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Smoothing and Filtering the Power Supply
The power supply is the most important part of the amplifier, and is usually
designed first, based on whatever power transformer has been chosen. A rough idea
of what the HT will be at each point in the amp is then known, and the design of the
amplifier stages can begin. There will of course be some mutual adjustment as the
design process continues, but ultimately the power supply dictates the limitations of
the amplifier as a whole.
When the AC signal from the power transformer is rectified to DC it will have
severe ripple. It must be smoothed into a more pure DC supply or the amp would be
very noisy indeed.
The design of power supplies can be a lengthy and taxing affair, especially when
considering regulated power supplies or choke input filters. Luckily, most guitar amps
do not require these features and use a very basic power supply topology. Most use
only one stage of smoothing to supply to the output stage, followed by simple
resistor-capacitor (RC) filters for the other stages in the amp.
The first capacitor after the rectifier is the most important and is known as the
'reservoir capacitor' (C1). Each rectified cycle will charge the capacitor to a peak
voltage, this will then decay as current is draw out
by the amp until the next rectified cycle charges it
up again. For most of the time then, the amp is
drawing its current from this capacitor and not
from the power transformer;- it acts as a reservoir
for current, as well as smoothing the DC voltage.
ESR: The capacitors used in the power
supply are large and so should have low
reactance at all frequencies. However, they are
not perfect, and actually have an Effective Series
Resistance (ESR) and this sets the output
impedance of the power supply. Some 'high
ripple current' capacitors are designed specially
to have a very low ESR, but these are expensive
and unnecessary for guitar. Even so, for the
power supply you should use the best quality
capacitors you can afford, since they are mostly what you end up listening to,
because they are where the amp draws its current from most of the time. Replacing
old and tired electrolytic capacitors with new ones usually results in a phenomenal
improvement bass response and tone.
The reservoir capacitor needs to have a low reactance at frequencies up to 40kHz,
because the HT at this point will contain high frequency harmonics caused by
rectification. Its performance at these frequencies can be greatly improved by adding
a capacitor in parallel with it, ten times smaller in value. (Another can also be added,
ten times smaller than the second, but this would be usually be considered an
extravogance.)
Choosing the reservoir capacitor: One
reservoir capacitor cannot smooth all the ripple
voltage (unless it were infinitely large), so we
must decide how much HT ripple voltage we can
tolerate and choose a capacitor that can achieve
it.
The ripple voltage is expressed as a percentage of the total HT voltage, so a 100Vdc
HT with a 10V peak-to-peak ripple 'riding' on it has 10% ripple.
Push-pull amps can tolerate more HT ripple because they cancel common
mode noise. We could reasonably allow as much as 20% ripple voltage, and 10%
would be excellent.
Single-ended amps are more susceptible to HT ripple, so we would allow 10% at
most, and 5% would be better. Adding a second filter before the power valve is
sometimes done in single-ended amps, bringing the ripple voltage down to a very low
level. This is not so easy in high power amplifiers as they draw significant current and
we would need a very large power resistor or choke in the filter.
In the following example we have a 50W push-pull amp, and the HT after
rectification is 350Vdc. We also need to know the average current the amp will draw.
This will be the sum of the quiescent currents of all the valves.
Supposing this amp has two ECC81's, an EF86 and two EL34's, the average current
drawn by each triode will be about 5mA and there are four triodes in all, making
20mA. The EF86 will draw about 3mA, making 23mA for the pre-amp.
We will assume that the power valves are biased to their maximum dissipation of
25W each (if the amp is Class-AB they won't be biased that hot, but we should
assume worst case scenario). The average anode current they draw will be in the
region of:
I=P/V
I = 50 / 350
= 143mA.
The screen-grids will also draw a quiescent current: The data sheet for the
EL34 suggests a screen-to-anode current ratio of 6.5, so we can expect the screen
currents to sum to:
143 / 6.5 = 22mA.
Added to the pre-amp current this makes 188mA in total for the whole amplifier.
(Remember, this is the average current, and is not the same as the peak current that
the amp will draw. In an amp like this, depending on the class of operation, the peak
current might be 200mA for one EL34 (while the other EL34 goes into cut-off) plus
the 23mA pre-amp current which won't vary much. That makes 223mA peak, and the
power transformer would need to be rated for at least this much AC current,
preferably 1.5x more if the amp is to be run at full power for long periods.
We decide to allow 10% ripple. The HT is 350V, so 10% of this is:
(350 / 100) * 10 = 35Vp-p.
The reservoir capacitor can be found using:
C = (t * I) / V
Where:
I = average load current drawn
V = ripple voltage peak-to-peak
t = duration between charging cycles and is equal to: 1/twice mains frequency.
In Europe the mains frequency is 50Hz, so: t = 1/100 = 0.01 seconds.
The reservoir capacitor required will be:
C = (0.01 * 0.188) / 35
= 54uF.
The nearest standard is 47uF. This would give a 40V ripple which is 11%; perfectly
adequate. Most modern designs use a value from 33uF to 220uF at most. Hifi amps
may use much more, but don't be tempted to use very large values unless your
power transformer is designed to handle a lot more current than you actually need.
The low power-factor caused by using a huge reservoir capacitor can cause a
'borderline' transforer to overheat! If using a valve rectifier you MUST check the data
sheet to see if the value of capacitor is allowable [see below].
Ripple current and valve rectifiers: When the reservoir capacitor charges up
with each input cycle, it draws a large current pulse from the rectifier and power
transformer. This is known as the ripple current, and the larger we make the
capacitor the larger it will be.
Calculating the ripple current is fairly lengthy, but sufficed to say it is usually about
five times the average current drawn. In this case we can expect it to be nearly 1A!
Luckily this does not mean we need a transformer rated at 1A (which would be as
expensive as it would be huge) as the current pulses are of short duration, which the
transformer should be able to handle. The current rating on silicon diodes already
takes ripple current into account, so 0.5A diodes would still be safe to use. However,
this is why you should not use a ridiculously large value capacitor, as the increased
ripple current would put a great strain on the rectifier and power transformer.
(Although the diagrams shown here depict a full wave rectifier, it could also be a
bridge rectifier, it makes no difference.)
Silicon rectifiers can handle larger ripple currents well, but valve rectifiers
cannot. For this reason, if you are using a valve rectifier you must check the data
sheet for the maximum allowable reservoir capacitor. THIS VALUE MUST NOT BE
EXCEEDED or too much ripple current will flow and the valve rectifier will almost
certainly be destroyed. For the GZ34 the limit is 60uF, and it would be a good idea to
always use a value smaller than this, rather than push the valve to its maximum
limits, especially since there is rarely any need for a capacitor larger than 60uF.
In a low power or singleended amp we might add a filter
after the reservoir capacitor but
before the output stage, to give an
almost perfect DC supply. This is
known as a 'pi filter' because it
resembles the Greek letter pi. A
resistor or a choke can be used. A
resistor would normally be in the
region of 330R to 1k, and
remember to check how much
power it will dissipate.
A choke is a better option since it
will drop less voltage than a
resistor, since its DC resistance
will usually be less than 100 ohms.
Power chokes are normally rated
at between 1H to 10H, and
remember to check it can handle
the peak current you want to draw.
By adding this extra filtering, the reservoir can be made smaller in value allowing a
higher ripple voltage which is then smoothed yet again by the second capacitor. This
is useful as it means we can reduce the ripple current, and put less strain on a valve
rectifier if one is used.
The value of the second smoothing capacitor isn't critical. It will not cause heavy
ripple current so is usually made equal to, or larger than the reservoir capacitor. A
rule of thumb is that the sum of the two capacitors (in micro-Farads), multiplied by the
inductance of the choke (in Henrys) should equal 200 or more for excellent
performance. For example, using a 22uF reservoir capacitor with a 47uF filter
capacitor and a 5H choke:
(22 + 47) * 5 = 345
The screen-grid supply: The next filter in the power supply is usually for the
power valves' screen grids. Traditionally this
filter used a choke because they were more
readily available than high power resistors.
Nowadays high power resistors are cheap
and small while chokes are large and
expensive, so most modern designs do
away with the choke. A resistor will also
drop more voltage than a choke, placing the
screen-grids at a safer voltage level.
The value of resistor depends on the
voltage you want to drop for the screengrids. The current through the resistor will
consist of the screen current plus the
current drawn by the pre-amp (22m and
23mA making 45mA total in this case).
If we wanted the screen voltage to be 300V
we need to drop 350 - 300 = 50V. Use
Ohm's law to find the value of resistor:
50 / 0.045
= 1111 ohms.
So we would use a 1k resistor (wire-wound
resistors are commonly available only in the E6 range so bare this is mind.) This
would actually drop 1000 * 0.045 = 45V. The average power dissipated in it will be:
(0.045 * 0.045) * 1000 = 2W
So we would use a 5W resistor or better.
The value of the filter capacitor is not critical. Ideally we would like the power
supply to smooth out all frequencies down to 1Hz and below. Therefore it is desirable
for each RC filter section of the power supply to have a time constant of 0.16seconds
or more, that is, the value of capacitor may be chosen according to:
C = 1 / (2 * pi * R)
Since our dropping resistor is 1k, so we might choose a capacitor of:
C = 1 / (2 * pi * 1000) = 159uF or more.
However, particularly in the power amp, we often use rather smaller capacitors so as
to introduce some sag at lower frequencies, so we may often find values from 10uF
to 100uF.
The Pre-amp supply: The pre-amp stages in the amp will have additional RC
filters. This is necessary to provide an excellent DC voltage for the more sensitive
pre-amp, to reduce the HT for the pre-amp, and to makes sure each stage is
decoupled from the next so cross modulation, feedback and noise does not occur. It
is usual to add one filter for every two triodes, or one for every small signal pentode
(because they are more susceptible to noise).
The value of resistor used in each stage depends on the voltage you want it to drop.
The current flowing from the power supply through the filter resistors progressively
decreases toward the input stage. Usually, using the same value resistors throughout
works quite well, and the HT will fall successively toward the pre-amp, but you can
use different value resistors according to Ohm's law to obtain different voltage drops
as you see fit.
Values are usually around 1k or greater, and remember to check the power they will
dissipate.
Again, the value of the capacitors is not critical, although if they are too small the
amp may be prone to low frequency oscillation, or motorboating. This is a common
problem in older amp, especially if the cap's have dried up over time. For the stiffest
power supply we again use: C = 1 / (2 * pi * R). Smaller values are often used, but
the low frequency response of the associated preamp stages should be carefully
attenuated, using small coupling/cathode bypass cap's, if motorboating is to be
properly avided. Values from 22uF to 220uF are typical.
The diagram [above] shows a working design, but it does not yet contain any bleeder
resistor, fusing or power/standby switches. These is dealt with in the sections on
fuses and power and standby switches.
If we needed to drop the HT voltage to a single stage without
affecting the HT voltage delivered to the other stages in the amp,
we could add a separate filter just for the valve in question,
branching off the rest of the HT supply. The resistor drops the
necessary voltage and the supply is decoupled with the filter
capacitor [left] in the same manner mentioned previously.
Voltage ratings: All the capacitors
used MUST have a higher DC voltage
rating than the HT, and ideally not less
than 25% higher, also taking into account
any rise that may happen due to variation
in the mains voltage, Capacitors with
ratings up to 450Vdc are readily available since they are still
used in switch-mode power supplies in modern appliances.
However, if we were worried about running a capacitor too
close to its voltage limit, we can place two or more
capacitors in series to increase the voltage rating of the
whole. Equal value resistors should be placed in parallel with
each capacitor to ensure the HT is divided equally between them. The resistors
should be large so as not to pass significant HT current, but should pass at least five
times the expected leakage current of the capacitor, which is typically in the region of
0.05mA. Values of 220k to 470k are typical, and remember to check their power
dissipation. They can also act as a 'bleeder' network that will allow the power supply
capacitors to discharge when the amp is switched off.
The total capacitance achieved will be less than one capacitor alone (capacitors sum
in the opposite manner to resistors). If using two identical capacitors the total
capacitance would be half the value of one alone.
Capacitors can also be placed in parallel to increase the capacitance as a whole
(capacitances in parallel sum). The voltage rating does not change in this case.