XLR Pin#1 problem - Bridged LM4780 - diyAudio
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Old 25th August 2009, 10:56 PM   #1
GAH is offline GAH  Fiji
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Default XLR Pin#1 problem - Bridged LM4780

Iíve just built a Bridged LM4780 (Audiosector kits) two channel amp thatís being feed by balanced sources. To extract the positive and negative signals for each channel Iíve used XLR to RCA (4) splitter cables made by Peter Daniel of Audiosector. The bridged LM4780 sounds great!

However when I replaced the four RCA chassis mount connectors (two per amp) and splitter cable with two XLR connectors I get severe hum! (XLR chassis - Pin #1 is connected to both Signal Grds on the PCB and Pin #2 & #3 to respective Signal Inputs on the PCB). I did manage to rectify the hum by also connecting Pin #1 (both XLR chassis connectors) to the Chassis Grd as well as Signal Grds on the PCBs. When I disconnected the Signal Grds on the PCB the hum returned.

Although Iíve cleared the hum, I question the correctness of splitting Pin #1 to both the chassis grd and signal grd on the PCB. In theory isnít that suppose to cause a ground loop but in my case it doesn't?

Iím also puzzled that one set of XLR cables that I have cause severe oscillation when connected direct to the amp, even when using the above rectified hum wiring. When this cable is connected between the CD and Preamp I have no hum or oscillation problem. This set of XLR cables has no shield but uses three solid core cables (DNM Reson) for each pin. I did notice however that the casing on these connectors is insulated from the casing of chassis XLR connector. I donít understand why that would be problem but thatís the only differentiation I found between the two sets of XLR cables I have?

Please let me know if Iíve wired Pin #1 correctly to my amp and why would my insulated XLR cables be a problem when connected directly to my LM4780 amp.
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Old 26th August 2009, 12:07 AM   #2
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For correct balanced mode, pin 1 ought to be connected to the chassis only, and not to any circuit ground. Trying to mix balanced and unbalanced signals and sources is always an area for problems, UNLESS you use transformers.
Directly connecting XLR to RCA is an electrical fudge that *might* just work on *some* equipment. Good quality 600:600 transformers will solve all such problems.

The metal shell of XLR plugs is conventionally not connected to anything.

XLR-XLR cables with no shield are PA speaker cables, not suitable for line level signals.
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Old 26th August 2009, 01:37 AM   #3
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For reference, please check the schematic here: http://audiosector.com/lm4780%20amp.pdf

In bridged mode the amp needs reference to ground and severe hum will result if ground connection from a source is disconnected. I had the same problem in my setup.

With the unbalanced amp you also connect signal ground to chassis ground and when it's done properly ground loops are no problem.

For more info check the thread here: Commercial Gainclone kit- building instructions

Those amps are so called "minimized" circuits and are more sensitive to quality of connection then more "conventional" equipment
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Old 26th August 2009, 02:23 AM   #4
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Default XLR Pin #1 problem - Bridged LM4780

Thanks gentlemen. But I'm still not clear if I should be concerned with the way I've wired the XLR connector?

As mentioned everything works fine when using balanced to unbalanced connection (using 4 RCAs) but substituting chassis connectors to XLR (2) I get hum if I don't connect Pin #1 to Signal and Chassis ground. In other words the only thing I'm changing or eliminating is the RCA chassis connectors ( and XLR/RCA adaptor) for XLR connectors. The only way to get a hum free sound is to wire the XLR chassis connectors as I do with the RCA connectors. My sources (CD player and Preamp) are all earthed (at mains) and are interconnected using balanced cables only.

Incidentally I use a star power grounding where I connect a thick copper wire between the OGs of both amp boards. Mid point on this wire I connect PG+&- from each rectifier boards. A wire with a 10ohm resistor is connected from here to chassis/mains earth.

Peter according to your circuit diagram I need to connect to Signal Grd on the PCB. Is it proper to connect Chassis XLR pin #1 to Signal Grd and Chassis Grd?
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Old 26th August 2009, 05:53 AM   #5
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I've always had to connect Pin 1 to signal ground, and pin 2 and 3 to each input half on the bridged amp. A single connection at signal ground did it for me, no chassis ground was needed (I did not have a chassis). I was using Peter's boards in a BPA arrangement, 4 chips total, 2 per channel, running of balanced soundcards.

I did get some hum, but that was because the soundcard was not operating in true balanced mode, there was just a ground lift on the cold pin (pseudo-balanced). Once connected to a soundcard with an actively balanced output, all problems disappeared.
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Old 26th August 2009, 06:14 AM   #6
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XLR pin 1 needs to be connected to signal ground on amp board.
Chassis needs to be connected to Earth ground from your power entry module.

You may connect signal ground through 10R resistor to chassis ground, but but that should not affect the humming.
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Old 28th August 2009, 01:47 AM   #7
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Quote:
Originally Posted by Peter Daniel View Post
XLR pin 1 needs to be connected to signal ground on amp board.
Chassis needs to be connected to Earth ground from your power entry module.

You may connect signal ground through 10R resistor to chassis ground, but but that should not affect the humming.
Thanks Peter. What you suggested above is the wiring procedure I used when using unbalanced i/ps and now balanced i/ps. As mentioned the only thing that I've changed was to XLR chassis connectors which require me to link Pin#1 to chassis as well as PCB signal grd. In essence isn't this the same when using RCA chassis connectors and your XLR/RCA adaptors? Your XLR/RCA splitter cable is bridging pin 1 to chassis. It seems from a theoretical standpoint this isn't correct when using balanced connectors, but when using RCA chassis connectors and a XLR/RCA splitter cables its considered ok. I don't see that there is a difference.
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Old 28th August 2009, 02:15 AM   #8
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Quote:
Originally Posted by GAH View Post
Your XLR/RCA splitter cable is bridging pin 1 to chassis.
My XLR --> RCA adaptor allows to use stereo amp with balanced signal, which converts it into mono balanced operation mode; whatever I did with pin 1 was absolutely necessary: http://www.6moons.com/audioreviews/a...r/patek_4.html

This is basically the same idea as described on page 7 of F4 manual: http://www.firstwatt.com/downloads/f4_om.pdf (Mono Balanced Operation feature)
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Old 28th August 2009, 03:18 AM   #9
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Quote:
Originally Posted by Peter Daniel View Post
My XLR --> RCA adaptor allows to use stereo amp with balanced signal, which converts it into mono balanced operation mode; whatever I did with pin 1 was absolutely necessary: http://www.6moons.com/audioreviews/a...r/patek_4.html

This is basically the same idea as described on page 7 of F4 manual: http://www.firstwatt.com/downloads/f4_om.pdf (Mono Balanced Operation feature)
Thanks Peter. I have no doubt what you've done is correct, I just want to make sure and understand what I'm doing is correct as well. So connecting XLR Pin #1 to Signal Grd and Chassis Grd with this circuit should be considered absolutely necessary as well. It certainly gives me no hum as it does with your XLR/RCA adaptor.
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Old 28th August 2009, 02:28 PM   #10
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Connecting pin 1 to SG is absolutely neccessary (in this particular setup). Normally I do not connect SG to chassis.

The chassis is connected to power entry module Earth ground and the center of copper wire between the OGs (of both amp boards) connects to chassis through 10R resistor.
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