Pulsar Modular – R52 Baxandall v1.0.0 for WiN

A 1952 diagram published by Peter Baxandall

In October 1952, Wireless World magazine published a schematic of a dual-knob tone control with an equalizer inside the feedback loop of a vacuum tube amplifier. The industry retained the schematic’s form but redesigned everything else. R52 is the schematic itself: an SP61 pentode in the loop, the published component values, and the conditions Baxandall specified for them—the source impedance and the far-end amplifier, which is now at your fingertips.

One network, not two lanes

LOW AND HIGH FREQUENCIES are two legs of a single feedback loop, so each changes what the other is working against. Raise both controls, and the midrange will recede; lower both, and they will come forward. SCOPE displays the interaction as you move it.

The amplifier is located inside the equalizer

The valve isn’t a saturation stage placed after the filter. It’s in the loop that makes up the curve, so the level is part of the sound: the INPUT determines how loud you hear the valve, and the high-frequency boost reproduces the valve exactly where you raise it. Quiet sounds are clear, loud sounds are colored, and there’s no clipping point in between.

It all depends on your fingers

In 1952, the impedance of the source and the far-end amplifier depended on the equipment you had. R52 displays them on the panel. SOURCE determines what powers the network, LOAD determines how hard the amplifier drives the 15-ohm monitor, and SHELF determines the range between two published high-frequency ranges.

Technical specifications

CHANNEL

Any number of channels, from mono to immersive, with one modeled valve circuit per channel. In a stereo bus, CHANNEL MODE switches between STEREO, MID, SIDE, L, and R; the ISOL function allows you to listen to only one component.

SCHEMES

1952 – Original circuit using the SP61 pentode. 1957 – Later circuit using the ECC81 dual triode. 1957 ALT – 1957 circuit using the SP61 pentode.

TONE

BASS and TREBLE, continuously variable from -1 to +1 with a display in thousandths for accurate reproduction. SHELF, continuously variable; in 1952 it lay between the two published Baxandall treble values.

SOURCE

From 0 Ohms to 250 kOhms. The operating range of the 1952 ends at the 10 kOhms mark and is indicated on the dial. New insert: 0 Ohms.

LOAD

OFF, 0.7, 1.5, 2.5, 3.5, and 7.0 Ohms — Output impedance of the amplifier connected to a fixed 15 Ohm monitor. OFF — Full bypass of the stage. New insert: OFF.

AMP

Amplifier in the LOAD path. Requires LOAD to be enabled; stereo and mono instances only, not MID, SIDE, L, or R. Works in real time at session rates up to 96 kHz; at higher rates, freeze, render, and export modes are used, and the LOAD path is used for real-time monitoring. Saved with presets, sessions, and A/B slots. Requires more CPU resources than the standard path.

PASSIVE IN/OUT PROCESSOR

Ensures bit-perfect, click-free, and consistent latency, so active and passive modes have the same latency.

DELAY

91 samples at 44.1 and 48 kHz (about 2 ms), 71 samples at 88.2 and 96 kHz, 0 samples at 176.4 and 192 kHz. Reported to the host, compensation was applied automatically.

Scaling Power

Always enabled, automatically adjusts to the session’s sampling rate. No control, no low-latency analog.

PRECISION

64-bit floating point. When rendering the same session repeatedly, the files will be identical.

AUTOMATION

Bass, treble, bandpass filter, source impedance, input level in dB, output level in dB, input/output, and load are all automatically adjusted. MATCHING, VERSION, and CHANNEL MODE are saved in the session but are not automated.

PRESETS

Regular .R52Preset files in the shared folder, which the browser reads in real time: /Users/Shared/Pulsar Modular/R52 Baxandall/Presets on macOS, C:\Users\Public\Documents\Pulsar Modular\R52 Baxandall\Presets on Windows. The Default preset is loaded with each new insertion.