Overview:
The Fredenstein F660A Limiting Amplifier is a valve (tube) based compressor and limiter with the
same very short signal path as the famous Fairchild 660/670. it uses the exact type of tubes, the
6386 LGP, as the Fairchild. Fortunately, new production tubes are available today, since original
tubes are hardly available anymore. The four tubes form a single stage push-pull Class A amplifier
and the whole signal path consists only of an T-type input attenuator, input transformer, Class A
amplifier and finally an output transformer. At this point the similarities end. The F660 uses a DSP
to control the operating points of the tubes, guarantying perfect working conditions all the time. The
side chain amplifier is designed with semi-conductor components since it “only” generates the
control voltage for the tubes. The final result is a compressor with the compression characteristics
of the Fairchild, but with an extended frequency response, lower noise and lower distortion. The
perfect buss compressor for today's high resolution audio. Since this a “Variable-Mu” type of
compressor the relationship between anode currents and compression is almost quadratic, causing
the anode currents to be very small at high compression rate. As with all “Variable-Mu” is not
advised to go beyond 10dB of compression since audible distortion will be present. Setting
compression to around 5 dB yields the best results.
Installation:
Located on he front panel, there are the output level meter, the gain reduction meter, the quick
access buttons for Gain, Threshold and Release Time, the LCD display and the rotary main control
button.
Fig. 1 F660A Front Panel
On the rear panel, there are the XLR connectors for MAIN INPUT, MAIN OUTPUT, SIDECHAIN
INPUT and SIDECHAIN OUTPUT, as well as the RJ11 Analog and digital LINK Connectors, the
IEC MAINS POWER connector, MAINS SWITCH, and most importantly the MAINS VOLTAGE
SELECTOR.
Please select your local mains voltage first before connecting to power!
For basic operations, only the MAIN INPUT, MAIN OUTPUT and, of course, the MAINS
POWER have to be connected. The SIDECHAIN INPUT and SIDECHAIN OUTPUT can be used
to insert filters and other equipment into the side chain. In case of using two F660s, the supplied
link cables (RJ11) should be used to link two units. Please use the the cable marked “digital” to
connect the digital link connectors, and the one marked ”analog” to connect the the analog link
connectors.
Fredenstein F660A Manual Release 1.0 Oct 17, 2014 Page 1