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  1. #51
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    Quote Originally Posted by CJS View Post
    Actually Philippe writes something about Mono operation in the SqueezeAMP README on github (last paragraph under section "Usage comments"):
    "According to TAS57xx datasheet, if you short L+/L- and R+/R- you can use the chip in post-filtered mono PBTL mode. Sounds a bit strange and I've not tried as of this writing, but if it might be handy to do a mono (more powerful) speaker".

    In my writings below I have copied a few pictures from the TAS5780M datasheet, since that is the TAS57xx version that is used on SqueezeAMP PCB v3.11 that I have. The TAS57xx ClassD amplifiers can be configured for "Stereo Bridge Tied Load" (Stereo BTL) or "Mono Parallel Bridge Tied Load" (Mono PBTL) operation. By default the TAS57xx on the SqueezeAMP PCB is configured for Stereo BTL operation. I think that for mono operation Philippe is referring to the post-filtered mono PBTL configuration that is shown in Figure 71 in the datasheet:

    Attachment 31645

    However Figure 71 only shows the filter components at the outputs of the Class D amplifiers. When reading further in the datasheet, I found that there are seperate application schematics for stereo BTL and Mono PBTL operation. The schematics are almost the same, however there is a difference in connection of the input pins of channel B of the power amplifier.

    For Stereo BTL operation the DAC_OUTB output is AC coupled to SPK_INB+, and SPK_INB- is AC coupled to GND, see figure 84. This is the configuration that is used on the SqueezeAMP PCB:

    Attachment 31646

    For Mono PBTL operation the DAC_OUTB output is terminated with a resistor to ground, and SPK_INB+ and SPK_INB- pins are connected to ground, see figure 85:

    Attachment 31647

    So for mono PBTL operation one would have to modify the squeezeamp board as follows: replace C113 with a resistor, remove C114 (optional) and connect pins 37 and 38 to ground. And software configuration of the TAS57xx should be changed from Stereo BTL to Mono PBTL operation. This would be the preferred method, but it does involve some small hardware modifications. People who are handy with a solder iron could do these modifications using the existing PCB.

    However in principle it is also possible to use the existing PCB for Mono operation, without any hardware or software modification!. This can be done as follows:
    • Connect plus terminal of mono speaker to output L+
    • Connect minus terminal of mono speaker to output R-

    Explanation: when connecting the mono speaker this way, the signal over the speaker is +L - (-R) = L+R = Mono.

    I have tested this "poor man's" mono configuration with my SqueezeAMP board, and it is actually working. I played some tracks from a CD with test signals, and got following results:
    • Left only and Right only signals sound equally strong from the mono speaker
    • Equal signals on Left and Right channels give a louder sound (expected is +6dB)
    • A test signal with Left and Right signals in anti-phase gives zero sound

    These test result show that the mono speaker indeed gets the L+R = Mono signal.

    There are two small caveats however:
    1) I measured an increased DC offset voltage over the mono speaker terminals. In normal stereo operation the DC offset voltages at low volume setting between L+ and L- respectively R+ and R- of my SqueezeAMP are both less than 2mV. However I measured a DC offset voltage of circa 45mV between L+ and R-. When the audio is paused the DC offset becomes 0mV (seems like the Class D amplifiers are disabled for idle I2S input signal; I would need to measure with an oscilloscope to confirm this). This DC offset voltage results in an audible plop when pausing and when resuming playback.
    2) Compared to normal stereo operation I heard an increased noise level coming from the tweeter of the mono connected speaker when playing a track with volume set to 0 (i.e. with my ear close to the tweeter). It could be that this is just caused by the fact that an L+R mono signal has higher noise than a single L or R signal.

    You can easily test this poor man's mono solution, and check if the plop and increased noise level are acceptable for your use case.

    PS: If you are interested in the test signals that I used (Channel check and Balance and Phase check signals) let me know. I can upload these signals on request.
    Very good findings, thanks! I can if interested, think about building a mix-batch with some in mono mode. It's very easy, I can just change the BOM (capacitor/resistors/0Ohms)
    LMS 7.9 on Pi 3B+ & Odroid-C2 - SqueezeAMP!, 5xRadio, 3xBoom, 4xDuet, 1xTouch, 1 SB3. Sonos PLAY:3, PLAY:5, Marantz NR1603, Foobar2000, ShairPortW, JRiver 21, 2xChromecast Audio, Chromecast v1 and v2, Squeezelite on Pi, Yamaha WX-010, AppleTV 4, Airport Express, GGMM E5, Riva 1 & 3

  2. #52
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    Quote Originally Posted by philippe_44 View Post
    Very good findings, thanks! I can if interested, think about building a mix-batch with some in mono mode. It's very easy, I can just change the BOM (capacitor/resistors/0Ohms)
    Indeed the mono PBTL configuration only requires a small hardware modification, but I think it is not not only a BOM change. Referring to the part numbers shown in my post, one can replace C113 with a 49.9K resistor and C114 with a 0 ohm resistor, but then still pins 37 and 38 of the TAS5780M need to be shorted such that they are both grounded. Not sure if this justifies a PCB change, the short can be made relatively easy on the existing PCB.

    It is not immediately clear to me what change is required on the software side. I would expect that settings of the input mixer shown in figure 73 in the datasheet must be changed, since apparently the mono signal is routed to DAC_OUTA.
    | LMS 8.0 | Squeezebox Boom | RPi0W + pCP 6.1.0 + HiFiBerry DAC Zero | SqueezeAMP |

  3. #53
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    Quote Originally Posted by philippe_44 View Post
    Very good findings, thanks! I can if interested, think about building a mix-batch with some in mono mode. It's very easy, I can just change the BOM (capacitor/resistors/0Ohms)
    If it could be done with jumpers then great but I don't think I'd push for a hardware variant that was only mono.
    Even if I went for mono in the ceiling I can imagine that I'd like the flexibility to easily change it to real stereo if I moved it.

    So if my project goes ahead .. and I'm not sure yet ... I think it try out the simplistic wiring at the speaker terminal knowing that if it is switched on/off with the lights then any pop might not be noticed since it would not be paused ... or I go with single stereo speaker and run four wires.
    Paul Webster
    http://dabdig.blogspot.com
    Author of "Now Playing" plugins covering Radio France (FIP etc), KCRW, Supla Finland, ABC Australia, CBC/Radio-Canada and RTE Ireland

  4. #54
    Senior Member sle118's Avatar
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    Pre-crimped molex 8 pin suggestion

    Philippe,

    This might have been answered already, but I was wondering if you had any suggestion for pre-crimped molex connector to tap into the side of the board for a nice SPI display?
    LMS 7.9 - 1xRadio, 1xBoom, 5xDuet,3xTouch, 1 SB2. Sony PlayStation, Emby, Chromecast v1 and v2 and...
    6xSqueezeAmp!
    Contributing to Folding@Home : https://stats.foldingathome.org/donor/squeezelite-esp32

  5. #55
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    Quote Originally Posted by sle118 View Post
    Philippe,

    This might have been answered already, but I was wondering if you had any suggestion for pre-crimped molex connector to tap into the side of the board for a nice SPI display?
    Well, unfortunately DigiKey does not sell the 8-pins pre-crimped, AFAIK. So best option is to buy a set of 4 or 6 pins pre-crimped cables and extract the wires (very easy) and re-insert them in a 8-pin connector that you bought.
    LMS 7.9 on Pi 3B+ & Odroid-C2 - SqueezeAMP!, 5xRadio, 3xBoom, 4xDuet, 1xTouch, 1 SB3. Sonos PLAY:3, PLAY:5, Marantz NR1603, Foobar2000, ShairPortW, JRiver 21, 2xChromecast Audio, Chromecast v1 and v2, Squeezelite on Pi, Yamaha WX-010, AppleTV 4, Airport Express, GGMM E5, Riva 1 & 3

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