Hi, and Welcome!

If you are asking about playing your phone as well as the Alesis kit through the amplifier, then whether you need an external mixer or not depends on the model of the amplifier and what inputs it offers. Most of the amps of that sort have at least a couple of inputs, but quite often one of them is for a mono microphone, so would not work well for taking the stereo output from the earphone socket of a mobile phone.

However, it's likely that the Aux In socket on the Crimson II is stereo, so a simple 1/8" stereo jack-jack cable would enable you to play the phone through the Crimson, mix with the kit sound output, and into the amplifier. The headphone volume control on the phone would enable you to control the balance between the phone and kit outputs. I'm assuming you are using the mono/L output of the Crimson to go to your amp.

Ok, this is how bad I am with technology- I dont even know if I'm using the mono/L output so - I think I need to check this. I started out with my drum kit and played through the headphones and then as I've started to learn a couple of songs, I've though I will get an amp, so I've just plugged everything in where I though it will fit and when I've got sound coming through I've gone with it
Thanks for your reply !

If its just a few inputs, there are very simple and dirt cheap mixers out there. Recently I had a need for a small head amp. I wasn't really looking for anything special. I took a risk and bought a very cheap one on amazon, and was blown away at actually how decent it was.

What I bought...

Now This is not going to replace proper small format mixer like a Mackie or Soundcraft or even Behringer. It will not be robust and it will not be able except loud sources, but it did work quite successfully for my application.

If you are will to risk few bucks it might be an option to looks at something like this:

Cheap as can be line mixer

If you want something with a bit more reliability:

Another 10 Ten Mixers Page

Link - your last two links don't resolve. They have both an http and an https in them.

Boswell, post: 467804, member: 29034 wrote:
Link - your last two links don't resolve. They have both an http and an https in them.

Thanks I think I fixed it...

Imho you don't need a mixer.
If I'm not mistaken, the alesis has an Aux IN. You could connect your phone to the aux in and mix the backtrack on your Alesis.
On your Alesis console, you should be able to set the volume of the aux signal using a potentiometer or something similar. Hint: make sure the volume of your phone is at max before you turn up the aux volume on your alesis.
As the aux in signal is routed to the output of the Alesis, you'll be able to hear you drumming with the backtrack through the amp.
Same applies to your amp. It might have an aux in as well, to which you could connect your phone. In that case you'll need to mix the backtrack signal with the drum signal on your amp.

I hope this post is of any value as it's my first post on this forum ;-)