Tone Circuits, Part 3.

Fender controls vs hifi controls.
From: mgarvin@panix.com (Mark Garvin)
Date: 26 Nov 1995 03:55:23 -0500

I've received a couple more email queries about tone controls, so I figured it would be best to answer them here.

Fender-style controls are used by many amp manufacturers because most guitarists prefer treble and bass boost (which is to say they prefer midrange cut). The Fender-style tone stack CANNOT provide treble or bass cut (in other words, no true mid boost).

One way to regard boost circuits is that all frequencies are attenuated--then that attenuation is cancelled for a range of frequencies. The amount of broad-band attenuation is called insertion loss, and is a necessary 'evil' of any circuit that provides a boost.

The standard Fender midrange control does not really boost just the midrange--it boosts ALL frequencies. In other words, it decreases the insertion loss. This is why the value of the midrange pot cannot be increased much past 10k or so: it will render the treble and bass controls less effective. The control works as mid-boost because it does cancel some of the midrange cut, but it cannot be a true boost, because there is no way for the mid control to elevate midrange levels any higher than bass or treble levels. In fact, increasing mid control will actually boost the bass and treble a bit.

Most tone circuits are centered around a midrange frequency at 300hz to 500hz. The Fender controls' center frequency (approx 300Hz) is close to the fundamental pitch of a guitar's open high E string.

By contrast, hifi amp tone circuits are usually centered around 1KHz--well over an octave higher. For guitar, the 1KHz tone circuit would provide a more effective control of bass frequencies, but at the expense of the rich, hard-hitting spectrum under the control of the treble knob. A 1KHz treble control would affect only harmonics of guitar frequencies.

However, the standard hifi tone circuit DOES provide treble cut and bass cut. Conversely, this provides an apparent midrange boost. When I refer to the 'standard' hifi tone circuit, this usually means the 'Baxandall' circuit, credited to inventor Peter Baxandall. Maybe this post is appropriate at this time, because Baxandall has just died recently. We have him to thank for much innovation in tube amp design.

Following is one of Baxandall's contributions to tone circuitry:

                                                                                
 In >---+-------------------------------                                        
        |                              |                                        
        /                              |                                        
        / R1                          ---                                       
        /                             --- C3                                    
        /                              |                                        
        |                              |                                        
        +--------                ----- | ---------> Out                         
        |       |                |     |                                        
        /      ---               |     /                                        
        / R2   --- C1    R4      |     /                                        
  Bass  /<------+------//////----+---->/ R5  Treble                             
        /      ---                     /                                        
        /      --- C2                  /                                        
        |       |                      |                                        
        +--------                      |                                        
        |                              |                                        
        /                             ---                                       
        / R3                          --- C4                                    
        /                              |                                        
        /                              |                                        
        |                              |                                        
      -----                          -----                                      
       ---                            ---                                       
        -                              -                                        
[NOTE: Use a DC-blocking cap to couple this circuit to a tube plate].

You've probably seen this circuit before in stereo amp schematics. A close look at this circuit will reveal that the bass and treble controls are almost opposite. The Bass pot (R2) and Treble pot (R5) are both large values compared to the impedance of surrounding components, so when viewing the circuit in simplified form, they can be ignored.

Visualize what is happening with the bass control: an R-C (resistor- capacitor) mid-cut (bass-boost) circuit is formed by R1/C2 when the bass pot is at max (wiper at the top). When the bass control is turned to minimum, C1/R3 form a bass cut circuit. Actually, it keeps about the same amount of midrange cut as before (flowing thru C1), but more bass frequencies are now shunted to ground thru R3.

The converse occurs with the treble control. This time, R4 serves as the resistance in both RC networks. At the max treble setting, C3/R4 cut mid and bass freq's (actually shunting them to the bass control which takes care of mid cut, etc). High frequencies pass straight thru C3 unimpeded. At min treble, C4/R4 still shunt bass and midrange freq's to the bass control, but now treble frequencies are shunted to ground thru C4.


Bass and Treble Boost (Mid Cut)

Consider what the circuit looks like with bass and treble at max. Keep in mind that both circuits use relatively high value pots, so they can be ignored for purposes of illustration. I've also omitted other components which are incidental to this setting.

 In >---+-------------------------------                                        
        |                              |                                        
        /                              |                                        
        / R1                          ---                                       
        /                             --- C3                                    
        /                              |                                        
        |                R4            |                                        
        +--------------//////----------+--------> Out                           
        |                                                                       
       ---                                                                      
       --- C2                                                                   
        |                                                                       
        /                                                                       
        / R3                                                                    
        /                                                                       
        /                                                                       
        |                                                                       
      -----                                                                     
       ---                                                                      
        -                                                                       
It is usually easier to read tone control circuits from right to left, so start at the 'Out' side.

C3 allows treble to pass straight thru, but diverts bass and mid freq's thru R4. At the junction of R1/C2, the bass frequencies take the high road, while most of the mids are shunted to ground thru C2/R3.

This circuit can be related directly to the Fender tone circuit: C3 is the 250pf treble cap, R4 is the 250K treble pot, R1 is the 100k fixed resistor, C2 is the .047 mid-cut cap, and R3 is the 6800 ohm resistor which prevents too much mid-scooping. All other components are incidental when controls are at max.

By the way, assuming that the 'left' and 'right' R-C sections do not interact (left RC net is low impedance compared to right-side RC net), the center frequency (and therefore the attenuation) can be approximated as:

fc = 159000 / (R1 * C2 * R4 * C3)0.5
where Cap values are in microfarads. R3 is ignored, since it is usually small compared to R1.


Bass and Treble Cut (Mid Boost)

This is where the similarity to the Fender control ends. When the Baxandall controls are set to min, the following circuit results. This has no equivalent Fender setting.

 In >---+                                                                       
        |                                                                       
        /                                                                       
        / R1                                                                    
        /                                                                       
        /                                                                       
        |                                                                       
       --- C1                                                                   
       ---                                                                      
        |        R4                                                             
        +------//////-------+--------> Out                                      
        |                   |                                                   
        /                  ---                                                  
        / R3               --- C4                                               
        /                   |                                                   
        /                   |                                                   
        |                   |                                                   
      -----               -----                                                 
       ---                 ---                                                  
        -                   -                                                   
Now, C4 immediately shunts treble freq's to ground. Bass and mid pass thru R4. At the junction of C1/R3, mid frequencies pass thru the cap, but bass freq's are diverted to ground. Treble and Bass cut == Mid Boost.

Pretty much the same here, ignoring R1:

fc = 159000 / (C1 * R3 * R4 * C4)0.5


Why don't guitar amps use the Baxandall circuit? Some do, but again, most guitarists prefer bass and treble boost. Also higher component count == $. And it's best to minimize the number of components in the signal path. The Fender-type circuit is simple, and if it does what you need, then stick with it. Some jazz or blues players and those looking for a bit more control may find the treble cut or 'mid boost' of the Baxandall circuit useful.

Please try to post follow-up queries. No problem with email generally, but I will not have as much time to reply to separate email questions. This post has already taken long enough. Darn ASCII diagrams.

Anyway, I hope this has helped to answer a few of the email queries. Sorry for the delay.

Mark Garvin
Composer/Design Engineer
New York City


Q & A

                                                                                
In <49df1h$87h@nnrp1.news.primenet.com> miker@primenet.com (Mike Rejsa) writes:
>*Many* thanks to Mark for his in-depth treatment!
                                                                                
>Minor question: Is there any effective difference between the 250pf/0.1/.047
>Fender setup and the 250pf/.022/022 Marshall version? (Pots are different
>too, of course...)
Hi Mike, If the 'slope' resistor (100k in the Fender circuit) and the treble pot (250k) stay the same, then the center frequency of the:

250pf/.047 circuit is around 295 hz with about a 19.7 db mid cut
250pf/.022 circuit is around 430 hz with about a 16.8 db mid cut

The numbers above are derived using simplified versions of the tone circuit which I outlined in a follow-up post. No guarantee on accuracy, since I'm rushing this.

The .022 circuit gives the bass control wider range, because like all simple R/C nets, these are limited to 6db per octave. Placing the notch frequency a bit higher affords about 2-1/2 octaves down to the guitar's low E string (82.4 hz). The standard Fender .047 circuit's mid notch is less than two octaves up from the low E string, so the low E cannot be 'boosted' be even 12 db.

The other (.1) cap (at the top of the bass pot) is primarily DC-blocking, since the reactance is still relatively low down to bass frequencies. Changing to .022 doesn't make that much difference, esp in the Marshall circuit with a 1 meg bass pot.

Nor does the value of the bass pot enter into the center frequency calculation very much. It DOES work as a voltage divider against the 100k slope resistor, so values over 250k don't do a whole lot of good. In fact, if a 1 meg pot is used there, best make sure that it's a true log taper (1/10th of the value at half-rotation) rather than a semi-log (usually around 1/3rd at half-rot.). Otherwise all the bass boost will happen in the low range of the pot...no audible effect with further rotation.

Mark Garvin

Editors Note:

If anyone has any of the other two parts of this series, I'd appreciate it greatly if you could send me a copy.
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