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Old 15th June 2014, 10:08 AM   #671
ruffrecords is offline ruffrecords  United Kingdom
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Quote:
Originally Posted by mm7 View Post
Currently I am considering to use an output transformer at preamp output.
But reading article "Jensen AN-003 INTERCONNECTION OF BALANCED AND UNBALANCED EQUIPMENT" http://www.jensen-transformers.com/an/an003.pdf
I found following:
There is a part 2.2 - "BETTER" uses an Output Transformer to Improve Balance
and part 2.3 - "BEST" uses an Input Transformer to "Fix" the Input Stage

Difference I see is OT at output side of cable, or IT at input side of cable.
Why the second is better?
Should I consider to use an input transformer at E-MU 1212 input to "fix" input stage?
It's complicated, but it is partly because the OT they use in that example is really an input transformer and it does not work too well as an OT. If you use a 600:600 OT then its performance will be fine.

Cheers

Ian
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Old 15th June 2014, 11:02 AM   #672
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by mm7 View Post
...
Should I consider to use an input transformer at E-MU 1212 input to "fix" input stage?
You would have to re-build the E-MU 1212. Simply placing a transformer between the preamp and the audio interface does not change the audio interface it would still have a differential amplifier inside that is made of op-amps.
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Old 15th June 2014, 05:09 PM   #673
mm7 is offline mm7  Canada
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Quote:
Originally Posted by ChrisA View Post
You would have to re-build the E-MU 1212. Simply placing a transformer between the preamp and the audio interface does not change the audio interface it would still have a differential amplifier inside that is made of op-amps.
They also say that a rebuilt will be more efficient, but placing IT right in front of input also works. The input transformer isolates cable capacitance and possible cable imperfectness, that can cause unbalance, from an input stage.

I assume the difference between "OT interface" and "IT interface" performance will be more pronounced when a cable is long. With short cables difference should be small.

Here is another article that describes IT and OT differences http://www.jensen-transformers.com/an/an002.pdf.

Last edited by mm7; 15th June 2014 at 05:22 PM.
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Old 15th June 2014, 05:50 PM   #674
mm7 is offline mm7  Canada
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Quote:
Originally Posted by ruffrecords View Post
It's complicated, but it is partly because the OT they use in that example is really an input transformer and it does not work too well as an OT. If you use a 600:600 OT then its performance will be fine.
Hi Ian,
Hmm. Why do you think that JT-11-EM is IT?
They have now JT-11-ELCF http://www.jensen-transformers.com/datashts/11elcf.pdf that is OT same as JT-11-EM except its different core.

I barely understand how it works, I think that they wrote that IT can do better then OT because it excludes unbalances of cable, caused by different capacities between wires 1-2 and 1-3 - secondary of IT has short leads to input stage with very low capacitance that can be ignored.
Also OT should have low impedance and resistance that means less turns of wire, that should decrease its self-inductance.
IT can have more self-inductance which is better for FR.

Is my understanding correct?

What do you mean by 600:600 OT? All 600:600 I've seen were cheap transformers for phone lines. I read that "600 Ohm" came from characteristic impedance of telegraph lines. Should we still use it in Hi-Fi or Pro audio?
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Old 16th June 2014, 03:15 PM   #675
mm7 is offline mm7  Canada
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Per Lundahl advises to use LL1588. It is interesting that it can be used as OT as well as IT.
Quote:
LL1588 is a high-level general-purpose transformer which can be used for microphone or line input, for line output and for galvanic isolation.
The windings are arranged to give perfect symmetry if the transformer is used in phase splitting input applications. The two coils structure also greatly improves immunity to external magnetic fields from e.g. power supplies and motors. Primary and secondary windings are separated by electrostatic shields. The core is a high permeability mu metal core. The transformer is housed in a mu-metal can.
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Old 17th June 2014, 10:05 AM   #676
ruffrecords is offline ruffrecords  United Kingdom
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Quote:
Originally Posted by mm7 View Post
Hi Ian,
Hmm. Why do you think that JT-11-EM is IT?
They have now JT-11-ELCF http://www.jensen-transformers.com/datashts/11elcf.pdf that is OT same as JT-11-EM except its different core.

I barely understand how it works, I think that they wrote that IT can do better then OT because it excludes unbalances of cable, caused by different capacities between wires 1-2 and 1-3 - secondary of IT has short leads to input stage with very low capacitance that can be ignored.
Also OT should have low impedance and resistance that means less turns of wire, that should decrease its self-inductance.
IT can have more self-inductance which is better for FR.

Is my understanding correct?

What do you mean by 600:600 OT? All 600:600 I've seen were cheap transformers for phone lines. I read that "600 Ohm" came from characteristic impedance of telegraph lines. Should we still use it in Hi-Fi or Pro audio?
The Jensen JT-11-ELCF IS a 600:600 transformer. You are right it is an output transformer. It is confusing because the JT-11-ELCF is a 600:600 output transformer and the JT-11P-1 is a 10k:10k input transformer. I mistakenly thought they were using the SAME transformer in both examples when in fact they were quite different. Sorry to have added to the confusion.

The bottom line is Jensen is right. If there is just a single transformer, it is best at the input end of an unbalanced connection. The ideal situation of course is a transformer at each end.

Cheers

ian
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Old 17th June 2014, 01:11 PM   #677
mm7 is offline mm7  Canada
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Ian thanks for clarification!
Quote:
The ideal situation of course is a transformer at each end.
I also thought about it, may be even OT is 1:4 and IT is 4:1 will be more efficient. But as a less expensive alternative, will be an impedance balancing network at output side and IT at input side work better than just unbalanced output with IT?
Balancing network would comprise of 20uF capacitor and 70 Ohm resistor serially between ground and wire #3.
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Old 18th June 2014, 10:14 AM   #678
ruffrecords is offline ruffrecords  United Kingdom
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Quote:
Originally Posted by mm7 View Post
Ian thanks for clarification!

I also thought about it, may be even OT is 1:4 and IT is 4:1 will be more efficient. But as a less expensive alternative, will be an impedance balancing network at output side and IT at input side work better than just unbalanced output with IT?
Balancing network would comprise of 20uF capacitor and 70 Ohm resistor serially between ground and wire #3.
Yes, impedance balanced output and an input transformer should work well and be relatively cheap. I think a lot of this is academic. The main thing for your original hum problem is to break the signal ground connection between the mic pre and the PC.

Cheers

ian
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Old 19th June 2014, 05:28 PM   #679
mm7 is offline mm7  Canada
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I've simulated the OT, IT with and without Output Impedance Matched Passive Network in LTSpice.
Cable is 6-ft, R 25mO/f, C 50pF/f.
Noises: sinusoidal signals 60,120 and 180Hz, 1V, common mode to both wires.

OIMPN -> 6ft cable -> IT
It shows up to 40dB lower buzz comparing to simple unbalanced output. It shows just little spikes of buzz a little bit above -140dB.

The result is very very close to results for OT -> 6ft cable -> IT.
However with OT it does not have any little spike of buzz at all.

OT -> 6ft cable -> bal load R10k - gives random noise floor around -120dB

So, the best result was, of course, OT-to-IT coupling. Very close to it, but 2 times cheaper - OIMPN to IT.

It is interesting, direct connection with floating pin 1 on output side shows very clean result - full common noise rejection (however in real life this allows some buzz).
Same results with direct connection from OIMP with grounded pin 1.
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Old 19th June 2014, 09:10 PM   #680
mm7 is offline mm7  Canada
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Originally Posted by mm7 View Post
It is interesting, direct connection with floating pin 1 on output side shows very clean result - full common noise rejection (however in real life this allows some buzz).
Same results with direct connection from OIMP with grounded pin 1.
Actually picture become more realistic, after I've added buzz voltage between MicAmp ground and PreAmp ground.
OIMPN cancels EMI and buzz very efficiently even without IT.

Here are schematics and FFTs of:
1 - unbalanced output to servo-balanced input
2 - OIMPN balanced output to servo-balanced input
3 - OIMPN balanced output to IT balanced input
Attached Images
File Type: png 6s45p_Dir_R.png (35.0 KB, 79 views)
File Type: png 6s45p_Dir_R_FFT.png (20.7 KB, 78 views)
File Type: png 6s45p_OIMPN_R.png (37.4 KB, 78 views)
File Type: png 6s45p_OIMPN_R_FFT.png (12.0 KB, 77 views)
File Type: png 6s45p_OIMPN_IT.png (35.7 KB, 74 views)
File Type: png 6s45p_OIMPN_IT_FFT.png (12.0 KB, 18 views)
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