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The sidechain transformer in my DIY'ed 660-ish compressor does that, and the audio from that transformer is distorted, too - it sounds kinda like it looks on the 'scope, the worst case of crossover distortion that I've ever seen or heard. That's because there is a separate threshold and ratio (the AC and DC threshold) settings, and the DC threshold generates a dead zone at zero-cross that looks just like an extreme case of crossover distortion. In a Fairchild design, that's ok because the sidechain amp is only used for that - the audio coming out is not meant to be listened to.
There's lots of signal in transformers in a tube comp - it not only sends the output back to some whimpy solid-state console (though most tube comps are capable of driving a 600 ohm terminated cable to levels in the +25 or +30 dBm range), but the transformers also need to deal with the power to charge the sidechain attack/decay capacitor. I don't know how the Manley is designed inside, but the Fairchild (and the Gates Sta-Level) uses something like a ten watt or so power amp for its sidechain, and even an Altec has a couple watts available. The Gates share the sidechain and output power amp - the sidechain comes off the primary of the output transformer, the Fairchild has a separate sidechain amp. That's enough power to make some audible transformer noise.
-Dale
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