Quick Guide

R52 Baxandall

Negative Feedback Tone Control

Peter Baxandall’s original 1952 two-knob tone control, rebuilt as the circuit he published: the tone network sits inside a valve amplifier’s feedback loop, so BASS and TREBLE steer one curve, and INPUT is a tone control before it is a level control. SOURCE and LOAD put the conditions of a 1952 installation under your fingers. CIRCUIT swaps in his rarely seen 1957 network.

1952SP61 pentode, the original circuit

1957ECC81 triode, his second voicing

1957 ALT1957 network on the SP61, an R52 extension

64-bitfloat throughout, oversampling always on

—Signal chain

Where each control sits

dry signal

▸

INPUT

drive into the valve

▸

SOURCE Ω

driving impedance

▸

TONE LOOP

BASS · TREBLE · SHELF inside the valve feedback path (CIRCUIT)

▸

LOAD / AMP

amplifier + 15 Ω monitor

▸

OUTPUT

final level only

▸

OUT

to the host

The EQ is the amplifier. The tone network sits between the valve’s output and input. Turning a knob tilts what the amplifier is asked to do; what you hear, even at flat, is the amplifier’s output. Drive and tone are one system, not an EQ followed by a saturator.

SOURCE is the first condition, LOAD the last. SOURCE sets the impedance feeding the network; LOAD sets the output impedance of the amplifier holding a fixed 15 Ω monitor. AMP replaces LOAD’s fixed colour with Baxandall’s own 1948 ten-watt amplifier, solved live. Both sit after the tone control, so AMP’s character is identical under every CIRCUIT.

MATCH compensates internally. It references the input before the circuit and holds sustained loudness close to the dry level, using the same curve SCOPE draws. The OUTPUT knob and the SCOPE trace do not move; turn MATCH off and the compensation transfers to OUTPUT without a jump.

—Five-minute walk

First session with R52

Put it somewhere broad

A full mix, drum bus, bass, acoustic guitar, vocal: anywhere you would reach for a tone control rather than a scalpel.

MATCH on first

So your first judgments are about tone, not level. Turn it off when you are ready to commit the level change.

Big moves on BASS and TREBLE

Make decisive moves and listen to the whole sound. Small changes are hard to judge in context; move clearly, then bring it back.

Lift both, listen to the middle

The two are arms of one network, so lifting both pushes the middle back. Open SCOPE to see the combined curve against the individual traces.

Glassy treble? Add SHELF

If a large TREBLE lift gets glassy, turn SHELF clockwise rather than backing off the lift. The presence stays, the top-octave glare goes.

INPUT for character, OUTPUT for level

Hold SHIFT while adjusting INPUT to counter-move OUTPUT and compare drive at the same loudness. With AMP active, INPUT also sets how hard the amplifier is driven.

Match the levels first. This applies to any comparison, including R52 against its own bypass or another EQ. Otherwise you are judging loudness, not tone.

—Interactive panel

Hover or tap any control

24 hotspots. Hover for a summary, click to pin the full description; click again or tap outside to release.

—Controls reference

Every control, its range, its job

Control	Range / states	What it does
BASS	−1 to +1	Low shelf that keeps growing (1952) or turns over into a hump (1957). Automatable as Bass.
TREBLE	−1 to +1	Treble lift whose reach grows with the amount. 1952 gives roughly three times the lift of 1957 at the same position. Automatable as Treble.
SHELF	CCW … CW	Clockwise = the lift corners into a shelf, less reach, a little more low-end weight. Zero is the published position. Automatable as Shelf.
SOURCE Ω	0 Ω to 250 kΩ	Driving impedance into the network. Default 0 Ω. Accepts typed values such as 10k. Automatable as Source Ohms.
INPUT	−12 to +12 dB	Valve drive: the tone control before the level control. SHIFT-drag links OUTPUT inversely. Default 0 dB. Automatable as Input dB.
OUTPUT	−24 to +24 dB	Final level only, after everything. Receives MATCH compensation when MATCH is turned off. Default 0 dB. Automatable as Output dB.
IN / OUT	on / bypass	Latency-matched, click-free, bit-exact bypass. Never touched by presets or A/B. Automatable as In/Out.
CIRCUIT	1952 · 1957 · 1957 ALT	Replaces the circuit and valve. Default 1952. Click forward, right-click back. Automatable as Circuit: a change crossfades over about 15 ms, so a step on a lane is safe mid-take.
LOAD	OFF · 0.7 · 1.5 · 2.5 · 3.5 · 7.0 Ω	Amplifier output impedance driving a fixed 15 Ω monitor. Default OFF (true bypass). Under AMP it becomes the feedback pot. Automatable as Load.
AMP	off / on	Baxandall’s 1948 amplifier solved live behind LOAD. Requires LOAD active; bypassed in component modes; heavy CPU. Not automatable.
MATCH	off / on	Holds sustained loudness near the dry input while you decide. Internal; transfers to OUTPUT when switched off. Not automatable.
CHANNEL MODE	STEREO · MID · SIDE · L · R	Which part of the image this instance processes; output is always full stereo. MONO / MULTI on non-stereo buses. Not automatable.
ISOL	off / on	Audition button in component modes: hear only the processed component, on both speakers, at its in-context level. Saves with session and presets; not a host parameter.
SCOPE	window	The solved curve, red dot, FLAT, ZOOM (±24/12/6/3 dB), PIN, RMS meter rail (DRY or IN, OUT, MATCH line).

Fresh insert: BASS and TREBLE flat, SHELF at the published position, SOURCE 0 Ω, LOAD OFF, CIRCUIT 1952, INPUT and OUTPUT 0 dB, MATCH off, CHANNEL MODE STEREO. The 1952 circuit and nothing else.

—CIRCUIT

Three voicings, two circuits, two valves

CIRCUIT replaces the circuit, not just the valve. The SP61 pentode has far more gain and distorts less; the ECC81 double triode lets roughly three times as much of the same character through. That difference is most of what separates the two 1957 positions.

Position	Circuit	Valve	Character
1952	Original 1952	SP61 pentode	A broad treble shelf that keeps climbing and a bass shelf that reaches into pure weight. Reach for this when you want the instrument’s full, wide-open reach.
1957	1957 network	ECC81 triode	A redrawn network with added components. The treble acts more like a presence control, and the bass turns over into a defined hump, cleaning up mud below. Fuller, thicker, more harmonically coloured: the published 1957 circuit.
1957 ALT	1957 network	SP61 pentode	Same 1957 curves on the 1952 valve. The pentode distorts about a third as much as the triode, so note attacks stay defined where 1957 thickens them. A Pulsar Modular pairing Baxandall never built.

Do not transfer knob positions between voicings. The same position produces different dB in each circuit; read the help strip. Switching to a 1957 circuit and the top is not where you expect it? Check SHELF before touching TREBLE.

—SOURCE, LOAD and AMP

The conditions around the circuit

SOURCE Ω: what feeds the network

Higher impedance makes boost and cut curves sag, the bands interact more heavily and the treble reach diminish. Set SOURCE first, then reach for BASS and TREBLE. SOURCE gives up a little level as it rises, so match levels (SHIFT-drag INPUT or MATCH) before you judge.

Range	Character
0 Ω – 600 Ω	Clean, modern baseline. The circuit operates wide open and transparent.
1 kΩ – 10 kΩ	Low-mid boxiness around 300–600 Hz gently dips away while the low-end weight stays firm. Punch and clarity without an active boost.
10 kΩ – 39 kΩ	Transient softening: highs begin to roll off. Softens the attack of snare hits, picked guitars and kick beaters while keeping the body.
up to 250 kΩ	A heavily loaded chain, typical of vintage high-impedance gear. Output drops, transients blur, the spectrum tilts. A worn-in colour.

LOAD: the amplifier’s grip on a 15 Ω monitor

The ohms on the dial are the amplifier’s output impedance, never the loudspeaker’s. The monitor stays nominally 15 Ω at every position. Lower impedance: higher damping, tighter bass, controlled transients. Higher: the cone keeps moving after the signal ends, giving low-end bloom, a low-mid dip and softer highs. Left-click steps forward, right-click back; every new instance starts at OFF.

Setting	Modeled condition	Sonic character
OFF	Complete bypass (infinite damping)	Transparent and linear. A modern solid-state amplifier with total cone control.
0.7 Ω	Tightest grip	Subtly dips 200 Hz and softens top-end glare.
1.5 Ω	Tight grip	Cleans up boxy low-mids, firms up the fundamental bass, smooths the upper registers.
2.5 Ω	Moderate-damping tube amp	A distinct hollow at 200 Hz and a shelved-down top above 10 kHz.
3.5 Ω	Low-damping character stage	Pronounced low-mid dip and audible top-end softening. For aggressive tonal shaping.
7.0 Ω	Historical loose-damping profile	Maximum low-end bloom and a deep low-mid hollow. Picks up a seven-ohm feed proposed in the LS3/5A’s own research report, published but never adopted.

AMP: the amplifier behind the speaker

AMP swaps LOAD’s fixed colour for Baxandall’s own ten-watt amplifier from January 1948, solved live as a circuit. The output valves are a deliberately imperfect 6L6G pair, and the transformer core keeps a slight lean after a loud bass passage, so the amplifier is program-dependent. Under AMP the LOAD dial becomes the amplifier’s feedback pot: 0.7 Ω is the amplifier as published, 7.0 Ω the least feedback, and every step is level-matched at 1 kHz. Less feedback also means harder drive: at 7.0 the amplifier overloads at roughly a sixth of the INPUT that 0.7 needs. AMP needs LOAD active, is bypassed in MID, SIDE, L and R, runs live up to 96 kHz, and uses real CPU. Its load rises when it is driven harder: higher INPUT, more-open LOAD settings and higher sample rates all cost more. Spend it on bass, drums and key buses; if playback stutters, reduce INPUT, return LOAD closer to 0.7, raise the host buffer, or freeze or bounce once the setting is final. Renders are deterministic, and an offline bounce renders cleanly even where real-time playback glitched.

AMP lamp	Meaning
Amber	AMP is active and running.
Purple	The session is running above 96 kHz. AMP prints in freeze, render and export; live monitoring uses the standard LOAD path. Freeze the track to hear it in context.
Red flash	AMP is being overdriven. Lower INPUT (SHIFT-drag keeps the level) for more headroom, or keep the drive if it serves the material.

—Tips & techniques

Eight recipes from the listening sessions

None of these is a setting, only a starting position and a direction. Each names a CIRCUIT, because the circuit decides more than any knob position does.

Punch on a bass without boosting

1952

Leave BASS and TREBLE alone and reach for SOURCE: come up from 600 Ω into the low thousands. Listen for the low-mid boxiness thinning while the weight underneath holds firm. By 10 kΩ the focus is still moving upward; past it the pluck attack blurs, and you have left definition and entered colour. Bring the level back before you judge.

Weight

1952

SOURCE at 600 Ω, then push BASS decisively. The control stays musical right up near the top of its travel, and at 600 Ω the circuit is unloaded and open. Listen for richness below the notes: a shelf that keeps growing under the material rather than a bump sitting on it, arriving clean.

Punch with a bounce

1957 / 1957 ALT

Give BASS a small lift. Small is the recipe: the character is there almost immediately. Listen for a defined hump on the fundamentals with the mud pulled out from underneath. The same small move in 1952 grows the whole low end instead; flipping between them with the lift in place is the fastest way to hear what CIRCUIT is for.

A big treble lift and how it finishes

1952

Take TREBLE well up, around +0.7, then let SHELF choose the top. Counterclockwise the lift stays bright: glossy, crystal, the published sliding lift. Clockwise it corners into a shelf: smoother, a touch more presence, far less top-octave glare at the same TREBLE position. Listen to hi-hats and cymbal tails; both ends are Baxandall’s and neither is the correct one. Choose per job.

Keep the top when leaving 1952

1957 / 1957 ALT

At the same knob position the 1957 circuits deliver roughly a third of 1952’s treble lift, so the top reads as gone after a flip. It is not gone: the control has different authority. Push TREBLE further and read the dB in the help strip rather than the angle on the dial. The 1957 lift arrives higher up and more gently, presence rather than reach.

Compare the circuits, not the drive

1952 vs 1957 ALT

At equal INPUT the 1957 circuits run their valve much harder than 1952, so an A/B between eras hears two things at once. To hear the networks alone, flip between 1952 and 1957 ALT, the pair that shares a valve, and trim INPUT down in 1957 ALT until the two lamps match. Equal INPUT compares the instruments as built; equal glow compares the tone networks. Leave the lamps out of any judgment between the two 1957s.

Starving the source impedance

1952

Run SOURCE out past the 10 kΩ fence, toward 39 kΩ and beyond, and let it misbehave: attack blurs, the balance tilts, the level falls away. Bring the level back with OUTPUT, or with INPUT if you want the valve working harder as well. A different instrument at the same loudness: softer-fronted, tilted, period. Gain cannot undo a shape change, which is why the far side of the fence is a palette rather than a fault.

The tighter of the two 1957s

1957 ALT

When the 1957 network is the right circuit but the bass is thickening, switch the valve. 1957 ALT runs its low end about three times cleaner, so note attacks keep their articulation; 1957 thickens them and carries more colour everywhere. Listen to the front of bass notes. The axis is more and less, not harsher and softer.

—Modifier keys & shortcuts

Windows / macOS / action

Windows	macOS	Action
SHIFT + drag	SHIFT + drag	INPUT: OUTPUT counter-moves by the same amount, so drive changes without a loudness change.
Ctrl + Alt + hover	Cmd + Opt + hover	Hear the plugin as if that control were at its resting position (SOURCE at 0 Ω, LOAD at OFF). The knob does not move, nothing is written to automation or the preset.
Click readout	Click readout	Enter an exact value. BASS and TREBLE accept the faceplate range or thousandths (0.5 = 500); a bare 1 means full travel. SOURCE accepts values such as 10k.
Click / right-click	Click / right-click	State controls (CIRCUIT, CHANNEL MODE, LOAD): click moves forward through states, right-click moves backward. The help strip names the current state.
Right-click preset name	Right-click preset name	Rescue my “preset”: restores the panel as it stood before the last load. Also on the menu at the right end of the top bar.
Hover	Hover	Bottom help strip: the primary quick reference. Changes with the control and, for state-based controls, follows the selected state. Shows actual dB for BASS and TREBLE in the selected circuit.

—Preset manager

The browser reads the folder

The browser keeps no copy of anything. It reads the shared preset folder, to any depth, and shows what is actually there. Right-click the preset name in the top bar, or use the menu at its right end, for Rescue my “preset”. Folder and preset names below are placeholders.

Preset browser: hover any element

★ Favourites (0)

⚲ Search (type above)

All / root

Folder A

Folder B

Folder C

Folder D

☆Preset 01

☆Preset 02

☆Preset 03 MID

☆Preset 04

right-click on a preset

Preset 01

Load preset

Save here…

Rename…

Edit description and tags…

Plain files in a shared folder

Every copy of R52 on the machine reads the same shared folder, so a studio computer shows one bank to everyone who sits at it. Each preset is a single .R52Preset file. If the shared folder is not writable, R52 falls back to an equivalent folder in your own user area.

macOS

/Users/Shared/Pulsar Modular/R52 Baxandall/Presets

Windows

C:\Users\Public\Documents\Pulsar Modular\R52 Baxandall\Presets

Linux

~/.local/share/Pulsar Modular/R52 Baxandall/Presets

What a preset carries

Stored	Which is
BASS, TREBLE, SHELF	the tone
SOURCE, LOAD	the conditions around the circuit
INPUT, OUTPUT	the drive, which is part of the character
CIRCUIT	the circuit itself
MATCH	whether compensation is engaged
CHANNEL MODE	where the tone lands: whole track, or MID, SIDE, L or R
ISOL	whether the component is the plugin’s output

What a preset never stores

IN/OUT. A preset that switched the plugin off as it loaded would look exactly like a preset doing nothing, so loading never touches bypass.
A/B slots. They are working memory, not part of any sound. Loading a preset lands it in both slots at once.
Window state. Loading a tone preset never opens, moves or closes your SCOPE window.

Sessions and automation

The session saves every parameter as values, not as a reference to a preset file, so renaming or deleting presets never affects a session. It also remembers the window size, the SCOPE state (open/closed, position, PIN, ZOOM, FLAT, DRY/IN), ISOL and the preset name. Both A/B slots come back holding the saved sound. Automatable: Bass, Treble, Shelf, Source Ohms, Input dB, Output dB, In/Out, Load, Circuit. Circuit crossfades between circuits over about 15 ms when automated. Not automatable but saved: Match, Channel Mode, Amp. ISOL is not a host parameter at all. Renders are deterministic: the same session rendered twice gives identical files.

Backup and moving between machines

Backing up means copying one folder; restoring means copying it back. Presets travel between macOS, Windows and Linux: drop the files anywhere inside the other machine’s preset folder. Installing a new version of R52 leaves your preset folder exactly as you left it; factory presets are refreshed unless you deselect that option.

—Pro Tools

Pro Tools preset management

Pro Tools handles plugin presets differently from most DAWs. Two settings let R52’s internal preset system and Pro Tools’ own management work hand in hand.

Set plugin default behavior

In the plugin header bar, open the Preset drop-down and go to Settings Preferences ➔ Set Plug-In Default to ➔ User Setting. R52 then recalls your most recent or user-defined settings instead of reverting to the factory default on every insert.

Save presets to the session folder

Again in the Preset menu: Settings Preferences ➔ Save Plug-In Settings to ➔ Session Folder. Custom R52 settings are then stored inside the current session folder rather than the global root settings directory, so they travel with the session when collaborating or moving between systems.

Enabling both options gives consistent recall and a smooth preset workflow across sessions.

—Uninstalling

Remove the plugin files

Your presets live inside the shared R52 Baxandall folder as plain files. Copy that folder somewhere safe before deleting it if you want to keep them for a reinstall or another machine.

Windows

VST3

C:\Program Files\Common Files\VST3\Pulsar Modular

Delete the R52 Baxandall.vst3 folder.

AAX

C:\Program Files\Common Files\Avid\Audio\Plug-Ins\Pulsar Modular

Delete the R52 Baxandall.aaxplugin folder.

Shared

C:\Users\Public\Documents\Pulsar Modular

Delete the R52 Baxandall folder (user guide and presets). If no other folders exist under Pulsar Modular, that can go too.

macOS

/Library/Audio/Plug-Ins/Components

Delete the R52 Baxandall.component file.

VST3

/Library/Audio/Plug-Ins/VST3/Pulsar Modular

Delete the R52 Baxandall.vst3 file.

AAX

/Library/Application Support/Avid/Audio/Plug-Ins/Pulsar Modular

Delete the R52 Baxandall.aaxplugin folder.

Shared

/Users/Shared/Pulsar Modular

Delete the R52 Baxandall folder (user guide and presets). If no other folders exist under Pulsar Modular, that can go too.

Linux

VST3

~/.vst3 (or /usr/lib/vst3)

Delete the R52 Baxandall.vst3 folder.

LV2

~/.lv2 (or /usr/lib/lv2)

Delete the R52 Baxandall.lv2 folder.

.deb

sudo apt remove r52-baxandall

If installed from the .deb package, remove it with your package manager.

Shared

~/.local/share/Pulsar Modular

Delete the R52 Baxandall folder (user guide and presets).

—Specifications

Latency and CPU

Session rate	Latency
44.1 / 48 kHz	91 samples (about 2 ms)
88.2 / 96 kHz	71 samples (under 1 ms)
176.4 / 192 kHz	0 samples

R52 reports its latency to the host for automatic delay compensation; IN/OUT is latency-matched. Oversampling is always on, sized automatically for the session rate. R52 processes each audio channel separately, so a stereo instance uses roughly twice the processing of a mono one; there are no quality modes or hidden performance settings. AMP is the main exception: its CPU use rises with higher INPUT, more-open LOAD settings and higher sample rates. If playback stutters, reduce INPUT, return LOAD closer to 0.7, increase the host buffer size, or freeze or bounce once the setting is final. Formats: VST3, AU, AAX (macOS, Windows), VST3 and LV2 (Linux, .tar.gz and .deb; prefer LV2 in Harrison Mixbus).

—Troubleshooting & FAQ

Common questions

Where is the clean or transparent mode?

INPUT is it. The valve sits inside a feedback loop and stays well-behaved at ordinary levels. Drive it gently for a transparent sound; there is no separate saturation stage to bypass.

I turned SHELF up and lost top end. Is it backwards?

No, that is correct: clockwise is more shelving, and more shelving is a tamer top.

The same knob position sounds much weaker after changing CIRCUIT.

It is weaker: at the same position 1952’s TREBLE gives roughly three times the lift of either 1957. Read the dB in the help strip, not the angle.

The readout says +700, but the knob only goes to 1.

Same position: the printed scale runs −1 to +1 and the readout counts thousandths for exact recall. +700 is 0.700 of full travel. Type 700 or 0.7 and you land in the same place.

Engaging LOAD changes the level.

LOAD models a downstream amplifier and loudspeaker; loosening the grip costs some level. Turn MATCH on to compensate while comparing.

Is 7.0 the speaker’s impedance?

No. The modeled speaker stays 15 Ω nominal. The dial sets the amplifier’s output impedance: how tightly it grips, and smaller is tighter.

One valve lamp is brighter than the other.

Your material is asymmetric. The lamps show drive per channel, not distortion, and they do not compare across the two 1957 voicings.

Where is the second set of knobs for Mid/Side?

There is none. One instance processes one component; run two instances, one on MID and one on SIDE (or L and R), each with its own circuit, SOURCE, LOAD, drive, MATCH, SCOPE and A/B.

The AMP button is unavailable.

AMP requires LOAD to be active: select any LOAD setting other than OFF. AMP works on stereo and mono instances and is unavailable in MID, SIDE, L and R modes.

The AMP lamp is purple instead of amber.

The session is running above 96 kHz. AMP prints during freeze, render and export, while live monitoring uses the LOAD path. Freeze the track to hear the AMP result in context.

I saved a preset in MID mode and it moved another instance into MID.

Every preset carries the channel mode it was saved in, shown as a suffix in its name. Loading an ordinary preset afterwards puts the instance back on the whole track.

Is there a zero-latency version, or an oversampling control?

No. There is one R52; oversampling is always on and sized for the session rate, and the host is told the latency and compensates automatically.

Deleting a preset: is it gone?

No, it moves to the Trash or Recycle Bin, and R52 asks first, naming the file.