PL 700 II Clair Bros Rising from the Ashes
- Thread starter Peter S
- Start date
Thought I would post this to the benefit of others following this thread. I shared this with Peter earlier today.
The PL400 I measured was connected to the Acurus Preamp which has its DC ground connected internally to its safety ground via a 3 prong plug. That is my system ground which is established at the pre. It is important to have only one safety ground connection point in any system for prevention of ground loops.
When I disconnected the preamp from the PL400, I measure ~36.9VAC on the chassis which is connected to the DC ground (transformer center tap) at the one point within the system. When I shunt the chassis to a safety ground through a 1M resistor, it drops to ~19VAC or about 19 uA of coupling current flowing down to the preamp safety ground. The system has zero noise at the speakers and the noise floor is down in the 100uV region.
When the preamp cables are plugged in to the amp, the chassis potential drops to 1mVAC from the 36.9VAC that is observed when no connection exists.
Hope this helps.
The PL400 I measured was connected to the Acurus Preamp which has its DC ground connected internally to its safety ground via a 3 prong plug. That is my system ground which is established at the pre. It is important to have only one safety ground connection point in any system for prevention of ground loops.
When I disconnected the preamp from the PL400, I measure ~36.9VAC on the chassis which is connected to the DC ground (transformer center tap) at the one point within the system. When I shunt the chassis to a safety ground through a 1M resistor, it drops to ~19VAC or about 19 uA of coupling current flowing down to the preamp safety ground. The system has zero noise at the speakers and the noise floor is down in the 100uV region.
When the preamp cables are plugged in to the amp, the chassis potential drops to 1mVAC from the 36.9VAC that is observed when no connection exists.
Hope this helps.
Joe, where is the other probe when you measure the ~36.9VAC on the chassis of the PL400? What is the source of this AC voltage? Does it come from the transformer as a form of electromagnetic induction or is it parasitic capacitance?
If you disconnect all the interconnecting cables between your Acurus Preamp and PL400, plug them in to AC power, and turn them on; what would the voltage be if you put one AC probe on the Acurus chassis and one AC probe on the PL400 chassis? Could you determine if the AC voltage originated in the Acurus Preamp or the PL400? Does it completely go away if you power either unit off?
If you disconnect all the interconnecting cables between your Acurus Preamp and PL400, plug them in to AC power, and turn them on; what would the voltage be if you put one AC probe on the Acurus chassis and one AC probe on the PL400 chassis? Could you determine if the AC voltage originated in the Acurus Preamp or the PL400? Does it completely go away if you power either unit off?
If this leakage (39 V on Joe's PL400 and 60 V on my PL700) is due to capacitance or insulation issues between the primary and secondary windings, then my crazy theory is that it should be there or measurable even with no current flowing through the primary. I will connect one side of the primary to line Hot and leave the other side of the primary not connected. I don't have access to a Hi Pot testing, maybe this will tell me something?
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A glimmer of hope. I connected the PL to my signal source (miniDSP) with the chassis of the PL NOT grounded by my hair-brained connection of the centre-tap bus to IEC jack ground. I was preparing myself for a blown-up DSP but the system worked without any increased hum.
There is still a low level hum (more of a hum/buzz) even with nothing connected to the amplifier.
Yet another confession; my ground connection from the transformer centre--tap to the backplanes was removed for previous testing. Instead of of re-soldering them to the mass of wires in the middle of the centre bus strap, I put a solder lug on each of these two wires a put one under each capacitor screw. Hold on to your "I told you so's"-----the amp is going back to the operating table to have this reduced.
This link was given to me from another forum; Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
As mentioned, with amp not connected to any source equipment, there is a buzz (less than 1mV) with volume min or max....and a 1mV hum ontop of this at volume 50%. I think part of this could be resolved by by-passing the vol controls and run a short coax from input jacks to PL14_20
There is still a low level hum (more of a hum/buzz) even with nothing connected to the amplifier.
Yet another confession; my ground connection from the transformer centre--tap to the backplanes was removed for previous testing. Instead of of re-soldering them to the mass of wires in the middle of the centre bus strap, I put a solder lug on each of these two wires a put one under each capacitor screw. Hold on to your "I told you so's"-----the amp is going back to the operating table to have this reduced.
This link was given to me from another forum; Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
As mentioned, with amp not connected to any source equipment, there is a buzz (less than 1mV) with volume min or max....and a 1mV hum ontop of this at volume 50%. I think part of this could be resolved by by-passing the vol controls and run a short coax from input jacks to PL14_20
Hi Peter
Have you paid careful attention to the wire routing (especially of the AC) in the chassis with respect to the shielded input cable runs which are long and subject to pickup? Proper wire dress cannot be overemphasized to achieve the lowest possible noise floor in your amp.
All AC should all be along the chassis top rear and closely paired for field cancellation. All DC wiring should be along the chassis bottom rear and the input wiring should move directly to the front panel along the right chassis edge, turn at the front panel and down to the controls.
Had you thought about your snubber caps being too big in size? Usually buzzing noise (instead of hum) at 60Hz is associated with the high frequency produced by the bridge diode snap off when reverse biased. If you are getting buzzing, I would be suspicious about this.
That article is informative but ends by saying 'good luck'
Have you paid careful attention to the wire routing (especially of the AC) in the chassis with respect to the shielded input cable runs which are long and subject to pickup? Proper wire dress cannot be overemphasized to achieve the lowest possible noise floor in your amp.
All AC should all be along the chassis top rear and closely paired for field cancellation. All DC wiring should be along the chassis bottom rear and the input wiring should move directly to the front panel along the right chassis edge, turn at the front panel and down to the controls.
Had you thought about your snubber caps being too big in size? Usually buzzing noise (instead of hum) at 60Hz is associated with the high frequency produced by the bridge diode snap off when reverse biased. If you are getting buzzing, I would be suspicious about this.
That article is informative but ends by saying 'good luck'
Joe,
Thank you for making these measurements/observations.
What do you think of this unit/units like this?
https://www.jensen-transformers.com/product/ci-2rr/
Would this Ground Isolator allow both the preamp and the main amp to be tied to safety ground without a ground loop caused by the interconnects?
Mark
Thank you for making these measurements/observations.
What do you think of this unit/units like this?
https://www.jensen-transformers.com/product/ci-2rr/
Would this Ground Isolator allow both the preamp and the main amp to be tied to safety ground without a ground loop caused by the interconnects?
Mark
Joe, Rod is in Australia where 220V mains are the norm. I believe I used a 50 Ohm resistor.
From Rod: By breaking the loop with the 10 Ohm resistor, the current is now less than 200mA, and the voltage across the interconnect will be very much smaller, reducing the hum to the point where it should no longer be audible.
From Rod: By breaking the loop with the 10 Ohm resistor, the current is now less than 200mA, and the voltage across the interconnect will be very much smaller, reducing the hum to the point where it should no longer be audible.