[sdiy] “tilt” circuit

Mattias Rickardsson mr at analogue.org
Wed May 20 14:56:34 CEST 2020

On Wed, 20 May 2020 at 14:36, Paul Perry <pfperry at melbpc.org.au> wrote:

> Rod Elliott has this covered. Section 10 on this page:
> https://sound-au.com/articles/eq.htm

A meatier description of this clever circuit is available in Douglas Self's
book Small Signal Audio Design (very recommended), where it's attributed to
Ambler, 1970:

"Tone-Balance Controls

A tone-balance or tilt circuit is operated by a single control, and affects
not just part of the audio spectrum, but most or all of it. Typically the
high frequencies are boosted as the low frequencies are cut, and vice
versa. It has to be said that the name is unfortunate as it implies it has
something to do with interchannel amplitude balance, which it has not. A
‘stereo tone- balance control’ alters the frequency response of both
channels equally and does not introduce amplitude differences between them,
whereas a ‘stereo balance control’ is something quite different. It is
clearer to call a tone-balance control a tilt control.
Tone-balance controls are (or were) supposedly useful in correcting the
overall tonal balance of recordings in a smoother way than a Baxandall
configuration, which concentrates on the ends of the audio spectrum. An
excellent (and very clever) approach to this was published by Ambler in
1970 [6] (see Figure 10.14). The configuration is very similar to the
Baxandall’s – the ingenious difference here is that the boost/cut pot
effectively swaps its ends over as the frequency goes up. At low
frequencies C1, C2 do nothing, and the gain is set by the pot, with maximum
cut and boost set by R1, R2. At high frequencies, where the capacitors are
effectively short-circuit, R3, R4 overpower R1, R2 and the control works in
reverse. The range available with the circuit shown is +/-8 dB at LF and
+/-6.5 dB at HF. This may seem ungenerous, but because of the way the
control works, 8 dB of boost in the bass is accompanied by 6.5 dB of cut in
the treble, and a total change of 14.5 dB in the relative level of the two
parts of the spectrum should be enough for anyone. The measured frequency
response at the control limits is shown in Figure 10.15; the response is
not quite flat with the control central due to component tolerances.
The need for one set of end-stop resistors to take over from the other puts
limits on the cut/ boost that can be obtained without the input impedance
becoming too low; there is of course also the equivalent need to consider
the impedance the op-amp sees when driving the feedback side of the network.
The input impedance at LF, with the control set to flat, is approximately
12 kohm, which is the sum of R3 and half of the pot resistance. At HF,
however, the impedance falls to 2.0 kohm. Please note that this is not a
reflection of the values of the HF end-stop resistors R3, R4, but just
a coincidence. When the control is set to full treble boost, the input
impedance at HF falls as low as 620 ohm. The impedance at LF holds up
rather better at full bass boost, as it cannot fall below the value of R1,
i.e. 6.8 kohm.
The impedances of the circuit shown here have been reduced by a factor of
10 from the original values published by Ambler, to make them more suitable
for use with op-amps. The original (1970) gain element was a two-transistor
inverting amplifier with limited linearity and load-driving capability.
Here a stabilizing capacitor C3 is shown explicitly, just to remind you
that you might need one.
A famous example of the use of a tilt control is the Quad 44 preamplifier.
The tilt facility is combined with a bass cut/boost control in one quite
complicated stage, and it is not at all obvious if the design is based on
the Ambler concept. Tilt controls have never really caught on and remain

------ Original Message ------
> From: "ulfur hansson" <ulfurh at gmail.com>
> To: "synth-diy mailing list" <synth-diy at synth-diy.org>
> Sent: 20/05/2020 10:22:46 PM
> Subject: [sdiy] “tilt” circuit
> >hello list,
> >
> >i have been wanting to elaborate on a synth master output stage for a
> while, and these strange times have cleared up space for prototyping time...
> >
> >i have been thinking about a “tilt” circuit that could shift weight
> between volume on a hipass and lowpass circuit, effectively making it
> brighter or darker as you turn a single knob.
> >
> >of course an xfade with a hi/lopass derived from an svf could be the
> answer, but it seems rather huge for a simple idea - are there any simpler
> ways to achieve this that come to mind?
> >
> >hope you all are safe and well,
> >-ulfur
> >
> >Sent from outer space
> >_______________________________________________
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