The fourth in a series of product reviews of outboard DSP hardware, this article explores the miniDSP Flex digital signal processing (DSP) unit. The Flex is a two channels in/four channels out, Sharc-based audio processor that offers a more cost-effective model than the previously featured miniDSP processors in the series, with a retail price of $570 for the TRS balanced IO model. Besides the competitive price, the review confirms how the miniDSP Flex manages to measure and impress.
The fourth outboard digital signal processing (DSP) unit to be examined in this series of currently available outboard DSP processors is the two channels in/four channels out miniDSP Sharc-based Flex audio processor from Hong Kong-based miniDSP. Founded in 2009, miniDSP is a technology company focusing on DSP platforms for audio applications. While its primary market is DIY users, miniDSP does have some OEM customers, with limited customization ability for its products, such as the Flex processor. For example, one OEM customer laser engraved his logo onto the main control knob of a miniDSP Flex processor, which works since miniDSP does not have its own logo on the front panel.
Photo 1: This is the miniDSP Flex DSP audio processor with power supply, remote, and Bluetooth antenna.
The two-in/four-out miniDSP Flex balanced input/output version is depicted in Photos 1-4 and is a more cost-effective model than the previously featured DSP processors, with a retail price of $570 for the TRS balanced IO model. The feature set, however, is similar to the some of the other processors in this series and is quite extensive
The miniDSP Flex uses a 32-bit floating-point 400MHz Analog Devices SHARC DSP, and features a multi-core USB audio processor from XMOS. Inputs include Stereo digital (USB Audio, SPDIF, Optical, and Bluetooth), stereo analog (balanced TRS). Outputs include balanced analog (TRS), and Digital (TOSLINK & SPDIF). It supports LDAC, aptX HD, aptX, AAC, and obviously SBC Bluetooth wireless streaming. The company offers an interesting optional upgrade to install Dirac Live, allowing for full-range room correction (20Hz~20kHz). Factory specifications detail an SNR at 120dB(A), and THD+N at -110dB (0.0003%).
The design supports use for USB audio recording with 4× channel processed output via USB audio to record inputs (e.g. for Vinyl). One of the most positive features is the highly visible white on black OLED front panel display, which guides users on all the features and while using the IR control from a distance. The design is also compatible with miniDSP’s Device Console control software, which is perhaps one the most user friendly DSP GUI tools I have used in my consulting business (Audio Weaver gets the award for most powerful and versatile).
Photo 2: This is the front panel layout of the miniDSP Flex DSP audio processor.
Photo 3: This is the rear panel layout of the miniDSP Flex DSP audio processor.
Photo 4: Here is the miniDSP Flex’s internal construction.
Figures 1-5 give the various menu windows available in the miniDSP control software. Figure 1 depicts the configuration screen for the two in four out architecture of the device. Note that the input channels also feature a 10-band PEQ (parameter equalizer).
Figure 2 gives the output channel functions for each channel. This includes a “fader” gain setting slider, 10-band PEQ, Crossover, Compressor settings, and the FIR development function. Each channel can also be inverted (180°) and muted.
Figure 3 depicts the 10-band PEQ function, while Figure 4 shows the crossover filter detail. Filters include Butterworth 6dB/octave to 48dB/octave, Linkwitz 12, 24, and 48dB/octave, and Bessel. There is also an Advanced setting for individually programming the biquads. Last, Figure 5 gives the compressor controls for each channel.
Figure 1: This is the miniDSP Flex’s menu screen for configuring both the input and output channels.
Figure 2: This is the miniDSP Flex’s output control screen.
Figure 3: This is the miniDSP Flex’s parametric EQ screen for the output channels. The input PEQ screen is identical.
Figure 4: This is the miniDSP Flex’s output crossover design menu.
Figure 5: The miniDSP Flex’s output compressor control screen.
Specifications for all three versions of miniDSP Flex are detailed in Table 1 And besides listing the manufacturers' specifications, all the outboard DSP units in this series were measured on an Audio Precision flagship APx555B analyzer courtesy of my friends at Audio Precision (AP), with a special “Thank You!” to AP engineer Charlene Chow who performed all the measurement sequences for this article.
Table 1: Full specifications table comparing all three versions of the miniDSP Flex.
Photo 5 shows the basic setup for the series of tests. Test categories that were performed included total harmonic distortion plus noise (THD+N), signal-to-noise ratio (SNR), frequency response at 0dB, crosstalk, and last, intermodulation distortion (IMD). Starting with THD+N (Figure 6), the 1kHz @ +4dBu performance of channel 1 is 106.74dB, and although not shown, the numbers were 105.58 @ 10kHz. Figure 7 depicts the SNR graph, which resulted in 116.32dB for channel 1 (both channels driven).
Photo 5: Audio Precision’s APx555 front/panel test setup is shown with the miniDSP Flex’s setup.
Figure 6: Graph of the APx555 THD+N (1kHz) measurement for the miniDSP Flex.
Figure 7: This graph shows the APx555's signal-to-noise ratio (SNR) measurement for the miniDSP Flex.
Frequency response for the miniDSP Flex (Figure 8) indicates the unit is flat 20Hz to 20kHz, and 3dB down at 33kHz. This test was level was set to reference the result to 0dB. The last test (Figure 9) compares the IMD response of the miniDSP Flex with the background IMD of the APx555B.
Figure 8: This is the frequency response graph for the miniDSP Flex DSP audio processor.
Figure 9: The last test compares the IMD response of the miniDSP Flex with the background IMD of the APx555B.
Taken all together, the miniDSP exhibited very good performance when measured with the APx555B, especially the spectacular 116.32dB SNR. With two outputs per channel, the applications would be for two-way system designs or two-way passive designs with an external subwoofer, both formats work well in either home audio or the studio monitor markets. However, using two units each Flex processor configured for 1 channel and four outputs, a more complex four-way active design would be possible. For more information, visit www.minidsp.com. VC
This article was originally published in Voice Coil, February 2025
About Vance Dickason
Vance Dickason has been working as a professional in the loudspeaker industry since 1974. A contributing editor to Speaker Builder magazine (now audioXpress) since 1986, in November 1987 he became editor of Voice Coil, the monthly Periodical for the Loudspeake... Read more
