Buffalo II & transformers

[Ryssen]

Now let me understand this: I connected directly to the output of 30mV rms (with the 3 ohm resistors as shown in the diagram) my amp, but even at low volume there is distortion. Why?

If you take the signal before the tubestage,is there distortion there to?


Another question,can I use the 330 ohms resistors even if I use the "unballancer" with differential tube input.(without a trafo)

 

[zz1969]

Interesting question, but simple answer. Since the Secondary load is relatively high, the DAC hardly sees the DC resistance of the transformer, if it is reasonably low. I used a 1:1 transformer with a lot higher DCR than that and yet was under 100 Ohm.

The DAC will see the 330R at all times and very little in parallel.

Lundahl 1517 will be OK.

BTW, hate to be pedantic, there is no such thing as Ohms, it is always Ohm (no "s") whether singular or plural.

Cheers, Joe

Hi Joe,

if using 2 DACs in dual mono, should we use 3 Ohm or 1.5 Ohm resistors? (in a variant with transformers and a tube stage)

Rgds,
Evgeny

 

[Joe Rasmussen]

Hi Joe,

if using 2 DACs in dual mono, should we use 3 Ohm or 1.5 Ohm resistors? (in a variant with transformers and a tube stage)

Rgds,
Evgeny

Often that is the case, if you are dumping twice the current, it will produce double the voltage.

 

[zz1969]

Hi Joe, thanks for a quick reply.
I am using your D setup at the moment: dual mono B-III with 1.5 Ohm resistors, Sowter 3603 transformers and an Aikido tube stage with 6N2P input and 6CG7 output tubes. Sounds good, for me better than any transistor output stage I tried. Thanks.
Evgeny

 

[Vingborg]

Morning good folks

I would like to bring life into this old thread again.

I have read this thread over and over again and there is still so many questions left unanswered!
I have a Buffalo III dac which I experiment with (like every one else in this thread )

For now I have finished a tube gain stage and I'm ready to experiment with Joe Rasmussen's suggestion to load the Sabre dac with 3R to ground or even lower (I have borrowed 2 Lundahl 1984 1:1) but: First of all I was thinking: Why use a transformer 1:1. The signal has to be gained after 3R of course like 50-70 times..
Would't it be enough to load the dac, put in a high quality cap between dac and tube gain stage?
What role does this 1:1 transformer actually have if the if the signal has to gained pretty much?

I had this theory - Why not use an Lundahl ribbon mic transformer like : LL1927A :http://www.lundahl.se/pdf/1927A.pdf
Or LL2913 :
http://www.lundahl.se/pdf/2913.pdf

And together with the exstremly low dcR of the primary it would be possible to let the dac see a very low impedance without "killing" the signal to ground with these very small resistors! And the gain after transformer would not have to be as high like in a riaa preamp - You guys follow me?

Ok, I send a mail for the swedish distributor of Lundahl with my theory Mr. Peter Buddee and he friendly replyed that he would have to ask Mr. Kevin Carter at K&K.
Kevin provided a quick respond: Ribbon transformers do not have enough induction in the primary to make a good bass respond from a source of 195 ohm like the ES9018!!
All right I can understand that - But where is the limit then? Lundahl dosen't provide us with the primary induction in there hole program.
Some people in this thread have tried the LL1941 and LL1933 both mc trannys with a pretty low dcR on primary and in my world it meens that the primary induction also would be low! And according to the datasheets they have to work from a source below 50 ohm

As you folks problerbly can see, I'm a little confused..
It seems like this thread has turn out from transformers as output stage to transformers with a gain output stage - So why use a 1:1 when we still load the dac with small resistors and gain after?
It would make sence to me to use a 1:16 or greater transformer to raise the signal from dac which must around 5mV with 0,5 ohm resistors to ground. That will give arround 80mV there just have to be gained about 25 times to reach 2V output - Please correct me if I'm on totally wrong path here

Anybody who want's to kick in with some useful facts are more than welcome

Joe Rasmussen - Marek - Gluca - RollE2K - Mars2
I can see that you guys have tried step-up's.. What do you think of my thoughts ?

If money was of no concern I would buy 2 pcs. of these LL1674 1:4 - LL2913 1:37 - LL1927A 1:55 - LL1941 1:16 - LL7902 1:4 just to try them all out!!

But a daytime carpenter job in Denmark, well it does not make you financially rich

Please let me hear from you all again
Best regards
Henrik

 

[Marek]

What role does this 1:1 transformer actually have if the if the signal has to gained pretty much?

desymetization, HF filtering and ground separation.

Lundahl dosen't provide us with the primary induction in there hole program.
Some people in this thread have tried the LL1941 and LL1933 both mc trannys with a pretty low dcR on primary and in my world it meens that the primary induction also would be low! And according to the datasheets they have to work from a source below 50 ohm

You're right, inductance od primary winding has to match output impedance of DAC. So in case of ES9018 you have to use at least 200:XXX trafo, not 50:XXX.
And you right, my LL1931 trafo has too low inductance for ES9018 - as you mentioned it is designed for 50ohm source - I got great sound BUT with lack of lower bass...

My goal for using step-up trafo (LL1931 in 1:8 turns ratio config) was to ommit one stage of active amplification, driving power tube amplifier ( 2 stage: VT-25+GM70 ) directly with DAC+step-up.
With standard amplifier you don't need such gain so you may consider lower (1:1 max to 1:4) turns ratio.
Note that with higher turns ratio there is a problem with output impedance because O_imp= I_imp * (turns_ratio)^2.

I'm not using any load resistors letting ES9018 to work with "voltage mode" because I really can't see any real advantages to work with "current mode" except some better -dB numbers which are negligible to me.

If money was no object I would order a custom supermalloy trafo from SAC Thailand with SILK Nano winding technology SAC Thailand

Marek

 

[AndriyOL]

I'm not using any load resistors letting ES9018 to work with "voltage mode" because I really can't see any real advantages to work with "current mode" except some better -dB numbers which are negligible to me.

Than why not to use direct voltage output from the dac?

 

[Marek]

Because:

desymetization, HF filtering and ground separation.

 

[AndriyOL]

What do you mean by desymetization? Is there a need to filter HF, does the signal from dac is 'dirty'? Perhaps you mean HF cut off?

The main reason to use transformer more likely is to convert balanced to unbalanced output signal.

 

[Marek]

desymetrization = balanced to unbalanced conversion.
Yes, signal from delta sigma DAC is 'dirty', it contains lot of HF noise (mainly from noise shaping process) which unfiltered will demodulate on nonlinearities into the audio band. It must be filtered out.

 

[AndriyOL]

What is the difference if you take just Positive and GND as unbalanced output from the dac and after transformer in regards to desymetrization?
About noise\nonlinearities, it's 108db without transformer against 120db in current mode, if I'm not wrong. You wrote this is not significant for you, than why bother with filtering, only if you are not getting higher -db in transformer voltage mode?

 

[Marek]

What is the difference if you take just Positive and GND as unbalanced output from the dac and after transformer in regards to desymetrization?

Then there is no desymetrization because there is only SE signal. Its obvious.

About noise\nonlinearities, it's 108db without transformer against 120db in current mode, if I'm not wrong. You wrote this is not significant for you, than why bother with filtering, only if you are not getting higher -db in transformer voltage mode?

Because HF noise is on the same level as audio signal, not -110dB...

An externally hosted image should be here but it was not working when we last tested it.

If you are able to hear and rest of your sound system are albe to reproduce audio with -100dB dynamic I'm jealous...Not saying about -110 and -120dB..

AndriyOL, it isn't thread about D/A converion basics. Lets focus on thread topic.

Thanks.

 

[rolls]

Lundahl MC transformers

You're right, inductance od primary winding has to match output impedance of DAC. So in case of ES9018 you have to use at least 200:XXX trafo, not 50:XXX.
And you right, my LL1931 trafo has too low inductance for ES9018 - as you mentioned it is designed for 50ohm source - I got great sound BUT with lack of lower bass...
Marek

I don't understand this. This transformer is not designed for a 50 Ohm source,
just measured this way:
quote: "Lundahl Frequency response (serial connection, source 50 Ω,
no load / secondaries open):
8 Hz -- 100 kHz +/- 1.0 dB"

Remember, this is a moving coil transformer, suitable for cartridges from about 1 to 15 Ohms, the highest would be the Denon 103, 40 Ohms.
Because of this they are perfectly suitable for our task, if their distorsion is low even at these higher output voltages, around a few hundred of millivolts.
My favorite would be 1941Ag, I use a silver output transformer the other way round, makes 1:25.

 

[Marek]

It is really not important that it is for 50 or 40 or even 15ohm.
Note that DAC output resistance and inductance of primaries makes high pass filter. Transformer suitable for ES9018 has to be at least 200:xxx because that transformer is designed to work with 200ohm source impedance.
I got my LL1931 on my desk for couple of years and unfortunetally despite its very very high performance (great mids, suberb details, 3d imaging) it has too low primary winding inductance to handle low fequencies properly...
With LL1941 things would get ever worse because of even lower primary inductance than LL1931.

 

[rolls]

It is really not important that it is for 50 or 40 or even 15ohm.
Note that DAC output resistance and inductance of primaries makes high pass filter. Transformer suitable for ES9018 has to be at least 200:xxx because that transformer is designed to work with 200ohm source impedance.
I got my LL1931 on my desk for couple of years and unfortunetally despite its very very high performance (great mids, suberb details, 3d imaging) it has too low primary winding inductance to handle low fequencies properly...
With LL1941 things would get ever worse because of even lower primary inductance than LL1931.

I have reread your postings and I think I have found the problem:
You don't see an advantage to use the dac in the current mode, because of just slighty better figures. I can't see the advantage as well, but I can hear it, it is quite a difference.
Now If you use it in voltage mode, source resistance will be 195 Ohm or so,
completely wrong for a Lundahl MC transformer.
So try current mode with two resistors of best quality, below 1 Ohm, I have chosen 0.6, use very thin copper wire, silver would be better, midpoint to ground. Then use 1:16. If the volume is too low, use 2 Ohm or even more, but the lower the better for a 9018. Try!

I have just looked into the LL 1941Ag specs:

Static resistance of each primary: 0.5 Ω
Static resistance of each secondary: 95 Ω
Frequency response (serial connection, source 10 Ω,
no load / secondaries open):
10 Hz -- 100 kHz +/- 1.0 dB

So they take 10 ohm, because 1:32 ratio is possible.
So for our task, about 1 Ohm source, it will be absolutely perfect,
and I have heard very good things from these.

 

[Marek]

Yes, I talked about voltage mode.

Sure I can try use low ohm resistors to put ES9018 in "current mode", but even with 1:16 config I'll need to additional gain stage - additional element that will affect SQ. Maybe I can use SRPP or mu-follower on some decent tube...will think about it.

What is the difference in SQ in "current mode" in comparsion to "voltage mode"? What kind of output stage you used in both cases?

 

[rolls]

Yes, I talked about voltage mode.

What is the difference in SQ in "current mode" in comparsion to "voltage mode"? What kind of output stage you used in both cases?

Difficult to write about the difference, it has just more clarity, power, dynamic.
But you are right, we have to talk about the output stage. I am lucky, I have 105db horn speakers, no need to amplify the 180mV output from the transformers more, even with passive slagleformers.
But as you have told it, the transformer has already solved 3 tasks:
filtering, conversion and unbalancing. This is very important, because I/V conversion is still the most difficult task for an active stage, especially for opamps. For unbal conversion you need another stage.
So after the transformer you need just a simple gain stage, tube or solid state,
my favorite would be the discrete opamp sold by Weiss engineering, they use it in their own dac, an ESS chip by the way.
For a cheaper solution I would use an OPA 134, because my Job amp sounds so good, I nearly could not believe, when I noticed half a year later, that it
has two 134 at the input!
So try current mode, even if it is not loud enough you can evaluate it!
Using a few Ohm is still better than voltage mode!

 

[Marek]

One thing is not clear to me.
When I use ES9018 in "voltage mode" with trafo I don't connect center tap of primaries to ground because of 1,65V output bias, and due to diffirential outputs IMO there is realy no need to.
In "current mode" with passive I/V you connect low ohm resistors to GND and because of 1,65V bias you make constant 8mA current flowing from DAC to GND from each of complementary output.
Now, it wouldn't be better to connect those resistors to 1,65V voltage source of very low impedance?

 

[AR2]

Vingborg,
I quite well understand your dilemma. I have been playing with transformers in output from DACs quite some time (as you could tell from included picture). Your thinking is correct. Kevin Carter from K&K is really awesome guy, helpful and with great knowledge about transformers. He originally turned me on Lundahl 1674, when I was trying to do output on AK4393 DACs. I did not try transformer usage as suggested in this thread - with resistors to turn DAC into current mode and transformers. Many people reported that they like it. I found the best success in using jFet buffer (balanced and no caps in a signal path) and Lundahl 1674 transformer, but used as 1:2 not as 1:4 . That way I was able to provide a proper loading to the DAC. Keep in mind that secondary loading reflects on primary and that is why that buffer works well. I have been working with ZenMod who designed that balanced buffer based on Nelson's buffer and who helped me a lot in understanding the transformer concepts.
Although there are much benefits in using transformer such as removing DC and rejecting HF noise, it is also important to know that there are disadvantages. The very fact we provide DC to the transformer is suboptimal and not best use of transformer. Also distortion in lower frequencies is relatively high - good thing is that distortion is very low in high frequencies. Even with that we like detailed and clean highs and that is the reason many people including myself enjoy it so much. Having transformers in DAC output for so many years, I decided to try just caps(very good ones) and buffer and I have to say I like it so far. I will need some time to really have some critical listening but so far nice.

 

[rolls]

One thing is not clear to me.
Now, it wouldn't be better to connect those resistors to 1,65V voltage source of very low impedance?

I am glad that I am not the only one who has sometimes problems to fully understand the output situation of this chip. I will try to explain it,
but probably there is someone who can do it better:
If you want current mode, it is mandatory, that there is always output current.
At bipolar zero, 1.65V, you have 4 * 2.112mA to ground, I can call this bias current.
If you had your bias at 1.65 V, there would be no bias current.
If you have an opamp at the output, you connect the current output always to the neg. input, which is a virtual ground. Theoretically this near zero situation is a big advantage, and this is the reason the tranny users have often been told that they are wrong!
We can not compare this dac with a differential amplifier output stage, because there is no negativ supply.
When I first tried current mode, I did nearly the same mistake: No midpoint on primary, that way the louder the music, the more current mode! Rubbish!
Everything unclear??!!