[BUILD] NU FEDERAL AM-864 PCB OFFICIAL BUILD & HELP THREAD

[dmp]

Here is a short video with the NU Federal on drums.

https://www.youtube.com/watch?v=0eJVCCYApOw

The video plays in fast forward when I try to watch it today.  Does this happen to anyone else?

 

[scott2000]

Yes....

I thought it was some new drum & bass stuff......

 

[bernbrue]

Oh, need to check what’s wrong there. At least a few people watch the video.
Cheers
Bernd

 

[bernbrue]

Fixed the video demo!! I love this compressor!

 

[hazel]

I just got a pair of v1.0 boards and some tubes.

I'm planning a stereo AM864 with one set of controls and I have some questions:

- When stereo linking what parameters are still individual operating? i assume it's Input and Output gain/att? I'd do a complete stepped control for everything

- It'd be great to have a unique 200uA GR meter. It's a crazy idea to just link both meter pads on PCB and feed meter?

 

[bernbrue]

Honestly I wouldn’t do that. For a mono channel  you already need a dual deck input potentiometer. The link between two channels guarantees that attack and release times are more or less identical. All other parameters need to be adjusted by hand. This unit is a compressor with lots of character and definitely not a precision tool for a mastering chain. You may check our “NU VARIMU 436” with steppend controls for each parameter.
Bernd

 

[hazel]

Honestly I wouldn’t do that. For a mono channel  you already need a dual deck input potentiometer. The link between two channels guarantees that attack and release times are more or less identical. All other parameters need to be adjusted by hand. This unit is a compressor with lots of character and definitely not a precision tool for a mastering chain. You may check our “NU VARIMU 436” with steppend controls for each parameter.
Bernd

Thanks for answering!

Sorry I didn't explain myself right.

I know Federal is a very colored compressor so not for mastering but stereo processing.

It would be feasible to make all parameters stepped stereo ganged rotary switches? I know I will be using 4 deck ones but is not an issue as long as I'm starting a Barry Porter Net EQ.

Channel matching is just measuring resistors and matching tubes and voltages accordingly

 

[bernbrue]

Basicly it is possible, although matching will be a tough job here.
Bernd

 

[fiat_cc]

Sorry if this is astupid question or has been asked before...

I bought Vintage Meters VM-B10R meters, but I can’t find any specs on the LED to illuminate them, hence I am not sure how to power them. Any ideas?
I have a 9v rail on my transformer. Tap off that after the rectifier diodes? Will I need a series resistor?

 

[Rob Flinn]

Sorry if this is astupid question or has been asked before...

I bought Vintage Meters VM-B10R meters, but I can’t find any specs on the LED to illuminate them, hence I am not sure how to power them. Any ideas?
I have a 9v rail on my transformer. Tap off that after the rectifier diodes? Will I need a series resistor?

Probably, unless it has a resisitor already in series.  Yes, tap after rectifier diodes, and after smoothing. Without a spec most led's can take 5mA.  V=IR will give a resistor value.

 

[fiat_cc]

I opened the meter up and had a look.  The little LED board in there is loaded with several surface mount LED's and resistors.  The back of the meter says 12VDC so I hooked it up to the relay V+ rail, just post filter cap, and I get 12V and they light up beautifully.

I built this for a friend who found this page online.  I don't currently have any balanced XLR gear at home in order to test things properly, so I completed the unit, and gave it to him to have a play with.  There is a little bit of hum, which I suspect is due to filament wiring.  I didn't notice until after I had assembled everything, that the filaments swap polarity from tube to tube.  Mine are all wired with the same polarity, so that will be my first port of call to try and correct the hum.  He also thinks he is not getting enough output.  With input and output both close to full, it is sounding nice, but he thinks it is too quiet.  Voltages are all pretty good, if a little on the high side (the transformer is rated for 230V on the primary, and my wall voltage was 253V when doing measurements).  I can't quite get B and C points to match.  They are within 2 volts, but that is as close as I could get them.  Everything else matches up.  The controls appear to do what they should, but at some settings the unit starts to motor boat, and yes, I did include the fixes to try and remove that.  Any thoughts?

 

[fiat_cc]

Another photo, although I hadn't quite finished in this photo.

 

[bernbrue]

Nice build. 
In order to reduce hum you should move the transformer a bit away from the XLR. You could also test another pair of 6AV6 tubes. I had problems finding a good pair. Of course you should check grounding and polarity of the heater as well.
I suppose you’ve used a 12AX7/ECC83 for output?
Cheers
Bernd

 

[fiat_cc]

Thanks Bernd.
Strangely the channel whose XLR is closest to the transformer is actually quieter in terms of hum than the other channel.
I agree though, that it is not an ideal layout.  I didn't realise the chassis was pre-cut.  It's difficult to find somewhere else to put the transformer. I could move it closer to the front of the case, down that channel in the middle, but I'm worried then that I'll start getting too close to the tubes and the signal chain on the board.  I had wanted to put it vertically on the back panel of the chassis, over near the IEC inlet, but it wouldn't fit in a 2RU chassis.

I will look at heater polarity and check grounding as a starting point. I think I have another pair of 6AV6 tubes, so I'll try those too.  Yes, I used reissue Tung Sol 12AX7 tubes.  I have plenty of 12AX7's around as I work on guitar amps quite a lot, so it's easy to try different tubes there too.

 

[fiat_cc]

OK, so I rewired the heaters and that did help marginally in reducing the hum, but it is very much still there.  One question I have is around 'ground' and 'PE' on the schematic.  Obviously PE is the physical chassis earth, and ground is the ground return path for the circuit.  I currently also have 'ground' attached to the chassis.  I've tried at the same point, and separate points to the physical earth, but I'm wondering whether perhaps I should lift that from the chassis?  Also, in the schematic, it shows 'J5' connected to 'ground'.  What is J5?
Another interesting thing is I can't seem to see the noise at the output on my scope.  There is a very small amount of noise visible, but it is not effected by either volume control (which the hum is), and it seems to be quite high frequency, where the hum is sounding to me like 50Hz (but possibly 100Hz). This makes me think it is some sort of ground loop issue, but I'm not really sure.  Another odd symptom is that putting the lid back on the chassis actually increases the hum, rather than reducing it, which is another reason I suspect grounding.  Any thoughts?

I tried unbolting the power transformer and moving it further from the XLR's but there was no discernible difference in noise. I have also identified a very microphonic 6BA6 in one channel which seemed to be causing some of the motor boating symptoms, but I tried swapping it from one channel to the other and there was no real discernible effect on the hum either, although the motor boating did seem to follow that tube.  Also, just how closely do I need to be able to match those 6BA6 tubes?  I seem only to be able to get within about 2V, which is less than 1% discrepancy, so I figure it is OK, but I'm curious.

Last couple of things... I just noticed trawling through the BOM document.  It mentions using thermal paste and an isolator for the LM317. I used thermal paste, but missed the isolator.  Could that be leading to hum somehow? Also, R27 is getting massively hot.

 

[bernbrue]

The Isolator for the LM317 is mandatory!! Also check the wiring of the output transformer.  j5 is a jumper that needs to be connected to ground. I haven’t got the pcb layout in front of me. It should be on the psu pcb.
Bernd

 

[fiat_cc]

Thanks Bernd.
The isolator is easy.  I can sort that out no problem.  Then I need to figure out this J5 business.  I can't see it anywhere.

 

[bernbrue]

Make shure that both, PE and GND are connected to star ground. That’s a point near the IEC connector,  where all ground wires are connected to chassis and IEC main ground. So, each pcb has two ground wires going to star ground!!  I can’t find J5 either.  Seems to be a „leftover“ of  our other projects.
Cheers
Bernd

 

[fiat_cc]

Yep, PE and Ground are currently both connected to the star ground.  I double check all my off board connections to pots and transformers.  Whatever is causing the hum is the same on both boards.  Would the lack of isolation on the LM317 cause hum somehow?

 

[fiat_cc]

No, isolating the LM317 didn't solve the hum issue.  I'm starting to tear my hair out with this.  The hum is present with all 3 of the 7 pin tubes removed, but disappears when I remove the 12AX7, but I've no idea where it's getting in.  I've removed the output transformer from the chassis on one channel, and tried moving it around as much as the leads will let me, but that makes no discernible difference. Both PE and Ground connections measure well under 1 ohm from the board to the chassis, so the earth connections are good.  What else can I be missing here?