Power Supply Isolation Question...

[Curtis]

I'm building my own version of a home theatre system with bi-amplified
speakers. I will be using several amplifier modules to power the various
drivers.
I want to power the amp modules off a single central power supply. This
supply
will provide +/- 35 volts with some basic filtering. Some amp modules will
be in
the same case as the power supply while others will be remote mounted in
speakers.
For these remote amps I will run a DC power cable with the +/- 35 volts &
GND.

However, I want each local amp module to have its own reserve of filter
capacitors even though it gets its power from the main supply. Some modules
will have a large reserve (bass amps) and some will have smaller reserves
(tweeter amps).

What I don't want is to have one amplifier draw power from the capacitors in
another amp module during music peaks. What I want to know is the easiest
way to
isolate the local power supply filter caps in each amp from other amps so
they don't
rob each other of power. I know how to do this with a single supply
(diodes), but
how do I do this with a split supply?

Any help much appreciated.

 

[John Woodgate]

I want to power the amp modules off a single central power
supply. This supply will provide +/- 35 volts with some basic filtering.
Some amp modules will be in the same case as the power supply while
others will be remote mounted in speakers. For these remote amps I will
run a DC power cable with the +/- 35 volts & GND.

However, I want each local amp module to have its own reserve of filter
capacitors even though it gets its power from the main supply. Some
modules will have a large reserve (bass amps) and some will have smaller
reserves (tweeter amps).

What I don't want is to have one amplifier draw power from the
capacitors in another amp module during music peaks. What I want to know
is the easiest way to isolate the local power supply filter caps in each
amp from other amps so they don't rob each other of power. I know how to
do this with a single supply
(diodes), but
how do I do this with a split supply?

In one word, DON'T. You are extremely liable to run into utterly
insoluble crosstalk and distortion problems. You are bound to compromise
the grounding, because you need one common ground for the input signals
and with a common power supply you must have a common point at the other
end, where all the large distorted currents lurk. Guess how I know.

Use a separate power supply for each amplifier. It may cost a bit, but
it will be well worth it.

You can use a central power supply for line-level stuff but NOT, without
a LOT of hassle, for power amplifiers.

 

[Ken Smith]

I'm building my own version of a home theatre system with bi-amplified
speakers. I will be using several amplifier modules to power the various
drivers.
I want to power the amp modules off a single central power supply. This
supply
will provide +/- 35 volts with some basic filtering. Some amp modules will
be in
the same case as the power supply while others will be remote mounted in
speakers.
For these remote amps I will run a DC power cable with the +/- 35 volts &
GND.

Let me see if I understand what you are planning. Is the cable to a
remote box like this:

Main Remote
Box Box
---- ------
! +35V !
O!----------------------------!O
! -35V !
O!----------------------------!O
! GND !
O!----------------------------!O
! Signal !
O!----------------------------!O
! GND !
O!----------------------------!O
---- ------


If this is what you are trying, there is likely to be trouble. Some of
your return current will flow down the signal's GND line. This will
create offset voltages in the signal that depend on output circuit
current. This is going to be trouble.

If this what you intend:

You really should send the signal differentially and have the amplitude as
large as practical. The CMRR of the differential input section in the
remote box has to be quite good. Beware of the high frequency (~20KHz)
CMRR it has to be good too. This signal should go on a "bell foil"
shielded twisted pair.

The output section in the remote box should be designed as to pass current
between the +35V and -35V and not return much (if any) current on the
ground. Using a full bridge or half bridge output does this.

One of the really troubling parts of the design is the fact that you will
have a lot of distortion products in the power wires. The power really
needs to be a twisted pair also and all other anti-crosstalk measures
observed in this area.

However, I want each local amp module to have its own reserve of filter
capacitors even though it gets its power from the main supply.

This filter needs to include a series impedance to reduce the high
frequency AC currents flowing in the cable. The power amplifier will have
some power supply rejection ratio. If the high frequency PSRR of the
amplifier isn't high enough and the ripple on the capacitors isn't low
enough, the harmonics on the power supply will show up as harmonics in the
output signal.

[...]

What I don't want is to have one amplifier draw power from the capacitors in
another amp module during music peaks. What I want to know is the easiest
way to
isolate the local power supply filter caps in each amp from other amps so
they don't
rob each other of power. I know how to do this with a single supply
(diodes), but
how do I do this with a split supply?

Each remote box should have rectifiers in its power input connections.
This prevents the current from returning down the wires. It does not
completely protect you though.

Assume an amplifier is drawing, lets say, 1A through the diode, If
another amplifier suddenly draws enough power to reduce the main supplies
voltage, the current through the diode stops. This means that the tweeter
boxes have to contain extra capacitance to carry through the power dips
caused by the bass amplifiers.

Having the inductors at the inputs more or less forces you to also include
rectifiers across the supply lines. Imagine that someone (you would
never) unplugs the connector with the power on. The inductors prevent the
current from stopping and can make quite a large voltage spike. If you
don't take care in the design, this spike could break the electronics or
arc weld the end off connector pins.