Manufacturer's Specifications:
Infrasonic Filter: -3 dB at 20 Hz, 18 dB/octave.
Ultrasonic Filter: -2 dB at 20 kHz, 12 dB/octave.
Distortion: 0.025% at 2 V output.
Impedance: Input, 47 kilohms; output, 1.5 kilohms.
Gain: Unity.
Maximum Output: 7 V.
Signal-to-Noise: 90 dB, at 2 V.
Dimensions: 4 1/2 in. (113 mm) W x 6 in. (155 mm) H x 2 1/4 in. (58 mm) D.
Weight: 0.9 lb. (0.4 kg).
Price: $83.00 kit, $108.50 wired--ordered direct, sent postpaid.
The Ace Audio Model 4100 is a band-limiting unit that incorporates both infrasonic and ultrasonic filtering; in other words, it serves to roll off both the high end and the low end. This is handled by a neat little box with Left and Right input and output jacks, making four in a row --which are a touch too close together for some phono plugs. On the back is an accessory a.c. outlet, a worthwhile convenience in that the outlet the 4100 uses is replaced. The interior layout is open and straightforward, with good separation between the power sup ply section and the signal filters. For those who like construction, the kit version would be a good choice with a $30.00 lower price.
Performance
The noise measured at the output jacks with the inputs terminated in 1 kilohm was less than 10 µV with A weighting. The signal-to-noise ratio referred to 1 V was over 100 dBA, or over 106 dBA referred to 2 V. The frequency response of the 4100 filters is shown in Fig. 1.
The-3 dB points were at 17 Hz and 29 kHz. The response was down 2.5 dB at 20 Hz and 1.7 dB at 20 kHz. Between 30 Hz and 10 kHz, the deviations from zero were very small. The high-pass filter is down about 17 dB at 7 Hz, with a slope of 18 dB per octave with decreasing frequency. This design should gain good reduction of record warp effects with very small effect on most music.
The low-pass filter serves as a small-signal roll-off device to reduce the possibility of slew limiting in the power amplifier, as well as removing miscellaneous high-frequency garbage. The attenuation on music energy is very small, but there is a reduction of 17 dB at 100 kHz, where the final 12-dB per octave slope is al ready well established.
At 1 kHz, the output was just 0.15 dB lower than the input, which is very close to the specified unity gain. At 1 V in/out, THD + N was 0.006% or less, from 20 Hz to 20 kHz, with a good part of the energy from 60- and 120-Hz hum/buzz components. The actual distortion products were down around 0.0015% to 0.0036%, and they did not increase until a lever of about 3 V. With 3 V in at 100 kHz, the output amplitude was reduced almost down to 0.3 V, from the filter roll-off, with absolutely no evidence of slew limiting.
Fig. 1 --Frequency response of 4100 filters.
The maximum voltages were 7.7 V at 20 Hz, 8.1 V over most of the band and 6.9 V at 20 kHz, plenty high for any nor mal usage. The input impedance varied quite a bit across the band: 115 kilohms at 20 Hz, 67 kilohms at 1 kHz, and 19 kilohms at 20 kHz. In most cases, the 19-kilohm figure is more than high enough, and the change in impedance would have little or no effect on the signal delivered from connected units. As was expected with the 1.5-kilohm output impedance, there was a drop of a few dB in output when switching loads from 100 to 10 kilohms. The lower load also caused increased attenuation from the high-end filter--a small change, but worth keeping in mind if very flat, extended response is important.
Use and Listening Tests
To check out the Ace Audio filter unit, I inserted it in the normal signal path from the preamp to the power amplifier.
First of all, it can be stated that there was no observed deterioration, such as hum or added distortion. It did prove its worth with a few records which have warps, as my present system doesn't have a sub sonic filter. I did not get any listening proof-of-performance of the ultrasonic filter as I don't have a slew-limiting problem, but I didn't notice any music missing. For those who need to add such filtering to their present (or future) sys tem, the Ace Audio Model 4100 per forms very well and has an acceptable price.
--Howard A. Roberson
(Source: Audio magazine, Mar. 1982)
Also see:
High-Quality Moving Magnet (MM) IC Preamp
Ultra-Low Distortion Phono Preamp: The LP-797
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