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Old 10th January 2014, 11:23 AM   #1
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Default mp3 --> ? Input Transformer ? --> class d

Hi,

A while back I was advised on 41Hz forum to use an OEP A262A3E audio transformer to go between an m3p player and class t amp driving a pair of monacor sph-30x speakers. This was to provide gain and ground loop isolation.

I've experimented and wiring the primaries like this seems to work for the ground loop isolation.

Click the image to open in full size.imgur: the simple image sharer


I'm guessing I need two of these for stereo.
I'm just interested in what the secondaries are actually for. I see references to people wiring them in series and placing capacitors and resistors across them which is currently all voodoo to me. Any pointers appreciated. Thanks.

Charles
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Old 10th January 2014, 12:52 PM   #2
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I have since abandoned this avenue of research due. Sorry to clog up the forum. This thread can be deleted if need be. Cheers.
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Old 11th January 2014, 08:59 AM   #3
Dr_EM is offline Dr_EM  United Kingdom
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That is a step-up (or down) transformer and you need to use the secondary to get any gain. I'm not sure how much gain you need but I'd recommend this wiring:

Click the image to open in full size.

This wiring puts the primaries in series and the secondaries in parallel, so based on the turns ratio of 1+1:6.45+6.45 I believe this produces 1:3.225, so your voltage gain can be 3.225 times, which is around 10dB (20log 3.225).

Connecting the shield to transmit ground helps reduce capacitive coupling effects at higher frequencies, i.e, blocks RF junk to some extent.

The output RC helps improve high frequency response, you tune it by passing a square wave (try 1kHz) and adjusting to eliminate overshoot and ringing. For this transformer try 3.3k and 6.8nF as a start.

I must stress the gain is voltage gain, if you have a low source impedance (an MP3 player designed to drive headphones should be) and high input impedance (the input to the T-amp is probably ~47k) then this virtually holds true.

BTW, if this is a battery powered MP3 player you won't have any ground loop issues anyway
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Old 12th January 2014, 03:54 PM   #4
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Hi Dr_EM,

It'll run off a battery some of the time, and others off a mains converter, and inputs will be a laptop plugged into the mains, iPads and Android tablets.

It hums unpleasantly when it's plugged into the mains (12volt / 1.5amp cheapo adaptor from an external hard drive), and is silent off the battery. The battery sounds much better too. The mind cancels out the hum but it's there. Clearly I need a better power supply too.

Interestingly it also hums and pops and clicks when it's running off the battery and a battery powered mp3 player, and I plug in the 12volt adapter to the wall and touch the ground on the power jack. I seem to have RF interference coming from all angles.

Your circuit diagram is fascinating and much appreciated. I don't understand how it works at all. It's going to take me some time to figure out whats going on before I build it, which is fine as I'm off travelling this week and am out of time on this project. I shall pack my little audio transformers and a selection of capacitors and resistors based on your design and see if I can build myself a little input box, along with a better power supply and RF copious shielding.

Charles
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Old 21st February 2014, 09:23 AM   #5
SAC is offline SAC  United Kingdom
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Did you have any luck with this? I can post a photo of my set-up using an amp9 with this exact transformer. I also have the recommended cap/resistor in place which was suggested to avoid pre-ringing. I did not use the full step up potential, going about half-way, since my preamp was not up powerful enough.

I would post the thread where this is discussed except the 41Hz forum has been down for ages.

P.S. Bit of a hijack, I found this thread searching for a way to pass two channels through this transformer (actually will need to but another pair for my 4way speakers). Now I would love a picture describing how that can be done. I do not need gain, but desperately need isolation due to the noise I get since I dropped my PLLXO after the transformer, and now have a digital Xover in front for 4way speakers.

Last edited by SAC; 21st February 2014 at 09:32 AM.
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Old 21st February 2014, 09:45 AM   #6
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You cannot pass two signals through this transformer. It might have two primaries (and two secondaries), but they cannot be treated as isolated because of the magnetic core inside.
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Old 21st February 2014, 10:07 AM   #7
SAC is offline SAC  United Kingdom
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Good to know, thank you.

Hopefully I can fix this hum/whining without using these costly transformers, assuming they would work at all.
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Old 21st February 2014, 10:15 AM   #8
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If your source is a laptop running from power brick or PC, there is probably nothing you can do running unbalanced interconnects. You would have to move to a double insulated (non earthed) amplifier unit. But this is not a novice DIY proposition really.

You could add balanced line drivers and receivers to your system, but the cost of that is probbaly similar to those transformers. They are actually very cheap transformers, really good quality ones are several times more expensive.
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Old 21st February 2014, 10:35 AM   #9
SAC is offline SAC  United Kingdom
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After two days of plugging things in and out, using cheat plugs, etc., I cannot say anything conclusively regarding this noise issue since it occurs so inconsistently. I'll leave it for now.
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Old 21st March 2014, 10:48 AM   #10
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Hi, no I didn't build it, left the parts in England and amp has blown up. Going looking for an amp in Delhi should be fun.
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