miniDSP Flex HTx Digital Audio Processor Review - HomeTheaterHifi.com (2024)

Hong Kong-based miniDSP has been manufacturing affordable, high-performance audio DSP (digital signal processing) products for the past fifteen years.

Since its founding, miniDSP has been a presence in the DIY community. With a reputation for value and high performance, their products have begun to move into the audiophile mainstream. The miniDSP Flex HTx is perhaps the most ambitious and versatile product miniDSP has produced to date, and it may forever change expectations of what a single product can do and how much performance can be delivered for under $1,000 USD.

The miniDSP Flex HTx is an eight-channel Dirac Live-capable DSP with digital (USB, Toslink, and S/PDIF), analog, and HDMI eARC inputs. At the price point, I can’t think of a single DSP product that has as many channels, is as versatile, or sounds as good. The miniDSP Flex HTx is a fantastic bargain that can be either the core of a simple 7.1 home theater system or can provide versatile forms of augmentation to an existing system.

Highlights

Introduction

The miniDSP Flex HTx is a slim component with a clean design and impressive connectivity. In terms of operation, the miniDSP Flex HTx can be used as a preamplifier, or it can act as a dedicated, high-resolution digital crossover for active loudspeakers. You can leverage the miniDSP Flex HTx to add Dirac Live room correction to an existing 7.1 home theater processor, or if your HDMI sources provide multi-channel bitstream to PCM conversion, you can use the miniDSP Flex HTx as the core of a multi-channel surround system, eliminating the need for a home theater processor. The miniDSP Flex HTx delivers this versatility with a level of measured noise and distortion that bests all currently available home theater processors.

miniDSP Flex HTx Digital Audio Processor Specifications

Design

The front panel of the miniDSP Flex HTx features a control knob and a large, legible, white-on-black display that automatically turns off a few seconds after input stops. This auto-off featureand the all-black color scheme make the miniDSP visually stealthy. The included infrared remote adds power controls and one-button access to up to four “presets” allowing users to switch between four collections of settings that can make subtle to fairly radical changes to the way the miniDSP Flex HTx operates.

On the rear of the unit, you will find a dense but well-arranged set of connections that includes:

● 8 unbalanced RCA inputs
● 8 TRS balanced inputs
● 8 analog RCA outputs
● 8 TRS balanced outputs
● an HDMI eARC “output” (more on this later)
● a 12v trigger for remote turn-on of accessories like amplifiers
● USB, S/PDIF, and Toslink digital inputs
● a DC power input

DC power is supplied by an external switching power supply with a detachable power cord. On the inside of the miniDSP Flex HTx, we find the following key components:

● 1 AKM AK5578EN 8-channel A/D Converter
● 1 Analog Devices ADSP-21489 SHARC DSP Processor
● 1 XMOS U11692C20 Multi-core Microcontroller
● 4 ESS ES9017S 8-channel DACs (run in stereo mode with groups of 4 channels combined in sum/difference to improve S/N from 120dB to 124dB)

As with most miniDSP products, configuration is accomplished via miniDSP-supplied software (in this case the miniDSP Device Console) that runs on Windows or Mac machines. Connectivity is accomplished via either a direct USB connection or an optional WI-DG Wifi/Ethernet to USB bridge – I used both during my evaluation.

Setup

The miniDSP Flex HTx was shipped double boxed with foam padding that provides more than adequate protection given the light weight of the unit. After unpacking, I placed the miniDSP Flex HTx on top of my existing miniDSP 4x10HD and found that the footprint was similar, with the Flex HTx being slightly less deep and standing slightly taller due to the more substantial feet on which it stands.

I downloaded the latest version of the miniDSP Device Console software from the “User Downloads” section of the miniDSP website, connected the included USB cable, and was greeted with a different user interface than the one I had grown accustomed to using with my 4x10HD.

Whether connecting to the unit via USB or the optional WI-DG, you must select your device on an initial “Discover Devices” screen. After selecting your device, the miniDSP Device Console displays a set of screens that allow you to configure the miniDSP Flex HTx. Though the screens looked different from the “plugin” user interface used in earlier products, much of the functionality was similar which lessened my learning curve to start configuring the Flex HTx. The screens are “progressive” in nature, with the first screen giving monitor levels of the input channels and the last screen giving monitors of the output channels. This is both intuitive and helpful in case you’re not sure how many active channels your HDMI source is providing, or if the configuration you’ve created has resulted in unexpected output or levels on the output channels.

The screens in between, as expected, let you control what happens between input and output and are largely where the DSP “magic” happens.

The “Bass Management” screen allows you to steer low-frequency content from some or all of your speakers to one or more subwoofers and allows you to choose the crossover points for each speaker and each subwoofer, giving a great deal of control at the cost of a steep learning curve.

The “Channel Routing” screen allows you to associate input channels with output channels. The simplest, and the default, configuration sends each input channel to its corresponding output channel with no EQ or processing applied. You can override this behavior to map 2-channel input to multi-channel output for active loudspeaker crossovers or to map bass management to multiple subwoofers. I have used this capability for several years to implement the active crossover for my DIY active loudspeaker system, and it’s great to see this functionality being carried over to the Flex HTx.

The “Dirac” screen does not allow control of the miniDSP’s Dirac functions as you might expect but rather shows the results of any previous Dirac calibration that has been stored to the current preset on the unit. This is not entirely intuitive, and we’ll talk more shortly about some of the challenges encountered during the Dirac calibration process.

The thing you expect to be able to do on the Dirac screen is to control which channels map to Dirac processing, what types of speakers are connected to each of those channels, and which channels are not used in Dirac processing. Those capabilities are actually accessible via the “Dirac Channel Selections” button that is available in the left column on all screens.

The “Matrix Mixer” screen is a seemingly superfluous view that allows the assignment of Dirac channels to physical channels. I’m unsure of the value of this capability unless it allows you to simplify and shorten the Dirac calibration process by running measurements once on groups of combined channels in an active loudspeaker crossover configuration.

The following equipment was used with the miniDSP Flex HTx in this evaluation:

Upstream:

● LG OLED77A2PUA 4K UHD TV
● Meridian G68 AXD
● Apple TV 4K
● MeLE Quieter Fanless PC (as a Roon Endpoint)
● Sony UBP-X800M2 4K Ultra HD Blu-ray Player

Comparison:

● Marantz AV10

Downstream:

● EAD Powermaster 500 Power Amplifiers (x 2)
● miniDSP 4×10 HD
● 3-way custom-built active loudspeakers with Morel and Scan-Speak drivers

Measurement:

● MiniDSP UMIK-1
● MOTU Ultralite mk-5 Audio Interface
● Picoscope 2204a
● REW (Room EQ Wizard)

In Use

Because the miniDSP Flex HTx is so versatile, I chose to limit my evaluation to the following three scenarios which I think represent common ways a miniDSP Flex HTx would be used:

1. As the centerpiece of a basic home theater
2. To add Dirac Room Correction to an existing home theater
3. To add an active crossover or bass management to an existing system

Act 1 – Basic Home Theater

The default 8-channel “straight-through” configuration of the miniDSP Flex HTX lends itself to 7.1 home theater use, so I decided to start there. I connected my Apple TV 4k, Sony Blu-Ray Player, and other HDMI sources to my LG 4k TV and connected the miniDSP Flex HTx to the eARC input of the TV, allowing HDMI audio to be “returned” to the Flex HTx as input. The “Input Channels” screen of the miniDSP Device Console software identifies which home theater channels map to each numbered input of the miniDSP Flex HTx, so I used this as a guide for making connections. I connected the miniDSP Flex HTx’s analog outputs to the balanced input of my active crossover for the front left and right channels, and to the subwoofer, center, and surround inputs on my amplifiers for the other channels. Because my amplification and crossovers have a mix of balanced and unbalanced inputs, I really appreciated that the Flex HTx provides balanced and unbalanced output for every channel making optimal connection a snap (assuming you have TRS cables or adapters for your balanced connections). The initial test of the connected system failed due to a bad HDMI cable, but when I replaced the damaged cable with one my cats hadn’t chewed on, I observed multi-channel audio being successfully passed from my Apple TV 4k through the TV to the miniDSP Flex HTx.

I used REW (Room EQ Wizard) to generate test tones to verify proper leveling to all of my speakers and then gave a listen. My first impressions are that the sound of the miniDSP Flex HTx is clear and uncolored and generally produces output that is as good as the input. What I also realized is that my trusty Meridian G68 home theater processor was adding some “opinions” in the form of multi-channel up mixing, surround modes, and other digital processing that I missed when using the pure multi-channel audio coming from my sources through the miniDSP Flex HTx. That said, what I heard with good sources was very good, and I found it on par with the multi-channel audio I had heard on the Marantz AV10 I recently acquired for in-home evaluation when not using the AV10’s up mixing or surround modes. The Marantz was perhaps a bit softer in its presentation and the miniDSP Flex HTx a bit “crisper”, but both seemed accurate to my ears.

Intermission – Dirac Live Calibration

Now that I had tried the miniDSP in my system in its purest form, I thought it was time to add Dirac Live calibration so I could compare the default presentation to the frequency response-leveled and time-and-phase-corrected Dirac Live experience. It was at this point that I hit a snag: there was no “Start Calibration” button in the miniDSP Device Console. I referenced the manual and found that Dirac Live calibration could not be done via the WI-DG connection, so I connected the unit directly via USB and saw that the missing button had returned.

Upon clicking “Start Calibration”, I was presented with a cryptic error message and the option to purchase a Dirac Live license. This unit was purchased with a license (and had the sticker on the back to prove it), so something still wasn’t right. After about a week of back-and-forth with miniDSP’s support (made more difficult due to the time difference between Hong Kong and San Francisco), we were able to determine the problem was that I needed to download the Dirac Live application from the Dirac website and install it before the calibration flow would work and the license would be detected. This recommendation worked, and I was finally back on track.

The third problem I encountered was during the Dirac Live calibration process – I’ll spare you the lengthy troubleshooting process and get right to the punch line. My system is a 5.1 home theater, which left two channels of the Flex HTx unused. When the Dirac calibration program tried to measure these non-existent speakers, I got an error message stating the signal-to-noise ratio was too poor for measurement (because non-existent speakers produce no “signal”.) After solving this riddle, I disabled the two unused channels in the somewhat hidden “Dirac Channel Selections” dialog in the miniDSP Device Console and tried again. This caused Dirac to ignore the two unused channels and calibration ran without further issue.

Act 2, Scene 1 – Dirac Live with the miniDSP

After about a week of fiddling, I now had a fully Dirac-enabled miniDSP Flex HTx, and I had managed to fill three of the four presets with a flat curve and two house curves with varying degrees of tilt (+4dB bass boost, -2dB treble cut and +6dB bass boost and -3dB treble cut). The Flex HTx has a wonderful feature that allows you to, with a single button, toggle Dirac processing on and off. This proved extremely valuable for quick A/B comparisons.

With Dirac processing enabled, there was a noticeable tightening of the bottom octaves. Bass integration between my full-range mains and the subwoofer was much better and bass transients seemed to be cleaner. The part I wasn’t so certain about was the processing in the higher frequencies. Midrange and treble also seemed to tighten up in a way that wasn’t entirely positive or natural. On the plus side, impulse response was improved with sounds from drums and percussion instruments having a more defined “leading edge”. The negative impacts on the sound were subtle but presented in two ways that I noticed in both the miniDSP and the Marantz AV10 I had recently evaluated.

1) The sound stage was less airy and open – midrange and high-frequencies seemed to collapse into a much smaller perceived image.
2) Mids and highs sounded almost over-damped. There was a certain strained quality to higher frequencies that was somewhat unpleasant. My wife commented that the Dirac Live correction when applied to higher frequencies made things sound louder and slightly more irritating.

Because this result was repeatable across multiple products, I think this is more a function of Dirac’s handling of high frequencies and not related to the miniDSP or Marantz hardware.

Act 3 – Active Crossover Duty

Though many audiophiles may not have used active loudspeakers in their systems, the “active crossover” use case is one of the special powers of miniDSP products and has been a primary driver of their popularity among the DIY community. In my system, I have custom-built three-way speakers with no internal crossovers – all crossovers (and some digital delay) are implemented by an active DSP between my home theater processor preamp outputs and the amplifier inputs. The amplifiers are then directly coupled to the speaker drivers without any additional passive components in the signal path to color the sound. For the past several years, that duty has been handled by the miniDSP 4x10HD and it has performed very reliably over that time. Given that the miniDSP Flex HTx appears to be improved in almost every regard, I was very interested to see if it might do even better in this capacity.

To ensure an apples-to-apples comparison, I set the Flex’s configuration to match that of my miniDSP 4x10HD by manually copying the parametric eq “biquad” and crossover settings from my 4×10 to the Flex HTx using the miniDSP Device console. Once configured, I gave the system a listen using the “direct” mode on my Meridian G68 and a variety of high-quality stereo material streamed from Qobuz via my MeLE Roon endpoint. The sound was clean with perhaps a tiny bit less “air” on the top end than I experienced from my 4x10HD. This difference was subtle, and discernible only at the upper limits of the audible frequency range with high sample rate source material. I expect that this small difference might be attributed to the 48kHz internal sampling frequency of the Flex HTx vs the higher 96kHz sampling rate of the 4x10HD. I reached out to miniDSP for comment, and the support team indicated that the 48kHz limit applied to both the Dirac and non-Dirac versions of the Flex HTx and that higher sampling rates would increase the cost of the Dirac Live license, which would drive up the overall cost of the product. Since an affordable price point is always an integral part of miniDSP’s formula, something had to give, and this was deemed a reasonable tradeoff. To make things interesting, the miniDSP dev team mentioned that feedback from users was considered, but that they did not make comments or commitments about future functionality. That means that this 48kHz limitation might be removed in the future, or not. Only miniDSP knows.

On The Bench

For bench testing the miniDSP Flex HTx, I used the following:

Oscilloscope (used for square wave signal generation and capture):

· Picoscope 2204a

THD+N Measurements:

· MOTU Ultralite mk5 Audio Interface
· Room EQ Wizard

The first test I ran was the usual 1kHz distortion test and I quickly learned that the distortion and noise results published by miniDSP were no joke – the Flex HTx is very quiet and clean. The measurements are so good, that I think my MOTU Ultralite is likely the weak link.

The next test I performed was a distortion sweep up to the 10kHz limit where we started to lose harmonics due to the low-ish 48kHz internal sampling frequency and the corresponding 24kHz filter. You can see that harmonic distortion is quite low and rises in a predictable, linear way up to the point where the harmonic is in the cutoff range of the digital filter. Excellent results, and the lowest I’ve measured in any equipment I’ve been able to test. Especially impressive is the absence of any power supply noise at 60Hz and harmonics above that frequency which definitely can be audible if present. Balanced connections likely help quite a bit here.

Next up are the usual intermodulation distortion tests. I ran both the CCIF 19kHz+20kHz and the SMPTE 60Hz+7kHz dual-tone tests, and both showed almost non-existent IMD – these are truly excellent results, again the best I’ve seen in my testing.

I did run square wave tests, but those results are not as useful for digital equipment as they always exhibit “wavy” square waves due to the omission of the higher-order harmonics required to correctly reproduce waves with clean edges unless end-to-end sampling frequencies are much higher than 48kHz.

Conclusions

I simply can’t think of another product that does everything the miniDSP Flex HTx can do with the same level of accuracy for any price, let alone for less than $1,000.

The miniDSP Flex HTx is an amazingly versatile and configurable product that can find a way to make almost any system better. It offers solutions to many common problems with measurements that are among the very best available at any price, delivered for a very reasonable price point that starts at just under $1,000.