Hey gang !
Let's further examine some guitar amp tone control circuitry. I'll show you results from a few of my Winter 2021-2022 experiments. Our context = clean, jazz guitar-focused amplifiers.
After publishing Gibson GA-50 Inspired Guitar Preamplifier Tribulations
I improved the basic tone circuit to reduce resistor shot noise and show the schematic below:
Although enjoyable, after experimenting with different capacitor values in the 2 shelving circuits, I abandoned this basic circuit. Why? Too much knob fiddling; plus I found I only liked a single 3 dB cutoff frequency for both the high and low shelf. Why bother with all this circuitry when a fixed high and low shelf frequency will do? I also wanted to focus on middle frequency circuits.
I learned that I prefer shelving tone equalizers over peaking or resonant types for both the low and high frequency. For mid range control, I seem to prefer peaking type albeit only if the resonant circuit Q is less than ~1.5
Above — A bass, middle, treble tone circuit taken from the out of production Carvin Sx-2000 preamp. The trio of equalizers use a single, unity-gain inverting op-amp stage and all 3 are peaking types. Please view the series capacitor(s) coming off each tone control potentiometer's wiper.
Thus each active tone circuit uses 2 signal capacitors that convert it from shelving to peaking. The design formulae for peaking tone equalizers gets considerably more complex than shelving designs, however 1 capacitor establishes a 3 dB point below and the other above a centre frequency which give the bell shaped response of a band-pass filter.
Above — Another circuit that provides a peaking equalizer response; the Wien bridge design. This particular design offers a different time constant for each half of the Wien band-pass filter. You'll see this often in guitar amps since it allows the potential of a slightly higher Q (and a sharper response). I really enjoyed the 753 Hz 3 dB frequency version as a low middle control on my bench test guitar amp. All 3 scaled designs use standard value caps and the 3 dB frequency may also be manipulated by tweaking the 22K resistor value.
Guitar amps often feature a single middle frequency tone control; or perhaps bling out and offer 2 middle frequency tone controls such as low and high midrange. Which middle frequencies should I choose vexes many amplifier designers. This is likely the reason we may go with adjustable midrange frequencies, however as aforementioned, I want to move away from that. I built a guitar preamp with 2 separate Wien tone circuits and painfully tried many different time constants to see what worked best for me.
To avoid potentiometer interdependence, each tone stage gets its own op-amp. You'll occasionally see this in high-end console mixers. Since most of us are just making 1 home brew guitar amp and not a production run where a higher parts count costs your company money, perhaps we can afford to bling out and put in as many op-amps stages as we choose?
To keep design simple, I chose identical time constants for the low-pass and high-pass circuits of each Wien band-pass stage. Thus, the standard Frequency = 1 / (2 pi * R * C ) formula is in play.
1 design consideration = what order do I put the bass middle treble circuits in? I did some experiments and with active tone circuits ,you may simple choose what sounds best to your ears. Some amps, however, run the middle, then low + high frequency circuits in order and this worked OK in my experiments.
Lower Middle 883 Hz Peaking
My middle frequencies were chosen for standard value capacitors. To my ears, choosing a low middle of 800-900 Hz offers the optimum single frequency to tailor the lower midrange. Using a Fender Telecaster + a Gibson ES-175 as my test guitars, I preferred the low middle frequency slightly scooped on the neck + bridge pick up combination; or with the treble pickup alone -- and slightly boosted when playing the front/ neck pickup alone.
I tested this board through my Popcorn PA with 3 different 8 Ω speakers: A 10 inch speaker in open-back mounting, a 12 inch speaker in a open-backed cabinet + another 12 inch speaker in a closed-back, ported cabinet.
Higher Middle 1540 Hz Peaking
I seem to prefer a higher middle frequency between 1200 and 1600 Hertz. Most often, I tended to boost this frequency in my listening tests, but occasionally left it flat. 1540 Hz is still low enough in the spectrum to adds some punch to your sound while avoiding the nasal sounding (when boosted) 1 KHz frequency. Boosting around 1500 Hz added some grit to my neck position humbucker pickup guitars, although too much boost sounded a little tinny, but not especially, since the Q only lies around 1.5 at maximal boost.
Low and High Tone Controls Shelving
Simple shelving circuits boost or cut the low and high frequencies. The low-pass filter uses the familiar topology of the low-pass variable frequency shelving stage shown in the first schematic of this blog post. Bass response rolls on & off smoothly and capacitor values of 0.39, 0.47 and 0.56 µF were tested. I prefer the really low 72 Hz turnover frequency at this point in time. You may also tweak the 3K9 resistor value slightly, or put in a temporary trimmer resistor to find your dream low frequency 3 dB point.
Chosen for a 12 KHz 3 dB turnover, the high-pass circuit follows the standard Baxandall design & added considerable shimmer to my guitar tone when boosted. Some builders may prefer a 10 KHz cutoff. Build and test stuff! You're the King on your bench.
On most of my experiments, I ran factory original Texas Instruments 5532 op-amps. On the low and high tone circuits, the 1.0 K end-stop resistors may be changed to limit the boost or cut as you prefer. They don't have to be symmetrical.
The 20K pots could easily be 10K potentiometers to lower resistor shot noise, however, at extreme settings of 10K control pots distortion may arise & you may have to increase your end-stop resistor values. As it goes, Baxandall circuits offer low input Z when boosting hard and heavy loading via the negative feedback path when cutting hard. Evidently, the 5532 performs better than many other popular op-amps in these extreme situations.
Further, conventional wisdom purports we use FET input op-amps since DC flows through our tone control pots.The TL072, or OPAx134 series come to mind. Bipolar input op-amps may drop noise and boost distortion performance if you don't mind adding a few DC blocking capacitors.
Other Experiments
Above — An active equalizer with all stages centred at 500 Hz with 4 different Q factors to allow listening tests. I also performed this maneuver at 190 Hz and 1.6 KHz. I placed 2 or more capacitors in parallel to get as close to each non-standard value design capacitance as possible.
I felt amazed how differently the same frequency band sounded when changing its Q. Obviously, moving between a Q of 0.85 and 1.0 wasn't staggering, however, I heard a clear difference. I liked a Q of 1 and 1.7 best. However, between these 2 values, I preferred a Q of 1 better as a boost and liked a Q of 1.7 better as a cut. I never imagined such a observation. LTSPICE did not inform me about my preferences either — gotta do listening tests. When Q = 5.1, my boosted or cut tone sounded unnatural @ 500 Hertz.
Above — A simple middle range Baxandall design. I've shown capacitor values for 3 frequencies, but I also scaled the cap values to test at 350, 400, 573 and 4000 Hertz. I preferred the Wien bridge middle range circuits over Baxandalls for midrange when both are centred on the same frequency. Likely, peaking sounds better than shelving to my ears for middle tones. Admittedly, by adding a capacitor to any Baxandall frequency band you may convert it to a resonant filter. The math poses a little more difficult than a Wien circuit, but its definitely an option for you to consider.
I think this might prove useful in a bass-middle-treble tone circuit sharing a single op-amp stage. You could make the low and high shelving, and then add a series cap to convert the middle control to a peaking type.
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