diff --git a/Software/components/core/src/BiquadDesign.cpp b/Software/components/core/src/BiquadDesign.cpp
index 3fd63fc..db76b15 100644
--- a/Software/components/core/src/BiquadDesign.cpp
+++ b/Software/components/core/src/BiquadDesign.cpp
@@ -82,12 +82,20 @@ BiquadCoefficients designPeakingEq(FrequencyHz center, GainDb gain,
     const float a1 = -2.0F * cosOmega;
     const float a2 = 1.0F - alpha / a;
 
+    // RBJ's cookbook form subtracts the feedback terms
+    // (y = ... - (a1/a0) y[n-1] - (a2/a0) y[n-2]); the ADAU1701 Param EQ
+    // cell's A0/A1 registers add them instead (y = ... + A0 y[n-1] +
+    // A1 y[n-2]), so the normalized RBJ a1/a2 must be negated when they
+    // land in A0/A1. Without this, any nonzero gain applies positive
+    // instead of negative feedback at the band's pole, so the biquad's
+    // state diverges and rails to a constant (inaudible DC) value --
+    // root cause of the 2026-08-25 total-silence-on-any-EQ-change bug.
     return BiquadCoefficients{
         .b0 = b0 / a0,
         .b1 = b1 / a0,
         .b2 = b2 / a0,
-        .a0 = a1 / a0,
-        .a1 = a2 / a0,
+        .a0 = -(a1 / a0),
+        .a1 = -(a2 / a0),
     };
 }
 
diff --git a/Software/components/core/test/biquad_design_test.cpp b/Software/components/core/test/biquad_design_test.cpp
index dd771cb..de2a4e1 100644
--- a/Software/components/core/test/biquad_design_test.cpp
+++ b/Software/components/core/test/biquad_design_test.cpp
@@ -54,6 +54,55 @@ namespace {
     return EXIT_SUCCESS;
 }
 
+/*
+ * The ADAU1701 Param EQ cell's A0/A1 registers ADD the feedback terms
+ * (y = ... + A0 y[n-1] + A1 y[n-2]), unlike the RBJ cookbook's native
+ * subtractive form. Denominator 1 - A0 z^-1 - A1 z^-2 = 0 has poles at the
+ * roots of z^2 - A0 z - A1 = 0; by the Jury test for a monic real quadratic
+ * z^2 + c1 z + c0 (c1 = -A0, c0 = -A1), both poles lie strictly inside the
+ * unit circle iff |A1| < 1, A0 + A1 < 1, and A0 - A1 > -1. A missing
+ * negation when mapping the RBJ a1/a2 into A0/A1 (2026-08-25 total-silence
+ * regression) fails this for real gain/Q combinations, so this guards
+ * against that class of bug rather than just checking magnitudes.
+ */
+[[nodiscard]] bool isAdau1701Stable(const core::BiquadCoefficients &c) noexcept
+{
+    return std::fabs(c.a1) < 1.0F && (c.a0 + c.a1) < 1.0F
+        && (c.a0 - c.a1) > -1.0F;
+}
+
+[[nodiscard]] int runPeakingStabilityTest()
+{
+    const struct
+    {
+        std::uint32_t hz;
+        float dbGain;
+        float q;
+    } cases[] = {
+        {100U, 9.0F, 0.9F},   // bass-enhance band 1 at max level
+        {400U, 3.0F, 1.0F},   // bass-enhance band 2 at max level
+        {1000U, -1.5F, 1.0F}, // stereo-enhance band 3 at max level
+        {3000U, 2.0F, 1.0F},  // stereo-enhance band 4 at max level
+        {8000U, 4.0F, 1.0F},  // stereo-enhance band 5 at max level
+        {1000U, 12.0F, 10.0F}, // GainDb::kMaxDb at high Q
+        {1000U, -96.0F, 0.2F}, // near GainDb::kMinDb at low Q
+    };
+
+    for (const auto &tc : cases) {
+        const auto center = core::FrequencyHz::tryFromHz(tc.hz);
+        const auto gain = core::GainDb::tryFromDb(tc.dbGain);
+        const core::BiquadCoefficients peaking =
+            core::designPeakingEq(*center, *gain, tc.q);
+        if (!isAdau1701Stable(peaking)) {
+            std::cerr << "unstable ADAU1701 biquad for " << tc.hz << " Hz, "
+                      << tc.dbGain << " dB, Q=" << tc.q << ": a0=" << peaking.a0
+                      << " a1=" << peaking.a1 << '\n';
+            return EXIT_FAILURE;
+        }
+    }
+    return EXIT_SUCCESS;
+}
+
 } // namespace
 
 int main()
@@ -64,5 +113,8 @@ int main()
     if (runFlatBiquadTest() != EXIT_SUCCESS) {
         return EXIT_FAILURE;
     }
+    if (runPeakingStabilityTest() != EXIT_SUCCESS) {
+        return EXIT_FAILURE;
+    }
     return EXIT_SUCCESS;
 }