Condor Cab
Sim
professional-quality frequency response in a DIY-friendly
circuit
by runoffgroove.com

This project is rated 2 out of 3 for the level of
complexity.

Typical DIY speaker cabinet simulator designs are simple builds, but often
fall short of a convincing frequency response due to the oversimplified designs.
Our goal was to design a speaker cabinet simulator suitable for direct recording
and headphone playing, capable of outperforming existing designs, but still
within the reach of do-it-yourself builders.

The starting point was the published response of a single 10 inch Jensen P10R
speaker. We chose to emulate a 4x10 cabinet, so low frequency response was
shifted down slightly from the published curves to account for the larger
cabinet and the combined cone area of four speakers.

The circuit is composed of the following functional blocks:

JFET amplifier. Amplifies the signal to drive the filters. Also
adds some overdrive at higher Gain settings, which emulates the behavior of a
real speaker operating at high volume. We liked using a J201 best, but an
MPF102 or 2N5457 will provide more treble-content and different overdrive
characteristics.

Bridged-T notch filter. Produces the 400Hz notch found in the P10R
response. Please note that the extra resistor in parallel with the capacitor
to ground is not only for biasing the op-amp, but it also produces a high-pass
shelving effect. High frequencies are then 6dB above low frequencies, as shown
in the speaker response curve. To reduce the notch depth, you can replace the
1n and 15n caps with 1n5 and 10n, or even 2n2 and 6n8, respectively.

High-pass filter. This is the network between the first two op-amp
stages. It simulates the low frequency rolloff of the speaker cabinet, plus
the 90Hz peak. For deeper bass, you can replace the two 47nF caps with 56nF or
68nF. For less bass, use 39nF or 33nF.

Low-pass filter. These are the two identical networks between
second and fourth op-amp stages. They simulate the steep high frequency
rolloff of a guitar speaker, in addition to the peak near 3kHz. For more high
frequency content, you can reduce the four 22k resistors to 18k or less. To
cut more highs, increase them to 27k or more.

A frequency response graph of the Condor is shown below. For comparison, the
Bulldog Cab Sim response curve is shown with a dotted red line. Note how the
steeper roll-off of the Condor removes more high frequency content. When those
are present, the resulting sound can seem harsh and unrealistic - especially
when using a distortion pedal.

Marshall reponse curve mod

The Condor can be modified to reproduce the frequency response of the
Marshall speaker simulator that is included on their amps with an XLR
output (as traced by Ed Rembold, available at Aron's
Selected Schematics). Substitute the following components:

Bridged-T notch filter: replace the 1n and 15n caps with 2n2 and 6n8,
respectively.

High-pass filter: replace the 18k and 100k resistors with 12k and
120k.

Low-pass filters: replace the 3n9 caps with 4n7, and replace the four
22k resistors with 15k or 18k (16k is the nominal value, if you can get
it; otherwise use 15k for slight extra treble, or 18k for some less
fizz).

The below graph compares the original Condor frequency
response with the Marshall mods.

Notch Depth Switch mod

While the above modifications will accurately reproduce the Marshall
speaker simulator's response curve, so many components differ from the
stock Condor that using a switch to toggle between the two is not
feasible. However, a compromise is available if we focus our efforts on
the Marshall notch depth. By adding and removing capacitors in parallel
with the notch caps, we can produce a notch depth that approximates the
notch produced by the Marshall speaker simulator. Best of all, a DPDT
switch can be used to toggle between the two modes - or a 3PDT can be used
to allow LED indicators for each response.

As shown in the partial schematic below, the 1n "bridge" cap is
unchanged while the stock 15n cap is changed to a 6n8. A 1n2 and 8n2,
both with a single-pole switch in series, are placed in parallel with
those caps, respectively. By using a DPDT switch for the two single-pole
switches, we can open a switch while closing the other. When the 1n2
"bridge" cap switch is open and the 8n2 switch is closed. The 6n8 and 8n2
caps are in parallel, which creates a total capacitance of 15n. Those
values produce a notch as found in the stock Condor.

As the DPDT switch is activated, the 1n2 cap is added in parallel with
the 1n, forming a 2n2 cap. Simultaneously, the 8n2 cap is switched out,
leaving just the 6n8 cap to ground. The notch depth is now roughly half
of the stock circuit, which is close to the Marshall response.

In addition to the added midrange content produced by the decreased
notch depth, more treble frequencies are present and the overall output is
increased. Remember, this modification does not reproduce the Marshall
response, but is a good "halfway point" that adds to the usefulness of the
stock Condor. Refer to the below schematic for the necessary
modifications. A 3PDT switch wiring diagram is also shown, but you can
ignore the middle column and use a DPDT if you don't want LED
indicators.

The Notch Depth Switch mod was implemented in the official
runoffgroove.com Condor build, shown below. The blue LED indicates that
the stock notch is active, while an amber LED is used to indicate the
Marshall notch.

We've named the circuit Condor after the national bird of Chile, the
home of Sebastian Tepper (a.k.a. STM), who collaborated with runoffgroove.com
on this project. We are very thankful for Sebastian's incredible knowledge of
filter design and enthusiastic commitment to the circuit. Without his
involvement, we would not have enjoyed the same excellent results that we can
now share with the DIY effects community.

Listen
to it!

Pablo De Luca (aka Gringo) contributed a PCB
layout for the Condor (PDF, 154k)

Condor by runoffgroove.com is licensed under
a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported
License.
Permissions beyond the scope of this license may be
requested.

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