DAC ouput using Transformer

[WithTarragon]

Originally Posted by panomaniac

A resistor load across the primary does present a more even impedance to the DAC. As to whether or not the DAC cares, well........

As a general statement, yes the DAC may care.
The DAC will have a spec sheet where it will state the minimum load. With out a sufficient impedance load, the amp (or whatever) may try to draw current. The DAC will not be designed for that. What you are looking for is a transfer of voltage not power.
Additionally, the DAC may have problems driving a capacitor (usually a necessary part of the the anti-aliasing circuit).
Have a look on the 'scope when the DAC drives a capacitor with and without a resistor. The signal may become very distorted ( I am not referring to the RC filtering).

[jkeny]

Originally Posted by Bill Fuss

Hi John,
Did you have the opportunity to look at the responses on a scope? My Sescoms have a prominent resonance up around 200khz that does show up in the top couple of octaves of the audio band. The Jensen secondary RC filter completely eliminated it.
You might try 1kohms across the primaries. That would be in parallel with the dac chip output circuitry so the trafos would see around 500ohms depending on the series resistance you are using.
Do you think the shimmer you refer to might be an artificial modulation artifact or actually be in the source material.

Best, Bill

Hi Bill,
BTW, thanks for pointing me to the Sescoms on eBay - fantastic value-for-money at $50. I will put a scope on them - as a noobie in scope use, do I want to output a sine wave from the laptop through the DAC & trafos, at what frequency, 200KHz?

I'm not using any series R yet so I'll try the 1K across primaries.

When I say shimmer, I mean in a good way like a crystalline clarity not a modulation. But I'm aware that odd order harmonics can give rise to a mid-range/HF forwardness that can seem like enhanced clarity. I'll scope the Slagles too!

[jkeny]

Originally Posted by WithTarragon

As a general statement, yes the DAC may care.
The DAC will have a spec sheet where it will state the minimum load. With out a sufficient impedance load, the amp (or whatever) may try to draw current. The DAC will not be designed for that. What you are looking for is a transfer of voltage not power.
Additionally, the DAC may have problems driving a capacitor (usually a necessary part of the the anti-aliasing circuit).
Have a look on the 'scope when the DAC drives a capacitor with and without a resistor. The signal may become very distorted ( I am not referring to the RC filtering).

Thanks Tarragon,
The PCM1793 DAC wants a minimum 1k8 load so I guess a 1-2K across the primaries would be a good starting point. Do I need to put some series R in there also? Just need to get the clock mod working on the unit before I can proceed M2TECH Hiface USB->SPDIF 24/192Khz asynch

[Bill Fuss]

Just put your signal gen across the primary with a 1K resistor in series with it and sweep the frequency.

The dac chip and the trafos both need some series resistance to operate properly. Looking from either direction, the series resistors are part of the load they see. The trafo sees the dac chip source impedence plus the series Rs as a whole, as a parallel circuit placed across the primary. The dac chip sees the trafo impedence and the Rs as a parallel load across it's outputs. Optimum values are probably not attainable, any workable value would be a compromise unless the trafo was designed expressly for the chip output.

[jkeny]

Originally Posted by Bill Fuss

Just put your signal gen across the primary with a 1K resistor in series with it and sweep the frequency.

The dac chip and the trafos both need some series resistance to operate properly. Looking from either direction, the series resistors are part of the load they see. The trafo sees the dac chip source impedence plus the series Rs as a whole, as a parallel circuit placed across the primary. The dac chip sees the trafo impedence and the Rs as a parallel load across it's outputs. Optimum values are probably not attainable, any workable value would be a compromise unless the trafo was designed expressly for the chip output.

Thanks Bill,
I'll be doing the signal gen on my PC & running this through this USB DAC which contains the PCM1793. So connecting the scope to the secondary trafo outputs & doing a freq sweep should do it.

I understand your second paragraph, thanks!

[jkeny]

Double post - removed

[jkeny]

Success with the clock mod - too late now to do anymore but listen to some music - Herbie Hancock River (the Joni Letters) - fantatstic!

[jkeny]

Sescoms in circuit - white top is the tube octal socket that the Sescoms plug into - no load Rs in circuit yet

[revintage]

Hey jkeny,

The PCM1793 DAC wants a minimum 1k8 load

Where did you get those figures from? The TI datasheet clearly states 2*1,7k. Their example includes 2*1,8kohm.

[jkeny]

Lars,
You're correct it's 1.7K on each differential leg - that's the problem in quoting from memory - my DAC used 1.8K on each leg as per their example.