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Old 18th January 2009, 10:15 AM   #1
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Default Input transformer: floating input = distortion

So I bought a couple of lundahl transformers, the LL1540. They are 1+1:1+1 high impedance units. They are used as input transformers for a PP amp, performing the phase splitting duties. I connected them as per the schematic: 12k on each secondary winding and the center tap grounded. The primaries are connected in series, and a RCA jack is connected to them, in first instance 'floating'.

Turning the amp on without anything connected to the jacks reveals a 'dead' quiet amp through my test speakers. As soon as I connect a RCA interconnect it starts humming. The interconnect is then plugged into the headphone output of a desktop CD-player. There is still a hum...when I turn on the CD-player the thing starts: heavy audible distortion at low levels. With an alligator clip I shorted the ground bar of the amplifier to the ground of the input jacks: the volume raised a bit and distortion was, apparently, gone. Hitting the pause reveals, again, a dead quiet amplifier.

Wonderful, isn't it? Still, I would actually prefer the input floating, just to create the galvanic insulation between CD player and amplifier. This is one of the main reasons for installing this input transformer anyway. So, what can be going on?

The CD player has a two pin connector (double insulated) and is not in any way grounded. The prototype's main earth is grounded to the 'central heating'.

I can draw it, even take some pictures (which would also reveal details about the new borg collective ) Still I think that the 'cure': non-floating primaries is not an unknown for most of you: I already saw some schematics with a switch to select between 'floating' and 'non-floating'. The switch is the easy cure: I would like a solution which really floats the primary!
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Old 18th January 2009, 12:46 PM   #2
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Try a 100k resistor from RCA hot to earth, and a 100k resistor from RCA common to earth. What does that do?

Sounds like the CDP output is cap coupled to the chassis.
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Old 18th January 2009, 01:13 PM   #3
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Thanks for the contribution! I am now working on a school project, but I will try it out tonight.

The CD player is an original Sony 227ES-D player. I have no idea how the input is done. This is the first time I read that the source can have an influence... I though about trying to feed the power amp from the output of my headphone amp, but this is a parafeed design which is therefore also cap-coupled.

I will try the two resistors and let you know the outcome!
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Old 18th January 2009, 02:04 PM   #4
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With a single ended source, you may be doomed to having to ground one end of the input transformer. Your input may be balanced, but do you really think that (the two wire power connection notwithstanding) the impedance to ground of both the output conductors is the same?

FWIW, I float the input to my preamp with no hum issues.
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Old 18th January 2009, 02:33 PM   #5
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Hi Stuart

It was exactly that little piece of iron on the schematics of your preamp and your poweramp that inspired me in going for the Lundahl at the input. Also this recent thread made me think that some galvanic insulation would not hurt.

That cap-coupled outputs can cause this problem I didn't know. Also I do not understand why it matters, but Zigzaflux has quite some experience with 'iron', so I accept and will try his recommendation.

Sure my CD-player is single ended output, but if I connect ground and signal to the balanced primary off the transformer there is no unbalanced impedance to ground...well, there is no 'ground' at the transformers input (I would think): all the primary sees is the output impedance of the CD-player.

Maybe different transformers react different to this?

Erik
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Old 18th January 2009, 02:39 PM   #6
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Have you measured the DC potential at the output of the CDP?


/Peter
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Old 18th January 2009, 02:43 PM   #7
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Quote:
Also this recent thread made me think that some galvanic insulation would not hurt.
It shouldn't. I'm not sure about the cap coupling hypothesis- my CD player has a cap-coupled output and no hum.
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Old 18th January 2009, 05:15 PM   #8
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Is "E" grounded?

Also, try ungrounding the secondary center tap. It is not necessary, and not necessarilly a good thing. Look at how Cinemag has theirs: http://cinemag.biz/line_input/CMLI-15-15B.pdf
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Old 18th January 2009, 06:26 PM   #9
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FWIW, I'm not talking about a cap-coupled output in the traditional sense, to remove DC offset from the CDP output. What I'm referring to is if (and I don't know your CDP) your source is fed with a 2 wire power cord, its output is completely isolated, free to float around wherever it wants to. Sure, it may be bonded to the chassis of the CDP, but this chassis would not be earthed if you only have a 2-wire power cord. Further, they may be coupling the negative side of the RCA jack to chassis with just a cheap ceramic cap. Either way, your output is still floating around.

By throwing in some high R values from each lead to ground, you are forcing a reference to earth, eliminating a possible source of noise injection through the capacitance to chassis / neutral / hot, whatever. By definition, a balanced system has equal impedances to ground, regardless of signal balance. Adding some balanced resistance will help swamp out the unequal capacitive reactance I suspect you currently have. I've experienced this before when connecting my waveform generator to my input transformer; it wanted to have a reference.

Anyway, this is just a guess / trial and error thing. Slapping some cheap resistors from your parts bin as recommended doesn't cost anything.

BTW, my experience with iron has been quite mixed, but I still pursue it. When it works, it's beautiful. When it doesn't, it's grounds for depression. Still trying to resolve my current interstage iron issues; making some good progress.
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Old 18th January 2009, 06:30 PM   #10
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Exactly, make the output impedance balanced and make sure there is no DC at the output that could casue saturation of the transformer core.

Should be silent and low distortion then.


/Peter
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