To avoid massive build errors, make sure the BOM Revision you follow matches the Revision of the PCB you are building!!
The FC526-XFMR Entire Bundle is a complete kit bundle that contains most all of the parts and components required to
complete a FC526-XFMR FET Peak Limiter module. The FC526-XFMR is physically and electronically fully VPR 500 series compatible.
The FC526-XFMR requires one 2520 style opamp for operation. The only item not included in this kit is the discrete opamp.
Those can be added below.
This kit ships with a EA2623-1
output transformer. Kits with
Litz wire 2623-1's
are also available.
FC526 vs FC526-XFMR: The biggest difference is the input stage, hence the XFMR acronym. The 2520 receiver opamp has been
replaced by the CA2622 input transformer and two transistor voltage follower. The middle 2520 "signal preamp" opamp has
been changed to a vertical DTO5 opamp. The THD nullification section has been simplified meaning that no THD measurement or
adjustments are required. The peak detector circuit has been simplified to reduce parts count and overall simplify the build.
Lastly, the VVR FET's conductive range bias has been changed from a -.5dB loss to a -1dB loss.
The FC526-XFMR is a somewhat unique compressor/limiter module based on commonly used circuit blocks. The audio path is comprised
of a CA2622 input transformer, a two transistor voltage follower, a DTO5 five transistor opamp, one 2520 style opamp and one
2623-1 output transformer. The receiver stage is the CA2622 input transformer and the two transistor voltage follower. The 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 and/or the signal preamp opamp.
The FC526-XFMR 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
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
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-XFMR'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
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:
(1) 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.
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. I will be creating a pictorial build guide but haven't been able to get to that as of yet. 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.