper to allow for the evaluation of
their individual sonic impact. Stemming from a project a while back, the Author does not recall how the inductance values were determined, although measuring inductances alone is fairly straight forward. As different, the Author has found that measuring the inductance in a series resonant circuit leaves a lot of lee-way for interpretation, which is why published numbers should be taken with a grain of salt in general[4], particularly if the exact measuring procedure is not declared. Measurements
have shown that with the bigger inductance setting
("high") treble control peaks around 10 kHz, whereas
the smaller one ("low") peaks around 12.5 kHz. A
bigger treble resonance capacitor (>1.8nF) changes
the control's frequency range towards low-mids. The
SRB layout uses 1.8 nF, whereas the Sabre II Bass'
layout uses 2.2 nF. Allegedly some later instruments
were seen with smaller values. Given some
imponderabilities, this choice probably remains a
matter of taste. The front buffer effectively separates the (generic) bass instrument from the SRB pre-amp, while the latter feels comfortable with the resonant circuit ahead. This way it can function reliably and repeatable as one would expect from a contemporary stomp-box effect unit. It will technically cooperate with any stock bass while maintaining full control functionality. As a matter of fact, it may even work with a passive SRB (custom) bass, as the guy shows it here. Playing With The EQ Stage Component Values Amongst the different schematics that are deemed “original”[5], there appear to be subtle variations in the pre-amp components as far as input resistance and feedback resistance go. Although it can be safely assumed that their effect was subtle overall (EB surely would not want to tarnish the image of his product by a unwise move), the author has experimented with those. Starting with
220k for input and 100k for the feedback resistor made
the whole effect more pronounced, but rather
unnatural. Unfortunately it messed with the control's
centre frequencies too. Changing to 100k / 220k was
slightly different. The basic character was present in
all instances, but the best version has been
found to be 220k /
180k. 180 k for the feedback resistor
produces enough gain that any subsequent amplification
(see below) is superfluous. Keeping product variability in mind,
the SRB’s pickup's parameters have been described as
1.5k (some say 2k), 0.7H, 120pF (on the
simulation, this peaks at 12 kHz) Note what a Music Man promotional site mentions regarding that subject:
The Author has
tried 1.2 k (together with the choke's DC resistance
this would be ca. 2k) and 330 pF for the pickup
emulator, which results in a 10 kHz resp. 12.5 kHz
peak. Close enough. Both have no influence on the
pre-amp's control frequencies. Those are directly
related to the feedback resistor and input resistor. Note that using a different transformer might ask for different values here due to its different inductance and DC resistance. For this reason, the Author clearly refrains from claiming that the devised unit sounds exactly like a genuine original bass. Please take note that not even two seemingly equal instruments are said to sound the same. Although finding
the overall result very pleasing at this point, the
author has played with the treble filter capacitance
of 1.8nF. Another 160pF that were at hand were wired
in parallel but did not make any perceivable difference. The treble pot
uses a pretty uncommon anti-log (C-suffix)
pot. This stems from the fact that by far the most
effect is crammed into the 0-100k range of the pot,
which by the antilog resistance track gets spread over
a bigger rotational travel for the user to appreciate.
A resistance of zero means maximum treble, which is
why any tolerance spread of the pot is totally
inconsequential. The awkward
resistor R12 in the feedback network that is only seen
in the Sabre, does not do anything apparent
neither on the simulation nor in rehearsals. It
appears to be necessary if the bright function
is wanted. However, the bright function does a little
on the simulation, but is inaudible in real listening
experiments, which coincides with observations from
musicians. Nobody could discern if this switch was
doing anything.
For those interested in details concerning the pre-amp's bass and treble controls and messing with their component values, there is some good information under the above link. Gain Considerations, OPA Choice And Optional Post-Gain Other tinkerers
have felt that the SRB circuit has an overall gain of
less than one, which for an on-board pre-amp is
inconsequential, but is highly annoying for a true
bypass setup (something that was neither possible nor
necessary in the original on-board unit). Gain depends
on the choice of the above resistors, and it also
rises the question of “concerning what frequency
range?”, taking the huge tonal variation into account
the circuit potentially introduces and the subjective
perception of loudness with respect to frequency
content.
Should the kind
reader want to change one of the aforementioned
resistors, a small
utility post-amplifier as the one depicted in
the following may become necessary to achieve a
perceived gain of unity upon bypassing.
Conversely, a different input
stage (e.g. using an OPA) with a gain of 1.5
or 2 can be used instead. By the way, the linear
trim pot that was used as the prototype's volume pot
works very well for setting the volume.
On the circulated schematics, the SRB's complete circuit has been copied and with it the log pot. A log pot makes sense in an instrument, otherwise nothing much would happen for the most of the pot's travel. However, stuffed into a stomp-box, this application is not expected to have more than a matching loudness. In this case the volume control reverts to a set-and-forget control, where an expanded physical travel around a centre position is welcome for precision adjustments. Indeed, with the above resistor values used, the centre position is about equal gain. So no real need for a log pot. The Author prefers this, but using a log pot of course does no harm.The original operational amplifier is long obsoleted, so an OPA703 was originally targeted, which had excelled on the L-2000 project. Regardless, the amplifier finally used is an LT 1097, which is a low-power precision op-amp akin to an OPA07. This was not chosen for a particular reason except it was one of the few possible candidates they had on stock coming in a DIP-8 package. Low power may
not strictly be a design goal in an external
stomp-box, because these are nowadays mostly powered
by an external supply. However, this OPA still has a
very low current consumption but introduces very low
noise compared to its heavily current-optimized
siblings and it performs excellent in this position.
Indeed the venerable TL07x may be used, too. Back To Index The Effect Of Pickup Location And Promising Pickup Combinations Although the above circuit applied as an external stomp-box brings us a lot closer to SRB tone (probably as close as it gets from the electronic part), the question of the contribution of the pickup's location remains. The author feels that a neck pickup’s tone (by itself reminding of a P-Bass tone) is not necessarily complemented by such an EQ change, while the bridge pickup’s tone (resembling a J-Bass with its gnarly tone) is much better suited. To be fair, the control's settings were not changed on the prototype during those experiments, because the trim pots used for evaluating the prototype PCB were linear. Changing the setting particularly of the treble control, which asks for a special pot, was nearly impossible. Not surprising,
the most promising combination (on all basses with
similar configuration tried) is a mix of both
positions, whereof an all-series combination provided the
heft needed.
A truly thunderous, authoritative and clear tone, that
cannot easily be replicated with a stationary
graphical EQ. The Author’s power amp is a respectable device equipped with an 8-band graphic EQ. Tweaking this yields a very similar tone, but the SRB post-EQ clearly sounds superior. This may be attributed to the resonant behaviour. Resonant filters always sound different than a bell type filter. Sometimes worse, but beneficial in this case. Generally speaking, fat (series)
tones work better than single coils. This is a bold
word out of the Author’s mouth considering his
otherwise pronounced preference for single coils. So what has been expressed by fellow musicians, that P-Basses and J-Basses should be left the way they are, rings true. The L-2000’s neck +
bridge serial tone and also its parallel tone are very
convincing together with the SRB post-EQ. The Author’s
L-2000, being
vastly hot-rodded[3],
has the added capability of selecting pickup
combinations like
inner pair or outer pair. Those tend to come out
overly nasal with the SRB post-EQ. String noises are
very much amplified to the point of being obnoxious,
at least if using the same control setting that worked
well for other modes. So, although being eminently
useful by themselves, those positions are best used as
they are. Note that those options are non existent on a SRB by definition and likely not on a stock L-2000 anyway. Take note that the original SRB we are talking about here, is a one-trick pony.Basses that have those positions available for switchable combinations, or even basses like the Ibanez ATK, which has the pickup in the right spot to start with, are perfect candidates. It does not make SRBs out of them overall, but will get you very close to it. In those cases, the stomp-box can add a very strong flavour of SRB to the arsenal. One word about the ATK´s native “Stingray” position (centre switch position): compared to the stomp-box, their SRB mode falls way short despite the rumour of being a Stingray killer. However, listen to the other switch positions in conjunction with the SRB-EQ! The author recently read a web article on John Entwistle, who is renowned for playing full volume, full treble. He uses a vigorous finger style with what was later known as typewriter playing. After reading
this article, the Author realizes to be playing a
similar way. This approach probably comes from the
roots as a guitar player in both cases. This way of
playing adds a lot of the typical string noise the
SRB is associated with. Maybe (this is a
speculation...) the SRB's haptics or the trousers moving
bottom end
invites the musician to dig in more? Who knows... The Author uses cobalt flats by the way, and not roundwounds, the latter which are frequently deemed a key component of the recipe. Trying a bass with reputable P/J pickups using roundwounds did not make any difference beyond that, except sounding more metallic right from a start. Beware of overpowering the post-EQ with overly bright tones, like active basses tend to present them. Pull the highs back somewhat on the instrument for a much more pleasing result. Back To Index Verdict The result has been a 100% success technically
and also sonically, because the circuit was not
falsely expected to transform lead into gold - there
is still something unfathomable around that basses
tone... However, it is not to be expected
that any future unit will be able to fill that gap
either.
That all said, the outcome is eminently
usable in that it can help clean up the mid-range of
your bass and introduce some heft in the
best of SRB-style,
if one can depart from the notion of perfection. It
surely is a worthwhile extension to your arsenal of
tones, although it is, much like its idol, a
one-trick-pony. Which is OK. A warning... First you think, whoa, what a
big tone. After a while you forget it is there - until
you switch the equalizer off again and your
tone is suddenly feeble and dull. Too much of it and
you get sick of it eventually. These are a typical
signs of addiction, and it has happened many times
with effects that were popular for a while. So use
with discretion. Back To Index Sound Samples The subsequent recordings have been done using the following setup and no further processing:
On the recording the difference between the three modes is less spectacular than in reality (played over a genuine bass enclosure), but the difference between the unaltered tone and the equalized tone is clearly audible. Note how the last recording, which uses the coil closest to the bridge, has string noises amplified. Back To Index Random Musings This sections
contains random musings in the sequence of appearance.
Overall, this EQ is nothing extraordinary from a frequency perspective. It is a low peak and a high peak resonant EQ. A functional equivalent plus some more functionality could be relatively easily realized using gyrator based full parametric EQ circuits. Ready-to-use boards are available for example from TH Custom Effects. A very similar approach has been described by Rod Elliott in Project 28. Note that the latter lacks some glue logic that makes it a fully usable pedal. However it outlines a way to add shelving capability to low- and high EQs at very little extra effort. There is quite a bit of circuitry that could be eliminated or at best remain a trim-and-forget section, such as the centre frequencies. The Author thinks that a full-blown parametric EQ will overwhelm the majority of users anyway and should thus remain reserved for specialized cases. Due to their complex nature devices like that are seldomly used as an effect for performance switching, like in a live performance. Both of those approaches consume so much more OPA and PCB estate which is in no relation to their yield, so the original circuit shines in its effectiveness and efficiency. A genius piece of work. That all said, the Author found that a little low-mid boost can be useful sometimes. As mentioned earlier, something like a 6 dB boost at 400 Hz adds some nice growl. A circuit that readily lends itself to this, is Craig Anderton's Freqency Booster. This fits seamlessly into the above project, however it was left as an on-off-toggle rather than a variable boost control. An attempt to design such an add-on has been described here. If preceding the SRB-EQ stage, the latter's input buffer stage could be eliminated entirely. This all could be most elegantly accomodated in a quad OPA like the TL074. At this point, the astute reader will start to realize that we have practically landed at the 3EQ circuit, albeit with a differerent quality. Since this is a streamlined piece of circuitry too, one might well choose that one from a start. The SRB-EQ's "flat" setting (on the Author's unit) is: bass and treble 11 o´clock, volume 1 o´clock - so all of them not quite at the center, which is just a matter of acknowledging. Set this way, the SRB-EQ may be left on permanently without noticeable coloration of the preceding bass. This way it can serve as an extension to the basses' built-in controls by actively adding a little low-end thunder or high-end sparkle á la Stingray. This is a superior use as compared to stomping on a box that changes tone radically. Back To Index Reference [1] AndysZeugs: MusicMan Stingray Classic pre-amp; https://andyszeugs.wordpress.com/musicman-stingray-classic-preamp/ [2] Guitar-Letters: a wealth of information on pickups and voicing (in German language), by Ulf Schaedla [3] Aquataur Music (H. Gragger): A Good Idea To Restore Frequency Balance And Why It Is Flawed; http://me.aquataur.guru/musicstuff/pickup_sim.html#A_Good_Idea_To_Restore_Frequency_Balance [4] Aquataur Music (H. Gragger): Measuring MFD Pickup Specs; http://me.aquataur.guru/musicstuff/L2000.html#Measuring [5] FSB (Bajaman): Baja Music Man guitar and bass onboard pre-amps; https://www.freestompboxes.org/viewtopic.php?t=431 [6] Nine Volt Nirvana (Joe Gagan) suite: e.g. tarpit meltdown;http://revolutiondeux.blogspot.com/2012/03/nine-volt-nirvana-fuzz-pedals-and.html Back To Index Update History
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