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Old 26th August 2010, 03:22 PM   #3031
Join Date: Oct 2005
Location: Central Florida
Did you download the chapter on audio transformers from the Jensen website. It will help quite a bit.

A perfect analogy are the step up trafos used with MC carts.

He mentions a 10 ohm load for the 1794, and that is probably a good ballpark figure for a working circuit.

Don't get confused by the trafo impedance ratings. A 600 ohm line trafo does not have a 600 ohm unloaded impedance, it is more like 100k ohms. The 600 ohm rating is the max load it will drive and still meet specs.

If you put 600 across the secondaries, the primary will also be 600. If you put 5k across the secondaries, the primary will also be 5k.

The impedance reflected from winding to winding will always be the square of the turns ratio X the load.

For instance, if you use 10 ohms across the primary of a 1/10 trafo the secondary will be 1k. The voltage will be multiplied by the turns ratio so the output voltage would be .780v P-P. If you could double the 10 ohms to 20 ohms then you would have 1.56v P-P.

The problem is, the dacs werent designed to generate voltage. They want to work into a dead short. That is the problem, what kind of load can you get away with. The dac designers are no help so everyone is on their own, but it is easy to see why you would want a 1/18-20 trafo ratio so you can keep the load as close to 0 ohms as you can.
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Old 26th August 2010, 03:31 PM   #3032
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Originally Posted by Ryssen View Post
Are you sure there was contakt on all the soldering points,the ground and so on?

I did some SMD soldering this evening,thats a small chip...

Click the image to open in full size.

what kinda trafffo you used to test the pmc1794?

how does the secondary filter (traffo) look like in you setup?
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Old 26th August 2010, 04:08 PM   #3033
Ryssen is online now Ryssen  Sweden
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I am going to use a http://www.lundahl.se/pdfs/datash/1571.pdf

I wont use a filter at first.
I will be a week or 2 before i test it,I think.
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Old 27th August 2010, 06:10 AM   #3034
danrap is offline danrap  Belgium
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Someone of you know these trannies?


They cost about 20 dollars each (for the 1:1 600 ohms); performances seems to be good (15 -30 khz freq range)

Now I stay with Monacor ltr110 but with a pair of double opa627, my dac seems to be better than transformers;
UTC a20 costs too much
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Old 27th August 2010, 09:36 AM   #3035
Join Date: Oct 2005
Location: Central Florida
The OEPs aren't any better than the Monacors but they will sound different. Neither of them give you all the specs you need to compare them to quality trafos, and I'm sure it's for a reason.

It's not surprising the opa627s sound better than the Monacors. I would keep searching for some good trafos on Ebay. The A-20s are not actually that good, you can do much better for less.
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Old 27th August 2010, 11:37 AM   #3036
danrap is offline danrap  Belgium
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Thank you Bill..
I'm sure that must exist some good trafos that can do better for less bucks..
Have you some brand in mind? like Lundhal,Hammond..or what?
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Old 27th August 2010, 12:30 PM   #3037
Join Date: Oct 2005
Location: Central Florida
I have heard 5 different brands of decent quality output trafos with the cs4398. They all sounded different in one way or another, and in many cases are not subtle differences.

A published spec of 30-30khz is meaningless unless a reference to level and distortion at its lowest specified frequency is also listed.

Keep in mind that if you buy a good used pair of trafos and they are not to your liking you can easily resell them. That is not true when you buy cheap stuff at retail price.

Lundhals are problematic for the unknowledgable, generally because of the termination requirements.
Units like the A20s are very old technology but still are pleasant sounding. Another old unit to try would be the Altec/Peerless 15356, they pop up on Ebay all the time.
Cinemag makes good, inexpensive trafos, $40 and up.
Sowter has a few good inexpensive trafos
Jensen has models of all prices and specs.
I am currently using the Jensen JT 11-DM, I bought them off of Ebay for $26 a piece. They are the second best that they make and are my best, by far.
The Tamura GA80080 are also very good, Ebay bought for cheap.

The list is endless. In general, stay away from anything listed as an input trafo unless you have the patience and test equipment to get them set up right. Input and output trafos are not interchangable.

I hope I helped a little. Please go to the Jensen website and read the writeup about audio transformers by Bill Whitlock, it's great.
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Old 27th August 2010, 02:11 PM   #3038
danrap is offline danrap  Belgium
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Join Date: Jul 2008
you helped me really very much.
Thank you for your post
Now I will start to take a look at ebay...
Do you know if the PCM1798 dac board can use the same trafos of cs4398 (1:1)?
I have 2 Dac board, mine is with cs4398 and another that will arrives soon with a pcm1798 dac..
Thank you again,
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Old 27th August 2010, 02:16 PM   #3039
Join Date: Oct 2005
Location: Central Florida
No, the 1798 is completely different. It requires current to voltage conversion.
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Old 27th August 2010, 05:31 PM   #3040
Mush is offline Mush  United States
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Location: ann arbor, mi
Default Behringer SRC2496 and UT A-20

Hi All,
My cheap Chinese Ebay DAC has died and I am now considering a Behringer SRC2496. They are about $200 from Amazon. I have my UTC A-20 transformers and am wondering if anyone has any experience with this kind of combination. Do you have any advice?

Lastly, what is a good way to connecting the transformers to the DAC output? I have seen several approaches. I am no engineer but know how to solder and experiment.

Thanks and I hope all is well with you.

Ciao - Mush
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