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Buffalo II & transformers

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T

tubeing

Member
Joined 2009
Hi everybody,
I'm waiting for my BII DAC looking around for some nice transformer to use in the output stage. I contacted Jensen, Sowter and Lundahl asking for the best choice for what I'm doing. They told me that my bigger problem is the input impedance of my preamp (I'd like to use the following connections: DAC->transformer primary->transformer sec ->preamp input) that's "only" 4K...but I can see that Lundahl LL1527 works at its best with 4K load, so I think it could be the best choice for me...but what kind of problems could arise with low preamp impedance? This is my first design with transformers, so I'm asking you if my calculations are right or not. With LL1527A, primary windings in parallel and sec windings in series, I have 90 Ohms in parallel with 4K and the DAC will see (dynamically) something like 22Ohm reflected by the secondary. But in this calculations, where must I consider the primary impedance? In series with the 195Ohms (equivalent series resistance of ESS9018)? How can I calculate the out voltage at the preamp input? In this case I have a 1:2 turn ratio...Last thing: I can see that there are people using a resistor across the secondary, to lower the reflected impedance in order to let the DAC work better (hopefully) in terms of THD+N. But, stated the best behaviour of LL1527 with 3-4K impedance, is it a good move?
Thanks in advance.
Regards

Stefano
 
markusA

markusA

Member
Joined 2009
I'm using the LL1588 with a 2:1 ratio at the moment.
No extra resistors what so ever and it stiull sound good.
Later I guess I'll hook it up to a tone generator and dial in the optimum load but for now I'm just enjoying the music. :D

I don't really know what the optimum load impedance is supposed to be for the LL1588?
 
M

Marek

Member
Joined 2003
Sabre32 DAC should be used in voltage mode when using transformers. I tried step-up trafos (expensive amorph LL1931 in 1:16 and 1:8 config) with current mode but effect wasn't good (high THD of low freq). Then I tried LL7901 (1:1, 2:1) with voltage mode - better, then I tried Jensen JT-11P-1 - best transformer effect (many people choose Jensens as the best trafo for Sabre32).
Anyway if you want to achieve THE BEST results forget transformers. Use your BuffaloII with tubes directly - results are incredible - every transformer give you worse results.
 
M

Marek

Member
Joined 2003
@Gluca

Did you make any THD measuremnts (espiecially for low frequency eg. 50Hz) with RMAA or similar?

Last days i tried this BuffaloII output stage configurations:
1. passive stage using Jensen JT-11P-1
2. Jensen -> SRPP based on 6N30P supertube
3. DAC directly to SRPP
4. DAC directly to tube diffirential preamplifier (PP->SE on 2 triodes+ current cource in cathodes a than cathode follower to lower output impedance)

all tested with 6C33C tube amplifier.
Results: definetely best and similar efect with 3 and 4, when using Jensens very good but there is something missing, some very low level details.
Seriously I did prefere passive stage because of simplicity no PSU, no bunch of cables etc but unfortunetally even the most sophisticated transformer degrade sound a bit..

Another option is DAC-> tube and transformer output from anode but I can't lose more money for transformers...
 
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