Adding a sound amplifier to a DCC chip.

Discussion in 'Talk' started by Scale7JB, 13 January 2019.

  1. Scale7JB

    Scale7JB Western Thunderer

    Hi all.

    I have posted quickly on the Zimo group, but thought I would check here too.

    I have three of the old Large Scale Zimo Mx690 chips, and the volume of the sound leaves a lot to be desired, I think they are on 1w output compared to 3w on more modern chips, and they just aren't loud enough for exhibition use.

    I wondered if something like this could be used in line to boost the volume, or would it cause damage to the chip?

    XPT8871 DC 3V 3.7V 5V Mono 5W Mini Amplifier Board Audio Amp Module One channel | eBay

    I was thinking the 5v power supply could be taken from one of the power outputs on the chip?

  2. Steph Dale

    Steph Dale Western Thunderer

    Input impedance is not quoted, but it's not very likely to be under 8ohm, so yes you could hook the input up to the speaker outputs of a decoder. In terms of powering it I'd use a suitable regulated 5v supply powered from the track, with a function switch to turn it on or off.

    Relatively simple to do, but it'd be worth checking the size of the PSU solution as any smoothing capacitor could be large, by model railway standards.

  3. simond

    simond Western Thunderer

    Hobbytronics supply a range of Polulu regulated 5V supplies which work fine as the input for an Arduino. Alternatively, there are single chip linear regulators available from RS & elsewhere which will give you 5V, but I’ve never used either of them in an audio application. I’d guess the Polulu supplies are very much higher frequency than you could hear, so I’d expect they’d be no problem. At least some of them have an external “enable” input.

    The linear regulator is cheaper, and I’d guess that would work fine too.

    Usual disclaimers etc.
  4. Steph Dale

    Steph Dale Western Thunderer

    Part of the problem is going to be reducing ripple on the power supply outputs at the DCC clock system; that certainly is audible. So somewhere in there a capacitor would be required to smooth out the power. I think.

  5. simond

    simond Western Thunderer

    Hi Steph,

    DCC signal has variable frequency but somewhere around 7kHz which is well inside the audible range of most folks. I’d imagine the regulated supply either needs to come off its own bridge rectifier from track supply, or could be supplied from the decoder itself. I guess the need for, and size of, any capacitors would be a subject for experimentation, but they’d be much smaller than a keep-alive anyway. A small 35V electrolytic would surely do, I guess.

    I agree with your thoughts on input impedance - but I wonder if an opto would be needed, given the supply and signal would be from the same source, so there’s a potential loop in there somewhere. Again, experimentation, with the potential for smoke...

  6. Steph Dale

    Steph Dale Western Thunderer

    Hmmm, yeah; that's why I was suggesting the amplifier have its own PSU straight off the track. My expectation would be that it's then far enough removed from the decoder to be able to do the job required without (destructive) feedback, but I see where you're coming from. And yep - you're right about the need for a 35v electrolytic, I'd need try and remember how to work out the minimum size necessary to counter 7kHz ripple. It's been a little while since I've needed to design a PSU!

    In the words of Vicky Pollard it's 'Yeah, but no, but, yeah, but no. But yeah': Certainly possible, but probably not as simple as plugging it onto the back end of your existing decoder. You may find that by the time you've got all the bits together it's no smaller or simpler than just using a larger or more modern decoder. What did the guys say on the Zimo forum?