diff --git a/src/io/manager.rs b/src/io/manager.rs
index 41746ea..02d0144 100644
--- a/src/io/manager.rs
+++ b/src/io/manager.rs
@@ -1,15 +1,15 @@
 use anyhow::{Context, Result};
 use crossbeam::channel::{Sender, bounded};
-use jack::{AsyncClient, Client, ClientOptions, contrib::ClosureProcessHandler};
+use jack::{AsyncClient, Client, ClientOptions};
 use log::error;
 
-use crate::io::processor::{ProcessHandler, Processor};
+use crate::io::processor::Processor;
 use crate::io::recorder::Recorder;
 use crate::sim::chain::AmplifierChain;
 
 /// Manages the audio processing chain and JACK client
 pub struct ProcessorManager {
-    _active_client: AsyncClient<Notifications, ClosureProcessHandler<(), ProcessHandler>>,
+    _active_client: AsyncClient<Notifications, Processor>,
     recorder: Option<Recorder>,
     /// GUI → audio thread: push a completely new preset
     amp_tx: Sender<Box<AmplifierChain>>,
@@ -24,18 +24,18 @@ impl ProcessorManager {
     /// Creates a new ProcessorManager
     pub fn new() -> Result<Self> {
         let (client, _) = Client::new("rustortion", ClientOptions::NO_START_SERVER)
-            .context("Failed to create JACK client")?;
+            .context("failed to create JACK client")?;
 
         let (tx_amp, rx_amp) = bounded::<Box<AmplifierChain>>(1);
 
-        let processor = Processor::new_with_channel(&client, rx_amp, None);
-        let handler = ClosureProcessHandler::new(processor.into_process_handler());
+        let processor =
+            Processor::new(&client, rx_amp, None).context("error creating processor")?;
 
         let sample_rate = client.sample_rate() as f32;
 
         let _active_client = client
-            .activate_async(Notifications, handler)
-            .with_context(|| "Failed to activate async")?;
+            .activate_async(Notifications, processor)
+            .context("failed to activate async client")?;
 
         Ok(Self {
             sample_rate,
@@ -62,7 +62,7 @@ impl ProcessorManager {
         }
 
         let recorder = Recorder::new(self.sample_rate as u32, record_dir)
-            .context("Failed to start recorder")?;
+            .context("failed to start recorder")?;
         let _ = recorder.sender();
 
         self.recorder = Some(recorder);
diff --git a/src/io/processor.rs b/src/io/processor.rs
index 846fdac..b05dba5 100644
--- a/src/io/processor.rs
+++ b/src/io/processor.rs
@@ -1,46 +1,62 @@
 use crate::io::recorder::{AudioBlock, BLOCK_FRAMES};
 use crate::sim::chain::AmplifierChain;
+use anyhow::{Context, Result};
 use crossbeam::channel::{Receiver, Sender};
-use jack::{AudioIn, AudioOut, Client, Control, Port, ProcessScope};
-use log::{error, info};
+use jack::{AudioIn, AudioOut, Client, Control, Frames, Port, ProcessHandler, ProcessScope};
+use log::{debug, error};
 use rubato::{
     Resampler, SincFixedIn, SincInterpolationParameters, SincInterpolationType, WindowFunction,
 };
 
-pub type ProcessHandler = Box<dyn FnMut(&Client, &ProcessScope) -> Control + Send + 'static>;
+const CHANNELS: usize = 1;
+const OVERSAMPLE_FACTOR: f64 = 4.0;
+const MAX_BLOCK_SIZE: usize = 4096;
 
 pub struct Processor {
-    /// Amplifier.
+    /// Amplifier chain, used for processing amp simulations on the input.
     chain: Box<AmplifierChain>,
+    /// Channel for updating the amplifier chain.
     rx_chain: Receiver<Box<AmplifierChain>>,
-    /// Optional recorder.
+    /// Optional recorder channel.
     tx_audio: Option<Sender<AudioBlock>>,
     in_port: Port<AudioIn>,
-    out_l: Port<AudioOut>,
-    out_r: Port<AudioOut>,
+    out_port_left: Port<AudioOut>,
+    out_port_right: Port<AudioOut>,
     upsampler: SincFixedIn<f32>,
     downsampler: SincFixedIn<f32>,
     /// Reusable buffer for input frames.
-    input_frames: Vec<Vec<f32>>,
+    input_buffer: Vec<Vec<f32>>,
+    /// Reusable buffer for upsampled frames.
+    upsampled_buffer: Vec<Vec<f32>>,
+    /// Reusable buffer for downsampled frames.
+    downsampled_buffer: Vec<Vec<f32>>,
 }
 
 impl Processor {
-    pub fn new_with_channel(
+    pub fn new(
         client: &Client,
         rx_chain: Receiver<Box<AmplifierChain>>,
         tx_audio: Option<Sender<AudioBlock>>,
-    ) -> Self {
-        let in_port = client.register_port("in", AudioIn::default()).unwrap();
-        let out_l = client.register_port("out_l", AudioOut::default()).unwrap();
-        let out_r = client.register_port("out_r", AudioOut::default()).unwrap();
-
-        let _ = client.connect_ports_by_name("system:capture_1", "rustortion:in");
-        let _ = client.connect_ports_by_name("rustortion:out_l", "system:playback_1");
-        let _ = client.connect_ports_by_name("rustortion:out_r", "system:playback_2");
-
-        let channels = 1;
-        let oversample_factor: f32 = 4.0;
-        let max_chunk_size = 128;
+    ) -> Result<Self> {
+        let in_port = client
+            .register_port("in_port", AudioIn::default())
+            .context("failed to register in port")?;
+        let out_port_left = client
+            .register_port("out_port_left", AudioOut::default())
+            .context("failed to register out port left")?;
+        let out_port_right = client
+            .register_port("out_port_right", AudioOut::default())
+            .context("failed to register out port right")?;
+
+        client
+            .connect_ports_by_name("system:capture_1", "rustortion:in_port")
+            .context("failed to connect to in port")?;
+        client
+            .connect_ports_by_name("rustortion:out_port_left", "system:playback_1")
+            .context("failed to connect to out port left")?;
+        client
+            .connect_ports_by_name("rustortion:out_port_right", "system:playback_2")
+            .context("failed to connect to out port right")?;
 
         let interp_params = SincInterpolationParameters {
             sinc_len: 256,
@@ -57,107 +73,151 @@ impl Processor {
             window: WindowFunction::BlackmanHarris2,
         };
 
-        let upsampler = SincFixedIn::<f32>::new(
-            oversample_factor as f64,
+        let mut upsampler = SincFixedIn::<f32>::new(
+            OVERSAMPLE_FACTOR,
             1.0,
             interp_params,
-            max_chunk_size,
-            channels,
+            MAX_BLOCK_SIZE,
+            CHANNELS,
         )
-        .unwrap();
+        .context("failed to create upsampler")?;
 
-        let downsampler = SincFixedIn::<f32>::new(
-            1.0 / oversample_factor as f64,
+        let mut downsampler = SincFixedIn::<f32>::new(
+            1.0 / OVERSAMPLE_FACTOR,
             1.0,
             down_interp_params,
-            (max_chunk_size as f32 * oversample_factor) as usize,
-            channels,
+            MAX_BLOCK_SIZE * OVERSAMPLE_FACTOR as usize,
+            CHANNELS,
         )
-        .unwrap();
+        .context("failed to create downsampler")?;
 
-        let input_frames = vec![Vec::with_capacity(client.buffer_size() as usize)];
+        let buffer_size = client.buffer_size() as usize;
+        upsampler
+            .set_chunk_size(buffer_size)
+            .context("failed to set upsampler chunk size")?;
+        downsampler
+            .set_chunk_size(buffer_size * OVERSAMPLE_FACTOR as usize)
+            .context("failed to set downsampler chunk size")?;
 
-        Self {
+        let input_buffer = vec![Vec::with_capacity(buffer_size)];
+        let upsampled_buffer = upsampler.output_buffer_allocate(true);
+        let downsampled_buffer = downsampler.output_buffer_allocate(true);
+
+        Ok(Self {
             chain: Box::new(AmplifierChain::new("Default")),
             rx_chain,
             tx_audio,
             in_port,
-            out_l,
-            out_r,
+            out_port_left,
+            out_port_right,
             upsampler,
             downsampler,
-            input_frames,
-        }
+            input_buffer,
+            upsampled_buffer,
+            downsampled_buffer,
+        })
     }
+}
 
-    pub fn into_process_handler(mut self) -> ProcessHandler {
-        Box::new(move |_client: &Client, ps: &ProcessScope| -> Control {
-            if let Ok(new_chain) = self.rx_chain.try_recv() {
-                self.chain = new_chain;
-                info!("Received new chain");
-            }
+impl ProcessHandler for Processor {
+    fn process(&mut self, _c: &Client, ps: &ProcessScope) -> Control {
+        if let Ok(new_chain) = self.rx_chain.try_recv() {
+            self.chain = new_chain;
+            debug!("Received new chain");
+        }
 
-            let n_frames = ps.n_frames() as usize;
-            let in_buf = self.in_port.as_slice(ps);
+        let n_frames = ps.n_frames() as usize;
+        let input = self.in_port.as_slice(ps);
 
-            let out_buf_l = self.out_l.as_mut_slice(ps);
-            let out_buf_r = self.out_r.as_mut_slice(ps);
+        self.input_buffer[0].clear();
 
-            {
-                let input_channel = &mut self.input_frames[0];
-                input_channel.clear();
+        debug_assert!(
+            self.input_buffer[0].capacity() >= n_frames,
+            "input_buffer too small; buffer_size callback missing an allocation"
+        );
 
-                if input_channel.capacity() < n_frames {
-                    input_channel.reserve(n_frames - input_channel.capacity());
-                }
+        self.input_buffer[0].extend_from_slice(input);
 
-                input_channel.extend_from_slice(in_buf);
+        let (_, upsampled_frames) = match self.upsampler.process_into_buffer(
+            &self.input_buffer,
+            &mut self.upsampled_buffer,
+            None,
+        ) {
+            Ok(data) => data,
+            Err(e) => {
+                error!("Upsampler error: {e}");
+                return Control::Continue;
             }
+        };
 
-            let mut upsampled = match self.upsampler.process(&self.input_frames, None) {
-                Ok(data) => data,
-                Err(e) => {
-                    error!("Upsampler error: {e}");
-                    return Control::Continue;
-                }
-            };
-
-            let chain = self.chain.as_mut();
-            for s in &mut upsampled[0] {
-                *s = chain.process(*s);
-            }
+        let chain = self.chain.as_mut();
+        for s in &mut self.upsampled_buffer[0][..upsampled_frames] {
+            *s = chain.process(*s);
+        }
 
-            let downsampled = match self.downsampler.process(&upsampled, None) {
-                Ok(data) => data,
-                Err(e) => {
-                    error!("Downsampler error: {e}");
-                    return Control::Continue;
-                }
-            };
-
-            let final_samples = &downsampled[0];
-            let frames_to_copy = final_samples.len().min(n_frames);
-
-            out_buf_l[..frames_to_copy].copy_from_slice(&final_samples[..frames_to_copy]);
-            out_buf_r[..frames_to_copy].copy_from_slice(&final_samples[..frames_to_copy]);
-            for i in frames_to_copy..n_frames {
-                out_buf_l[i] = 0.0;
-                out_buf_r[i] = 0.0;
+        let (_, downsampled_frames) = match self.downsampler.process_into_buffer(
+            &self.upsampled_buffer,
+            &mut self.downsampled_buffer,
+            None,
+        ) {
+            Ok(data) => data,
+            Err(e) => {
+                error!("Downsampler error: {e}");
+                return Control::Continue;
             }
+        };
+
+        let final_samples = &self.downsampled_buffer[0][..downsampled_frames];
+        let frames_to_copy = final_samples.len().min(n_frames);
+
+        let out_buffer_left = self.out_port_left.as_mut_slice(ps);
+        let out_buffer_right = self.out_port_right.as_mut_slice(ps);
 
-            if let Some(ref tx) = self.tx_audio {
-                let mut block = AudioBlock::with_capacity(BLOCK_FRAMES * 2);
-                for &s in final_samples.iter().take(BLOCK_FRAMES) {
-                    let v = (s * i16::MAX as f32).clamp(i16::MIN as f32, i16::MAX as f32) as i16;
-                    block.extend_from_slice(&[v, v]);
-                }
+        out_buffer_left[..frames_to_copy].copy_from_slice(&final_samples[..frames_to_copy]);
+        out_buffer_right[..frames_to_copy].copy_from_slice(&final_samples[..frames_to_copy]);
+        for i in frames_to_copy..n_frames {
+            out_buffer_left[i] = 0.0;
+            out_buffer_right[i] = 0.0;
+        }
 
-                if let Err(e) = tx.try_send(block) {
-                    error!("Error sending audio block: {e}");
-                }
+        // If the recording channel is available, handle sending audio blocks to the recorder.
+        if let Some(ref tx) = self.tx_audio {
+            let mut block = AudioBlock::with_capacity(BLOCK_FRAMES * 2);
+            for &s in final_samples.iter().take(BLOCK_FRAMES) {
+                let v = (s * i16::MAX as f32).clamp(i16::MIN as f32, i16::MAX as f32) as i16;
+                block.extend_from_slice(&[v, v]);
             }
 
-            Control::Continue
-        })
+            if let Err(e) = tx.try_send(block) {
+                error!("Error sending audio block: {e}");
+            }
+        }
+
+        Control::Continue
+    }
+
+    fn buffer_size(&mut self, _c: &Client, frames: Frames) -> Control {
+        debug!("JACK buffer size changed to {frames} frames");
+        let new_size = frames as usize;
+        let cap = self.input_buffer[0].capacity();
+
+        if cap < new_size {
+            self.input_buffer[0].reserve_exact(new_size - cap);
+        }
+
+        if let Err(e) = self.upsampler.set_chunk_size(new_size) {
+            error!("Upsampler cannot grow to {new_size}: {e}");
+        } else {
+            self.upsampled_buffer = self.upsampler.output_buffer_allocate(true);
+        }
+
+        let needed_down = new_size * OVERSAMPLE_FACTOR as usize;
+        if let Err(e) = self.downsampler.set_chunk_size(needed_down) {
+            error!("Downsampler cannot grow to {needed_down}: {e}");
+        } else {
+            self.downsampled_buffer = self.downsampler.output_buffer_allocate(true);
+        }
+
+        Control::Continue
     }
 }
diff --git a/src/main.rs b/src/main.rs
index 574b8d7..895d536 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -53,19 +53,19 @@ __________                __                 __  .__
     for &key in &required_vars {
         match std::env::var(key) {
             Ok(val) => info!("{} = {}", key, val),
-            Err(_) => anyhow::bail!("Environment variable '{}' must be set.", key),
+            Err(_) => anyhow::bail!("environment variable '{}' must be set.", key),
         }
     }
 
     let mut processor_manager =
-        ProcessorManager::new().context("Failed to create ProcessorManager")?;
+        ProcessorManager::new().context("failed to create ProcessorManager")?;
 
     let chain = create_mesa_boogie_dual_rectifier(processor_manager.sample_rate());
 
     if recording {
         processor_manager
             .enable_recording(&args.recording_dir)
-            .with_context(|| format!("Failed to enable recording in '{}'", args.recording_dir))?;
+            .with_context(|| format!("failed to enable recording in '{}'", args.recording_dir))?;
     }
 
     processor_manager.set_amp_chain(chain);
@@ -77,7 +77,7 @@ __________                __                 __  .__
         info!("Ctrl+C received, shutting down...");
         shutdown_flag.store(false, Ordering::SeqCst);
     })
-    .expect("Error setting Ctrl+C handler");
+    .expect("error setting Ctrl+C handler");
 
     while running.load(Ordering::SeqCst) {
         thread::sleep(Duration::from_secs(1));