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.
The Benchmark AHB2 power amplifier and HPA4 headphone amplifier both feature feed-forward error correction. This correction system is an important subset of the patented THX-AAA™ (Achromatic Audio Amplifier) technology. It is one of the systems that keeps these Benchmark amplifiers virtually distortion free when driving heavy loads. It is also the reason that these amplifiers can support 500 kHz bandwidths without risk of instability when driving reactive loads.
This paper explains the differences between feedback and feed-forward systems. As you read this paper, you will discover that you already understand the benefits of feed-forward correction because you use it instinctively to improve a feedback system commonly found in your automobile. If feed-forward correction can improve your driving experience, it may also improve your listening experience!
If you look at the back of any Benchmark product, you will find balanced XLR analog-audio connectors. As a convenience, we also provide unbalanced RCA connectors on many of our products. In all cases, the balanced interfaces will provide better performance.
We build our unbalanced interfaces to the same high standards as our balanced interfaces, but the laws of physics dictate that the balanced interfaces will provide better noise performance.
This application note explains the advantages of balanced interfaces.
Benchmark has introduced a new analog-to-analog volume control circuit that features a 256-step relay-controlled attenuator and a 16-step relay-controlled boost amplifier. The volume control has a +15 dB to -122 dB range in 0.5 dB steps and is a key component in the HPA4 Headphone / Line Amplifier.
Our goal was to produce an analog-to-analog volume control with the highest achievable transparency. We wanted to be able to place this volume control in front of our AHB2 power amplifier or in front of our THX-888 headphone amplifier board without diminishing the performance of either device. Our volume control would need to have lower distortion and lower noise than either of these amplifiers. Given the extraordinary performance of these THX-AAA amplifiers, this would not be an easy task!
This application note discusses the engineering decisions that went into the development of this new analog volume control circuit. The end result is a fully buffered volume control with a signal-to-noise ratio that exceeds 135 dB. THD measures better than the -125 dB (0.00006%) limits of our test equipment.