diff --git a/sndlock.cpp b/sndlock.cpp
index 4470ea3..8fb2ae1 100644
--- a/sndlock.cpp
+++ b/sndlock.cpp
@@ -108,7 +108,7 @@ static void dsp_thread()
                         sample += (double)buf_in[SND_RCHAN*j+k];
                     std::complex<double> rotated;
                     rotated = sample*std::polar(scale, phase_in[i]*2.0*M_PI/(double)UINT32_MAX);
-                    phase_in[i] += ftw[i];  // wraps on underflow
+                    phase_in[i] -= ftw[i];  // wraps on underflow
 
                     lpf_y[i] += (rotated - lpf_y[i])*lpf_k[i];
                     double mag = std::abs(lpf_y[i]);