Installation manual

MYTHS, MAGIC AND FACTS
Damping Factor - This amplifier specification has been blown out of all proportion. What
it means is the ability of the amplifier to resist a change in it’s output voltage. The formula
is DF= Speaker Z / Amplifier output Z (where Z is impedance). So many manufacturers
have claimed ridiculous, and often false damping factors. A damping factor of 1000
implies that the output impedance of the amplifier is 0.004 ohms (4 ohm load). The only
way to attain this figure is to apply masses of negative feedback (or use positive
feedback) and too much feedback makes amplifiers sound harsh and clinical. Also
damping factor changes with frequency. The lower the frequency the higher the DF
number. Typically the DF can be ten times larger at higher frequencies.
Let us take this amplifier whose output impedance is 0.004 ohms (Zout). The speaker
circuit is a series circuit and the following impedances(resistances) are in series with this
0.004 ohms. Let us assume that this DF measurement was made at the amplifier’s
speaker terminal. The first extra contact resistance is the speaker wire to the speaker
terminal (WT ohms). Then there is that of the wire itself for two conductors (W). Next is
the contact resistance of the wire to the speaker terminal (WS). Next there is the contact
resistance of the wire from the speaker terminal to the voice coil (WV) and lastly there is
the DC resistance of the voice coil itself (DCR). So what we have is a series circuit with
the following resistances in series and adding up. WT+W+WS+WV+DCR+Zout.
WT,W,WS,WV and Zout are very small indeed. Certainly less than 0.1 ohms. Whoa, look
what has happened the EFFECTIVE DAMPING FACTOR has been reduced from 1000 to
40 by just taking into account those pesky unavoidable contact resistances. Now for the
cruncher, remember that the DCR is also in series and is typically 3.2 ohms for a nominal
4 ohm speaker. So we must add 0.1+3.2 = 3.3 ohms and now EFFECTIVE DAMPING
FACTOR is now a magnificent 1.212! (4 divided by 3.3)This is the real world. We see that
the DCR of the speaker swamps all other resistances in the speaker circuit and the 0.004
ohms amplifier output impedance is almost meaningless. It has been found that a DF of
about 20 is quite sufficient to dampen the voltage spikes from the speaker. An eye opener
this one is it not? Good tube amps sound marvelous - low damping factors!!
Output Power of Amplifiers - This spec has been so badly abused it is not even funny.
Peak power, Maximum power, Transient power, RMS power these are titles that have
been given to the power spec of amplifiers. The above all mean nothing. Peak power
needs to be associated with a time period, Maximum power is just nonsense, Transient
power is even more nonsense and . The ONLY
meaningful way to specify an amplifier’s output power in watts is CONTINUOUS POWER.
The formula for power is: (RMS volts x RMS amps) or (RMS volts x RMS
volts/Impedance) or (RMS amps x RMS amps x Impedance). In each of these formulae
there is an RMS number multiplied by another RMS number (or by itself) and RMS x RMS
cannot = RMS. RMS means
root mean square and it is the same as saying 4 x 4 = 4 Which we know is not true.
The answer is just 4 with no root sign attached.
RMS power is just not a specification
So THERE IS SIMPLY NO SUCH THING AS RMS POWER.
Page 11