A “choke” is the common name given to an inductor that is used
as a power supply filter element. They are typically gapped iron
core units, similar in appearance to a small transformer, but with only
two leads exiting the housing. The current in an inductor cannot
change instantaneously; that is, inductors tend to resist any change in
current flow. This property makes them good for use as filter elements,
since they tend to “smooth out” the ripples in the rectified voltage waveform.
Why use a choke? Why not just a big series resistor?
A choke is used in place of a series resistor because the choke
allows better filtering (less residual AC ripple on the supply, which means
less hum in the output of the amp) and less voltage drop. An “ideal” inductor
would have zero DC resistance. If you just used a larger resistor, you
would quickly come to a point where the voltage drop would be too large,
and, in addition, the supply “sag” would be too great, because the current
difference between full power output and idle can be large, especially
in a class AB amplifier.
Capacitor input or choke input filter?
There are two common power supply configurations: capacitor
input and choke input. The capacitor input filter doesn’t necessarily have
to have a choke, but it may have one for additional filtering. The choke
input supply by definition must have a choke. Capacitor input filters are
by far the most commonly used configuration in guitar amplifiers (in fact,
I can’t think of a production guitar amp that used a choke input filter).
The capacitor input supply will have a filter capacitor immediately
following the rectifier. It may or may not then have a second filter composed
of a series resistor or choke followed by another capacitor. The “cap,
inductor, cap” network is commonly called a “Pi filter” network. The advantage
of the capacitor input filter is higher output voltage, but it has poorer
voltage regulation than the choke input filter. The output voltage approaches
sqrt(2)*Vrms of the AC voltage.
The choke input supply will have a choke immediately following the rectifier.
The main advantage of a choke input supply is better voltage regulation,
but at the expense of much lower output voltage. The output voltage approaches
(2*sqrt(2)/Pi)*Vrms of the AC voltage. The choke input filter must have
a certain minimum current drawn through it to maintain regulation.
The voltage difference between the two filter types can be quite large.
For example, assume you have a 300-0-300 tranny and a full-wave rectifier.
If you use a capacitor input filter, you’ll get a no-load max DC voltage
of 424 volts, which will sag down to a voltage dependent on the load current
and the resistance of the secondary windings. If you use the same transformer
with a choke input filter, the peak output DC voltage will be 270V, and
will be much more highly regulated than the capactor input filter (less
variations in supply voltage with variations in load current).
How to select a choke:
- DC current
- DC resistance
- Voltage rating
Chokes are typically rated in terms of max DC current, DC resistance,
inductance, and a voltage rating, which is the max safe voltage that can
be applied between the coil and the frame (which is usually grounded).
If you are using a choke-input filter (not likely, unless
you are trying to convert a class AB amp to true class A and need the lower
voltage, or if you are designing an amp from scratch and want better supply
regulation), the choke must be capable of handling the entire current of
the output tubes as well as the preamp section. Note that this doesn’t
mean just the bias current of the output tubes, but the peak current at
full output. This usually requires a choke about the size of a standard
30W-50W output transformer, since the choke must have an air gap (just
like a single-ended OT) to avoid core saturation due to the offset DC current
flowing through it, and the choke also must have a low DC resistance, to
avoid dropping too much voltage across it, which will lower the output
voltage and worsen the load regulation. This combination of low DCR, air
gap, and high inductance (more on that later…) usually results in a substantial
sized choke. To calculate the required current rating, add up the full
power output tube plate currents, screen currents, and the preamp supply
currents, and add in a factor for margin. For a 50W amp, this may be 250mA
If, on the other hand, you are selecting a choke for a capacitor input
supply (such as the typical Marshall or Fender design), then the requirements
are relaxed quite a bit. The purpose of the choke in these type supplies
is not for filtering and voltage regulation, but just for filtering the
DC supply to the screen grids of the output tubes and the preamp section.
The screens typically take around 5-10mA each, and the preamp tubes draw
about 1-2mA or so (for the typical 12AX7; 12AT7’s are usually biased for
around ten times that). This means that you can get by with a much smaller
choke, and, in addition, the preamp supply current doesn’t vary that much,
so you can get by with a higher DC resistance, which means smaller wire
can be used to wind the choke, which means higher inductance for a given
size core. Just add up the current requirements of the screens and preamp
tubes, and add a bit more for margin. For a 50W amp, a typical value might
For a typical choke input supply, you need a choke with no
more than 100-200 ohms or so DCR. A capacitor input supply typically might
use a choke with a 250 ohm – 1K DCR. The higher the resistance, the more
voltage drop and the poorer the regulation, but the cost will be lower.
As for the inductance value, this depends on how much filtering
you want. The inductance, in conjunction with the filter capacitance, forms
a lowpass filter. The larger the inductor, the lower the cutoff frequency
of the filter, and the better the rejection of the 120Hz (if full wave
rectified) or 60Hz (if half wave rectified) AC component of the rectified
DC. In general, the larger the better, within reason (larger inductances
at low DC resistances mean larger chokes, which cost more money). Typically,
5-20 Henries is a good choice with the standard 32-50uF electrolytic capacitors.
The inductance and capacitance values also determine the transient response
of the supply, which means the tendency for the supply to overshoot or
“ring” with damped oscillations whenever a current transient is applied
(such as at startup or on a heavy current surge, such as a hard “E” chord
at full power!).
The voltage rating must be higher than the supply voltage, or the insulation
on the wire may break down, shorting the supply to the frame.
I highly suggest going to Duncan Munro’s website (http://www.duncanamps.com/)
to download his power supply calculator program. It will allow you to experiment
with different inductance and capacitance values and see the resulting
residual AC ripple and transient response of the supply filter. Both capacitor
input and inductor input filters can be simulated. It is a great educational
Randall Aiken. May not be reproduced in any form without written
approval from Aiken Amplification.