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Well-known member
Mar 13, 2006
Wolfenbüttel - Germany
I would like to provide you with some files, information and recommendations for the NU FEDERAL 864 built based on our new developed pcb [Zayance & bernbrue].

The NU FEDERAL is our version of the famous Federal Limiter  AM 864/u. We replaced the original 6SK7/6SN7/6SQ7 tubes with 7-/9-pin glass tubes (2x 6BA6, 12AU7, 6AV6), changed the power supply and added a few options like input pad, variable output pad,  balancing trimpots, relay based true hardwire bypass, variable attack and release time, ratio and threshold pots, stereo link and so on. Here is a short video with the NU Federal on drums.


PCB's are available!! Click below.

Finished unit


Stuffed  PCB

BOM and Schematic

BOM: https://docs.google.com/spreadsheets/d/1494R_j2GwApoZcIZ743OuftWG7Q_4nS62gMaNTXq2zE/edit?usp=sharing

Schematic & BOM as PDF



fpd files will be available soon.

3HE Frontpanel  available here:

2HE frontpanel (+ enclosure, IEC, XLR etc.) available here:


Powertransformer (stereo unit)
Primary: 2 x 115V
Secondary: 2 x 170-200 VAC 100mA, 2 x 6,3 VAC 2A, 1 x 9 VAC 1A

available here:

any decent 1:1 or 1:4 transformer with secondary centertap  (test what you've got at hand)

4:1 (10K/600R) transformer with primary centertap, typically suitable for double Triode push pull circuit. The Edcor XSM 10K/600R works, the XPP 10K/600R might be better (not tested)

!!! Forum member AVDO makes fantastic and price worthy transformers  for this project. Contact him and support his good work!!,!
Here you go: https://groupdiy.com/index.php?topic=62494.0
My personal review of his transformers: https://groupdiy.com/index.php?topic=65692.0

Heater wiring
Before you solder the tube sockets you have to solder the screw connectors for the heater supply from underneath. Only the heater connector on the top right is soldered from component side.

Fellow member “Fiat_cc” found out, that a 12AU7 as output tube works fine when changing the heater wiring for this tube.

fiat_cc said:
For anybody who is interested in running this unit with the 12AU7 tubes (as a closer match to the original Federals 6SN7GT) with correct heater voltages, the mod is very simple! Just put a jumper wire from each terminal of the 12au7/12ax7 heater terminal block (this ties pins 4 and 5 together), then solder one leg of the heater supply directly to the pcb pad for pin 5, and the other to pin 9. A 12ax7 will provide too much gain running this way I think, so try the 12au7.

Measuring points
The small pads with letters from A to H are test points for measuring voltages against ground (G). Next to the tube sockets there are dots indicating pin 1 of the tube.
Here are the voltages from my prototype:
B+ = 215 V

A= 8,35 V
B= 208 V
C= 208 V
D= 155 V
E= 0,58 V
F= 0,58 V
G= 210 V
H= 210 V
Use the trimpots for aligning/balancing the anode/cathode voltages. Trimpot R9 (500R) is a bit small and should be increased to 2,5K or 5K.

Reducing potentiometer  range
The original Federal Limiter was a unit with fixed settings for attack/release and trimpots on the back for threshold and ratio, set and forget so to say. The calibration procedure is described in the original manual. When turning threshold and ratio to extreme settings (fully clockwise),  the unit (and I suppose the original unit as well, maybe someone can confirm) will start to motorboat.  These settings are just ridiculous and completely unusable. So the range of useful settings can be found more or less in the first quarter of the pot range. I strongly advise to "tame" the pots for:

Threshold Limiter: 100 K log pot plus a 400K resistor in series
Current control: 1K log pot with a 1K3 resistor strapped across legs 1 and 3. (resulting in ~ 600 R total resistance).


2x 6BA6 (EF93)
6AV6 (EBC91)
12AU7 (ECC82)*

* The 12AU7 is supposed to be very similar to the 6SN7 used in the original unit. Personally I recommend  the 12AX7 (ECC83)  in this circuit. Please check the “heater wiring “ passage when using the 12AU7 !!

Any 200uA to 1mA DC meter will work. I successfully tested a Sifam R32 standard VU meter as well.

Reading stuff

Original manual: http://www.preservationsound.com/wp-content/uploads/2014/02/Federal-AM-864-U-Manual-copy.pdf


Update 5th of March:
Has anyone built it yet? I have my second channel running. Really nice what it does. Wiring needs to be cleaned up and then I'll close the lid.
Some observations:
- buy a bunch of 6BA6 tubes. I had a NOS pair in and the meter barely moved up.  With another pair the meter moved up as it should.
- when linking two channels the already slow attack/release time doubles.
- the meter trimmer is a must, since the meter is very sensitive to the threshold/ratio settings. It's recommended to find suitable settings first, switch in bypass and trim the meter to zero.

update 17th December
This compressor is indeed very special. I tested it for a few days now on different musical material (bass, drums, vocals etc.). It takes some time to understand how it reacts and what it does, since threshold and current control interact somehow. The meter needs to be recalibrated when using extreme settings. It's recommended to use a 6x2 or 4x3 Lorlin for the current control in order to have reasonable fixed settings. The threshold pot needs to be "tamed" as well.  A 200uA DC meter works fine with R10(240 Ohm). Revision 1.1 will have a 1K pot in series with R10 in order to calibrate the meter. An ordinary VU meter without an internal rectifier can be used as well.  The variable time constants (attack & release) we implemented are very useful, especially the shorter release times. I used a 1:1 input transformer (Haufe/Malotki) without input pad. Output transformer is a 10K/600R Edcor.  While I'm testing the unit and searching for some improvements, Tony includes these in the pcb layout revision 1.1.  Boards will be available after Christmas. Stay tuned!
Today I had some time to measure the NU Federal. I used RMAA in loopback mono mode, inserted the NU federal into my ADT-5MT mixer and tried to get unity gain with the input/output controls.  The threshold and ratio pots were turned fully counterclockwise so that at least the meter showed no gain reduction.

As you can see, the results for noise are very convincing for a tube unit. The frequency response with sidechain disconnected (pulled the 6AV6) is absolutely flat. The frequency response shown here is with 6AV6 inserted. The values for THD are quite high, mainly caused by the Edcor XSM transformer. Yes, it's a colour box.
Wow cool project!!
Just finished mine on turret board from scratch... And I am not 100% happy with the sound yet.

Will you version be cleaner than the original?
What transformers would you use?
We spend about 3 month in designing and optimising the pcb layout.  The pcbs are still untested. I'll get them tomorrow and build the Limiter  in the next few weeks.  We hope that everything will work out fine and that our rev. 1.0 boards will be the last revision (as we  had it with the sta level pcbs)