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Old 25th October 2012, 08:05 AM   #371
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Quote:
Originally Posted by audiodesign View Post
Joe see this:

DAC-END R (ES9018)
I see, the D option was modified, and you did what I would have done, mentioned the source.

Cheers, Joe R.
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Old 9th December 2012, 04:35 PM   #372
rolls is offline rolls  Switzerland
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Quote:
Originally Posted by Joe Rasmussen View Post
It creates an offset current, something you can only do with Sabre DACs and also lowers the Primary impedance - always a good thing.

Try it - you already have the resistors in place -then listen and report back here. Others have said that it improves the tonal balance, making it sound more solid, less light.
Cheers, Joe
I would like to thank Joe Rasmussen, because his idea lo charge the dac before the transformer is simple but brilliant. I have done my homework regarding this mod. For more than 2 years I have used a 1:10 transformer, loaded secondary with 1kOhm. Then I saw Joe's
drawing and began to think about it, followed by some work:
- I soldered a 1 Ohm to the primary, things opened up.
- I have a silver winding tube output transformer made by Audio Consulting. I use it the other way round, and this way it is a 1:25 step up. I have loaded my buffalo (TP24bit, battery driven, no regs) with a 0.07mm wire, about 25cm long, resistance is about 1.2Ohm. I measured the resistor with a ruler, not a meter.
This was a big step forward, and it performed very well, when we had a dac contest in my appartment, contenders were:
buffalo32, Metrum six, Meitner M1.
- A week after the meeting I thought about the discussion regarding grounding the primaries. Yesterday I soldered a ground wire to the middle point of the I/V wire, according to Joes drawing. Wow!!!:
Incredible resolution power, more micro dynamic, this is Sabre passive in current mode!
It is a pity that The Sabre chip sounds o.k. even in voltage mod, so often people use 1.1 trannies, which is still much better than most of the active I/V stages, but sounds just broken compared to (nearly) passive current mode, but there are people paying 1000 Dollars for a transformer to run a Sabre in voltage mode....
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Old 9th December 2012, 05:07 PM   #373
rolls is offline rolls  Switzerland
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I will give you ideas to run a Buffalo in (nearly)current mod with transformers:
Buy the best transformer you can afford, you will be rewarded!
It should be from 1:10 up to 1:30.
The most interesting item at the moment is Lundahl's new MC step-up with silver winding, if it can stand the higher voltage.
Use the best I/V resistor: a piece of thin silver wire. Now the math:
I have 1.2Ohms, middlepoint to ground is mandatory, then we'll have: 7mVolt x 25(1:25)= 175mV output, 1.2x625=750Ohms (about) on the secondary side. Very low output indeed, no problem for my TVC and 105db speakers. If you need more output, you can increase the I/V a bit, but think of the output resistance. If you sill need more, you choose a "mild" active stage, gain about 12db, I think nobody really needs 2 Volts!

regards André
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Old 9th December 2012, 07:21 PM   #374
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Quote:
Originally Posted by rolls View Post
- A week after the meeting I thought about the discussion regarding grounding the primaries. Yesterday I soldered a ground wire to the middle point of the I/V wire, according to Joes drawing. Wow!!!:
Incredible resolution power, more micro dynamic, this is Sabre passive in current mode!
I much prefer grounding load resistance, too - same as Joe Rasumussen's "C" schematic
Without that, I feel like losing a little focus and dynamics (but still sounds good)


I use two 22R load resister at primary of feastrex line transformer.
22R load lowers output voltage a lot, but isn't a problem as I have more than enough gain at following audio chain.
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Old 9th December 2012, 11:41 PM   #375
RollE2k is offline RollE2k  Sweden
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Is it extremely big difference in sound when loading the dac with ~0,8R vs 2R or 3R for each phase?
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Old 9th December 2012, 11:44 PM   #376
rolls is offline rolls  Switzerland
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Originally Posted by RollE2k View Post
Is it extremely big difference in sound when loading the dac with ~0,8R vs 2R for each phase?
I can not tell you, I have about 1.2 Ohm totally, a soldered wire.
Try it out and report!
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Old 9th December 2012, 11:57 PM   #377
RollE2k is offline RollE2k  Sweden
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Sorry, but i was just wondering if i should go for SUT's or to buy the Legato's to my Preamp/Dac-project (same box for everything). And since i don't even have the Buffalo yet it is hard.
I just wondered if anybody had tried, and if it was a big difference. Which would help me choose if i go for SUT's vs Active.

Thinking about using Lundahl LL1941 with 2,7R I/V and 1:16 step-up, which would be ~1400R output resistance (Should be approx ~1850R incl. SUT internal impedance) and approx 0,69V on the secondary - and i would end with approx 1,75VRMS after line-stage gain.

My third alternative would be even smaller I/V resistors with a pair of tubes between the dac and my linestage, but this i will probably skip for other reasons.

Last edited by RollE2k; 10th December 2012 at 12:23 AM.
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Old 10th December 2012, 12:42 AM   #378
rolls is offline rolls  Switzerland
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Quote:
Originally Posted by RollE2k View Post
Thinking about using Lundahl LL1941 with 2,7R I/V and 1:16 step-up, which would be ~1400R output resistance (Should be approx ~1850R incl. SUT internal impedance) and approx 0,69V on the secondary - and i would end with approx 1,75VRMS after line-stage gain.

My third alternative would be even smaller I/V resistors with a pair of tubes between the dac and my linestage, but this i will probably skip for other reasons.
If you have a linestage, then the Lundahl without an active stage will be perfect, but ask if it can stand the higher voltages involved here.
about 2k output is O.K., your linestage has probably 47k Ohms input. I would not be worried about the higher I/V resistor. Build it this way, and if there is sufficient gain, you can reduce the resistor. A silver wire is even better than a Texas resistor, but it would be difficult to get it in 0.07mm. It has to be thin, otherwise you need a meters! I have taken copper until I find silver.
Build it and enjoy it!

regards André
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Old 10th December 2012, 12:09 PM   #379
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While this is about transformers, I think the schematic below works along similar principles - that you can choose a step-up transformer for the necessary "gain" or an actual gain stage. They key is to understand what is done to the Sabre DAC itself: This, in my view, is the way to get the most out of it, bring that load value as low as you can get away with.

Cheers, Joe R.
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Old 10th December 2012, 12:21 PM   #380
RollE2k is offline RollE2k  Sweden
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But, since my line-stage is balanced with 5,4dB gain on each phase - then wouldn't it be better to connect output from each phase with a 2.7R resistor to GND, then maybe the cap between both phases (what does this do in this specific case?) and then into the step-up transformer.
In my case the output from the transformer would go straight into the volume attenuator (50k R2-R log attenuator between the phases not down to GND) with approx 15cm wire, twisted and then maybe 15cm wire into my tube-line-stage.
Thare shouldn't be any trouble running from the step-up into the volume right? since impedance matching seems quite fine ~1.85k into 50k.
Also Joe, wouldn't it be better skipping the GND connection on the output, at least when i run transformer - to keep DAC-GND from my Line-stage GND?

Thanks alot for taking your time helping in this thread so far :-)
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