diff --git a/amodem/equalizer.py b/amodem/equalizer.py
index a8f3570..c14cef1 100644
--- a/amodem/equalizer.py
+++ b/amodem/equalizer.py
@@ -7,11 +7,11 @@ from amodem import sampling
 import itertools
 import random
 
-_constellation = [1, 1j, -1, -1j]
-
 
 class Equalizer(object):
 
+    _constellation = [1, 1j, -1, -1j]
+
     def __init__(self, config):
         self.carriers = config.carriers
         self.omegas = 2 * np.pi * np.array(config.frequencies) / config.Fs
@@ -20,7 +20,9 @@ class Equalizer(object):
 
     def train_symbols(self, length, seed=0, constant_prefix=16):
         r = random.Random(seed)
-        choose = lambda: [r.choice(_constellation) for j in range(self.Nfreq)]
+        # Use low-level randomness for cross-version compatibility.
+        random_symbol = lambda: self._constellation[r.getrandbits(2)]
+        choose = lambda: [random_symbol() for j in range(self.Nfreq)]
         symbols = np.array([choose() for _ in range(length)])
         # Constant symbols (for analog debugging)
         symbols[:constant_prefix, :] = 1