Power Amplifier: RCA + XLR Inputs thru isolation transformer

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siox

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Nov 6, 2012
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Hi,

I'm restoring a Yamaha PC2200 power amp. i use (isolated) rca jacks instead of the unbalanced input jacks.

This amp is "class II" equipment (european version), no safety ground involved. with that in mind:

i have two sowter tranformers, type 3575, which i intend to use as input transformers.

does this make sense:

Primary:

XLR Pin 1 -unconnected
XLR Pin 2 -RED
XLR Pin 3 -ORANGE

RCA Hot - RED
RCA GND - ORANGE

Secondary:

WHITE - Signal Input's CH A+B
YELLOW/BLACK - Signal GND's CH A + B

I am not sure if this works because with no safety ground involved the Signal Inputs on the Amp CH A+B would be "floating" with no reference to ground - right?

Appreciate your help / thoughts,

Best Georg

3575.gif




 

gridcurrent

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siox said:
I'm restoring a Yamaha PC2200 power amp. i use (isolated) rca jacks instead of the unbalanced input jacks.
assuming you mean P-2200:
the 0V from the circuit card finds chassis via the shield of the connecting cable, a 2200 will not function otherwise.
note that XLR pin 1,  1/4" jack sleeve, and the pin 2/pin 3 switch all find chassis at the sheet metal screw.
the low or "ground" of any insulated phono jack must likewise be connected to chassis. 
siox said:
This amp is "class II" equipment (european version), no safety ground involved. with that in mind:
that should give some folks here fits.
siox said:
i have two sowter tranformers, type 3575, which i intend to use as input transformers.
they look fine.
I might prefer vintage 31267 or the modern Carnhill VTB9046 wired 2:1 (9600:2400),
mounted in an external box.
 

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abbey road d enfer

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gridcurrent said:
"This amp is "class II" equipment (european version), no safety ground involved." 
that should give some folks here fits.they look fine.
Class II, also known as double insulation, has specific safety rules regarding insulation. The mains xfmr has its primary and secondary wound on separate bobbins, and capacitance between any leg of the mains and the chassis is less than 120pF.
proper operation requests that all conected equipment have continuity between chassis.
 

gridcurrent

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North American bound Yamaha P-2200 amplifiers, units with toriodial power transformers,
typically measure:
approximately 45 uA  leakage current,
and less than 12 uA  with the primary windings reversed,
consistent over a range of serial numbers.

would like to see what the Class II transformer looks like;
better yet, leakage current.
 

abbey road d enfer

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gridcurrent said:
would like to see what the Class II transformer looks like;
http://docdif.fr.grpleg.com/general/MEDIAGRP/NP-FT-GT/GT18001%20-%20Transformateurs%20BT.pdf
Sorry, they don't have an english version (or couldn't find it).

better yet, leakage current.
Less than 10uA worst case.
 

siox

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abbey road d enfer said:
Class II, also known as double insulation, has specific safety rules regarding insulation. The mains xfmr has its primary and secondary wound on separate bobbins, and capacitance between any leg of the mains and the chassis is less than 120pF.
proper operation requests that all conected equipment have continuity between chassis.

according to that: it is no allowed to lift signal ground (or low) in this case, beside the fact that the circuit will not work with no ground reference from the incoming signal (thank you gridcurrent).

so my only option is to use the transformer on the XLR's, connecting pin 2+3.

maybe someone can explain:

-why designed as "class II" device - the only explanation for me is to avoid ground loops. looking at the schematics only the european version is "class II".

-according to the class II definition i found in german, this device would no get "class II" certification these days (i think). some parts of the main's wiring is not double isolated after entering the chassis.

- in a proper installation, the amplifier chassis will get safety ground thru grounded rack rails. in the moment i screw this amp in such a rack this device would become class I....?


best,
georg
 

abbey road d enfer

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siox said:
-why designed as "class II" device - the only explanation for me is to avoid ground loops. looking at the schematics only the european version is "class II".
Apparently, there are still places in Europe where domestic installations don't have an earth.

-according to the class II definition i found in german, this device would no get "class II" certification these days (i think). some parts of the main's wiring is not double isolated after entering the chassis.
Which parts? The wires, fuses, switches don't need to have "double insulation", for what it would mean...

- in a proper installation, the amplifier chassis will get safety ground thru grounded rack rails. in the moment i screw this amp in such a rack this device would become class I....?
Since class 2 is generally considered a better option, no reason to change it.
Belt and braces...
Actually, racking class 2 equipment is often a source of headaches, as it creates ground loops.
 
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