Power Supply
Introduction
A dual-rail (positive and
negative) D.C. power supply is needed for the audio cards (Dual Input Card,
Dual Output Card).
As there are various options
for this, no power supply PCBs or designs are available. The requirements will
also depend on the actual mains voltage, and its stability, in the country of
use.
Separate supply rails will
also be required for any logic/microprocessors used to drive the cards - which
may even be provided from a separate transformer. (If simple D.C. controls are
used for the cards, then these can be derived from the audio supply rails if
needed.)
Power supplies meeting these
requirements can be designed/built or an off-the-shelf commercially available
unit can be used. The choice of power supply design will depend on
the:
- number of cards to be powered (i.e.
the total current consumption)
- user's desire for an unregulated or
regulated supply
- user's own electronics design
experience, etc.
If the audio cards are
integrated into a complex system, then the power supply will be a relatively
straightforward element for the designer to include.
Although it is assumed that
most users will opt to build a linear regulated power supply to feed the audio
cards in a subrack, it is recognised that this will not be necessary or the
chosen option for all users. Hence, the sections below discuss the outline
requirements of the power supply; some possible options and simple guidelines
are suggested.
Is The Power Supply
Important?
It is important for audio
ICs (op-amps etc) to be fed with low noise and stable D.C. power, even though
most audio ICs have a high power supply noise rejection.
Where ICs are fed directly
from the power supply rails, which may be feeding many ICs in different parts
of the audio chain, the power supply is clearly very important, as audio
signals in one part of the chain must not affect other ICs.
One of the design features
of this system is that each audio card (such as the Dual Input Card) has its
own on-card voltage regulators, reservoir capacitors and decoupling capcaitors.
This isolates the critical voltage regulation/supply design to each card and
allows a more conventional (and less "over the top") main power
supply.
Why Use
Regulators?
In a linear (conventional)
power supply design, voltage regulators help to smooth out and, usually,
completely remove changes in the output D.C. voltage which would otherwise be
caused by:
- variations in mains supply voltage
(both long term changes and short term dips/"brownouts")
- remnants of a.c. ripple on the D.C.
output due to the diode bridge and reservoir capacitor charging/discharging
cycle
If designed correctly, a regulated power
supply will help to make the audio system perform consistently on a day-to-day
basis regardless of the state of the mains supply.
On-Card
Regulators
The audio cards (such as the
Dual Input Card) have voltage regulators on each card to guarantee low noise
and stable D.C. supplies. This also allows either an unregulated (but fairly
stable) supply or a (pre-) regulated supply to be used.
In steady state at an
ambient temperature of 22ºC, the Dual Input Card draws less than 80mA per
D.C. supply rail. It is recommended that 100mA is allowed per card (e.g. with a
rack containing 12 cards, the power supply must be capable of delivering at
least 1.2A minimum per supply rail - so possibly a power supply rated at 1.5A
to 2A). The current consumption of Dual Output Cards will increase slightly if
all outputs are being driven at high signal levels into low impedance (600 ohm)
loads.
The standard design assumes
that the on-card regulators are set to produce outputs of ±15V. The
LM317/LM337 regulators specified, will operate with a minimum input voltage of
1.8V greater than the required output voltage - so 17V rails are required.
Although the on-card regulators could be operated up to their maximum rating of
37V input, large heatsinks would be required - the standard card designs do not
allow for this! The regulators will operate at a sensible temperature without
any heatsinks over the 17V to 25V range (at 25V input and 22ºC ambient
temperature, the regulators will be hot, but should be touchable!). If a
regulated power supply is used, it is suggested that this is set in the range
18V to 20V.
The on-card regulators can
be set to operate at lower voltages if required - for example, ±12V if
±15V power supply rails will be used.
Power Supply
Options
Overview
The information above shows
that for ±15V D.C. regulated supplies on each card, the power supply
needs to produce positive and negative supply rails for audio use,
either:
- "un-regulated", but stable, in the
range 17V to 24V
- regulated to 19V (for example).
with any other rails for
logic/microprocessors as required for each design.
An unregulated supply can be
used as long as the stability of the mains supply voltage is
acceptable.
If a regulated supply is
desired, then this will either be truely "linear", a combination of transformer
plus switching regulators, or a fully "switch mode" supply (such as those used
in computers where high power, low voltage supplies are needed). Current audio
fashion is still, mainly, to use conventional linear supplies as they are
straightforward to understand and have few components. Switch mode supplies on
the other hand have many components working at much higher voltages - the
reliability can be poor unless high quality modules are used (but some very
high quality modules are available). Using a PC computer supply is probably not
the right choice as its fan will generate acoustic noise!
Warning!
Building a mains power
supply should only be carried out by those experienced in this as it will be
connected to the mains supply. Standard practices should be followed
for the physical construction (country dependent) such as earthing metal cases,
sleeving all mains voltage connections to prevent accidental contact, fitting
warning labels, installing fuses in the "live" and, possibly "neutral", mains
power feeds within the case.
All power supplies must be
enclosed in an adequate case to protect users from the mains voltage
connections inside. If a plastic case is used, the transformer/power supply may
need to be located further away from the audio cards (subrack) to prevent the
field from the transformer being induced into the audio circuits and being
audible as "hum".
Note that commercially
manufactured power supplies must comply with various legislation and standards
- for example in the European Union power supplies must not inject pulses and
other interference back into the mains supply.
Commercially Available
Power Supplies
There are a few options for
commercially available power supplies.
The lowest cost will
probably be a plugtop/wall wart style (or trailing lead equivalent) power
supply - regulated and unregulated types are available - which may be of
sufficient power to feed a small number of audio cards. Two single output units
can be joined together to create dual rails (positive and negative) if they are
"floating" (i.e. they have no earth pin). However, care must be taken to
ensure that the polarity is correct, as there are no reverse connection
protection diodes on the audio cards.
For a larger number of audio
cards with a higher total current consumption, various linear and switch mode
power supply modules are available which would need to be installed in a case
with mains power inlet, fuse(s) and D.C. outlet.
If you do not wish to
design/build your own power supply, then some companies will manufacture cased
power supplies for you (not a low cost option).
Unregulated Linear
Supply
The diagram above shows the
generic circuit for an unregulated dual rail D.C. supply (a higher resolution
drawing is available on the Photos/Downloads page),
with a fuse in the "live" feed only.
Any variations in the mains
supply voltage will be directly reflected (percentage wise) in the D.C. output
voltages.
For audio use, toroidal
transformers are normally used as they have low hum fields compared to frame
transformers - however, slow-blow fuses are needed as they tend to produce
surges at switch on (as they provide a low impedance connection between the
reservoir capacitors and the mains supply). Toroidal transformers are available
"potted" (encapsulated) in plastic with a single fixing thread - this type of
packaging guarantees that a fixing bolt cannot be accidentally connected to the
case earth at both ends of the bolt (creating a large bang...!). If an
off-the-shelf transformer cannot be found, some toroidal transformer
manufacturers will custom design one for an additional charge.
A larger transformer (higher
power rating, noted in "VA") than actually required based on the current load
may be needed, as this will offer better regulation with varying loads. This
will be important if an unregulated supply is being used as with a smaller
transformer, the output voltage will increase more as the load
reduces.
Do remember to consider the
ratings of the diode bridge rectifier - the peak inverse voltage and the peak
current (which may be significantly higher than the average current drawn due
to the brief charging cycle of the reservoir capacitors).
Regulated Linear
Supply
An unregulated supply (such
as that described in the section above) can be used as a basis for a regulated
design. For a regulated supply design, a regulator is "inserted" between the
basic power supply and the feeds to the audio cards.
This will require a higher
voltage from the unregulated section and, if linear regulators are used, will
require heatsinks, which may be large depending on the working voltages
chosen!
Linear power supply design
information is available in many electronics text books (such as
Horowitz and Hill : The Art of
Electronics).
RF Rejection
Chokes
If the power supply is
located in a separate case to the audio cards or some distance away from them,
it may be advisable to install RF rejection inductors (chokes) and/or ferrite
rings on the D.C. power supply inlet feeds within the audio card case. This
will prevent/minimise any RF signals from running along the
Motherboard.
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