Note: information in this article is provided for historical reference. The author currently uses DSP approach in cabinet simulation, which he strongly believes to be a superior solution in comparison to analog circuitry.


Cabsims - Cabinet Simulator (or Speaker Emulator) circuits are very useful not only at home studio, during guitar recording, night practicing using headphones, but also on the stage as an additional signal output, which can be plugged directly into the mixer's input. Cabinet simulator's job is to emulate the frequency response of a guitar amplifier's speaker cabinet, which is the last and one of the most important elements of guitar sound creation (with the exception of microphone and the listener's perception of sound).

As an electronic circuit a Cabsim is a set of active or passive filters that create a desired frequency response. In its simplest form it's a typical low pass filter with roll off at about 4-5kHz, since this is the usual bandwidth limit of a guitar speaker (-3dB roll off).

Such a simple solution was implemented in TRIAMP amp by H&K.

Schematic

Triamp Slave out frequency response

As one can see from the schematic, the cabinet simulator is composed of two low pass filters connected in series.

Similar idea (but based on LC elements) can be found in Record Out part of Triaxis preamp by MesaBoogie.

Schematic

Triaxis Record Out frequency response

One step further; if we examine the frequency response of a guitar cabinet, we will notice another important feature - low frequency roll off starting usually at 100-200 Hz. This is achieved by using a high pass filter. Many amps contain various devices based on these two filters:

Few examples - again by H&K - are cabinet simulators available in discontinued preamp series: Cream Machine, Crunch Master, Metal Master.

Schematic

Cream Machine Cabsim frequency response
Similar cabinet simulators are utilized in Marshall JTS series amps - they have an additional frequency boost close to speaker's resonant frequency.
Schematic
MarshallJTM series cabsim frequency response

Cabsim used in JMP-1 preamp.

Schematic

b_250_188_16777215_00_images_schm_Jmp1-f.gif

Its older brother, more complex, with a possibility of advanced sound tweaking, used in SE100 Speaker Emulation System.

Schematic

Marshall SE100 Frequency response (cabsim part)
Marshall SE100 frequency response (cabsim part)

The LANEY TF-200 amp has a VIBE switch, which turns on a filter that has a frequency response shown here:

Schematic

TF200 Viber filter frequency response

The next preamp by Mesa Boogie - V-Twin - has a simple filter, which can deliver purring blues tones. As usual for Mesa, the filter is based on inductance.

Schematic

V-Twin Rec Out frequency response

A very simple solution based on two multi-feedback filters and with an unconventional frequency response, was used in the Screamer50 amp by ENGL. Signal is collected after the output transformer.

Schematic

Engl Line Out frequency response

Back to H&K. There are several interesting cabinet simulators based on RLC filters. Let's take a look at two devices: Red Box and Tubeman II preamp. RedBox is a completely passive device, which can be plugged between the amp and the speaker. It can be also used with line level signals.

After extraction of a cabsim schematic out of Tubeman II we can see that both devices are almost identical. Tubeman's initial stage contains an additional second-order high-pass filter.

Schematic

Tubeman cabsim frequency response
RedBox frequency response

However, speaker cabinet's frequency response does not consist only of high and low frequency reduction and speaker's resonant frequency boost. In between we will find lots of "peaks" and "notches", which are results of many factors like: speaker type, speaker enclosure mechanics, or - very importantly - the cabinet surrounding. The actual frequency characteristic of a speaker-microphone arrangement is highly irregular and scattered. It is caused by surfaces surrounding the enclosure creating echoes of sound waves. There are simulators that try to implement this frequency response irregularity.

Boss TM3 cabsim is a faint attempt of such an implementation.

Schematic

TM-3 frequency response

MP2 preamp by ADA has a possibility to to switch on or off additional Notch filters. The schematic available in the Internet can be hardly read, but our colleague RR managed to draw the cabsim part. His schematic and modifications suggested by him can be found below:

Schematic

MP-2 frequency response
MP-2 frequency response

Another quite interesting circuit that provides vast modification possibilities is the Varicab module used in ADA amps. Electronically it's a set of gyrators + two additional low and high pass filters. According to the manufacturer, the circuit offers simulation of all the common guitar cabinet types: 1x12", 2x12", 2x10", 4x12".

Schematic

And here is a modified version for single supply, more common in guitar electronics:

Schematic

Varicab 1x12 Bright/Dark
Varicab 2x10 Bright/Dark
Varicab 2x2 Bright/Dark
Varicab 4x12 Bright/Dark
Varicab High band adjustment
Varicab Thump control

And finally two "cherries on the cake" - the most complex circuits that most accurately simulate the speaker enclosure's frequency response. These are devices designed by guys from LXH2 Web site. Please visit their Web site for details. The circuits simulate the sounds of two main brands: Fender and Marshall. Shown are the frequency responses of these simulators. Again, electronically the cabsims are constructed of various multi-feedback filters.

I personally tested only the "Fender" version and without changes it sounded too dark. It's no wonder, it was designed to work together with the preceding circuit that simulated the preamp. After few changes in high pass filters it sounded much better ( in my opinion :-)).

LXH Fender Cabsim Schematic

LXH Marshall Cabsim Schematic

LXH Fender Cabsim frequency response
LXH Marshall Cabsim frequency response

DSP based speaker simulation

Out of curiosity i decided to examine my Pandora PX4D to see what are the frequency responses of the filters:

1x8TWD
1x8TWD frequency response
2x10BLK
2x10BLK frequency response
4x10TWD
4x10TWD frequency response
1x12TWD
1x12TWD frequency response
1x12VOX
1x12VOX frequency response
2x12BLK
2x12BLK frequency response
2x12VOX
2x12VOX frequency response
4x12VOX
4x12VOX frequency response
4x12CLS
4x12CLS frequency response
4x12MDN
4x12MDN frequency response
4x12VIN
4x12VIN frequency response
LA4x10 Bass
LA4x10 Bass frequency response
MDN4x10 Bass
MDN4x10 Bass frequency response
MTL4x10 Bass
MTL4x10 Bass frequency response
CLS8x10 Bass
CLS8x10 Bass frequency response
UK4x12 Bass
UK4x12 Bass frequency response
STU1x15 Bass
STU1x15 Bass frequency response
JAZ1x15 Bass
JAZ1x15 Bass frequency response
AC2x15 Bass
AC2x15 Bass frequency response
US2x15 Bass
US2x15 Bass frequency response
UK4x15 Bass
UK4x15 Bass frequency response
LA1x18 Bass
LA1x18 Bass frequency response
  COMBI Bass
COMBI Bass frequency response