To squeeze the best performance out of BBDs, a "compander" circuit is recommended.
Compander is a portmanteau of compressor and expander. If signal is compressed prior to the BBD, and expanded after, noise levels can be noticeably improved.
The NE570/571 is a compander IC. It is a dual unit containing 2 identical companding "kits" on each side of the IC package. Each "kit" can then be connected with a few external components to form either a compressor or a expander.
The companding kit contains a "gain cell", an op amp, and a full wave rectifier. If you are familiar with VCAs from synthesizers, and/or the use of OTAs (CA3080, CA3094, LM13600/13700), then you are familiar with this "gain cell" as it is called on the NE570 datasheet. The gain cell is technically a current controlled current amplifier, so calling it a "VCA" (voltage controlled amplifier) is inaccurate, but you can mentally substitute "VCA" for "gain cell" and the overall idea is the same. The control current for the gain cell is created by the full wave rectifier.
Confusing Internal Connections
It's important to note two internal connections within our "kit" elements:
1) The output of the gain cell is internally connected to the inverting input of the op amp.
2) The full wave rectifier's output is internally connected to the gain control of the gain cell. We have some access to this gain control via the "rectifier cap" pin found at pins 1 and 16. This is the pin that is exploited by some Memory Man circuits for the "Squlech" (EH 1307 / EH 7550) and bypass functions (EH 7811). There are several application notes (Philips AN174, ON Semi AND8159) for more info.
Two Different Connection Patterns for either Compression or Expansion
Even with the internal "hardwired" connection between gain cell and op amp, we are able to form two quite different circuit topologies to achieve compression or expansion. This can be confusing when tracing a pedal, since each side of the NE570 will be connected differently, despite the mirror image pinout when the chip is raw.
Compressor Topology
Compression is achieved by connecting our input to the inverting op amp and taking our output from the op amp. We then have to send this output back into the rectifier input and the gain cell input. The gain cell then acts as a shunt feedback resistor that changes value with signal amplitude. (Remember that the gain cell output is always connected internally to the inverting op amp input.) More input signal = more negative feedback. Less input signal = less negative feedback.
Expander Topology
Expansion is achieved by connecting our input to the gain cell and the rectifier input. The op amp buffers the current output into a voltage output for putting back in our bigger circuit. More input signal = more output signal. Less input signal = less output signal. Pins 1 and 16 allow us to change the gain independently of the input signal to the rectifier. The Squlech and bypass special functions exploit this for an attenuation and muting effect, respectively. Forcing this voltage more negative will result in attenuated or cutoff delay output.