Past projects: ICEpower125ASX2 dual-mono amplifier

Another past project, a compact but very powerful amplifier based on a pair of ICEpower 125ASX2 modules. These class D-modules have a built-in switching power supply and are good for around 450-500W when running in bridge-mode as they do here. They also have very impressive specs for dynamic range etc. and since I have  “squeezed” two modules into a 1U case measuring just 230x280mm, this is a small amplifier with a big punch.

This amp was actually the first step in a “quest” to build a compact and high quality fully-balanced signal chain (the second step is the Retro RIAA-stage in the previous post) and I am now looking into building a matching preamp and DAC as well. I haven’t quite settled on the specific designs yet, but a balanced B1 buffer might be on the cards :).

The impressive specs of the ICEpower modules would not matter much if the sound was’t on par though, but fortunately I think it is. I have tried more or less all ICEpower-modules (except the 250ASX) and these are probably my favourite ones. The soundstage is big, there is impressive clarity and detail from top to bottom and of course lots of power on tap. In some ways the amp is a little “lean” in the presentation, so it does not necessarily suit all speakers, but even with the fairly bright Elacs I have the results are very good. If matched with something with a warmer sound signature (such as the Sonus Fabers I have my eyes on…) I am sure it would be really fantastic.

The modules are stock with no additional input buffering or similar, so I am actually also considering getting another pair and experimenting with different input configurations, including the transformer-based option used by many of the “pro” builders of ICEpower-based amplifiers. However, I might spring for a pair of 250ASX’es instead. If anyone has experience with both the 125ASX and the 250ASX please feel free to leave a comment on the differences in sound signature 🙂


17 Responses to Past projects: ICEpower125ASX2 dual-mono amplifier

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  5. Hi, Can you please tell me how to use this amp with a active crossover? Their connectivity diagram shows that the 2 outputs can go directly to LF and HF, but how will the cutover frequency be configured?

    • theslowdiyer says:

      That only works if you have an active crossover on the input which provides two outputs that have LF and HF cutoff respectively. Then you simply connect the HF output to the left input on the ASX and the LF output to the right input (or vice versa). Then you can simply connect the ASX outputs directly to the drivers.

  6. Philip Kerpen says:

    I just came across the post and I am going to build something pretty much identical. I’m going two build 2 four channel amps, with 2 125ASX2 modules in each. I would like to go for a compact build and was thinking of using the Galaxy GX243 230 x 230 x 40 mm cases. Do you think it’s feasible?

    And what about input buffering? I have seen some charts that suggest the low input impedance at higher frequencies results in significant roll-off, and since I will be using these amps for 4-way dipole speakers that might be worth doing.

    Do you have any suggestions or tips and tricks you would be willing to share? This will be my first amp build.

    • theslowdiyer says:

      Hi Philip, I think you can just about get away with the GX243 for just the modules, but there is not much space left over. If you want space for buffers a GX34x chassis is probably better. If it is your first build I would recommend just assembling the amps and get sound before you start experimenting with buffers. Then it is also easier to decide if you think they make a difference 🙂

  7. Simon Ravenscroft says:

    Hi I am so glad I found your post, I am also building the exact same project.
    I recently purchased a Nord Two ICE Power Stereo and am really pleased with the power, sound stage and great bass tones, this is connected to my PSB T3’s and couldn’t be happier, I am placing my old T2’s in my office and could buy another amp but where’s the fun in that so I have on the way a pair of 125ASX2 modules that I plan to build into a case.
    To be clear I am no electrical engineer I just love to build rather than buy, built a couple of nixie clocks from kits so I’m half decent with a soldering iron and following instructions and figuring out the rest. But I have never worked with amp’s so this is new. Your pictures will help me unbelievably when it come to wiring it up, I was a little unsure how to wire them as mono blocks as apposed to stereo, I was just going to use one but into 8 ohms I believe that will only give me about 60-70 watts. I see you have used balanced XLR inputs but RCA out, can I ask why you use balanced XLR input as opposed to RCA.
    Also I have seen a few other similar projects with their pictures and they use – I believe they are input buffers ?? what is their purpose and are they worth considering or at least use a case large enough to install later. Do they bring a lot to the party?
    Thanks again for your pics – a big help

    • Simon Ravenscroft says:

      Forgot to ad I am using a little Douk Audio 6N3 valve pre amp if that makes any difference to using a buffer or not

      • theslowdiyer says:

        Hi Simon, the balanced connections are needed if you want to get the full power from a balanced (BTL) module. In (very) simple terms: When you bridge two SE-amplifiers for BTL-mode, you are effectively asking one amp to swing positive while the other one should swing negative relative to ground – this gives you twice the voltage across the load and in theory four times the power. However, if you feed the BTL amp an SE-signal, the negative signal is at ground-level, meaning the one channel doesn’t deliver any output and so just sits idle.

        You therefore either need a “real” balanced signal from the preamp or you need to cheat by inverting the positive SE signal from the source and feeding the negative equivalent to the second channel to make it deliver power. This additional circuit can be an inverting buffer or a real bal/se-converter or a line transformer and that is most likely what you will have seen on commercial kit.

  8. Simon Ravenscroft says:

    I think I understand now, so if I install 2 x XLR inputs for balanced pre amps and connect straight into P102 signal input and connect a pair of RCA inputs for unbalanced and run through a buffer and then into P102 input ?
    Can you suggest a buffer ? some have suggested a BUF634 Module ?

    Now the power for the buffer can be taken from P103 24v aux supply that I will also use to run the power on/off switch but the BUF634 Module I have seen states a power range of 18v +/- 2.5 so do I need a different buffer module or different power supply or will the P103 be ok?

    Amusing I am correct up to now and I have the signal input from the balanced XLR and the signal from the buffer both connected into the same P102 on the 125ASX2 module is it wise to install a balanced/unbalanced switch to switch off power to the buffer boards when using a balanced pre amp input ?

    Again many thanks for your help from a newbie that is trying not to bite off more than he can chew (can’t turn round now I have ordered the 125ASX2 modules and a nice chassis)

    • theslowdiyer says:

      You’re moving in the right direction, but still not quite there 😉 The “buffer” you need isn’t a normal buffer, but a separate stage that generates a balanced input from the SE outputs on your preamp – it’s not the same and it’s not intended to be used in parallel with “real” balanced XLR inputs.Something like this would work, but nothing based on BUF634 is appropriate here

      Also, you can’t use the AUX-power on P103 to directly power this circuitry, partly because the voltage is too high and partly because it’s unregulated, meaning it isn’t as stable. You need a PSU stage to regulate the AUX-PSU down to +/-12-15V which is what most opamp-based circuitry is comfortable with and you also need to ensure that you do not damage the AUX supplies on the 125ASX.

      If you send me an email through the contact form I’ll point you to some relevant docs – that’s easier than continuing here 😀

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