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Fender/Alembic/BBE Tone Stack Explained

D

doctec

Mar 22, 2005
166
10
Beaverton, Oregon
#1
Many bass amps and preamps use the traditional Fender/Alembic/BBE "tone stack." This is a simple passive RC network that has stood the test of time even though it is not nearly as intuitive as a graphic or parametric equalizer. I recently posted this explanation to another thread and thought I'd add it here for further distribution.

I acknowledge immediately that I'm describing the "first order" aspects of the circuit. Engineers who wish to do a more detailed analysis of the circuit will find that things are somewhat more complex than what I've described, BUT for the purposes of a musician "turning the knobs," this description should provide some additional insight that IMHO has been missing from previous descriptions of this circuit.

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I've used the Fender/Alembic/BBE tone stack for a long time as a player and also used it (and variations of it) in several designs that I've done. When you look at the actual circuit, the interactions make more sense -- to me anyway :-)

There are really two separate sections in the circuit. The first section involves the BASS and MID pots, two caps and a fixed resistor. When the BASS control is at 0, the MID control is essentially a level control for all frequencies for this section of the circuit. When the BASS control is turned up, more low frequencies are *added* to the output of this section, increasing the overall level.

The second section consists of the TREBLE pot and a cap. One side of the cap connects directly to the input of the tone stack. The value of this cap is such that only high frequencies will pass through it. The TREBLE pot provides the final output from the tone stack and is connected such that at one extreme setting (10) it passes *mostly* the output from the treble cap and at the other extreme setting (0) it passes *mostly* the output from the BASS/MID section.

Now here's the observation I haven't seen anyone else make...

The TREBLE knob is actually a *blend* control between the output of BASS/MID section and the output of the treble cap !

So when you turn up the TREBLE, you're actually *reducing* the level that's coming from the BASS/MID section at the same time you're *increasing* the amount of signal coming from the treble cap.

Realizing this made it easier for me to think about how the knobs work. For example, if you want to add more highs and keep the lows and mids the same, you need to not only turn up the TREBLE knob, but also turn up the BASS and MID knob to compensate. I've seen descriptions of this circuit that say the BASS and TREBLE controls are "boost only" and the MID control is "cut only", but that's somewhat misleading. I tend to think of the MID control as a "direct" level, the BASS as a low-frequency boost and the TREBLE knob as a "blend" control as described above.

Hope this is helpful and not too confusing :-)
 
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#4
In my view the tone stack is a low pass network and a high pass network in parallel. The low pass is slightly modified to become a shelving network by the addition of the mid control such that as frequency increases, a maximum amount of attenuation is reached and this point is controled by the mid control. The Treble control is not a blend control per se but as it is turned up the bass does reduce slightly by about 2dB. If it was truly a blend control, the Bass and Mid controls would have no affect when the Treble control is all the way up.

A neat (and free) program that graphically demonstrates what many common tone controls do can be found here: http://duncanamps.com/tsc/index.html

It's slightly nerdy and you might not be able to make much sense of it if you don't understand frequency response graphs, but it's pretty accurate. In fact when you go to the download page linked above, you can watch the graph change as the Treble control is adjusted from min to max.

Duff2 beat me to it !!!
 
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#5
Loudthud said:
In my view the tone stack is a low pass network and a high pass network in parallel. The low pass is slightly modified to become a shelving network by the addition of the mid control such that as frequency increases, a maximum amount of attenuation is reached and this point is controled by the mid control. The Treble control is not a blend control per se but as it is turned up the bass does reduce slightly by about 2dB. If it was truly a blend control, the Bass and Mid controls would have no affect when the Treble control is all the way up.
Click to expand...

Thanks for these observations. You're right about the TREBLE control not completely negating the BASS and MID controls when it's all the way up. I was describing the TREBLE pot as if it were panning between two voltage sources, which is making the analysis (much) simpler than it really is :-) The BASS/MID section of the circuit always shunts some high frequencies to ground through the 100K resistor regardless of the setting of the TREBLE control. The Tone Stack Simulator indeed shows that there's about a 2db reduction in the low frequencies when the the TREBLE control is maxed. 2db looks like a small number on paper, but to me, it "feels" like a significant change when I'm turning the knobs on my amp :-) So I'd hope that just realizing that turning up the TREBLE knob adds highs *and* reduces lows will be helpful for someone trying to figure out the interactive nature of the tone stack.

I've always been really impressed by the elegance of this circuit. I don't know who the actual designer was (certainly not Leo Fender himself), but he sure managed to get a lot of functionality out of a handful of inexpensive parts.

BTW, there's a really good technical analysis of the tone stack on the AX84 website: http://www.ax84.com/p1.html -- It's in the "P1 Theory Document"
 
#6
Leo and (I think) George 'borrowed' that circuit from an RCA mic preamp I believe.

I love that tone stack and I can't doubt your technical observations of it. My view of it is that you play with it a while to find your sound, if it's in there - it isn't for everyone of course - and add an EQ after the tone stack to adjust for narsty rooms ...

The beauty of that tone stack IS the quirky nature IMO. I consider it's quirks to be of a particularly musical nature. I can totally appreciate why some folks find those quirks frustrating though. You really have to spend time with it to appreciate it's nature. and making qwuick- on the fly adjustments in a live setting is not for the faint of heart. (That's why the additional EQ ...)

Here is another take on it. I used the pad on the GK and the EQ on the GK for room adjust. 1 side of the F2B was my 'dry' and the other siode was my 'wet'. I used the separate vol & EQ's to gain balance and to restore a little body that was robbed on the 'wet' side. Used the A+B output to drive the GK of course. This was a really fun setup while I was running an involved effects channel.

alleyoop.jpg
 
#7
Interesting. Now you guys have got me playing with software again.

Seems that most people recommend 2-10-2 or in this thread, 0-10-0 tone settings for a close to flat response with the Fender tone stack. After playing with Tonestack, I can get an almost ruler flat response by turning the mids to just a bit above 7, and adding just a tiny bit of bass while keeping the treble at 0. I realize this is theoretical but nevertheless...
 
#8
iammr2 said:
After playing with Tonestack, I can get an almost ruler flat response by turning the mids to just a bit above 7, and adding just a tiny bit of bass while keeping the treble at 0.
Click to expand...

This is true. If both the BASS and TREBLE controls are at or near '0', the frequency response if fairly flat and MID knob acts like a gain control.

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BTW, If anybody knows the name of the designer who invented this circuit, please let me know. I think he deserves some sort of recognition for his contribution to the art of guitar amplifier design. I just checked the RCA Radiotron Designers Handbook, which was the standard 'cookbook' for audio circuits in the 50's and while there were lots of tone control designs, this was not one of them. I'd guess he was an engineer working for Fender in the early 1950s. It'd be fascinating to find out if he had a clue how ubiquitous this circuit would be 50+ years after it was created!
 
#9
This may be of some interest (or not).

There is a thread on the Alembic Forum which discusses the tone stack found on the Alembic preamps (F-2B and F-1X), and also implementation of the "Duncan Tone Stack Calculator" in observing the response. One member of the forum states that when using the tone stack calculator program to emulate the Alembic preamps, you should select the "Fender" tab and then change the values of C2 (from 100 to 47) and C3 (from 47 to 100).

[Invalid or Expired Link Removed]
 
#10
great info. I can't wait to get home and play with my fender tbp-1 now. It does seem there's definitely some difference as to what's really "flat" on these things. I have tried plugging my bass directly into the amp vs 2-10-2 on the fender, and the fender definitely does not sound completely "flat" at those settings, at the same time it adds good amount of bass/mids and tons of warmth to the sound which I like. I think i might try 0-10-0 and maybe even playing around with the other options mentioned here just to see if it really is possible to get any flatter sounding, but its all good regardless.

I've played around a good bit with the settings including even plugging my stereo into it and getting rid of the mids/treble using my cab like a subwoofer kinda thing, it really is a versatile tone stack once you get the hang of it.
 
#11
Does anyone know if the old 1.5 rack space Demeter VTBP-201 uses a modified version of the Fender tone stack? The Demeter adds a 4th control called "presence", so I know it's not exactly the same. I've haven't been able to find any info online that explains how the Demeter tone controls react. From the sound of it I suspect the mid control is cut-only, because I like it turned a good bit past 12 o'clock, even though I'm not going for an extremely mid-heavy sound.
 
#12
Nope - Demeter has it's own design. 6 db boost or cut.

From their website

Treble: The treble control gives 6dB of boost or cut at 4kHz (6dB per octave, shelving type).
Middle: The middle give 6dB of boost or cut at 500Hz (wide Q, peaking type).
Bass: The bass control gives 9dB of boost or cut at a selectable frequency of 60Hz or 120Hz (6dB per octave, shelving type).
Presence Control: The presence control gives up to 12dB of boost at a selectable frequency of 2kHz or 4kHz (active type, 6dB per octave).
Bright Switch: The bright switch gives up to 6dB per octave of boost from 2kHz.

Lovely clear preamp - pretty much sonds like a DI with gain. The tone stack isn't worthless but it's too subtle for my uses.
 
#13
4Mal said:
Nope - Demeter has it's own design. 6 db boost or cut.

From their website

Treble: The treble control gives 6dB of boost or cut at 4kHz (6dB per octave, shelving type).
Middle: The middle give 6dB of boost or cut at 500Hz (wide Q, peaking type).
Bass: The bass control gives 9dB of boost or cut at a selectable frequency of 60Hz or 120Hz (6dB per octave, shelving type).
Presence Control: The presence control gives up to 12dB of boost at a selectable frequency of 2kHz or 4kHz (active type, 6dB per octave).
Bright Switch: The bright switch gives up to 6dB per octave of boost from 2kHz.

Lovely clear preamp - pretty much sonds like a DI with gain. The tone stack isn't worthless but it's too subtle for my uses.
Click to expand...

Thanks for the info. I had seen that page on the Demeter site, but it is talking about the current VTBP-201S unit, not the older VTBP-201 that I have, so I wasn't sure if they both used the same design. But I took a second look at some of the old review links, and they confirmed that you are correct.

I'm still not completely clear on how the Treble and Presence controls interact. It seems like they're saying the Treble control is shelving boost/cut while the Presence is non-shelving, boost only. That sounds a little weird (the boost-only part) so I may be mistaken. I've been using my LM II lately -- I'll have to break out my rack setup again and see if it sounds close to flat with all the controls at 12 o/clock but the Presence close to the minimum setting.
 
#14
I have a new Hartke LH500 head, and this type of tone stack is, I believe, what its eq section is built on.

I downloaded the program from duncanamps, and the Fender tone stack model is certainly pretty much what I hear going on with the Hartke. A very pleasant, and rather scooped, eq with knobs at 12 o'clock. Flat seems to be 9/2/9 o'clock, give or take.

Someone interested in this kind of eq setup, in a new head at a great price, can look to the Hartke LH Series. I find it very intuitive, and hard to get a sound that sucks. :)
 
#15
edbutler3 said:
It seems like they're saying the Treble control is shelving boost/cut while the Presence is non-shelving, boost only. That sounds a little weird (the boost-only part) so I may be mistaken.
Click to expand...

No, that's correct. It's common for amp heads to have a boost-only feature like that- sometimes "presence" which boosts the highs and high-mids, sometimes "contour" or "enhance" which boosts lows and highs but leaves the mids scooped.
 
#19
Thanks for helping me better understand my Bmax. Been setting the stack 2-5-2 for years and boosting the paramid 5 or so @900. Played around with it last night, and its making a lot more sense. Friday at the gig we'll see if I can fill the room with a better sound..
 
#20
Had to comment again.. Very pleased.. Downloaded the calculator and figured out visually where I wanted to go (treble 0, bass about 3, mids 5 to 8 depending on the room and how I was coming through) Gigged last night and that was very close. Followed a previous suggestion where to set the BBE and I was rewarded with a very good tone.. Now to trade my 2-12's for a Fitzmaurice cab...
 

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