AML ez1073 dc on input and output

[bobbeloud]

Hello all, new member here.

I recently purchased a pair of AML ez1073 500 series kits and built them this last week. I work at a studio and was able to take them in yesterday and listen to them against many top dollar preamps. They performed phenomenally when stacked up against Vintage 1073's, BAE's, Chandler TG-2's, API, and more. They sound great and I have no issues with them in terms of functionality.

One thing we noticed is they sounded like they had dc on them when patched, so we checked the units with a meter and they both had 1.2vDC on pin 2 and 3 of the XLR in/outs. When phantom was turned on, they had the proper 48v on the input. This was not a chassis issue, because I took them back home and they did the same thing in my rack and no other 500 preamps that we had at the studio or I have at home had dc on the ins/outs. I went through and checked all the cap and diode orientations and as many resistors as I could read the codes on but everything so far looks like it's in the right spot.

Just as a disclaimer; these were not my first builds, I've done other CAPI and Hairball stuff with no issues, and recapped consoles/taken on smaller tech projects. I feel pretty confident about my soldering skills etc. I did bias them to 90mV and calibrate them. I'm going to keep going through them and checking stuff but I'm curious if anyone else has ever had this issue and where might be the most likely places to check, I'm very happy with the sound of these preamps and even temped to just used them in spite of the issue, but I would like to get to the bottom of it before I order more.

Any help is appreciated, thank you!

Schem is in this document:
http://www.audiomaintenance.com/downloads/ez1073pre_colourbook.pdf

 

[Ilya]

Those are transformer coupled on both input and output. There should be no way for DC from inside the unit to the outer world.
How are you making a measurement? What is your reference ground point?
You can check if the chassis ground and signal ground are tied together. Besides that I can't see any immediate DC path to power rails from input or output.

 

[bobbeloud]

Those are transformer coupled on both input and output. There should be no way for DC from inside the unit to the outer world.
How are you making a measurement? What is your reference ground point?
You can check if the chassis ground and signal ground are tied together. Besides that I can't see any immediate DC path to power rails from input or output.

Thanks for the response, definitely a head scratcher. The weird thing is they're both doing the same thing. I'm referencing ground with pin 1 on the xlr, so measuring 2 or 3 to 1, but referencing chassis ground on the rack gives me the same vDC reading off either pin 2 or 3. And I'm still getting +/-16 on my rack power rails so everything is normal there.

Chassis ground and signal ground are tied together, I'm reading open between PSU common (pin 13) and Analog Ground (pin 5), and 10ohm resistance from either of those to Chassis ground (pin 1).

Another noteworthy development - The vDC that I'm reading on the xlr isn't consistent and drops while the unit is on, almost like a cap slowly discharging? I turned the units on about two hours ago and they read 4.2 vDC on one and 6.4vDC on the other. I've left them on and been checking them and they've both dropped about 2 or 3 Volts, and Cycling Phantom power doesn't put me back to square one, they go right back where they were and continue to slowly drop.

These could all be useless observations but I'm not really sure what I'm looking for, still went through and checked more components and everything I can see looks like it's in the right spot in the right orientation.

 

[Brian Roth]

I suspect some sort of measurement error, but no idea why. Those units are transformer coupled in and out. What do you read across XLR pins 2 and 3 (with phantom switched off)?

Bri

 

[bobbeloud]

I suspect some sort of measurement error, but no idea why. Those units are transformer coupled in and out. What do you read across XLR pins 2 and 3 (with phantom switched off)?

Bri

I’ve got 0 vDC across 2 and 3 on both units

Bobby

 

[Brian Roth]

I’ve got 0 vDC across 2 and 3 on both units

Bobby

That would be correct. So you're seeing an "offset" from XLR pin 2 or 3 to pin 1. Hmmmmm....

What meter are you using? Do you have an oscilloscope with capabilities to measure DC? I ask, because my first cheapo scope (Eico kit) when I was a kid offered only AC measurements.

What happens when you connect a smallish resistor (like 1000 or 10000 Ohms) in parallel with your meter probes and then measure from pin2 or 3 to pin1? IOW, cobble a method to add the resistor from red to black meter leads while also measuring the XLR "offset" voltage. In my world, I'd use two test leads with alligator clips on each end to be able to connect the "dummy load" resistor to the ends of the test probes.

Bri

 

[Ilya]

Chassis ground and signal ground are tied together, I'm reading open between PSU common (pin 13) and Analog Ground (pin 5), and 10ohm resistance from either of those to Chassis ground (pin 1).

I’m not familiar with 500 series grounding conventions, but what you wrote is strange. Looks like a ton of different grounds.
In the end, all grounds should be tied together. Maybe there’s some connection inside the 500 chassis, I don’t know.
Try connecting all grounds together and see what you get. Or you can use signal ground as a reference.

 

[bobbeloud]

That would be correct. So you're seeing an "offset" from XLR pin 2 or 3 to pin 1. Hmmmmm....

What meter are you using? Do you have an oscilloscope with capabilities to measure DC? I ask, because my first cheapo scope (Eico kit) when I was a kid offered only AC measurements.

What happens when you connect a smallish resistor (like 1000 or 10000 Ohms) in parallel with your meter probes and then measure from pin2 or 3 to pin1? IOW, cobble a method to add the resistor from red to black meter leads while also measuring the XLR "offset" voltage. In my world, I'd use two test leads with alligator clips on each end to be able to connect the "dummy load" resistor to the ends of the test probes.

Bri

Using Fluke meters at the studio and an Ideal meter at home. When I put a 1k resistor in parallel with the leads I’m only getting 22 mvDC, which seems way more normal. Without the resistor I’m getting around 3vDC at the moment.

 

[Brian Roth]

I can't explain your numbers, but the dummy load hints it's a measurement error of some sort.

The module has transformer in and out which should preclude any DC offset at in or out.

 

[3nity]

Could this be a problem related to the 24v converter circuit?

 

[bobbeloud]

I can't explain your numbers, but the dummy load hints it's a measurement error of some sort.

The module has transformer in and out which should preclude any DC offset at in or out.

Interesting. We could definitely hear the DC pops and crackles when patching to this preamp, and none of our other 500 units have this offset error so I’d still like to get to the bottom of it. Other than that I really love these things!

 

[bobbeloud]

Could this be a problem related to the 24v converter circuit?

We were wondering that as well. It looks like they regulate the +16 to 8vDC with the 7808 and then use -16 as “ground” to get 24v. So the offset could be the result of ‘ground’ not being 0 in the circuit? But I’m still getting the offset when I reference the chassis ground on the 500 rack itself and my rails are still reading +/-16 when referencing that same chassis ground so I would think the ground is right and the offset is a stray voltage on the unit?

 

[Ilya]

Remove the module from the chassis and check if you still get that offset symptoms.

 

[bobbeloud]

Remove the module from the chassis and check if you still get that offset symptoms.

I do not get the offset when the slot is empty, and I don’t get the offset with any other modules in that slot/any slot. And I’ve gotten the offset with these modules on two different racks so I feel safe so say I’ve eliminated the rack as a suspect.

 

[Ilya]

Chassis ground and signal ground are tied together, I'm reading open between PSU common (pin 13) and Analog Ground (pin 5), and 10ohm resistance from either of those to Chassis ground (pin 1).

I'm still not able to understand how this is possible.
On the schematic, the connection between pins 5 and 13 is clearly shown. Chassis is connected with RC in parallel. I can't imagine a situation where pin 5 and 13 don't show continuity, and at the same time both show 10R to chassis.
I repeat once again. Try tying 5 and 13 together and see what you get. Then try tying them both to chassis (short 10R resistor) and see what you get.

 

[bobbeloud]

I'm still not able to understand how this is possible.
On the schematic, the connection between pins 5 and 13 is clearly shown. Chassis is connected with RC in parallel. I can't imagine a situation where pin 5 and 13 don't show continuity, and at the same time both show 10R to chassis.
I repeat once again. Try tying 5 and 13 together and see what you get. Then try tying them both to chassis (short 10R resistor) and see what you get.
View attachment 97921

My mistake, 5 and 13 are tied together. I don’t know if ‘open’ was the right word there. I meant they show no resistance with each other, and they both show 10 ohm to chassis (pin 1), just like you said. I’ll jump that 10R resistor this evening and let you know what happens!

 

[Ilya]

I don’t know if ‘open’ was the right word there.

"Open" means no connection.