I’ve just built a Bridged LM4780 (Audiosector kits) two channel amp that’s being feed by balanced sources. To extract the positive and negative signals for each channel I’ve 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 I’ve cleared the hum, I question the correctness of splitting Pin #1 to both the chassis grd and signal grd on the PCB. In theory isn’t that suppose to cause a ground loop but in my case it doesn't?
I’m 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 don’t understand why that would be problem but that’s the only differentiation I found between the two sets of XLR cables I have?
Please let me know if I’ve wired Pin #1 correctly to my amp and why would my insulated XLR cables be a problem when connected directly to my LM4780 amp.
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 I’ve cleared the hum, I question the correctness of splitting Pin #1 to both the chassis grd and signal grd on the PCB. In theory isn’t that suppose to cause a ground loop but in my case it doesn't?
I’m 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 don’t understand why that would be problem but that’s the only differentiation I found between the two sets of XLR cables I have?
Please let me know if I’ve wired Pin #1 correctly to my amp and why would my insulated XLR cables be a problem when connected directly to my LM4780 amp.
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.
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.
For reference, please check the schematic here: http://audiosector.com/lm4780 amp.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: http://www.diyaudio.com/forums/showthread.php?t=123003
Those amps are so called "minimized" circuits and are more sensitive to quality of connection then more "conventional" equipment
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: http://www.diyaudio.com/forums/showthread.php?t=123003
Those amps are so called "minimized" circuits and are more sensitive to quality of connection then more "conventional" equipment
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?
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?
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.
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.
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.
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/audiosector/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.
Pin #1 problem
After further investigation I found the 10 ohm resistor had become disconnected from the chassis earth. This explains why I had to connect Pin #1 to Chassis and Signal Grd to eliminate hum from my amp. Now with the 10 ohm resistor properly connected to chassis earth (from power star) and Pin#1 connected ONLY to signal ground as you stated I'm hum free. All is sounding well.
Peter, thanks again for your help.
After further investigation I found the 10 ohm resistor had become disconnected from the chassis earth.
This explains why I had to connect Pin #1 to Chassis and Signal Grd to eliminate hum from my amp. Now with the 10 ohm resistor properly connected to chassis earth (from power star) and Pin#1 connected ONLY to signal ground as you stated I'm hum free. All is sounding well.Peter, thanks again for your help.
There was a similar discussion on a subject here: http://www.diyaudio.com/forums/showthread.php?t=117444
10R break resistor - Why is necessary?
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.[/QUOTE]
Peter, why is the 10R resistor necessary? By including such a resistor is there any compromise to safety should a power fault occur?
Regards
Glen
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.[/QUOTE]
Peter, why is the 10R resistor necessary? By including such a resistor is there any compromise to safety should a power fault occur?
Regards
Glen
Thanks Peter
Andrew T made a comment about safety when using a break resistor. See below. What's your opinion on this?
The 6ohm disconnecting resistor between Safety Earth (chassis) and Audio Ground (Star Ground) does not comply with the mains safety requirement of most countries.
The exposed conductive parts must be permanently connected to the Safety Earth.
The reason for this requirement is to ensure that the mains fuse blows quickly, if the mains live lead during a fault condition contacts and/or connects to other low voltage parts of the equipment.
If fault current prior to the fuse blowing passes from Live to Audio ground then through the 6ohm resistor to Safety Ground back to distribution board earth the voltage appearing on the audio ground will be high enough to kill anyone that happened to touch the equipment at the wrong time. Further, fault current passing through the resistor will more likely blow the resistor before it blows the fuse, leaving the Audio ground at Live voltage and no effective connection to Safety Earth.
I recommend either of two solutions.
The first and safest is direct connection from Safety Earth to Audio Ground. This generally gives rise to hum loops in most connected equipment.
The alternative is to use a Disconnecting Network between Audio Ground and Safety Earth. This Disconnecting Network can consist of a parallel combination of:- Power resistor, inverse parallel coupled Power diodes, Power Thermistor, HF capacitor.
The Disconnecting Network MUST survive LONGER than it takes the mains fuse to rupture and for the arc to extinguish.
Andrew T made a comment about safety when using a break resistor. See below. What's your opinion on this?
The 6ohm disconnecting resistor between Safety Earth (chassis) and Audio Ground (Star Ground) does not comply with the mains safety requirement of most countries.
The exposed conductive parts must be permanently connected to the Safety Earth.
The reason for this requirement is to ensure that the mains fuse blows quickly, if the mains live lead during a fault condition contacts and/or connects to other low voltage parts of the equipment.
If fault current prior to the fuse blowing passes from Live to Audio ground then through the 6ohm resistor to Safety Ground back to distribution board earth the voltage appearing on the audio ground will be high enough to kill anyone that happened to touch the equipment at the wrong time. Further, fault current passing through the resistor will more likely blow the resistor before it blows the fuse, leaving the Audio ground at Live voltage and no effective connection to Safety Earth.
I recommend either of two solutions.
The first and safest is direct connection from Safety Earth to Audio Ground. This generally gives rise to hum loops in most connected equipment.
The alternative is to use a Disconnecting Network between Audio Ground and Safety Earth. This Disconnecting Network can consist of a parallel combination of:- Power resistor, inverse parallel coupled Power diodes, Power Thermistor, HF capacitor.
The Disconnecting Network MUST survive LONGER than it takes the mains fuse to rupture and for the arc to extinguish.
If a resistor does not comply with the mains safety regulations of your country, use thermistor, like it's done in First Watt amps (CL60).
You can also connect directly safety ground to audio ground or use disconnecting network as Andrew recommends.
If you check my official GC building guide, you will notice that I actually recommend thermistor or Disconnecting Network:
http://www.diyaudio.com/forums/showthread.php?p=1518378
The resistor is for the brave ones only
You can also connect directly safety ground to audio ground or use disconnecting network as Andrew recommends.
If you check my official GC building guide, you will notice that I actually recommend thermistor or Disconnecting Network:
http://www.diyaudio.com/forums/showthread.php?p=1518378
The resistor is for the brave ones only
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