DAC working but AMP not working

I have a setup in which I have been using the DAC on a Pi3 quite happily using alsa with DMIX configured via the /etc/asound.conf file. The application is currently using two instances of MPLAYER to play simultaneously - one a silent file and one the real music - the silence is there to reduce the "click" at the start and end of playback hence the use of DMIX. Otherwise this would be a single player instance application.

I swapped the DAC for the AMP and set the /boot/config.txt file to use the AMP settings but there is an issue. With the DAC, I get an output amixer control called "Digital" and I had been using that to adjust the overall volume. But with the AMP installed, I only get Master and Channels.  Using MPLAYER to play a file I get an error 

[AO_ALSA] alsa-lib: pcm_dmix.c:989:(snd_pcm_dmix_open) unable to create IPC semaphore

I confess to having got the system to work with lots of googling and copy/paste etc. So I'm not really sure how I have it working for the DAC and hence do not know what I should address to make it work for the AMP. Any suggestions toward a solution would be much appreciated.

My DAC working asound.conf is:

pcm.hifiberry {

  type hw card 0



pcm.!default {

  type plug

  slave.pcm "dmixer"



pcm.dmixer {

  type dmix

  ipc_key 1024

  slave {

    pcm "hifiberry"

    channels 2




ctl.dmixer {

  type hw

  card 0





Thanks in advance


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