contact resistance of rotary switch ?

[Bernhard]

The advantage in my case is I can have an output with volume control from a current source DAC with a transformer, a switch and a resistor, and the lower the volume, the lower the distortion.
Currently my I/V resistor is 1K, but I chose that because I have a preamp with gain.

Doug

Exactly what I do, so you have 1 kohm I/V on the transformer secondary ?

Any link ?

 

[DougL]

Here is the DAC I have RAKK DAC
Here is the Schematic I followed.
RAKK DAC passive output
However, I prototyped it with a 1k resistor and an inexpensive 1:4 interstage 600:10K Edcor.

I was looking to replace the resistor in the drawing with a shunt attenuator with 1.5 db steps.

Doug

 

[tedr]

Possible problems:

lowest volume step -60dB = 0,031 ohm

+ 0,005 ohm variation in contact resistance = -58,8 dB

second lowest volume step-57,5 dB = 0,043 ohm

- 0,005 ohm variation in contact resistance = -58,3 dB is very close to -58,8 dB, so the last steps could become very uncertain.

If the variations are that huge. In that case I would use two switches with one 0,062 ohm in parallel with each switch.

Also as mentioned before, I can use 62,5 ohm for 0 dB and 0,062 for -60 dB if I want

When the attenuation is 40 dB or more it may not matter if the value is not so precise because you are not listening with such care.
I like your picture of the switch "in Seimens I trust" it is a pretty machine.
Good Luck

 

[Bernhard]

However, I prototyped it with a 1k resistor

The transformer behaves like a low pass filter, so the DAC chip does see a low impedance for audio frequencies but what happens if the impedance goes up above 30 kHz ? HF level will go up but will again be filtered by the transformer, so maybe it does not matter ?
I feel a bit unconfortable with I/V resistor on secondary.

 

[DougL]

The transformer behaves like a low pass filter, so the DAC chip does see a low impedance for audio frequencies but what happens if the impedance goes up above 30 kHz ?

My understanding is the HF roll off is due to the stray capacitance's in the windings, and that the Z will drop, not rise at HF.

I feel a bit unconfortable with I/V resistor on secondary.

There are some advantages, such as a 16 fold reduction in the effect of contact resistance. (in the case of a 1:4 step up) I am happy with mine on the secondary, but YMMV.
Remember that the resistance of the primary is effectively in series with the resistor on the secondary.

Regards;


Doug

 

[Bernhard]

Maybe I stay with fixed resistors for I/V and use a TVC on the output, that gives the lowest possible output impedance at any volume setting.