New Chinese miniamps…

A few weeks ago I stumbled across a post on diyaudio mentioning the latest in (the long line of) ultra-mini class D chipamps offered on eBay/Aliexpress. The boards looked interesting and so I “splurged” for a pair which turned up this past weekend. Although in all honesty “splurged” probably isn’t the right term here as I paid a grand total of 12.50 EUR for the pair – including shipping (!!).

Like many of the other small amp boards, this one uses the TPA3118d2 class D IC from TI. The TPA3118 chip has a fairly good reputation on diyaudio and it will give a reasonably high power level when used as a mono amplifier (called PBTL-mode). The chip has a thermal pad on the underside and so uses the PCB copper as the heatsink surface. Unlike many other similar boards though, these boards are pure poweramps (no onboard volume pot) which is fine for me. They are also tiny at 35 by 46 mm – which means even two channels side-by-side are still visibly smaller than a credit card (!!)

So, very small, very cheap, quite powerful, and decent sound quality – what’s not to like? Not a lot actually. It would have been nice if the chip had been the TPA3116 which has the thermal pad facing upwards. Even through there isn’t a lot of space, it would still have been possible to fit a small heatsink on top of the chip and thereby (probably) improve the thermal performance and get a bit more output power.

Another slight negative is that the input capacitor is an X7R ceramic type which really isn’t the best choice for audio. Because it’s a 0603 SMD it’s also tiny and very difficult to replace/exchange with something better. The boards seem to be set to the highest gain (36 dB) which is good if the source is a smartphone or similar, but on the high side if it’s a typical hifi source like a DAC. Again, adjustment of the gain would require replacing 0603 resistors which is not that fun a pain in the … but overall it’s a minor downside.

The boards in stock form don’t have any connectors for inputs and outputs, but I’ve added some to my pair to make testing a bit easier. I need to do a bit more listening with these “prototypes”, but if the sound quality is any good I do have a specific project in mind for them for later 🙂

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4 Responses to New Chinese miniamps…

  1. Evenslowerdiyer says:

    Hi Slowdiyer,

    did you find some time for additional listening to those?

    There ist one option for a gain change on these amp boards. Removing R27 should give the default gain of 20dB. I have read some reports of this being manageable with a standard soldering iron.

    Do you have any ideas for a volume control matching these cheap amps? A plain pot might not be the best option. Simple (Chinese) preamp maybe?

    Best wishes,
    Evenslowerdiyer

  2. theslowdiyer says:

    Hello,

    I don’t have any listening impressions yet – one of the boards had a small but very serious accident just after posting the article 😉 But I do have a couple of new pairs on the way so I can continue experimenting later 😀

    As to whether you need a preamp it depends a bit on the gain, because from looking at the datasheet (p. 14) for the TPA3118 there is a relation between gain and input impedance:

    At 20 dB gain the input impedance is 60kOhms, meaning you can probably just about get away with a 10k pot on the input (the rule of thumb is 1:10 or greater between the impedances) as long as it is fed by a low-impedance source.

    At higher gains the input impedance drops and some sort of active circuit is a good idea. Technically I don’t think that much is required and so a simple opamp circuit should do it, but you can of course experiment here. In keeping with the “cheap ebay” theme and the tradition that tubes + class D is a good combo (everyone knows that… :D) then maybe a small low-voltage tube design is worth trying?

    • Evenslowerdiyer says:

      Hi Slowdiyer,

      thanks a lot for your long and detailed answer.

      As the TPA3118’s input impedance is varying with gain and my soldering skills are limited, I will have two options: 20dB (60kOhms) and 36dB (9kOhms). Let’s hope that these will not translate to “too low” and “too high”. 🙂

      I am thinking about using these amp boards for building compact active speakers. As I plan on using only computers as audio sources, an USB DAC will also be integrated. It is a PCM2704-based model from China and will be the source of analog audio.

      I like the idea of using a simple 10k pot between DAC and amp. If it does not work well due to the currently unknown DAC board’s output impedance, I can still start searching for a simple opamp circuit as you suggested.

      As I plan on hiding all that electronic stuff in a tiny compartment at the speaker’s base, I feel a tube might not be the best solution here.

      Thanks again for your help. You might be slow at diying, but you are really quick at giving valuable advice. 🙂

      Best wishes,
      Evenslowerdiyer

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