In artificial reverb algorithms, gains are commonly varied over
time to break up temporal patterns, improving quality. We propose
a family of novel Schroeder-style allpass filters that are energypreserving under arbitrary, continuous changes of their gains over
time. All of them are canonic in delays, and some are also canonic
in multiplies. This yields several structures that are novel even in
the time-invariant case. Special cases for cascading and nesting
these structures with a reduced number of multipliers are shown as
well. The proposed structures should be useful in artificial reverb
applications and other time-varying audio effects based on allpass
filters, especially where allpass filters are embedded in feedback
loops and stability may be an issue.