pl400 ground loops and hum

T-wrecks

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#1
Hello all! well I have been on this community before, some may remember, and im just looking for advice and/or help. Mine is a (400) series I, eight fin. It has had the outputs replaced long ago (all mj21196g) and rca410 drivers. I have Don's relay board fitted, as well as a pl14-20 rev.e board installed. I bought the board stuffed. The big PS caps, bridge rectifier and current sharing resistors are all original.

Im trying to figure why I always have so many ground loop issues with this amp. Seems like whatever I connect to it creates a ground loop of some kind. A 60 hz hum that is sometimes very faint but persistent. I use a behringer rackmount line mixer as a preamplifier. This by itself causes no hum. Playing either of my keyboards through the stereo doesn't cause any hum either. Plugging the tv through the mixer does produce a hum. very noticeable. Removing the ground lug from my mixer solves this, yet a ratshack ground loop isolator does not. As a matter of fact the ground loop isolator introduces its own very strange popping and static issues! Sometimes tripping the relay board completely and requiring the amplifier to be shut off and reset.

Anyways I finally relented and disconnected the tv from the stereo permanently and re-attached the ground plug to the mixer and things were great until recently when I tried to listen to some records. No matter how I plug in my record player it produces a faint 60hz hum. Obviously my line mixer doesn't have a phono pre-amp, so I have tried a cheap BBE phono preamp. it hums. then I tried a old hafler preamp and it hums as well. This is a faint hum, not as loud as the one from my tv. Weirdest is that my ground loop isolator has basically no effect! At least I know it worked years ago, maybe is broken? I dunno.

So what gives is the pl400 just prone to grounding issues? or maybe I got something screwy inside the old git, and I need to replace something or move a ground wire
 
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VSAT88

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#2
Hello all! well I visited this site a bit when I first got my phase linear, and im just looking for advice and/or help. Mine is a (400) series I, eight fin. It has had the outputs replaced long ago (all mj21196g) and rca410 drivers. I have Don's relay board fitted, as well as a pl14-20 rev.e board installed. I bought the board stuffed. The big PS caps, bridge rectifier and current sharing resistors are all original.

Im trying to figure why I always have so many ground loop issues with this amp. Seems like whatever I connect to it creates a ground loop of some kind. A 60 hz hum that is sometimes very faint but persistent. I use a behringer rackmount line mixer as a preamplifier. This by itself causes no hum. Playing either of my keyboards through the stereo doesn't cause any hum either. Plugging the tv through the mixer does produce a hum. very noticeable. Removing the ground lug from my mixer solves this, yet a ratshack ground loop isolator does not. As a matter of fact the ground loop isolator introduces its own very strange popping and static issues! Sometimes tripping the relay board completely and requiring the amplifier to be shut off and reset.

Anyways I finally relented and disconnected the tv from the stereo permanently and re-attached the ground plug to the mixer and things were great until recently when I tried to listen to some records. No matter how I plug in my record player it produces a faint 60hz hum. Obviously my line mixer doesn't have a phono pre-amp, so I have tried a cheap BBE phono preamp. it hums. then I tried a old hafler preamp and it hums as well. This is a faint hum, not as loud as the one from my tv. Weirdest is that my ground loop isolator has basically no effect! At least I know it worked years ago, maybe is broken? I dunno.

So what gives is the pl400 just prone to grounding issues? or maybe I got something screwy inside the old git, and I need to replace something or move a ground wire
I am not the most knowledgeable dude to speak to here on site by far but have you poked around in there and checked for bad / loose / cold solder joints ? Have you tried the amp on another outlet in another room ? Do you have any pictures ?? EDIT: BTW, from what I have seen the PL 400 is not any more prone to ground loop issues than any of my other amps. EDIT again...I just re read your post. Those filter caps are what 40 years old ? I could easily see where they could be suspect. WO them (http://www.whiteoakaudio.com/estore.aspx) !
 
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marcok

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#4
I suggest you to connect your PC to PL 400 via headphones connector , using it as cd player or as streaming tuner .
At this point you can understand if the guilty is PL 400 .
Ciao
Marco
 

T-wrecks

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#5
cap

are these suitable for the power capacitors?
s-l500.jpg
Well I ran my iPhone headphone out through my mixer and into my trusty 400. had a great time listening to music, no hum or anything. Before anyone asks, yes I have tried flipping plugs on everything that wasn't polarized plugs. suppose I should go buy a pack of cheater plugs
 
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T-wrecks

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Hi Lee! Yes I have looked at White Oaks capacitor stacks. when I bought this amp I originally was going to WOPL the whole dang thing and be done, nowadays I appreciate the fact that it isn't "perfect" amp. Historically its important to me that some of it remains "phase linear" I know that's silly. Slightly less capacitance I understand, still improved over the original I believe.
 

Northwinds

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#8
You either have a hum or not, it's not just half the time. The culprit IMO is your mixer, not the amp. Maybe poor gain matching in the channels vs the source you use?
 
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Netfly

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#9
Hum can be a tuff one. Its most likely external to the amp. Here's a few thing you can try:
For now remove all but one source.
Change interconnect cables.
Check for external noise like light dimmers or other equipment on the circuit.
Ensure all equipment is plugged into same circuit.
Ground strap component chassis together with 18 gauge or heavier wire.
 

mlucitt

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#13
Wondering if T-wrecks ever got his hum fixed? Anyway, I am trying to catch up and hoping for some help from the folks on the Phoenix Board.

I see Joe's new schematics reference SIG_GND (signal ground), which is pretty obviously the input coax shield. Then there are many references to AGND, which I assume is Audio ground and can be traced back to the star ground. However, on the schematic for the Control Board, pin 5R, the source of ground for the board, is also called AGND, which I thought does not exist off of the Control Board.

To add to my confusion, on the Control Board Wiring Chart v2, pin 5R is listed as Chassis Ground, which I always thought was Earth or the Green Wire on the 120V main power cord. The only things I have connected to the chassis of the PL400 are a couple of EMI AC capacitors and the ground wire on Don's DC Protection board because that circuit is not in the signal path. Everything else referenced to ground (including the meter circuit) goes to Star Ground, which is the copper bar between the power Capacitors. Is that the accepted practice?

Comments please...
 

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#14
Wondering if T-wrecks ever got his hum fixed? Anyway, I am trying to catch up and hoping for some help from the folks on the Phoenix Board.

I see Joe's new schematics reference SIG_GND (signal ground), which is pretty obviously the input coax shield. Then there are many references to AGND, which I assume is Audio ground and can be traced back to the star ground. However, on the schematic for the Control Board, pin 5R, the source of ground for the board, is also called AGND, which I thought does not exist off of the Control Board.

To add to my confusion, on the Control Board Wiring Chart v2, pin 5R is listed as Chassis Ground, which I always thought was Earth or the Green Wire on the 120V main power cord. The only things I have connected to the chassis of the PL400 are a couple of EMI AC capacitors and the ground wire on Don's DC Protection board because that circuit is not in the signal path. Everything else referenced to ground (including the meter circuit) goes to Star Ground, which is the copper bar between the power Capacitors. Is that the accepted practice?

Comments please...
AGND is analog ground. 5R and 5L are connected to the copper bus between the bulk caps. You only have to connect either 5L or 5R since you have installed the bus wire jumpers on the back side of the board. With the backplane kit, these bus wire jumpers are omitted as the power feed for each channel comes independently from the backplane boards

There should be one and only one connection from that copper bus to the chassis and only at one place (usually the middle leg of the terminal strip closest to the bulk caps.
 
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mlucitt

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#15
Joe, thanks. I have that single wire from Star Ground connected to the middle leg of that terminal strip closest to the bulk caps via a 2.2 Ohm resistor to allow the Star Ground to float slightly away from any potential on the chassis. I do this because I am using a 3-wire power cord with the green wire connected to the bare chassis for safety.

What do you think about the idea of taking the unused Control Board ground pin, say 5L, and use that as the ground for the meter circuit? It would eliminate one connection to the Star Ground, and I don't see a path for a ground loop. For the Backplane amplifiers, it would not make much of a difference.
 

Gepetto

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#16
Joe, thanks. I have that single wire from Star Ground connected to the middle leg of that terminal strip closest to the bulk caps via a 2.2 Ohm resistor to allow the Star Ground to float slightly away from any potential on the chassis. I do this because I am using a 3-wire power cord with the green wire connected to the bare chassis for safety.

What do you think about the idea of taking the unused Control Board ground pin, say 5L, and use that as the ground for the meter circuit? It would eliminate one connection to the Star Ground, and I don't see a path for a ground loop. For the Backplane amplifiers, it would not make much of a difference.
The meter connection is likely a safe connection Mark.

With the AC safety ground connected to the chassis, you will develop ground loops if any other component in your system also has a safety ground connection to its chassis. This includes connections to your pre that emanate from cable connections such as a TV etc. To avoid ground loops, there can be one and only one component with the safety ground to chassis connection in your system.
 

mlucitt

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#17
The meter connection is likely a safe connection Mark.

With the AC safety ground connected to the chassis, you will develop ground loops if any other component in your system also has a safety ground connection to its chassis. This includes connections to your pre that emanate from cable connections such as a TV etc. To avoid ground loops, there can be one and only one component with the safety ground to chassis connection in your system.
True, and I have experienced this myself with guitar amps and mixing boards that all used 3-prong AC plugs. Currently, my turntable, preamp, and CD player all have 2-prong AC plugs. The turntable does have a ground wire separate from the coax shields and this ground wire connects to the preamp chassis, not sure how effective this is in combatting hum, but I have not heard or measured any 60 Hz hum.

Thanks for your comments.
 
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#18
Joe, thanks. I have that single wire from Star Ground connected to the middle leg of that terminal strip closest to the bulk caps via a 2.2 Ohm resistor to allow the Star Ground to float slightly away from any potential on the chassis. I do this because I am using a 3-wire power cord with the green wire connected to the bare chassis for safety.

What do you think about the idea of taking the unused Control Board ground pin, say 5L, and use that as the ground for the meter circuit? It would eliminate one connection to the Star Ground, and I don't see a path for a ground loop. For the Backplane amplifiers, it would not make much of a difference.

A 2.2 ohm resistor is a bit light for separation of the star & chassis ground's. I would shoot for 10 ohms or so. Also a resistor is not a good choice in this location in an amplifier, a preamp would probably be fine. I know a lot of manufacturers have used resistors for this in the past however it wouldn't take much current in a failure scenario to open a resistor, unless your using a big wirewound anyway. For safety purposes you'd typically want to ensure the device you use would not fail open before the main fuse does. I use an NTC inrush protector most of the time and a few times I have used anti-parallel diodes with a resistor in parallel with the diodes. This surge suppressor at Mouser is cheap and capable of 9A continuous and much more surge current and is 10 ohms cold. The only negative is that its about 7/8" diameter.
 

mlucitt

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#19
A 2.2 ohm resistor is a bit light for separation of the star & chassis ground's. I would shoot for 10 ohms or so. Also a resistor is not a good choice in this location in an amplifier, a preamp would probably be fine. I know a lot of manufacturers have used resistors for this in the past however it wouldn't take much current in a failure scenario to open a resistor, unless your using a big wirewound anyway. For safety purposes you'd typically want to ensure the device you use would not fail open before the main fuse does. I use an NTC inrush protector most of the time and a few times I have used anti-parallel diodes with a resistor in parallel with the diodes. This surge suppressor at Mouser is cheap and capable of 9A continuous and much more surge current and is 10 ohms cold. The only negative is that its about 7/8" diameter.
I appreciate your comments. My thoughts on floating ground...
For the PL 400, I calculated about .5 V possible static imbalance between Star Ground and Chassis Ground. That gives me about 225mA of current through a 500mA resistor. And a grounding casualty/failure will trip the power panel circuit breaker because the chassis is grounded by the third wire of the power plug, probably before the 8 amp line fuse fails. I could go up to 4.7 or 10 Ohms, that makes some sense to me.

For in-rush current control (a whole different animal and not really needed, but it makes us feel good)...
For my WOPL 700B, I did use power line anti-parallel diodes in a full-wave bridge, never had a problem with that one. Still running strong in my brother's house at 472 Watts per channel.

The NTC thermistors are common in a lot of electronics, and they are in the surge-protected power strip supplying all my test equipment. I used two 5 Ohm thermistors in series in a Hafler DH-220 and they seemed to work for in-rush current control. For the PL 400, I will use a relay-activated in-rush circuit with two 10 Ohm resistors in series rather than thermistors because thermistors may allow in-rush currents when the power is cycled without allowing the thermistors to cool down. There is also an urban legend of thermistors failing with the amplifier idling, because the current flow is not high enough to get the thermistor so hot as to reduce its higher resistance, so the thermistor cycles hot and cold, until it fails. This also causes the power supply voltage to go up and down slightly. I'm not sure of this, but it sounds plausible.
 
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#20
I think that I confused the point I was trying to make by using the term "inrush" protection. I wasn't talking about inrush protection. I was specifically talking about using an NTC thermistor in place of the resistor you're using to isolate the star ground and chassis ground planes. This is not my concept, I'm standing on the shoulders of others for this one :iconbiggrin:. This has been used in commercial products, I included a schematic of an older Pass Labs Aleph amplifier's power supply to illustrate the thermistor break between circuit common (star ground) and chassis ground.

There was a discussion about the reason for using an NTC thermistor instead of a resistor between star and chassis ground many years back on DIYAUDIO. The basic reason is because there are failure modes in which current can exceed the capabilities of a standard resistor. If the resistor opens and you do not know about it then the safety of the device can be compromised. Not only this but the fact that in the event of a failure in which current begins to flow from Star to chassis ground the NTC thermistor will drop resistance which is what you want.

AlephPS.jpg
 
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