The FC526-Litz All Discrete Entire Bundle is a complete kit bundle that contains most all of the parts and components required to complete a FC526 FET Peak Limiter module. The FC526 is physically and electronically fully VPR 500 series compatible.
The FC526 requires three 2520 style opamps for operation. The only items not included in this kit are the discrete opamps. Those can be added below.
This kit ships with a Litz wire 2623-1 output transformer. Kits with EA2623-1's are also available.
***Please note: The Litz 2623-1-L wire colors are different than the EA2623-1. To use the Litz 2623-1-L on this build, please follow the table on the bottom left of this datasheet.
The FC526 is a somewhat unique compressor/limiter module based on commonly used circuit blocks. The audio path is comprised of three 2520 style opamps and one 2623-1 output transformer. The first 2520 is used as a balanced transformerless receiver amp. This input stage can be hit with a +28dBu signal before clipping occurs. The 2nd opamp is known as the signal preamp. This is the stage where the audio signal is detected, rectified and then used to reduce the audio signal level, aka gain reduction. The final opamp is the output amplifier, which is followed by a 1:2 balancing transformer. This part of the circuit is very much the same as the 2nd stage of a VP28 or ML2. The output stage also clips just beyond +28dBu. The red front panel peak LED indicates clipping of the output stage.
The FC526 uses a common FET gain reduction method. This FET hangs just after the input amp and is employed as a voltage variable shunt resistor aka VVR, very much similar to the legendary 1176. The control voltage for this FET is a product of the FET's bias voltage and a rectified DC signal of the output of the signal preamp. By definition, this is a "feedback" style peak limiter. The sidechain detection circuit is a mash-up of an 1176 and an 1178 using two amps from a quad IC opamp to rectify the audio's AC to DC. This DCV is then mixed with the FET's bias voltage, varying the resistance of the shunt FET in a manor that is proportional to the incoming audio signal.
The gain reduction meter uses a FET with similar characteristics to the VVR FET so you get a visual representation of the gain reduction that is occurring. The LED meter string is very fast and accurate which is accomplished with LM339 quad voltage comparators.
The rotary ratio switch has the common 1176 ratios available as well as 2:1 and ABI, which stands for All Buttons In. ABI takes the VVR FET out of its conductive range so it is normal for the green gain reduction LED to turn off when switched to this ratio.
Via a front panel switch, the sidechain has an optional HP filter that rolls off at 6dB per octave starting 3dB down at approximately 180Hz.
The upper right switch is a hard relay bypass. This simply couples the module's input directly to the module's output, completely bypassing the FC526's circuitry.
The lower right switch turns the gain reduction on or off. This switch turns off the audio going to the sidechain detector amp allowing the input signal to still travel thru the all discrete transformer coupled path.
This kit features high quality components throughout like C&K mini toggle switches, a Grayhill rotary switch and Bourns potentiometers/trimmers.
Attack Time: ~20 microseconds adjustable to 800 microseconds
Release Time: 50 milliseconds adjustable to 1.1 seconds
Current Draw: Less than 118mA per bipolar 16V rail
Following is a list of additional components that are required to complete this build:
(3) 2520 style discrete operational amplifier
PS: DOA's can be added to this kit at the bottom of this page.
Here are required items that we do not carry at the store:
Solder, soldering iron and basic small hand tools
Very good assembly skills
A decent understanding of schematics and electronic components
This kit requires some detailed calibration steps not all that different from an 1176 build. You will need a way inject a sine wave tone into the module and accurately monitor the output level. You will also need a way to monitor the THD component of the output. I am not a DAW guy but assume there are free software plugins for this somewhere. The extension test jig will also help make these procedures painless.
***This is one of the more complicated builds we have at the store. I do not recommend this for a beginner or inexperienced builder. While there are no plans of a dedicated "Assembly Guide", our friend and ex-newbie Chunger will be documenting a photo-build-doc like no other can! This may take a while so keep that in mind before purchasing. As always, I will have a build/support thread running at the GDIY forum.
