I have ONE Boston Acoustics Micro90X Speaker (for 15-125 watt amps), and a Topping VX1 2x25W Class T Amplifier (notionally 2 x 25 watts) that I don't currently use.

I thought both of these gave terrific performance for the price when I had two of the speakers (plus a decent subwoofer) in my hifi STEREO system. The amp was actually a little underpowered, but sound quality at non-ear-splitting volumes was way better than I'd have expected.

What I'd really like to do now is drive that speaker as a MONO system. I don't know much about electronics, but I found this in an article titled The Mighty T-Amp: You’ll Be Blown Away...

Being BTL [bridge-tied load], these amps have some other interesting characteristics, including the ability to output a mono audio output from a stereo input by simply connecting the load from the + terminal of one channel and the – terminal of the other.

Can it really be that easy? I guess all I'm really asking for here is someone who sounds like they know what they're talking about to say Yes!, and give me the confidence to give it a go. Also to reassure me there's no significant fire risk (I'd shed a tear if my Brave Little Amplifier simply died, but I wouldn't want to go down with it! :)

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    \$\begingroup\$ Afaik those already have balanced outputs (i.e. they are already bridged) so I don't think you can tie the + output of one channel to the - of the other to get more voltage swing. I think what the author meant is that basically you mix the two channels together with that method, but you will not get more power. And even though it might work I'm not sure if its a good idea. I had some of those Topping amps and I used to tie the inputs together and drive a 2 way speaker with a single amp (each channel of the stereo amp to either HF or LF of the speaker). \$\endgroup\$
    – Wesley Lee
    Mar 16, 2019 at 23:38
  • \$\begingroup\$ @Wesley Lee: Thanks for the feedback. But maybe I shouldn't have mentioned the fact that the VX1 couldn't really deliver very high undistorted output to a pair of Micro90X speakers (which had previously been easily handling the output from a 2 x 60 watt amp). Volume isn't an issue for the single-speaker setup I'm after now, so can you elaborate on why else you're "not sure if its a good idea"? \$\endgroup\$ Mar 17, 2019 at 1:25
  • \$\begingroup\$ Well, for one, you leave the outputs of each side floating. And then the output that you do connect will receive a signal that it wasn't supposed to. Let's say you have a "song" that is pure silence on one channel and a sine wave on the other. Plugged "correctly", the silent +- side will source and sink no current. The sine channel will source and sink that current. When you flip it, one of the positive outputs will be supposed to source no current, but the negative side will be sinking a sinewave from it. It might very well deal with this perfectly fine, (...) \$\endgroup\$
    – Wesley Lee
    Mar 17, 2019 at 1:33
  • \$\begingroup\$ but I dont think the complementary outputs of the same channel are designed to be so independent. \$\endgroup\$
    – Wesley Lee
    Mar 17, 2019 at 1:33

1 Answer 1


The TA2021 in the Topping amp is already BTL when driving loudspeakers.

So you best bet for using it in mono is just driving both inputs in parallel and connecting two loudspeakers. If you've only got one 8 ohm loudspeaker you can only get 14W from the TA2021

Two 4 ohm loudspeakers would be best.

Now about that mono interconnection:

If you connect the loudspeaker between the left+ and the right- output you'll get the sound in mono (an even mix of left and right inputs)

with that 8 ohm speaker you'll get up-to 14W

  • \$\begingroup\$ I'm not bothered about the output level (it's for late-night use at low volume). So can I take this as a Yes! (connecting ONE loudspeaker between the left+ and the right- output as per cited link will work and be safe)? \$\endgroup\$ Mar 17, 2019 at 14:16
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    \$\begingroup\$ yes! that will work and be safe leave the other outputs unconnected. \$\endgroup\$ Mar 18, 2019 at 2:26
  • \$\begingroup\$ Thanks very much. Out of interest, do you know if this way of connecting the speaker load would normally be practicable, or is it just something that happens to apply to some particular amp circuit designs (specifically, Class T amps, as implied in the link I put in my question)? And is there some reason why I couldn't find any examples of higher-powered (2x60+W) Class T amps reflecting what I consider to be the astonishing price/performance ratio of the VX1? \$\endgroup\$ Mar 18, 2019 at 14:11
  • \$\begingroup\$ any BTL stereo amplifier should support this. \$\endgroup\$ Mar 20, 2019 at 1:43

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