AML EZ3415 build trouble shooting

[dwrockdoctor]

Hi all, new member here!

For the past few weeks I've been trouble shooting what amounted to a rather difficult pair of AML Neve 3415 pre builds but through perseverance and a lot swearing I got it to the point where it's passing sound from microphone through to my DAW interface in its expected way but to get there I had bypass the output transformer entirely and take an unbalanced output from where the signal normally goes into the output transformer. I've built these with the six position gain boost rotary and external gain potentiometer (fader in the original circuit but I used a 5K pot for mine). Also put a switch in to switch between mic and line level.

With the output transformer in line the TP8 and TP9 bias test points swing wildly with its voltage readings and I can't get a proper reading to bias properly. Sound passes through okay but soon after powering up it starts making a low frequency pulsing oscillation. Unplugging the gain boost connector stops the oscillations and plugging it back in briefly stops it from happening but resumes pretty soon after again. The oscillation pulses faster at higher gain settings too. None of this happens with the output transformer out of circuit.

With the OT out of circuit setting bias is straight forward. I haven't set the the other trimmer yet using a 0dbu test tone because I haven't figured out the best way to do that so I set it by ear by singing into the mic until it sounds how it should. I plan to get some test tones and a way to measure the db's coming out of the output soon to dial it in perfect.

I've tested the OT's for resistance across the primary and secondaries out of circuit and got a reading 3.6 ohm on the primary and 34 ohm and 44 ohm on the secondaries. Not sure if maybe I over tightened the screws of the OT's when securing them to the PCB but they still pass sound through. The problem is the millivolt readings on TP8 and 9 with OT in circuit and the pulsing oscillation also when it's in circuit.

They sound great without the OT's in circuit using an unbalanced out (I actually went with an impedance balanced output using low ohm resistors) but I would ultimately like to get them to work with the OT's in circuit as intended. Is there anything I might be missing with what I've described, any help would be greatly appreciated.

 

[Bo Deadly]

So is this the circuit?


Sounds like a feedback issue causing oscillation which is causing havoc on the supply which is why TP 8 and 9 are jumping around.

Double check R33, R34, C16, polarity of C16, and whatever is connected to Gain Boost 2. Disconnect Gain Boost 2 and check it's resistance to ground at various switching positions. If it's too low, the gain will be too high and you'll get instability.

Could also be a grounding issue like a bad solder joint. Are you an expert soldering machine?

 

[dwrockdoctor]

Thanks for that Bo Deadly!

I don’t know if I’d say a soldering machine haha but I’ve successfully made numerous guitar pedals and tube amps so I have a bit of experience under my belt. Mistakes do happen and I had a few dodgy solder points around the input transformer which brought the preamps back to life once fixed. But it only works well without the OT in place.

Makes sense that those components you mentioned could be causing an issue as the modules works work fine without the OT which effectively takes that component path out of circuit that were going to the tertiary winding of the OT. I’ll check all those ones with the multi meter and see if there’s anything dodgy going on.

Another thought was doing the few necessary changes to components and order the carnhill VTB9070 OT and change this into something closer to a 1081 circuit.

At least I have a working module without the OT’s at this point if all else fails! Still sounds really nice as is.

 

[dwrockdoctor]

Also, in the calibration notes for setup of the 3415 module it says apply a 0dbu signal to the input and adjust the trimmer until theres 0dbu on the output but how should the external controls be set? To the lowest gain setting on the gain boost? And where to set the gain fader/pot too? Fairly vague instructions there.

I've only calibrated by ear so far by talking and singing into a microphone and there's plenty of gain on tap even with a Shure SM7b which is notorious for needing a lot of gain and when I dial in the right amount of gain for an SM7b there's is still heaps of gain left on the boost and fader controls so maybe it's set way too hot on the trimmer that sets the gain at the moment. Still not sure why it only oscillates with the OT in circuit but like Bo Deadly mentioned, the wiring around the tertiary winding might have something not right with it also. I really need to figure out the right way to put a 0dbu signal through this thing to calibrate properly.

The original Neve 3415 schematic notes also mention a 1n2 capacitor and 10k resistor needs to be wired across the gain fader/pot when gain exceeds 70db. This isn't mentioned in the AML colour book but I thought it was worth mentioning, maybe the extra capacitor and resistor work to control oscillation in the circuit above 70db? Might be worth a try to see if it helps.

 

[dwrockdoctor]

Thought I'd touch base on how I'm going with the build for anyone that might stumble across this who is having trouble building one of these. I've got it working with the output transformer in circuit, albeit, without the tertiary NFB winding hooked up. A lot of gain no matter how I set the gain trimmer adjustment so I did some more googling and found out about the 3045 module that basically used the same output transformer but it didn't have the tertiary winding that the 3415 had and had a 1k5 resistor and 47pf cap in the nfb circuit between the inverting input and output of the BA440 card in place of the the tertiary nab winding. I got out the alligator clips and quickly strapped across the nearest resistor value to the inverting input and output and now the gain is at a much more sane level and I think I'm getting somewhere. I'll experiment tomorrow and do some more gain biasing and hopefully this build can come to a close and I can get to making some music again hahaha.