diff --git a/dsp/fn529fe0.cpp b/dsp/fn529fe0.cpp
index 87f8cc1..5fdc60a 100644
--- a/dsp/fn529fe0.cpp
+++ b/dsp/fn529fe0.cpp
@@ -9,9 +9,13 @@
 
 namespace fn529fe0 {
 
-void iir1(float* x, const double* A, const double* B, size_t nbin, double /*acc0*/) {
+void iir1(float* x, const double* A, const double* B, size_t nbin, double acc0) {
     // leaky first-order: y = A*acc + B*x ;  acc = y  (B = 1-A from live tables)
-    double acc = 0.0;
+    // State persists across calls via static accumulator (per-thread).
+    static thread_local double acc = 0.0;
+    static thread_local size_t last_nbin = 0;
+    // Reset if nbin changed (new config/resize)
+    if (nbin != last_nbin) { acc = 0.0; last_nbin = nbin; }
     for (size_t i = 0; i < nbin; i++) {
         double y = A[i] * acc + B[i] * static_cast<double>(x[i]);
         acc = y;