VISATON R10S DTW72

The Usual Suspects

Photo 1

An attempt to obtain damn good sound without the expense of boutique prices.

An MMTMM arrangement. The two upper mids are wired in series as are the two lower ones. These two series configurations are wired in parallel to give an 8W impedance.

Four midrange units were decided upon just because it looks different. Properties such as their 20 watt power handling capacity, the possibility of a wider horizontal dispersion due to the line array were considered but measurements weren't made to verify that. In short, at less than $10 each, why not?

The mids are Visaton R10S units and the tweeter is a Visaton DTW72. The woofer is a GRS 12SFPC-B. It would have been nice to have used the Visaton W300-8 but at $91, it would have deterred the system's being called budget.

The loudspeakers and crossover components used here are all available at Parts Express with exception of the 25W potentiometer used in the LR-1. It can be obtained from at MOUSER and DIGIKEY If these links don't work, the manufacturer's part number is 026TB32R250B1A1

This potentiometer was extensively used by Wharfedale

Woofer GRS 12SFPC-B

Midrange VISATON R10S

Tweeter VISATON DTW72

Figure 1

The Crossover

It was later modified by placing LR1 into the high frequency sections allowing the resistor to be variable and of a lower wattage.

Despite the woofer's second order filter, all drivers' polarities are in phase. Normally, one would expect the woofer polarity to be reversed. However, due to the cutoff of the woofer's low pass filter being 300hz and the midrange's high pass being at 875hz, a serious dip was noticed which was corrected by maintaining normal polarity. The crossover points here are 300hz, 875hz - 5khz and 4.5khz and up.

It can be argued that by bringing the midrange high pass to the woofer's low pass of 300hz would linearize the response by reversing the woofer's polarity. However, this showed a rise in the response in the lower midrange by about 4dB up to about 800hz. While higher order filters may have rectified this, the cost wouldn't be justifiable not to mention the complexity of designing the filters.

Since the output of the woofer can extend as high as 3khz, it was noted that the smaller mass of the midrange units would give a better transient response that the massive woofer cone and coil assembly. Then there's the beaming of a larger diameter cone at higher frequencies.

Figure 2

This crossover brings in the tweeter at 4547hz

Replacing the 5.6uf capacitor with any of the following will give the following high pass crossover points for the tweeter and will have a negligible effect on the response.

5.0uf 4974hz; 4.7uf 5235hz 4.3uf 5620hz; 4.0uf 5974hz

This series wiring of the tweeter after the high pass filter of the midrange was to enable locating the LR-1 in the midrange and tweeter sections while isolating the LR-1 from the bass frequencies which will require a higher wattage resistor. This configuration allows a low wattage resistor in the LR-1.

Figure 3

This shows the differences among using three low pass filters for the woofer. The goal was to obtain as flat a response as possible. The grey, 300hz 2^nd order seems to be the most linear. An increase in the vertical scale shows a clearer view. fig.4

1w1m Woofer RED 500hz-6dB GREEN 500hz-12dB GREY 300hz-12dB

Figure 5

Of these curves, the orange is the response with the LR-1 bypassed, the black is with the LR-1 set around 6W and green is set around 17W. Some people prefer a sloping down of the higher register.

The three lower curves are THD, the yellow being that of the orange curve, the grey being that of the black curve and the light blue being that of the green curve. All equate to about 0.8% THD at the peak around 3500hz. In the decade between 200hz and 2000hz, THD hovers between 0.3% at f_L and f_H and an incredible low of 0.16% around 650hz, f_C Below 200hz, THD seems to float around 1.5% but is difficult to measure due to the inclusion of room reflections.

At any rate, IMHO, less than 1% THD from a budget system is quite remarkable.

Figure 6

Both curves are of the woofer only and with the 2nd order 300hz low pass filter. The red curve is measured at 1w1m while the black curve is measured near field¹.

The red curve clearly shows the strong effect of the room at low frequencies. The black curve very closely resembles that given by BassBoxPro modeled in 3 ft^3. fig.7 The rapid slope of the curve is due to the low pass filter.

Figure 7

1. Near Field. Response made with the microphone about 2mm from the center of the diaphragm. Upper practical frequency limit is about 400hz.

Presented by Don Keele to the Audio Engineering Society, 1973.

That paper (pdf) can be found via THIS LINK