Product Review - A Tale of Two Pots: The AU/RA D7A Stereo Volume Control and the Channel Islands Audio VPC-1 Stereo Volume Control - February, 2002
John E. Johnson, Jr.
Stepped Design, 33 Steps, 1.666 dB per Step, 22 kOhm Total
One Set of RCA Inputs, One Set of RCA Outputs
Capacitor Coupled Inputs and Outputs
Size: 3 1/2" H x 3 1/2" W x 10" D
Weight: 6 Pounds
MSRP: $175 EURO Plus Shipping; $199 USA Including Shipping
Channel Islands Audio VPC-1
Continuous Potentiometer Wiper Design, 10 kOhm Total
Two Sets of RCA Inputs, One set of RCA Outputs
Size: 2 1/2" H x 4" W x 4" D
Weight: 1 1/2 Pounds
MSRP: $249 USA
AU/RA v.o.s., Mala Tyrsovka 5, Praha 4, Modrany, Czech Republic, Phone and Fax 02 / 44 40 34 99; E-Mail email@example.com; Web http://www.auravos.cz/
Channel Islands Audio, 567 W. Channel Islands Boulevard, # 300, Port Hueneme, California 93041; Phone 805-984-8282; Fax 805-984-8283; Web http://www.ciaudio.com
Over the years, we have had a number of readers ask us about controlling the volume of a circuit without using a preamplifier, for example, between a CD analog out and a power amplifier, or reducing the output of a CD player before going into a preamplifier that would otherwise be overloaded.
Presented here are two such devices, one of which (AU/RA) is made in the Czech Republic (formerly Czechoslovakia) and the other (Channel Islands Audio) being a USA product. Both appear to be well made, and worked fine here in the lab. They are both about the same price. However, there are some differences, which might have application to various situations. First, let's discuss the design of each, and then we will cover the sound quality.
The AU/RA D7A is larger and heavier than the VPC-1. The metal cover is very thick. Inside, you can see that the mechanism is quite complex (photo below). The volume control knob on the front turns a shaft that runs a good distance into the chassis. When you turn the control, two "wipers" contact 33 different and separate surfaces. In the photo, red arrows point out the wiper making contact along the circular array of separate contact surfaces. All surfaces are gold plated. Each of the 33 contact surfaces in connected to a different resistor (1% tolerance) that is put in series with the circuit. The highest resistance is 22 kOhm. The lowest is 0, which gives a straight through connection. Steps are 1.666 dB apart, and the volume control knob is marked in 5 dB points. So, as you decrease the volume, more resistors are put into the series circuit. This is a very popular way of building stepped volume controls, but it does add a lot of solder joints as you decrease the volume more and more.
The output connections each have a 2 µF capacitor in series with the hot lead (center connection of the RCA jack). This does two things. One is that it isolates the inputs and outputs, reducing the effect of changing resistor values (i.e., changing the output impedance) as you change the volume. The output impedance of a good preamplifier might be 100 Ohms. Maintaining a constant low output impedance with changing volume is one of the functions of a preamplifier circuit. If it were not an issue, no one would bother using anything other than a good potentiometer for a volume control with CD players that had enough output voltage to drive the power amplifier. Secondly, the capacitors block DC from getting to the power amplifier input. The four capacitors are shown in the photo below (red arrows). This is not the same thing as having a "buffered" output. Buffering normally means an actual amplifier circuit, but the output equals the input, i.e., unity gain. The purpose of the buffering is to reduce the output impedance. Tube preamplifiers are particularly in need of something to reduce output impedance, and the buffer circuit accomplishes this.
The rear panel of the AU/RA has one set of input RCA jacks and one set of output RCA jacks (photo below), all gold plated. There is also a ground connector, so that you can connect the unit to the ground on your CD player, receiver, power amplifier, etc.
Channel Islands Audio VPC-1
The VPC-1 is a much different design than the AU/RA. First, it uses a continuous wiper in the potentiometer, meaning that regardless of the volume setting, the number of solder joints in the circuit does not change. It is not stepped, meaning that you cannot easily come back to exactly the same setting once you turn the volume control knob. There are no capacitors in the input or output circuit, which means the impedance changes directly with the volume setting. However, some people rail at the thought of adding capacitors to the signal path. The inside of the VPC-1 shows a very simple layout (photo below).
The potentiometer (10 kOhm) is in the upper right part of the picture. A toggle switch on the lower right lets you select between two different input sets, so you could connect a CD player stereo analog out and an FM/AM tuner stereo out to the VPC-1, with its output going direct to the power amplifier. In the lower left hand quadrant of the photo, you can see several sets of jumpers. This lets you fine tune each input set's output in relation to the other, in case input 1 is a lot louder or softer than input 2. A close-up of this circuit is shown below. To attenuate the volume of input set 1 or 2, you pull the jumpers out of the respective sockets. The photo at the top of this page shows the input selector toggle, which also has a mute position.
The rear panel of the VPC-1 is shown below. Pretty straightforward. If you can't figure this one out, try a different hobby. Wouldn't it be great if the rear panel of a receiver looked like this? Maybe they will one day, when each component connects to another with a single digital cable.
I tested the D7A and VPC-1 with a Toshiba SD-6200 DVD player, McIntosh MC-602 power amplifier, Carver Mark IV ribbon speakers, and Nordost interconnects and speaker cables.
I was actually quite surprised at how good these simple little units sounded. I have used a passive preamplifier in the past (McCormack Audio), but it is a much more complicated product, with buffered and unbuffered outputs. It is also a much more expensive component. The D7A and VPC-1 are basically just volume control pots in a chassis with input and output jacks.
OK, here are the differences. I played all sorts of CDs using the analog audio output jacks from the Toshiba DVD player. These outputs undoubtedly use inexpensive op-amps. So, I wanted to try the volume controls driving the input of the McIntosh, which has an input impedance of only 10 kOhms. This is about as low an input impedance as you are likely to find on a power amplifier. I felt that any deficiencies in the ability of an inexpensive op-amp in a mass market player to drive an input would show up most with a low impedance input.
With both the D7A and VPC-1, there was a slight laid back character to the high frequencies, but it was more pronounced with the VPC-1. This is due, I think, to the presence of up to 10 kOhms from the potentiometer, in the signal path. If you use really long speaker cables, you can get a similar result, namely, reduced high frequencies. With the D7A, the highs were not as quite recessed. However, both had recessed highs compared to any of my active preamplifiers, e.g., the Balanced Audio Technology VK-5i. But, as I said before, one of the purposes of a preamplifier is to keep a low impedance output regardless of the volume setting. The VPC-1 had a tighter bass, while the D7A bass was a bit floppy. The mid frequencies of both were a little recessed compared to my VK-5i. But, tube products, which the VK-5i is, have a lush midrange. That is part of their appeal.
These are tonal characteristics, not flaws. I found both volume control units really sounded quite acceptable, even with the high-performance power amplifier and speakers. The D7A allowed a little more volume when it was turned up all the way, but the op-amps in the DVD player began to fall apart in sound quality (clipping into the 10 kOhm input impedance of the McIntosh). Also, neither volume control unit would let me drive the McIntosh amplifier to full output, topping out around 60 watts per channel instead of the full 600 watts per channel. This is not the fault of the volume controls. Rather, it is an issue with limited output voltage of the DVD player and input sensitivity of the power amplifier. There are undoubtedly some player-amplifier combinations out there that would give you full output when the volume controls are turned all the way up.
When using a simple potentiometer for a volume control, it is imperative that you have short interconnects with as low a capacitance as possible. In our review case, we used Nordost, which typically have very low capacitance. They were one meter in length.
One serendipitous finding was that I had lots of turning room on the controls to get a convenient volume. This is characteristic of passive preamplifiers that I have tried in the past. With an active preamplifier, I would only be able to turn the control up to about 9 o'clock before it would be too loud for me. With the passive (D7A and VPC-1), I could turn it up to 12 o'clock or even more before getting really loud.
Being a purist, if I were to buy one vs. the other, I would opt for the VPC-1 because I don't like having anything in the signal path that is not absolutely necessary, and the capacitors in the D7A fall into that category, for me. I also prefer a tight bass, which the VPC-1 has. However, if you like more treble, then the D7A would probably be the better choice. In any case, my views on using a simple potentiometer for certain purposes are now fully revised. If you need to control a line-level volume, either one of these products will do the job for you.
- John E. Johnson, Jr. -
© Copyright 2002 Secrets of Home Theater & High Fidelity
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