Using the HPA-2 as a Long Lines Driver
Converting the HPA-2 to a long lines driver.
Introduction
The high current output configuration of the HPA-2 is ideal for use
as a transformerless long lines driver. Conversion of the unit for this
purpose, source, transmission line, and termination considerations will
be discussed.
Fig 1.0 1/2 HPA-2 Modified for Long Lines Driver Use
General
The HPA-2 is a two channel inverting amplifier card with current
boost buffers that provide an output current capability of 300 mA (each
side). Independent channels give the HPA-2 the configuration flexibility
needed for a long lines driver.
A Basic Configuration Change
As supplied, the HPA-2 has a variable gain output. Since we
desire to use the unit as a transformerless line driver we will
typically want a fixed gain. We will convert the gain of the HPA-2
to unity (be sure to convert both sides).
Figure 1.0 shows the modifications necessary, they are enclosed in
boxes. Replace the potentiometer with a jumper and feedback resistor
combination. The feedback resistor needs to be a 10 k 1% metal oxide
type. If you desire to retain variable gain, do so with careful thought.
In the unbalanced example to follow, the gain pot can be left in the
input side, but a unity gain inverter must be created on the opposite
leg. The gain pot may be retained for both sides of the HPA in both
balanced input examples.
We also recommend removing the TRS jack, replacing it with a XLR
type and mounting the board in a chassis, with double sided sticky foam
tape and a tie wrap.
Drive Configurations
Determine whether the source that will be driving the "Long
Line Driver" is a balanced or an unbalanced source and then use the
proper circuit shown below. Please note that these drawings specify the
now obsolete HPA-1. These drivers will perform with outstanding
clarity using the new HPA-2. The indicated 30 Ω output
resistors are not present in the new HPA-2 and will have to be added
externally to the HPA-2s circuit board.
Unbalanced Source
When driving from an unbalanced source, feed the signal into the
"Left" input. The other input is fed from the "Left"
output, prior to the build out resistor. I.e. tie the "Right"
input, post pin 1, to the "left" output (pin 7 of the buffer)
of the HPA-2. This can be done with a small jumper wire, located on the
bottom of the board. This jumper makes the right channel follow the left
creating a differential or "balanced" output. The balanced
signal output is taken from the two build-out resistors. Note the
polarity inversion: the + output comes from where the ring output
previously connected and output from the tip location.
Fig 2.0 Wiring of HPA for Unbalanced-Balanced Source
Electronically Balanced Source
Electronically balanced sources and transformer sources are very
easy to hookup. Follow the diagrams below. Note the polarity inversion:
the + output comes from where the ring output previously connected and
output from the tip location.
Fig 3.0 Wiring of HPA for Electronically Balanced Source
Transformer Drive
Fig. 4.0 Wiring of HPA-2 for Transformer Output Drive
Line Considerations
Transmission lines have capacitance that must be fed current. The
higher the frequency, and output voltage swing, the higher the current
requirement. Using a standard NE5532 operational amplifier, (commonly
used in output stages) the nominal length of transmission line (Belden
8451 @ 32 pF/foot) that can be driven to full output at 30 kHz is 300
feet. Longer lines require more than the 40 mA current limit of a
NE5532. This is the rational for the conversion of the HPA-2 to a long
lines driver. Additionally, by reducing the inter conductor capacitance,
longer lines may be driven with the same amplifier current limit. For
instance, by using Mogami 2574 cable with its 6 pF/foot capacitance
1000' of cable can be driven with the straight NE5532. A current boosted
NE5532 (modified HPA-2) can drive 2250' of Belden 8451 to full output
level at 30 kHz, and 7,500' of the Mogami cable to the same level @ 30
kHz, over six times the length of a straight NE5532.
Terminations
Terminations are not needed, nor are they desirable at cable lengths
out to 3000' (1/10 l @ 20 kHz). Beyond that distance a true transmission
line should be established. A 60 resistor across the line at the receive
end will terminate the line as a true power matched line. There will be
a 6 dB drop in level as a result of termination. High frequency
equalization may be necessary for extremely long lines.
Receive Amplifiers
Since the drive amplifier does not have an output transformer,
one may need to be present at the input of the receive device.
If the equipment does not have a transformer built into its circuit,
an external transformer will in all probability need to be added.
We recommend those manufactured by Reichenbach Engineering or
Jensen Transformers.
For additional information on transmission lines, see
"A Clean Audio Installation Guide™"
a Benchmark Media Systems, Inc. application note.
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