Gold

Testing Unbalanced Devices on AP Portable One
« on: September 04, 2018, 11:38:49 PM »
I rarely test unbalanced devices on my bench. I have a consumer stereo preamp that a friend asked me to look at. RCA In and Out. Two prong AC power cable. The shell of the RCA cables is connected to chassis. No chassis connection on the power cable.

I set up the Portable One as described in the manual. I made cables that have RCA tip to XLR pin 2 and RCA shell to XLR pin 3. Shield is connected to XLR Pin 1 with NC on the RCA connectors. The Portable One Generator is set to 40Ω Unbalanced. HiZ Input.

I was getting very strange results. It was like it wasn't grounded. When you touched the device there was static discharge and tons of 60Hz hum. I didn't believe what I was seeing was true so I pulled out a "bump box" bi directional -10dbV to +4dBu type thing. Electronically balanced. When I put that between the DUT and the Portable One everything behaved as I expected.

What's going on?


scott2000

Re: Testing Unbalanced Devices on AP Portable One
« Reply #1 on: September 05, 2018, 03:17:55 AM »
I know things can get weird when feeding unbalanced and not shorting pins 1 and 3 together if the source is transformer balanced..... but, if the manual says to wire up things like you say, that doesn't seem to be the case.....

Also....Are you sure the shield isn't exposed and maybe making contact at the rca end??? I always  shrink a tiny section when I leave the shield off on ends just to make sure....

Wonder if it has something to do with that pin 1 problem deal I've heard about.... I need to read up more on that actually ...



« Last Edit: September 05, 2018, 03:28:13 AM by scott2000 »

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #2 on: September 05, 2018, 04:54:04 AM »
I know things can get weird when feeding unbalanced and not shorting pins 1 and 3 together if the source is transformer balanced..... but, if the manual says to wire up things like you say, that doesn't seem to be the case.....

I tried grounding the chassis.  Which is the same as tying RCA  shell and XLR Pin 3 to ground.  Checked continuity everywhere.  No difference.

Quote
Also....Are you sure the shield isn't exposed and maybe making contact at the rca end??? I always  shrink a tiny section when I leave the shield off on ends just to make sure....

I’m sure.  Shrink up to where the conductors are soldered. Checked for continuity between shield and anything. Flexed the cable and connectors for a torture test.

EmRR

Re: Testing Unbalanced Devices on AP Portable One
« Reply #3 on: September 05, 2018, 10:12:05 AM »
Hmm, I don't think I've seen that with my ATS-1. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #4 on: September 05, 2018, 11:10:11 AM »
Hmm, I don't think I've seen that with my ATS-1.


Same situation, floating chassis with unbalanced signal return tied to chassis? Generator XLR pin 2 to signal +, XLR pin 3 to return, shell, chassis. Same thing for anylizer input. Just double checking.

I have the outputs in the back feeding a scope. Maybe I’ll try disconnecting those.

JohnRoberts

Re: Testing Unbalanced Devices on AP Portable One
« Reply #5 on: September 05, 2018, 11:22:28 AM »
I rarely test unbalanced devices on my bench. I have a consumer stereo preamp that a friend asked me to look at. RCA In and Out. Two prong AC power cable. The shell of the RCA cables is connected to chassis. No chassis connection on the power cable.

I set up the Portable One as described in the manual. I made cables that have RCA tip to XLR pin 2 and RCA shell to XLR pin 3. Shield is connected to XLR Pin 1 with NC on the RCA connectors. The Portable One Generator is set to 40Ω Unbalanced. HiZ Input.

I was getting very strange results. It was like it wasn't grounded.
it isn't...  2 wire line cord is double insulated...  Some consumer gear will internally cap couple between chassis and presumed neutral (fat blade on 2 wire plug). This cap can cause mischief especially if outlet is miswired with neutral and line swapped.

I recall lots of hum problems trying to interface with consumer VCRs back in the 80s-90 when I was doing cheap SMPTE lock between audio cassette and consumer VCR machines.
Quote
When you touched the device there was static discharge and tons of 60Hz hum. I didn't believe what I was seeing was true so I pulled out a "bump box" bi directional -10dbV to +4dBu type thing. Electronically balanced. When I put that between the DUT and the Portable One everything behaved as I expected.

What's going on?
If a balanced interface works, it sounds like your problem is solved.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #6 on: September 05, 2018, 11:26:55 AM »
This cap can cause mischief especially if outlet is miswired with neutral and line swapped.

The AC plug that was on there wasn’t polarized. I tried flipping it with no change . I installed a polarized plug. Brown wire to live Blue wire to neutral.

Now that you mention it they were re doing the power on my floor. My studio power is a separate run but the shop power was shared and wonky. I’ll double check that.
« Last Edit: September 05, 2018, 11:30:19 AM by Gold »

JohnRoberts

Re: Testing Unbalanced Devices on AP Portable One
« Reply #7 on: September 05, 2018, 12:06:34 PM »
FWIW 2 wire unbalanced consumer gear can't have safety grounded chassis, and proper balanced interfaces so audio ground (0V) is a bit of a trapeze act.

Good luck

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

scott2000

Re: Testing Unbalanced Devices on AP Portable One
« Reply #8 on: September 05, 2018, 01:37:22 PM »
I tried grounding the chassis.  Which is the same as tying RCA  shell and XLR Pin 3 to ground. 

Still wouldn't be the same as grounding pin 1 at the output xlr?

Kinda hard to visualize for me this second and, it really only matters if your AP rig is not electronically balanced I think.....

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #9 on: September 05, 2018, 02:00:02 PM »
Still wouldn't be the same as grounding pin 1 at the output xlr?

Kinda hard to visualize for me this second and, it really only matters if your AP rig is not electronically balanced I think.....

XLR pin 1 is ground. It is the same as the ground pin on the power cable as it should be. AP’s all have a fancy and famous transformer negative impedance thing going  on.  The inputs and ( I think) outputs float.

Same results with just the input cable connected or just the output cable connected.


scott2000

Re: Testing Unbalanced Devices on AP Portable One
« Reply #10 on: September 05, 2018, 03:13:40 PM »
XLR pin 1 is ground. It is the same as the ground pin on the power cable as it should be. AP’s all have a fancy and famous transformer negative impedance thing going  on.  The inputs and ( I think) outputs float.

Same results with just the input cable connected or just the output cable connected.

Sorry...I meant grounding pin 3......i.e, .... tying pin 3 and 1 together at the xlr outputs

....Please update on what you get sorted.... These ground things fascinate me....


Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #11 on: September 05, 2018, 03:31:03 PM »
Sorry...I meant grounding pin 3......i.e, .... tying pin 3 and 1 together at the xlr outputs

....Please update on what you get sorted.... These ground things fascinate me....

If there is a potential difference between XLR pin 1 and ground anywhere else that is the definition of a pin 1 problem. There is no pin 1 problem.

Audio1Man

Re: Testing Unbalanced Devices on AP Portable One
« Reply #12 on: September 05, 2018, 03:40:38 PM »
Hi Paul
You need to make a special cable.  The shield connects only to the SHELL of the RCA. AP makes this cable  and it come with RCA to BNC adapter.
XLR to RCA wire XLR 2 to center of the RCA
XLR to RCA wire XLR 3 to SHELL of the RCA
XLR to RCA wire XLR 1 to SHELL of the RCA
Duke

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #13 on: September 05, 2018, 03:52:18 PM »
Hi Paul
You need to make a special cable.  The shield connects only to the SHELL of the RCA. AP makes this cable  and it come with RCA to BNC adapter.
XLR to RCA wire XLR 2 to center of the RCA
XLR to RCA wire XLR 3 to SHELL of the RCA
XLR to RCA wire XLR 1 to SHELL of the RCA
Duke

Hi Duke,

So the cable example in the manual assumes a grounded chassis?

When I tried grounding the chassis of the DUT it didn’t make a difference.  RCA shell is tied to chassis. I didn’t use XLR pin 1 on the AP tonconnect  to because with the connector there is no access. I used a ground post on another piece of test gear and checked continuity between XLR pin 1 on the AP and the ground post. Unless I’m missing something that is equivalent to tying RCA shell to XLR pin 1. This is massive 60Hz I’m getting  not an elevated noise floor level.

I will try making the cable you suggest and report back.

Re: Testing Unbalanced Devices on AP Portable One
« Reply #14 on: September 06, 2018, 09:37:24 PM »
The pin 1 on the XLR _is_ the ground you want for signal references if you're unbalanced. Not sure why you can't access that pin - maybe buy a clean XLR and wire it the way it's intended?

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #15 on: September 06, 2018, 10:05:12 PM »
When I redo the cable I will bring the shield out  for access to XLR pin 1.  It’s not a few extra dB of hum. The hum is around -12dBV. With no signal input. The ground post I used measured 0 ohm with a 4.5 digit meter to XLR pin 1.

Audio1Man

Re: Testing Unbalanced Devices on AP Portable One
« Reply #16 on: September 06, 2018, 11:44:24 PM »
Hi Paul
I worked for AP for many years. The INPUT signal is pin 2 & pin 3 for BALANCED or UNBALANCED, only the cable end to the DUT needs to be changed. The generator source and BALANCED or UNBALANCED mode as well as the cable to the DUT must be changed as needed. Some time the generator GROUND may need to FLOAT.  As I stated the shield pin #1 should attached to the wire from pin 3 @ the RCA CONNECTOR only for DUT OUTPUT TESTING. From the “MASSIVE” hum is the product working? I sent you a pvt message you can call me. Duke

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #17 on: September 07, 2018, 02:02:58 AM »
Hi Paul
I worked for AP for many years. The INPUT signal is pin 2 & pin 3 for BALANCED or UNBALANCED, only the cable end to the DUT needs to be changed.

Yes, that’s what I have done but when I made the cables I clipped the shield at the RCA ends. That’s how it was drawn in the manual.  I will redo those and bring the shield out in a Teflon tube so it can easily be soldered and desoldered  to the RCA shell.

Quote
The generator source and BALANCED or UNBALANCED mode as well as the cable to the DUT must be changed as needed. Some time the generator GROUND may need to FLOAT.

I will do the same with this cable.

Quote
From the “MASSIVE” hum is the product working? I sent you a pvt message you can call me. Duke

When I put a “ bump box” active balanced to unbalanced converter between the DUT and the AP the DUT worked as expected. One channel is low in level which was the problem but no massive hum.

The bump box pin 1 of the XLRs is grounded to the power cable ground pin  as well as the shells of the RCA connectors. I checked the bump box grounding scheme with nothing connected to it.

Thanks for the offer of a call. I will check out a few more things before calling. I looked for my outlet tester but couldn’t find it. I’ll get another. My sniffer says it’s probably a crossed live and neutral power problem like John R said.
« Last Edit: September 07, 2018, 02:22:39 AM by Gold »

Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #18 on: September 07, 2018, 06:54:45 PM »
New test cables but no new tests. I only work in stereo. My studio is all wired with 5 pin xlrs. I have 3 pin to 5 pin XLR break out cables from the AP. Most of my stuff uses the 5 pin xlrs. When I need something else I use 5 pin XLR to whatever I need adaptors.


Gold

Re: Testing Unbalanced Devices on AP Portable One
« Reply #19 on: September 14, 2018, 02:33:54 PM »
Ding! Ding! Ding! We have a winner, John R.  Live and Neutral reversed at the AC receptacle. I reversed Live and Neutral in the extension cable, and put a hard to miss label on the cable. All is right with the world now.  The DUT is behaving as expected now. Thanks for everyone's help.


 

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