Reactive Speaker Load

From reaike–(at)–x.netcom.com Sat Dec 2 09:24:35 CST 1995
From: reaike–(at)–x.netcom.com (Randall Aiken )
Newsgroups: rec.audio.tubes
Subject: Reactive speaker emulator for guitar amplifiers
Date: 2 Dec 1995 04:59:01 GMT
X-NETCOM-Date: Fri Dec 01 8:59:01 PM PST 1995

I received e-mail asking me to post this to rec.audio.tubes, because
the reader’s server did not have alt.guitar.amps, so please ignore this
if you have seen it on alt.guitar.amps. The original question was how
to make a reactive load emulator for a guitar amplifier.

I also received e-mail informing me that Aspen Pittman of Groove Tubes
holds a patent for a reactive speaker emulator. I have not seen his
patent circuit, but I am told it consists only of an inductance in
series with a resistive load. If so, it will provide a reactive load
for the amp, but will not emulate the characteristic resonant impedance
peak associated with speakers. I am also told that there was an
article in EQ magazine a few years ago detailing his type of circuit
and tips on how to build one. Just beware that you cannot use this
circuit in any commercial application without the possibility of patent
infringement.

Following is a slightly edited version of the original post:

A speaker presents a varying impedance load to the amp. The stated
impedance is usually measured at 400Hz, and can vary widely over the
frequency range. Speakers generally have a large resonant peak that
can be as much as 5 times the rated impedance, or even higher. The
impedance also starts to rise upward following the trough impedance at
400Hz. This rise can occur at varying frequencies and have a varying
rate, depending upon the speaker and its enclosure.

Here is a copy of my design for a circuit that will simulate the
impedance variations you get with a 16 ohm speaker cabinet. The real
speaker will probably vary depending upon how hard it is driven, due
to motional restrictions of the cone, which this circuit will not do.

Note that for 100 watt power capability, the peak current that can flow
in the inductors (depending upon frequency) is around 3 amps, better to
be conservative. The capacitor must be rated for this ripple current
or it will overheat and explode. If you cannot find a bipolar cap of
sufficient voltage rating, you can connect two unipolar electrolytics
of 200uF in series back-to-back with a 10K resistor across each of them
to equalize the voltage drop. If you cannot get one of sufficient
current rating, you can connect two 50uF units in parallel to equal
100uF; the current capability will add, i.e. two 1 amp rated units in
parallel will withstand 2 amps ripple current. Series-parallel
combinations to achieve the proper voltage/current ratings are
acceptable. Low-ESR (equivalent series resistance) capacitors are
best. Current capability (and resistor wattage) can be derated if used
with a 50 watt amp (around 2 amps).

Also, the inductor capacitor values can be modified to move the
resonant frequency or the high frequency impedance breakpoint; the 68
ohm resistor can also be raised to achieve a higher resonant peak.
You can match the resonant frequency to the speaker you are trying to
emulate.

The impedance plot for this circuit looks like this (but smoother!):

___
84 ohms – – __
– – ___
_ _ _____
_ _ _____
_ _ _____
_ _ _____
16 ohms — ————-

| | | | |
| | | | |

DC 72Hz 400Hz 2kHz 10kHz

This circuit will make the amp react to the load, unlike purely
resistive dummy loads, but if you want to tap off the input and send
the signal to a board, or other equipment, you will need to attenuate
it with a voltage divider, and low-pass filter it to simulate the
frequency response rolloff of the amp.

I do not guarantee this circuit for any application and will NOT be
responsible if you blow up your amp!

******

One additional note: a simple low-pass filter will not sound very much
like a real speaker; you need to use combinations of lowpass, highpass,
bandpass, and bandstop networks to emulate the peaks and valleys of the
average guitar speaker cabinet. A fairly good combination is a peak of
about +3dB at 240Hz, and a dip of about -1dB at 1250 Hz, a lowpass
cutoff at about 4KHz with an 18dB/octave rolloff, and a highpass cutoff
at about 100Hz with a 12dB/octave rolloff.

Also, Dutch posted an excellent followup to this post where he
mentioned that he designed a similar circuit, but included an
additional resistor across the series inductor to limit the high
frequency impedance to about 22 ohms. This resistor would go across L1
in the above circuit. He also had slightly different resonant points.
I suggest you look it up on alt.guitar.amps.

Randy Aiken
reaike–(at)–x.netcom.com

Reactive Speaker Load

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