PMC twenty5 22i Bookshelf (Stand-mount) Speakers with Built-in Power Amplifiers Upgrade Review – BONUS: HOW TO IMPLEMENT AN ARYLIC BLUETOOTH S50 PRO+ RECEIVER

PMC speakers were used for music and post-production in such movies as Titanic, Mission Impossible, Captain Phillips, Pearl Harbor, and Game of Thrones. I got my hands on the twenty5 22i bookshelf (stand-mounted) speakers for review. They are the upgrade model from the original PMC twenty.22 speakers that we reviewed in 2015. I also implemented an Arylic Bluetooth S50 Pro+ Receiver with the PMC speakers.

Along with the speakers, I received a pair of their new Class D power amplifiers that replace the rear panel on the speakers so that they become fully active speakers that can be driven by RCA or XLR preamplifier outputs in your hi-fi system. As a bonus, I will describe how to use them via Bluetooth from your phone, laptop, or PC.

PMC is a UK speaker company, founded in 1991 by Peter Thomas. Their products have been, and still are, used as monitors in music and movie production, so it is obvious that they attend to great detail in the way the speakers sound (accuracy). Although we have reviewed PMC, I had never personally heard them until I obtained a pair for review. Apparently, I missed a lot. Along with the PMC twenty5 22i speakers, they included a pair of their new Class D power amplifiers that convert the speakers from passive to fully active.

The PMC twenty5 22i speakers are a very interesting design. They are angled up at 5 degrees to deliver better time alignment of the sound. The tweeter voice coil is then more in vertical alignment with the woofer voice coil with respect to the listeners’ ears. The photo below shows the tilt and a nice view of the oak finish.

The tweeter has a fabric cone, and the mid-woofer is a glass fiber weave, as a result, the sound is smooth in the high frequencies, as opposed to a metal dome tweeter that has more intense edge transients. That does not mean all metal dome tweeters sound harsh; it is just a matter of preference by consumers as to what they want their speakers to sound like.

The tweeter dome is covered with a metal mesh to protect it. Click on the photo to see a larger version where the mesh is more clearly shown. You will also be able to see the texture of the mid-woofer cone. At the bottom is PMC’s unique Advanced Transmission Line Technology (ATL) opening, which is technically not a “port”. It is rectangular, and it has a fluted insert with some thin foam at the rear. The ATL is responsible for the excellent bass output of this speaker.

Here is PMC’s description of this technology:

“A transmission line places the drive unit at the end of a long, highly damped tunnel which absorbs all the unwanted mid and high-frequency energy that is radiated from the rear of the driver, but allows the lowest frequencies to exit the line in-phase with the frontal radiation from the drive unit, greatly enhancing its bass performance. The transmission line is effective over a wide bandwidth of around two octaves centered on the main resonant frequency of the line, and over this region it controls the motion of the drive unit, massively lowering distortion and raising the maximum possible output level. The resonant frequency of the transmission line is defined by the acoustic length of the line and is the frequency at which one-quarter of a wavelength is equal to the acoustic length of the transmission line.

Transmission line speaker design requires the balancing of many complex parameters and variables, from drive unit parameters to line length, area, taper and profile, foam placement, thickness, specification, etc. All of these parameters affect each other in complex ways, meaning that they must all be considered and optimized together in order to achieve an ideal result. The cost and difficulty involved in successfully achieving this is why PMC is one of so few companies to successfully develop transmission line speakers.“

The tunnel from a transmission line is usually fluted, but not always so. The fluting means that it has a decreasing tunnel width from the driver to the opening, and it is called a “transmission line folded horn.” A cross-sectional diagram of the folded horn version of a transmission line is shown below. It is usually also filled with polymer foam or fiberglass to dampen it.

^© https://en.wikipedia.org/wiki/Transmission_line_loudspeaker

The transmission line can be quite complex, as shown in the following diagram.

^© https://en.wikipedia.org/wiki/Transmission_line_loudspeaker

A port is simply an opening in the enclosure near the driver. It is not in a tunnel. The port can be tuned, but that tuning is different from a transmission line. Ports extend a woofer’s response, and transmission lines lower a bass driver’s resonant frequency, allowing for less excursion to produce the same output. Transmission lines also reduce distortion and improve resolution.

The rear panel of the twenty5 22i  (without the amplifier upgrade) has two heavy binding posts that accept bare wire, lugs, or banana plugs.

The two Class D amplifier modules come as a pair along with two AC power cords in a separate shipping box (see photo below). Note that this type of product is not actually wireless. It needs the 120 Volt AC supply and either an RCA or XLR analog cable connected to your preamplifier on one end and the speaker on the other. So, in fact, there are two “wires” instead of one speaker cable. However, the goal is to allow you to have the speakers perhaps across the room from your hi-fi gear and use a much smaller and less noticeable RCA cable to connect them, all without tying up a pair of power amplifiers in your hi-fi setup. Also, as I will show, you can use Bluetooth transmission from your phone, laptop, or PC to a Bluetooth receiver next to the speakers which could sit in your dining room with your hi-fi gear in the living room driving them while you have dinner, with no wires between your hi-fi setup and the speakers. To me, the Bluetooth arrangement is what makes these speakers really nice in a home, without a bunch of wires going along the floor. That’s the way I would use them. More on this later.

Exchanging the two conventional binding post rear panels with the amplifier panels is straightforward.

You remove the six hex bolts with the included wrench. In the photo below, I have pulled the rear panel with the crossover network out, but the wires are still attached to the drivers. The amplifier panel is sitting next to the speaker on the left.

Now here is the tricky part. Pulling the white plastic plug assembly apart is difficult. They are tightly attached so as to keep them from coming apart by themselves inside the speaker. You can see the plug assembly just to the left of the white transformer.

There is a tab on the side of the plug assembly that has to be pressed, and then you pull the plug out of the socket. Then you set the binding post panel aside and attach the crossover network plug to the socket on the amplifier panel. The photo below shows the socket on the amplifier panel. There is a tab socket on the side facing the camera which is pointed out by an arrow in the top photo, and the bottom photo shows the tab socket on its end. There is a tab on the plug from the wires to the drivers that has to be inserted into that tab socket to lock the plug and socket together. There are four wires in the plug assembly, two for the tweeter and two for the woofer.

Here is the crossover board with an arrow pointing out the socket. You can see the difficulty. The tab socket is on a side where the opposite side is next to the inductor, so you can’t grip the socket while pressing the tab on the plug to remove it. However, while you mull this problem over, you can see that the crossover parts are of excellent quality.

And, the amplifier board (below). You can see that the crossover parts (at the bottom) are much smaller. This is because they occur before the amplifier stage. That way, no energy is wasted by power amplifier heat due to the reactive impedance of the output power having to go through the crossover network. The inductors and capacitors at the top are the low-pass filter that removes most of the switching frequency, which for these amplifiers, is 500 kHz. The low-pass filter is at 50 kHz. At the bottom left is a Model Selector jumper for the model of speaker the amplifier is to go with. You can see that it is in the 2522i slot.

In the passive twenty5 22i, both the tweeter and woofer are 2^nd order. In the active twenty5 22i, both the tweeter and woofer are 4^th order, and the crossover network is filtered actively and before the power amplifiers (as is the case in all true-active speakers).

I listened to the PMC twenty5 22i speakers using Qobuz high res streaming from my PC through a Lynx E22 laboratory-grade balanced soundcard, a PS Audio StellarGold preamplifier, XLR cables to the PMC Class D power amplifiers mounted on the rear of the speakers. I also used an Arylic S50 Pro+ Bluetooth receiver feeding the twenty5 22i speakers where indicated and in the bench test results (starting at Figure 15).

Here are the standard albums that I use for reviewing products. I don’t listen to all of them for any one review, but I listen to some of them.

Some new albums released in July 2024 that I also listened to.

Here is a summary of my findings:

1 – Plays loud without much audible distortion – Yes

2 – Violins sound clean and natural – Yes, smooth edge transients

3 – Sweet high-end rather than edgy, irritating sound – Yes

4 – Human voices natural – Yes

5 – Detailed – Yes

6 – Congestion at high volume – No

7 – Midrange prevalence – Somewhat

8 – Sibilance – Low/Medium

9 – Powerful, punchy bass – Yes, and it is clean

10 – Bass boominess – No

Bench test measurements were conducted with the speaker on a stand and the calibrated (to 30 kHz) microphone 1 meter from the front of the speaker, on-axis, pointed horizontally in between the tweeter and mid-woofer. I set the volume level at 1 kHz and left it that way for all the other tests. Measurements for Figures 1 through 13 were conducted with the Class D amplifier in place on the speaker. I used my PC to generate test signals, output from my soundcard’s XLR analog outputs, to a PS Audio StellarGold Preamplifier, whose XLR outputs were connected via XLR balanced cable to the amplifier panel on the speaker. The Sensitivity toggle on the PMC amplifier panel was set to +. Impedance/Phase (Figure 14) was measured with the original speaker binding post plate in place.

At 50 Hz and 82 dB SPL (Figure 1), THD was 0.47%, which is excellent for a 6.5″ driver. (I do not use THD+N as that includes room background noise.) The 2nd-ordered harmonic and 3rd-ordered harmonic are about the same level.

At 100 Hz, distortion rose a bit, to 0.60%, but for a woofer on a bookshelf speaker, that is still pretty good (Figure 2). Now, though, the 3rd-ordered harmonic is larger than the 2nd-ordered harmonic.

With a 1 kHz test signal (Figure 3), distortion went down to 0.20%.

Figure 4 is 5 kHz, the signal was coming from the tweeter (1.6 kHz crossover frequency), and distortion was a low 0.19% at 83 dB, with the 2nd-ordered harmonic being larger than the 3rd-ordered harmonic by a large margin.

And at 10 kHz (Figure 5 below), THD was low, at 0.07%, and again, the 2nd-ordered harmonic was higher than the 3rd-ordered harmonic. This is why the speaker sounded so smooth in the upper music frequencies and transients.

For the principal IM measurement (Figure 6 below), 60 Hz and 7 kHz at a 4:1 ratio, IM was 1.4%. Analog input Class D amplifiers often require a DAC and line driver amp on the application processor, which can add noise to the speaker output. It is difficult to say where the additional noise in the midrange here came from. It is in the mouse fart level of 20 dB SPL.

Looking at an expanded X scale for the IM test (Figure 7), we can see that there were four side peaks on either side of the 7 kHz sine wave peak. They are labeled d2, d3, d4, and d5 on either side. Four peaks is good for 90 dB SPL.

19 kHz and 20 kHz sine waves are another IM test, and for the test, not only are the side-peaks part of the data but the 1 kHz B-A peak. Here, in Figure 8, there are 6 side peaks (shown in more detail in Figure 9), and the B-A peak at 1 kHz, labeled d2L, is very low, which is good.

Figure 9 below shows the 19 kHz/20 kHz test with an expanded X scale. You can see the individual side-peaks.

The 1/1oth decade test (Figure 10) shows distortion is mostly around 1 kHz and 8 kHz to 20 kHz.

The Frequency Response is shown below in Figure 11. It shows that the midrange is a bit lower than the bass and high frequencies. Distortion becomes predominantly 2nd-ordered at about 2.3 kHz which is just above the crossover frequency of 1.6 kHz. So, the tweeter produces that type of distortion. Another nice thing is that distortion stays below 1% all the way down to 50 Hz. The PMC twenty5 22i is a relatively expensive speaker, and you get what you pay for.

The Step Response shows that the tweeter is wired with a reversed-phase, but the woofer is wired positively. The third peak on the right is the port. Wiring tweeters reversed is common.

The Cumulative Spectral Display is shown below in Figure 13.  Enclosure resonances are reasonably well controlled, and there are some driver resonances at 6 kHz and 17 kHz.

The Impedance/Phase plot is shown below. Obviously, this was measured when the standard rear plate with speaker binding posts was in place, not the new amplifier plate. The Impedance has a minimum of 6 Ohms at 8 kHz and a maximum of 46.5 Ohms at 1.7 kHz. This is reflected in the midrange attenuation of the sound. The Phase has a maximum of + 45⁰ at 15 Hz, and a minimum of – 59⁰ at 2.6 kHz.

I took the opportunity to acquaint myself with the latest Bluetooth technology. Arylic graciously supplied me with an S50 Pro+ Bluetooth Receiver for this purpose. It worked out really well!

Below is the Arylic S50 Pro+ Bluetooth Receiver. It sells for $149 direct from Arylic. All the modern codecs are supported, but the quality of the signal also depends on the Bluetooth transmitter. The path defaults to the best codec that both the transmitter and receiver can decode.

Features of the S50 Pro+ include an ESS9023 Sabre DAC, 6 Wireless Streaming Services Supported: WiFi, AirPlay, Spotify Connect, Bluetooth 5.0 aptX HD, DLNA, and UPnP, 3 Audio Inputs and 5 Audio Outputs (including an optical input/output and sub out), 15+ Music Services Supported such as Spotify, Apple Music, Amazon Music, Tidal, Internet radio, and NAS, Multiroom Streaming Supported.

The rear panel has RCA analog inputs and outputs, and there are also optical digital in and out and coax (RCA) out. There are hardwire inputs for wifi, USB, and LAN.

It comes with a compact remote control that has a lot of features, including a treble and bass tone control.

Bluetooth transmits and receives on the 2.4 GHz frequency that Wi-Fi uses as well. You might wonder how Bluetooth and Wi-Fi devices can all be used at the same time in your home. They work together due to Time Division Multiplexing (TDM) by taking turns along the transmission of the frequency stream.

For Bluetooth, the frequencies range from 2,400 MHz (2.4 GHz) to 2,483.5 MHz (2.4835 GHz). Bluetooth uses what is called the Frequency Hopping Spread Spectrum (FHSS). The Bluetooth device “hops” between 79 channels (in the range of 2.4 GHz to 2.4835 GHz), each channel being 1 MHz wide, at 1,600 times per second. Time-division multiplexing (TDM) assigns numerous data bitstreams into a single signal by separating the signal into segments, each having a very short duration. Each individual data bitstream is reassembled at the receiving end based on the timing.

For Wi-Fi, three frequencies are used: 2.4 GHz, 3 GHz, and 5 GHz. Wi-Fi employs the Direct Sequence Spread Spectrum (DSSS). Like Bluetooth, the spectrum is 83.5 MHz wide (see below).

For the Bluetooth tests, I set up the PMC twenty5 22i speakers with the Arylic S50  Pro+ receiver’s RCA outputs into the PMC Class D amplifier inputs. For this test, I turned on the Bluetooth circuit in my PC and wirelessly connected to the S50 Pro+. The Pro+ RCA analog outputs were connected via RCA cables to the RCA inputs on the two 22i speakers. (Obviously, the speakers’ Class D amplifier panels have to be connected to 120 Volts AC sockets.)

You have to pair your music source (Laptop, Phone, PC, etc.) with the S50 Pro+ before you can use it. This involves selecting the Pro+ through your WiFi network first. Power on the S50 Pro+ by clicking the power button in the top left corner of the remote control. Download the 4STREAM app to your phone, boot it, and follow the instructions. The Pro+ will show up as SoundSystem_XXXX where the XXXX is four numbers.

Once the Pro+ is paired, open Settings on your computer and go to Bluetooth. You should see the Pro+ listed there as SoundSystem. Connect to it.

If you are using a music streaming service such as Qobuz, open it and go to the speaker icon. It should list SoundSystem. Connect to it. Below is a screenshot of my PC settings using Qobuz. It is set to Speakers SoundSystem and DirectSound. Your settings may vary (use of Wasapi).

Using the remote control, click the BT button (Bluetooth). Turn the volume all the way down before you play any music. This is accomplished by pressing the – key on the circular button. The PMC twenty5 22i power amplifier sensitivity is 96.5 dB with the Sensitivity toggle on the rear panel set to -, and 106.5 dB when it is set to +. The maximum output of the Pro+ is 2 Volts, so you don’t want to start the music without setting the volume control all the way down. Use the 96.5 dB sensitivity setting on the amplifier panel to start with.

Select an album and click the play button. Then, slowly turn up the volume using the remote control.

There you have it.

Although I listened to the PMC twenty5 22i speakers with the stock binding post panels in place (no PMC Class D amplifier panels in place), this review is based on the speakers with the PMC amplifiers. They sounded great using my own amplifiers, but that is a second variable since the amplifiers are different, and the central point of the review is the new Class D amplifier upgrades to existing PMC speakers, along with using Bluetooth transmitters and receivers to eliminate having a wire (RCA or XLR cable) from a preamplifier to the PMCs.

I listened to the same albums as for the tests just using the PMC twenty5 22i speakers with XLR cables going from my computer to the PS Audio StellarGold preamplifier and from there to the XLR inputs on the amplifier panels on the rear of the speakers. In the case of the Arylic S50 Pro+ Bluetooth receiver, I used my phone connected wirelessly by Bluetooth to the S50, and the RCA line outputs from the S50 to the inputs on the rear of the speakers.

The sound quality was very similar to the direct hard-wired setup, except that it was a bit smoother and softer. This is because the signal is going through the Bluetooth codec which has a reduced bitrate compared to Qobuz by itself. This is an expected effect, but I still enjoyed the music greatly. The Arylic S50 Pro+ is an inexpensive option that allows you to make the most of the built-in amplifiers that the PMC twenty5 22i speakers have.

Some of the bench test results shown below are from using my PC and Windows Media Player playing test files via Bluetooth to the Arylic S50 Pro+ receiver sitting a few feet from my PC, and the RCA outputs from the Pro+ to the inputs on my soundcard which were analyzed by my PC bench test software (the PMC speakers were not in the signal path for these bench tests).

The first test result, shown below in Figure 15, is a 1 kHz sine wave at -0.1 dBFS. The volume control on the Pro+ was turned up all the way. The output was 3.55 dBV which is very close to 2 Volts. THD+N was 0.043%. The primary harmonic is 3rd-ordered.

The IMD test (60 Hz, 7 kHz) at -5 dBFS input yielded 0.004% IMD which is excellent.

The Frequency Response using a 16/44.1 test file shows that the response is flat until about 17.5 kHz and then is down 3 dB at 20 kHz. This suggests that there is a roll-off filter at 20 kHz. In this particular test, I used the Audio Check test album that is available on Qobuz. The little blips in the curve are due to FFT artifacts, not the Arylic’s response.

The Frequency Response using a 24/96 test file is shown in Figure 18 below. The response still is just to 20 kHz as a result of the analog roll-off filter. So, the frequency response of the S50 Pro+ is 20 Hz to 20 kHz regardless of the music sampling rate.

I also broadcast music and test files from my Samsung S24 Ultra phone from Qobuz streaming service. I wanted to see how the setup handled deep bass. Here are the first couple of minutes from Telarc’s release of Copland’s Fanfare for the Common Man. My phone was 4 feet from the Pro+ receiver. There are some very intense bass drum notes. You can see them at about 35 Hz. They came through loud and clear.

Using my phone again and Qobuz streaming, I played a 1 kHz sine wave from the Audio Check test files album on Qobuz, 4 feet from the Pro+ receiver. There was an order of magnitude more THD, but THD+N was nearly the same. So, playing from Windows Media Player gave less distortion than playing from Qobuz. A difference in bitrate between Windows Media Player and Qobuz was probably responsible.

With the same setup as in Figure 20, but with my phone upstairs and at the other end of the house, so passing through several walls and the floor, distortion was nearly the same. This is excellent. I can see that Bluetooth quality has improved a lot.

So, the Arylic S50 Pro+ Bluetooth receiver would make a useful addition to the PMC twenty5 22i speaker setup. I tried it out with the PMC twenty5 22i speakers near my dining room table, playing music via Qobuz from my phone. This is really cool stuff! I didn’t need to turn on my main system in the living room. Just the PMC speakers, Arylic S50 Pro+, and my phone, making for very enjoyable dinners.

“I’m an audiophile too. Any questions?”

PMC’s twenty5 22i bookshelf speakers are a knockout! Building on their “monitor” design, with low distortion, these speakers will sound as good in your home as they would in a music recording studio’s editing room.

Their new two-channel power amplifiers that enable the speakers’ ability to be driven by RCA or XLR cables instead of speaker cables allow you to send your music by Bluetooth directly to the speakers in another place in your home, such as your dining room. Arylic’s Bluetooth S50 Pro+ Receivers eliminate cables from your hi-fi system altogether. A very nice setup indeed!