Douk Audio & One Little Bear VU3 Audio Switcher Box & VU Meter
The Douk Audio & One Little Bear VU3 Audio Switcher Box with VU Meters was acquired for its appealing price of $140, intended for comparing amplifiers and speakers. While the VU meters were not a primary requirement, they add a pleasing aesthetic. The unit is constructed with what appears to be black anodized aluminum, giving it a solid feel. The front panel features buttons for switching between Amplifier A/B and Speaker 1/2, along with a Level Adjustment for the VU meters.
The included remote control mirrors the front panel switching functions and also controls the VU meter brightness, a PEAK LED function, and can power off the VU meters (but not the main unit). The rear panel is equipped with banana jack connectors for amplifiers and speakers, and a USB C power port. A 3-foot USB C to A cable is supplied.
The device's manual states that its ground connections are independent, preventing mutual interference. It incorporates a built-in protection load that automatically engages when amplifier A or B is not selected, ensuring the amplifier remains loaded and protected. The unit utilizes 5 Amp relays, capable of handling 100W (unspecified load). This is estimated to be 100W into 4 ohms or 200W into 8 ohms based on I²R calculations. A notable feature is that the VU3 retains the last selected amplifier/speaker combination after power cycling.
Testing and Measurements
Initial testing involved creating custom adapters to connect RCA outputs to banana jacks, using 8-inch long, 16-gauge speaker wire. The audio analyzer used for testing was a QuantAsylum QA402.
Measurements were conducted from 20Hz to 40kHz with a signal level of 0dBv (1vrms). The "loop-thru" measurement, utilizing the custom adapters and cables, yielded the following THD/SNR results:
| L: SNR dB | 102.24 dB | R: SNR dB | 102.20 dB | L: THD % | 0.00065% | R: THD % | 0.00066% |
| L: THD+N dB | -99.55 dB | R: THD+N dB | -99.52 dB | L: RMS Volts | 1.00 V | R: RMS Volts | 1.00 V |
| L: Gain dB | 0.01 dB | R: Gain dB | 0.01 dB |
The frequency response, after applying a weighting filter to account for cables and adapters, showed a relatively flat response across the tested frequencies.
Further testing involved measuring the THD/SNR from AMP A to SPEAKER 1. The results indicated a slight loss of 2.27dB and an increase in THD+Noise by 32dB, though the overall THD & SNR remained good for vintage equipment.
| L: SNR dB | 100.22 dB | R: SNR dB | 99.71 dB | L: THD % | 0.03990% | R: THD % | 0.04007% |
| L: THD+N dB | -67.94 dB | R: THD+N dB | -67.98 dB | L: RMS Volts | 769 mV | R: RMS Volts | 769 mV |
| L: Gain dB | -2.27 dB | R: Gain dB | -2.28 dB |
The frequency response for the AMP A to SPEAKER 1 path showed a roll-off at higher frequencies.
Testing the path from AMP B to SPEAKER 2 yielded similar THD/SNR results. The isolation between the Left & Right channels was measured at approximately 48dB at 20kHz, which was considered less than ideal.
Further testing focused on the isolation between AMP A and AMP B. A loop-thru measurement for the Left Channel (AMP A input to QA402 Left Channel Output) and a similar setup for the Right Channel (AMP A input to QA402 Right Channel Output) was performed. The plot indicated 65dB of isolation between the two amplifiers at 20kHz, which is deemed sufficient for a 100W/4ohms setup.
The VU meters were calibrated using a 0dBv input signal, showing accurate readings at -3dBv and -5dBv inputs as well. The PEAK LEDs illuminated correctly around 0dBv.
Manufacturer Communication and Conclusion
Following the testing, Douk Audio was contacted regarding a potential issue. They suggested that the problem might be related to older versions of the product and provided guidance on checking the circuit board for specific components, noting that newer revisions isolate the voltage transformer. The reviewer confirmed having the latest board revision.
The reviewer was unable to find a schematic for the VU3 but suspected the issue might lie with the transformers in the VU meter circuit. Prior to this unit, the VU3 had been used successfully for A/B testing between an older integrated amplifier and an Aiyima A07. Despite the slight frequency response deviation noted in the current testing, the unit was returned.
The review concludes by mentioning an interest in comparing the performance of the Van Alstine switcher.
