diff --git a/src/PortAudio.jl b/src/PortAudio.jl
index 776ead6..ca22b7e 100644
--- a/src/PortAudio.jl
+++ b/src/PortAudio.jl
@@ -1,9 +1,6 @@
 module PortAudio
 
-# simple interface
 export play
-
-# AudioNodes
 export SinOsc, AudioMixer, ArrayPlayer, AudioInput
 
 typealias PaTime Cdouble
@@ -18,13 +15,12 @@ _stream = nothing
 
 ################## Types ####################
 
-# A node in the render tree
-abstract AudioNode
-
 typealias AudioSample Float32
 # A frame of audio, possibly multi-channel
 typealias AudioBuf Array{AudioSample}
 
+# A node in the render tree
+abstract AudioNode
 
 # Info about the hardware device
 type DeviceInfo
@@ -32,99 +28,24 @@ type DeviceInfo
     buf_size::Integer
 end
 
+include("nodes.jl")
+
 type AudioStream
-    # TODO: this union may have performance penalties
-    root_node::Union(AudioNode, Nothing)
+    mixer::AudioMixer
     info::DeviceInfo
 
     function AudioStream(sample_rate, buf_size)
-        new(nothing, DeviceInfo(sample_rate, buf_size))
+        mixer = AudioMixer()
+        new(mixer, DeviceInfo(sample_rate, buf_size))
     end
 end
 
-function render(node::Nothing, device_input::AudioBuf, info::DeviceInfo)
-    return zeros(AudioSample, info.buf_size)
-end
-
-#### SinOsc ####
-
-# Generates a sin tone at the given frequency
-
-type SinOsc <: AudioNode
-    freq::Real
-    phase::FloatingPoint
-
-    function SinOsc(freq::Real)
-        new(freq, 0.0)
-    end
-end
-
-function render(node::SinOsc, device_input::AudioBuf, info::DeviceInfo)
-    phase = AudioSample[1:info.buf_size] * 2pi * node.freq / info.sample_rate
-    phase += node.phase
-    node.phase = phase[end]
-    return sin(phase)
-end
-
-#### AudioMixer ####
-
-# Mixes a set of inputs equally
-
-type AudioMixer <: AudioNode
-    mix_inputs::Array{AudioNode}
-end
-
-function render(node::AudioMixer, device_input::AudioBuf, info::DeviceInfo)
-    # TODO: we may want to pre-allocate this buffer and share between render
-    # calls
-    mix_buffer = zeros(AudioSample, info.buf_size)
-    for in_node in node.mix_inputs
-        mix_buffer += render(in_node, device_input, info)
-    end
-end
-
-#### Array Player ####
-
-# Plays a AudioBuf by rendering it out piece-by-piece
-
-type ArrayPlayer <: AudioNode
-    arr::AudioBuf
-    arr_index::Int
-
-    function ArrayPlayer(arr::AudioBuf)
-        new(arr, 1)
-    end
-end
-
-function render(node::ArrayPlayer, device_input::AudioBuf, info::DeviceInfo)
-    i = node.arr_index
-    range_end = min(i + info.buf_size-1, length(node.arr))
-    output = node.arr[i:range_end]
-    if length(output) < info.buf_size
-        output = vcat(output, zeros(AudioSample, info.buf_size - length(output)))
-    end
-    node.arr_index = range_end + 1
-    return output
-end
-
-#### AudioInput ####
-
-# Renders incoming audio input from the hardware
-
-type AudioInput <: AudioNode
-    channel::Int
-end
-
-function render(node::AudioInput, device_input::AudioBuf, info::DeviceInfo)
-    @assert size(device_input, 1) == info.buf_size
-    return device_input[:, node.channel]
-end
 
 ############ Exported Functions #############
 
-
 function play(node::AudioNode, stream::AudioStream)
-    stream.root_node = node
+    # TODO: don't break demeter
+    stream.mixer = vcat(stream.mixer.mix_inputs, [node])
 end
 
 function play(node::AudioNode)
@@ -196,7 +117,7 @@ function audio_task(jl_filedesc::Integer, stream::AudioStream)
     jl_stream = fdio(jl_filedesc)
     jl_rawfd = RawFD(jl_filedesc)
     while true
-        out_array = render(stream.root_node, in_array, stream.info)::AudioBuf
+        out_array = render(stream.mixer, in_array, stream.info)::AudioBuf
         # wake the C code so it knows we've given it some more data
         wake_callback_thread(out_array)
         # wait for new data to be available from the sound card (and for it to
diff --git a/src/nodes.jl b/src/nodes.jl
new file mode 100644
index 0000000..b5e3c18
--- /dev/null
+++ b/src/nodes.jl
@@ -0,0 +1,82 @@
+#### SinOsc ####
+
+# Generates a sin tone at the given frequency
+
+type SinOsc <: AudioNode
+    freq::Real
+    phase::FloatingPoint
+
+    function SinOsc(freq::Real)
+        new(freq, 0.0)
+    end
+end
+
+function render(node::SinOsc, device_input::AudioBuf, info::DeviceInfo)
+    phase = AudioSample[1:info.buf_size] * 2pi * node.freq / info.sample_rate
+    phase += node.phase
+    node.phase = phase[end]
+    return sin(phase)
+end
+
+#### AudioMixer ####
+
+# Mixes a set of inputs equally
+
+type AudioMixer <: AudioNode
+    mix_inputs::Array{AudioNode}
+
+    function AudioMixer{T <: AudioNode}(mix_inputs::Array{T})
+        new(mix_inputs)
+    end
+
+    function AudioMixer()
+        new(AudioNode[])
+    end
+end
+
+function render(node::AudioMixer, device_input::AudioBuf, info::DeviceInfo)
+    # TODO: we may want to pre-allocate this buffer and share between render
+    # calls
+    mix_buffer = zeros(AudioSample, info.buf_size)
+    for in_node in node.mix_inputs
+        mix_buffer += render(in_node, device_input, info)
+    end
+    return mix_buffer
+end
+
+#### Array Player ####
+
+# Plays a AudioBuf by rendering it out piece-by-piece
+
+type ArrayPlayer <: AudioNode
+    arr::AudioBuf
+    arr_index::Int
+
+    function ArrayPlayer(arr::AudioBuf)
+        new(arr, 1)
+    end
+end
+
+function render(node::ArrayPlayer, device_input::AudioBuf, info::DeviceInfo)
+    i = node.arr_index
+    range_end = min(i + info.buf_size-1, length(node.arr))
+    output = node.arr[i:range_end]
+    if length(output) < info.buf_size
+        output = vcat(output, zeros(AudioSample, info.buf_size - length(output)))
+    end
+    node.arr_index = range_end + 1
+    return output
+end
+
+#### AudioInput ####
+
+# Renders incoming audio input from the hardware
+
+type AudioInput <: AudioNode
+    channel::Int
+end
+
+function render(node::AudioInput, device_input::AudioBuf, info::DeviceInfo)
+    @assert size(device_input, 1) == info.buf_size
+    return device_input[:, node.channel]
+end
diff --git a/test/nodes.jl b/test/nodes.jl
new file mode 100644
index 0000000..a85acbe
--- /dev/null
+++ b/test/nodes.jl
@@ -0,0 +1,35 @@
+using Base.Test
+using PortAudio
+
+info = PortAudio.DeviceInfo(44100, 512)
+dev_input = zeros(PortAudio.AudioSample, info.buf_size)
+
+# A TestNode just renders out 1:buf_size each frame
+type TestNode <: PortAudio.AudioNode
+end
+
+function PortAudio.render(node::TestNode,
+                device_input::PortAudio.AudioBuf,
+                info::PortAudio.DeviceInfo)
+    return PortAudio.AudioSample[1:info.buf_size]
+end
+
+#### AudioMixer Tests ####
+
+# TODO: there should be a setup/teardown mechanism and some way to isolate
+# tests
+
+mix = AudioMixer()
+@test mix.mix_inputs == PortAudio.AudioNode[]
+@test PortAudio.render(mix, dev_input, info) == zeros(PortAudio.AudioSample, info.buf_size)
+
+testnode = TestNode()
+mix = AudioMixer([testnode])
+@test mix.mix_inputs == PortAudio.AudioNode[testnode]
+@test PortAudio.render(mix, dev_input, info) == PortAudio.AudioSample[1:info.buf_size]
+
+test1 = TestNode()
+test2 = TestNode()
+mix = AudioMixer([test1, test2])
+@test PortAudio.render(mix, dev_input, info) == 2 * PortAudio.AudioSample[1:info.buf_size]
+