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By Allen H. Burdick
Converting the HPA-1 to a long lines driver.
The high current output configuration of the HPA-1 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-1 Modified for Long Lines Driver Use
The HPA-1 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-1 the configuration flexibility needed for a long lines driver.
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-1 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.
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-1. The indicated 30 O output resistors are not present in the new HPA-1 and will have to be added externally to the HPA-1s circuit board.
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-1. 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 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
Fig. 4.0 Wiring of HPA-1 for Transformer Output Drive
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-1 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-1) 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 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.
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.
If an audio system is composed of multiple components, we may have detailed specifications for each component, but we will not know the performance of the combined system without doing some calculations. You may have questions such as these:
Use Benchmark's online audio calculators to find answers!
For example, if we know the output power of an amplifier, as well as the sensitivity and impedance of our loudspeakers, we can calculate the maximum sound pressure level that our system can produce.
This application note provides interactive examples that help to answer the questions listed above.
We have added an "Audio Calculators" section to our webpage. Click "Calculators" on the top menu to see more like these:
Secrets contributor Sumit Chawla recently caught up with Benchmark’s VP and Chief Designer, John Siau to get a little more in-depth on several subjects.
Q: "Benchmark is one of the few companies that publishes an extensive set of measurements, but you also balance that with subjective testing. Can you talk about the equipment, the listening room, and the process for subjective testing?"
Q: "Was there ever a time where you learned something from a subjective test that was not captured by measurements?"
Q: "You conducted some listening tests to determine whether distortion in the “First Watt” was audible. What test material did you use for this, and what did you find?"
Q: "The AHB2 amplifier incorporates THX Audio Achromatic Amplifier technology. When and how did the partnership with THX come about?"
Q: "Linear power supplies have been and remain quite popular in high-end devices. You favor switch-mode power supplies. When and why did you make this switch?"
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