NASWA Journal Columns · Equipment Reviews, September 1997

Alan Johnson, N4LUS • 2490 Sharon Way • Reno, NV 89509 alanjohnson◊

Equipment Reviews, September 1997

The Alpha Delta Variable Response Console

Let’s face it–shortwave broadcasting is not a high-fidelity medium. Most of the reason is the inherent noise and interference of the medium as well as the vagaries of ionospheric propagation. Some receivers can be part of the problem due to small speakers and weak, non-optimal audio amplifiers. There is a constant conflict between the desire for wide-band filter and audio response for “arm-chair” copy and the necessity of narrower selectivity for interference rejection with the accompanying muffled audio. I find that I am always on the look-out for products that can make SW reception sound better.

People have taken various approaches to try to mitigate this circumstance. One option is to tap the radio’s signal before it gets to the audio output amp and route the signal to an external amp and speaker. This works well for stations that are in the clear, plus the external amp’s tone controls can be used to tailor the sound to the listener’s preference. When more selectivity and noise/interference rejection are indicated, there are various outboard audio filters available. Grove Enterprises introduced their SP-200 a few years ago. This product combined an amplifier, tone controls, a peak/notch filter, noise limiter, squelch and a speaker, all in one enclosure. The new VRC (Variable Response Console) from Alpha Delta, follows along these lines. As far as I know, this is the first non-antenna related product from this Kentucky company.

The Box

The VRC is an eight pound cast aluminum black box measuring 8 x 8 x 6 inches (HxWxD). The majority of the box’s volume is used as a bass reflex (ported) enclosure for the 4 inch speaker. The front panel consists of a perforated grill for the speaker and the control panel for the electronics. The amplifier provides two watts of output and uses a push-pull design for low noise and distortion. Controls include rotary knobs for high and low frequency adjust, knobs for the peak/notch frequency and bandwidth and pushbuttons to activate the peak or notch filter. There is also a power pushbutton and a “set and forget” volume control (the manual recommends using the receiver’s volume control during listening). On the back of the box are a coaxial power input jack, a 1/8″ phone jack for audio input and a RCA jack for tape recorder output. There is a supplied AC adapter supplying 12 volts at 500 mA–if you use a different DC power source, pay attention as the center pin of the coaxial connector is minus DC volts. The VRC’s input is high impedance, which permits using a receiver’s speaker, headphone or line level output to drive the VRC. Using a receiver’s line level output affords the options of using the receiver’s usual speaker for monitoring with the VRC off and eliminating the radio’s audio amp from the signal chain, possibly reducing distortion and noise. The instruction sheet discusses connecting the VRC to radios such as the Drake SW8 that have an audio output that is floated above ground and mentions a plug adaptor that is supposedly furnished with the unit–I couldn’t find one in the packing of the review unit.

The Controls

On the left-hand side of the VRC’s front panel are the on-off pushbutton and the high and low frequency controls. The Low Frequency control performs identically to a “Bass” control on a high-fidelity amplifier, i.e. it serves as a low frequency boost or cut control , providing approximately +/-12 dB of adjustment at 100 Hz, with decreasing effect up to 1000 Hz. The High Frequency control does not function as a “Treble” control, but instead sets the cut-off frequency of a low-pass filter. This filter has a slope of 30 dB per octave and the cut-off frequency can be adjusted from 500 Hz to 10 kHz. Above these knobs are a verticallly oriented nine segment LED bargraph display that shows the setting of the low frequency control and a horizontal ten segment display that shows the cut-off frequency of the high frequency control. There is a mono 1 / 4″ headphone jack at the bottom middle of the panel (the instructions state that an adapter must be used with stereo headphones). The internal speaker is deactivated when headphones are plugged into the VRC’s phones jack. To the right of the headphone jack are the knobs that set the center frequency and bandwidth of the peak/notch filter and above these knobs are the pushbuttons that select either normal, peak or notch filtering. The peak can provide about 20 dB of effect over a few hundred Hz at its narrowest setting and several thousand Hz at its widest. The notch provides up to 40 dB of rejection over a similarly adjustable bandwidth range. The center frequency of the peak or notch can be adjusted over a range of 400 to 10,000 Hz. The peak and notch filters track each other exactly, which means that an interfering heterodyne can be localized in the peak mode and then eliminated by switching to notch mode–a technique that I find makes the process of notching a het much easier. At the far right of the front panel is the small rotary volume control. The instruction sheet provides graphs of the responses of the unit’s various modes.


The VRC provides an outstanding degree of flexibility for tailoring a signal’s audio. The VRC has equal applicability for the program listener, DX’er, ute enthusiast and ham radio operator. For program listening, where I try to use the widest IF bandwidth possible for fidelity, I could set the VRC’s high frequency cut-off to eliminate high-frequency whine and splatter without adversely affecting the treble content of the signal. For DX’ing, I could adjust the high and low frequency controls to maximize speech intelligibility–sometimes I would even use the peak filter at a relatively wide bandwidth to emphasize the speech frequencies. Using the peak control with a narrow bandwidth affords “single-signal” reception of CW and data modes. Despite this degree of flexibility, the VRC is very easy to use and the LED bargraphs show the user exactly what is going on with the low and high frequency controls. It would be very handy if the effects of the peak/notch controls could also be displayed. Another nice feature of the VRC is that the filters are infinitely adjustable (not in steps as with some digital filters) over a very wide range of frequencies. The VRC is also very quiet, contibuting very little noise of its own to the signal. If I had a VRC of my own, instead of a review unit, I would switch the bar knobs for round ones–I think this would make fine-tuning the controls just a bit smoother.

Dramatic effects can be achieved thanks to the wide range of the VRC’s controls, however, I often found that subtle adjustments yielded the best effects. I would most often use the receiver’s widest bandwidth and then just slightly roll off the VRC’s high frequency response to cut down on the whine and hiss in the signal without unfavorably affecting the fidelity for listening to music on stations “in the clear”. One interesting thing that I noted with the VRC was that it allowed me to distinguish between receivers and stations based on their sonic content. For instance, Radio Australia tends to have a very bassy signal compared to other stations, which I could compensate for with the VRC’s low frequency boost/cut control. My JRC NRD-535 tends to emphasize the bass frequencies–again, the VRC allowed me to counteract this.

The Competition

The only comparable unit to the VRC is the Grove SP-200 (now offered in the “B” version–I haven’t yet gotten the information concerning what changes have been incorporated in the “B”). The SP-200 sells for $50 less at $199 versus the VRC’s $250. The SP-200 offers a squelch to cut off the audio output when the receiver’s output drops below a certain level and a clipper-type noise limiter. The Grove unit also has a recorder activator which will switch on a tape recorder equipped with a “remote” jack when audio is present. On the other hand, the VRC offers a wider range of adjustment of the peak/notch and high frequency cutoff controls, with the top end being 10 kHz vs. 6 kHz for the SP-200. The SP-200 has the usual cut/boost treble control as opposed to the VRC’s variable lowpass filter. I felt that the VRC also offered a “richer” sound. One thing that I didn’t prefer in the VRC was the fact that the receiver’s output is not bypassed to the VRC’s speaker when the VRC’s power is off, a feature that the SP-200 has.

All in all, I think the Alpha Delta Variable Response Console represents a good value and a worthwhile addition to any listening setup. It provides a good speaker and an amplifier with more than sufficient output, plus highly effective tone controls and a very flexible peak/notch filter. More information can be obtained from Alpha Delta Communications, Inc., P.O. Box 620, Manchester, KY 40962; (606) 598-2029. Thanks to Universal Radio for providing the review unit.

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