I have for sale an excellent working Timewave DSP-599zx audio filter.
This is one of the best outboard DSP units and will work great with your older HF radios without the DSP features.

The DSP-559zx is an extraordinarily versatile digital signal processor
designed for amateur and shortwave radio voice, data and CW operation.
The DSP-599zx uses advanced digital signal processing technology to
implement algorithms that perform five basic audio functions:
 Random noise reduction
 Adaptive multi-tone and manual notch filtering (Tone noise reduction)
 Bandpass/Highpass/Lowpass filtering
 Signal generation including FSK RTTY modulation
 Signal detection and measurement including RTTY demodulation
The DSP-599zx combines these five basic functions to reduce noise and
interference and improve radio communication. The DSP-599zx hardware
and software architecture allow easy field upgrade with new features and
algorithms. The same hardware and software architecture also allow
ergonomic mode oriented operation of the DSP-599zx. The LCD alphanumeric
display provides a clear view of operating settings when switching
between operating modes. The quick-select push buttons and optical
encoders for filter tuning allow instant mode change with total recall of last
setting and memories. Front-panel selectable and adjustable inputs allow
you to quickly setup and adjust your DSP-599zx to wipe out noise and
QRM like never before!
Here are a few more highlights among the many other operating features of
the DSP-599zx:
 Selectable Automatic Gain Control
 Configurable bypass control
 Two selectable input channels
 Two configurable output channels
 Six memories for instant recall of user-defined configurations
 Test instrument mode for analyzing signals and other equipment

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More information on this unit can be found at;

Price is $265 shipped CONUS.