diff --git a/README.md b/README.md
index 95c9e72..94d06be 100644
--- a/README.md
+++ b/README.md
@@ -41,6 +41,7 @@ The following [infinite impulse response](https://en.wikipedia.org/wiki/Infinite
 * [2n-order Butterworth cascade of second-order low or high pass filters](https://en.wikipedia.org/wiki/Butterworth_filter)
 * [second-order notch filter](https://en.wikipedia.org/wiki/Band-stop_filter)
 * [second-order bandpass filter](https://en.wikipedia.org/wiki/Band-pass_filter)
+* [second-order allpass filter](https://en.wikipedia.org/wiki/All-pass_filter)
 
 ### [blockdc.hh](blockdc.hh)
 
diff --git a/biquad.hh b/biquad.hh
index 1ed1384..2d2d19e 100644
--- a/biquad.hh
+++ b/biquad.hh
@@ -86,6 +86,22 @@ public:
 		a1a0 = a1 / a0;
 		a2a0 = a2 / a0;
 	}
+	void allpass(int n, int N, VALUE QdB)
+	{
+		VALUE alpha = UnitCircle<VALUE>::sin(n, N) / (VALUE(2) * idecibel(QdB)),
+			cn = UnitCircle<VALUE>::cos(n, N),
+			b0 = VALUE(1) - alpha,
+			b1 = -VALUE(2) * cn,
+			b2 = VALUE(1) + alpha,
+			a0 = VALUE(1) + alpha,
+			a1 = -VALUE(2) * cn,
+			a2 = VALUE(1) - alpha;
+		b0a0 = b0 / a0;
+		b1a0 = b1 / a0;
+		b2a0 = b2 / a0;
+		a1a0 = a1 / a0;
+		a2a0 = a2 / a0;
+	}
 	TYPE operator()(TYPE x0)
 	{
 		TYPE y0 = b0a0*x0 + b1a0*x1 + b2a0*x2