DAC ouput using Transformer

[Steve Eddy]

I'm a bit confused.

What exactly is the goal here? To simply transformer couple the output of a PCM1793?

se

[jkeny]

Thanks WithTarragon,
I thought the DAC needed to see resistance - for PCM1793 min R is 1K8 but I built one channel with 1K8 (as part of LPF) in each differential leg to the primary windings & I built one channel with no series Rs just straight out to the primary - both sound the same. So I'm confused as the DCR of the 1:1 trafo I'm using is 400R (primary) & 400R (secondary).

I'm not sure what you mean by "right" transformer - is there not a quality ranking or are we talking about the dreaded & elusive "synergy" curse?

I just got some of the new Sabre Vout DAcs in the post (ES9022), so I'll be trying the transformers with these as well

[Pano]

Here is a little diagram. The top is for a normal amp hook up. You would just plug this into an integrated amp or a preamp.

The bottom is cap-less connection to a T-Amp. But - you will have no analog volume control. OK if you do it digitally. But if you need a normal volume pot you'll need another transformer. A TVC would be ideal here. (not the autoformer type).

[Bill Fuss]

Hi John and Everyone,
I wasn't the first in the other thread to try trafos, but I might have been the person that did the most research, and I have previous trafo experience with concert sound equipment going back more years than I would like to admit.

It's funny that the casual practice you have shown of tying a ground to a trafo primary or secondary is totally contradictory to the original need for their use in the first place, galvanic isolation.

John, your trafos are definitely not line out units if they measure 400 DCR, the nominal impedance is in the range of 10x to 20x the measured DCR so they are around 5k/5k. In your wiring diagram with one leg floating they are acting as coupling capacitors, not trafos. With a 5k trafo, try a 5k load across the secondary. Contrary to many opinions, 600ohm output trafos do not need a load. Look it up if you don't believe me.

I know nothing about T-Amps, but I know not to subject trafos to DC, it will magnetize the cores unless there is an air gap built into them. The only air gapped trafo I tried was the UTC A-22 which was fine sounding but missing a lot of low end weight.

That's my two cents, best, Bill.

[zigzagflux]

Originally Posted by Bill Fuss

Contrary to many opinions, 600ohm output trafos do not need a load. Look it up if you don't believe me.

And the times I have tried to point this out have been met with more naysayers than supporters. It appears to be mainly centered around a belief that transmission line theory applies for 20 - 20 kHz audio. So whether you are talking about a 10k:600 ohm transformer or a cat5 with its 100 ohm characteristic impedance, adding resistance/load is the better way to go.

Not sure how this belief continues to propagate among DIY enthusiasts, but they believe it very strongly.

[jkeny]

Originally Posted by Steve Eddy

I'm a bit confused.

What exactly is the goal here? To simply transformer couple the output of a PCM1793?

Steve, I guess it's what you state but also to apply LPF bandwidth limiting, DC de-coupling & convert Bal to SE without using Capacitors or OP- amps? Is there a better way? I have seen discussions on the Buffalo DAC thread about using a transformer as one of the best output stages. I also see Wavelength using transformer outputs on their Sabre implementation. These rae the motivations why I tried it. I also had a good Dave Slagle pre-amp trafo here which allowed me to experiment without shelling out for a decent transformer to try this out & I'm impressed.

Panomaniac, (Mike, isn't it?) - thanks for the diagram - I see now So I've attached my version which is different in the red circled area which just leaves one leg of the secondary floating (although now connected a 100R across this leg & ground). I'll experiment further trying to understand what's going on.

I see your 2nd Tripath diagram - I can see how the connection to biascap floats the signal at the 2.5V needed but I'm not sure how it will work without a ground - I will give it a go, thank you

[jkeny]

Originally Posted by Bill Fuss

Hi John and Everyone,
I wasn't the first in the other thread to try trafos, but I might have been the person that did the most research, and I have previous trafo experience with concert sound equipment going back more years than I would like to admit.

It's funny that the casual practice you have shown of tying a ground to a trafo primary or secondary is totally contradictory to the original need for their use in the first place, galvanic isolation.

John, your trafos are definitely not line out units if they measure 400 DCR, the nominal impedance is in the range of 10x to 20x the measured DCR so they are around 5k/5k. In your wiring diagram with one leg floating they are acting as coupling capacitors, not trafos. With a 5k trafo, try a 5k load across the secondary. Contrary to many opinions, 600ohm output trafos do not need a load. Look it up if you don't believe me.

I know nothing about T-Amps, but I know not to subject trafos to DC, it will magnetize the cores unless there is an air gap built into them. The only air gapped trafo I tried was the UTC A-22 which was fine sounding but missing a lot of low end weight.

That's my two cents, best, Bill.

Thanks Bill,
If I understand you correctly, I measured the resistance across the Primary as 400ohm DCR. The impedance is 10X to 20X this, so 4K to 8K?

Bill, I wasn't being casual about this. I had these transformers here & I decided to experiment with them & was just trying to get it to work without hum & this is how I found it to work but as I said all along, and why I started this thread, was to try understand what's going on & how to do this correctly. Of course, I lost the galvanic isolation along the way - is this the main benefit you see in using the transformer approach?

Here's what Dave Slagle said about these transformers

DCR per winding 200 ohms (400r end to end)
inductance per winding 130hy (520H end to end)
DC offset possible 1ma
10V@20hz gives you 3500 gauss (with ~7Kgauss being saturation for 80% nickel)

Driven from a 600R balanced source with a 13K resistor across each secondary and signal taken from CT to + i get bandwidth past 400K with the occasional wiggle of less than half a dB above the audio band.

dropping the load resistors to 500 ohms each gives a few dB peak at say 125K

And the transformers are bifilar wound

Bill, the Tripath use of the transformer does not put DC through the coil, or does it?

About the load needed on Transformers here's what Slagle says, for what it's worth:

Indeed transformers should be terminated on one side or the other with a low impedance otherwise the finite inductance will get you. I did come to the realization that since the bifilars do not like to be loaded, you could put your 10K (or whatever) load resistor across the primary with the same net results. (i think). The transformers will not need a load in order to behave so maybe the primary is the way to do it?? This is the same approach i have been taking lately with phono SUT's that often want to see more of a load than we like to present to iron that behaves properly in the first place. I have always found loading to hurt the sound of iron but sometimes it is unavoidable.

[Bill Fuss]

Originally Posted by jkeny

Thanks Bill,
If I understand you correctly, I measured the resistance across the Primary as 400ohm DCR. The impedance is 10X to 20X this, so 4K to 8K?

Bill, I wasn't being casual about this. I had these transformers here & I decided to experiment with them & was just trying to get it to work without hum & this is how I found it to work but as I said all along, and why I started this thread, was to try understand what's going on & how to do this correctly. Of course, I lost the galvanic isolation along the way - is this the main benefit you see in using the transformer approach?

Here's what Dave Slagle said about these transformers And the transformers are bifilar wound

Bill, the Tripath use of the transformer does not put DC through the coil, or does it?

About the load needed on Transformers here's what Slagle says, for what it's worth:

Hi John,
I completely understand your outlook, and that additional info from the designer is helpful, but you must be careful about the terms high impedance vs low impedance. In SS eqpmnt 5k is high, in tube stuff 5k is low.
The nominal impedance is generally the design parameter but does not reflect the value the source will see, that is directly reflected through the trafo from the secondary load to the source. Apparently the trafo likes a 10k load but your amp would see a 5k source impedance, possibly too high, don't know.
Getting back to basics, the designer has stated his source as 600ohms balanced which is in the ballpark of your dac chip output. The secondaries should be totally isolated from the dac board. If there is hum you might try grounding the ct of the primaries to the dac board, but I have never had to.
The basic wiring concept is simple, balanced out from the chip to balanced in on the trafo to either balanced or unbalanced out of the trafo. No grounds except for a needed reference ground for your downstream single ended equipment which is simply tying one side of the secondary to the shield of the RCAs and not the chassis. I have used both balanced and SE preamps with zero hum.
If the Tripath needs something else then you have some research to do. Try the basic setup on a different amp to gain some insight, if the Tripath is problematic.
If you are using a meter to verify the circuit output and have no signal as previously stated in the other thread then you have problems other than the trafo implementation. I don't mean to be denegrating in my statements, I just tend toward bluntness, a fault I have always had.

Best regards, Bill

[jkeny]

Originally Posted by Bill Fuss

Hi John,
I completely understand your outlook, and that additional info from the designer is helpful, but you must be careful about the terms high impedance vs low impedance. In SS eqpmnt 5k is high, in tube stuff 5k is low.
The nominal impedance is generally the design parameter but does not reflect the value the source will see, that is directly reflected through the trafo from the secondary load to the source. Apparently the trafo likes a 10k load but your amp would see a 5k source impedance, possibly too high, don't know.

Good point about the diff between SS Vs Tube. As I said these Slagle trafos were what I had at hand & just had a try with them - maybe they're not suited but the sound from them was excellent in my drawn configuration, even if they were only operating as capacitors. (I'll try capacitor output separately as an experiment)

Getting back to basics, the designer has stated his source as 600ohms balanced which is in the ballpark of your dac chip output. The secondaries should be totally isolated from the dac board. If there is hum you might try grounding the ct of the primaries to the dac board, but I have never had to.

I'll try grounding the primary CT.

The basic wiring concept is simple, balanced out from the chip to balanced in on the trafo to either balanced or unbalanced out of the trafo. No grounds except for a needed reference ground for your downstream single ended equipment which is simply tying one side of the secondary to the shield of the RCAs and not the chassis. I have used both balanced and SE preamps with zero hum.

This is the way I would also have understood trafos to work, that is before I actually tried it & got hum,no signal, etc. This threw me & I realised that I needed some expert help!

If the Tripath needs something else then you have some research to do. Try the basic setup on a different amp to gain some insight, if the Tripath is problematic.

It's just that the Tripath has 2.5V bias on it's inputs (It's a Class D amp) which is normally blocked by a capacitor - Panomaniac was suggesting a way to avoid using a capacitor by the schematic he drew.

If you are using a meter to verify the circuit output and have no signal as previously stated in the other thread then you have problems other than the trafo implementation.

No I now have signal but with the configuration I drew. Time for some more experiments

I don't mean to be denegrating in my statements, I just tend toward bluntness, a fault I have always had.

I like bluntness - it's unequivocal! There's too much double speak, spin & unclear communication around today, it's refreshing to get direct answers to questions - thank you!

Best regards, John

[rossl]

Originally Posted by jkeny

I see your 2nd Tripath diagram - I can see how the connection to biascap floats the signal at the 2.5V needed but I'm not sure how it will work without a ground - I will give it a go, thank you

I agree with Bill. The bottom diagram by panomaniac (post 13) should work without hum.

If you have hum them either something isn't hooked up right or you have a power supply fault. Or a bad transformer.

I say that without knowing what the amp input schematic looks like. Do you have a link to it?

isolating the grounds is the main benefit you will see in using this transformer approach. You shouldn't need to connect grounds between the DAC and amp.