UA 610 Preamp - Transformer Substitute

[Winston OBoogie]

OK good job, that looks more like it.
Give it a whirl

 

[Winston OBoogie]

P.S.  thanks for chiming in PRR . I'm sure you're right about it being leakage.

 

[sr1200]

@dawsonaudio the last schematic you put up, is this working?  I've been wanting to put one of these together for a while and haven't been able to find a viable schematic.  Good job btw.

 

[Winston OBoogie]

I've been wanting to put one of these together for a while and haven't been able to find a viable schematic.

The schematic contained in the 1st post of this thread is correct. 

 

[dawsonaudio]

I haven't had the chance yet to finish assembling the pre amp.  Been busy this week.  I will finish this weekend...everything seems to work as far as the voltage readings I'm getting.  I'll post some images.

Looking forward to testing it out.  Thanks again for the help here.

Nate

 

[dawsonaudio]

Here are a few images of my 610 mic preamp.  Just had some free time to get back to this project.

I plugged in a Shure sm57 and the sound is very thin, almost like an old am radio, there is a slight buzzing underneath the audio.  So I disconnected the eq section as previously suggested in an older reply to eliminate the eq section.  Wwhen I started to turn up the volume knob, 'shreaking' came through the headphones etc...quickly turned the knob back down.  Hmmm, not sure what I did wrong here.

Any ideas would be appreciated. 

Thanks,
Nate

 

[dawsonaudio]

Inside

 

[dawsonaudio]

Underneath

 

[radardoug]

Transformer in? You only have one leg of the input connected. You need to check the wiring on the input transformer.

 

[dawsonaudio]

+ side (red wire) is connected to the 47 ohm resistor as per the schematic.  It might be hard to see but that connects to the input transformer as well.

 

[dawsonaudio]

Just pulled electronics out of box...I noticed I didn't connect the secondary of the output transformer correctly.  It's a UTC A25.  Looks like pin 3 and 4 needed connecting to each other...is that correct?  Just one thing I noticed.

 

[Winston OBoogie]

... pin 3 and 4 needed connecting to each other...is that correct? 

Yes.

 

[dawsonaudio]

Not trying to hack into your original schematic.  Just trying to simplify the circuit without the eq at this time.  Will the circuit work this way as I've eliminated some of the eq components...probably too many! 

I've got the electronics out on the table with a couple xlr pigtails connected directly to the transformers.  I had wired both in/out transformers incorrectly.  So know they should be wired accordingly.  Just not sure about the ground connections from the pigtails...both those would normally be grounded to the xlr panel mount connector/s?

I've got the lf and hf switches disconnected at this time.  I'd like to just connect the 250k volume pot and test the mic preamp that way.    Only components not installed on the board are C6 and R22.  Those are on one of the switch assemblies.  I just didn't want to continue connecting things....sorry if I've hacked the schematic horribly.  Just trying to eliminate the eq section entirely to get things working in the most basic form if that's the correct way of saying it.

Thanks again for all your help.

 

[dawsonaudio]

Out of the box

 

[Winston OBoogie]

Hey Nate,
Yes that looks correct for an eq-less version.  Good idea I think to get it happening like this first.
Is your HA-100 a UA reissue type from the LA-2a?

 

[Winston OBoogie]

Regardless, I think a better way to hook up your input transformer would be to use the strapping for 200 ohms which would be: signal + on 2, link 3&4, signal - on 5.  And l would lose that 47 ohm resistor. 
Good luck

P.S.  secondary loading resistor could come down a bit if there's any ringing:  500k - 600k would be a good spot to start. 

 

[dawsonaudio]

It’s an original ha100x I’m pretty sure.  I’ve had it for about 20 years not sure when they were reinstated.

Audio is passing fine now just massive amounts of 60hz hum. Not sure if that is loop or heater induced. 

And I’ll try rewiring the transformer

 

[dawsonaudio]

I rewired the transformer but still have the hum.  I notice that when I drag the electronics away from the power supply and ammo box, the hum starts to dissipate.  I'm thinking the power transformer is the cause.

Would it be advantageous for me to build a separate power supply box to eliminate the hum?  Also, I'm not sure if the aluminum board that all of the components are mounted to might be attracting noise into the circuit. 

Or maybe my component layout needs to be better.

 

[Winston OBoogie]

That the hum decreases when you move the aluminium plate away from the ammo box is key.
The steel ammo box is a magnetic conductor and will couple leakage flux from the mains transformer into sensitive parts of the circuit unless there is a non-conductive spacer between the transformer and the box.  The radiated field can be reduced easily with distance.
I'd also look at the wiring of your heaters.  I'm not sure if your are running them AC or DC (I would suggest DC) but, either way, it'd be best to run these wires flat against the aluminium plate, beneath all your point-to-point signal wiring. 

 

[radardoug]

The wiring on that input transformer looks bad too. You cant just pick up a ground at the nearest point. you have to follow a proper grounding scheme. And no shielding on the wire to the grid.