Passive attenuator impedance question

07-06-15: Davehrab
The IC from your passive attenuator should be very low in capacitance and as short as possible to keep from rolling off the highs ... Al can expound on this

Thanks, Dave. Good point, which is certainly often a major consideration when a resistance-based passive attenuator is being used. In this case, though, given that the cable is being driven from an impedance of less than 1K (about 900 ohms, as I mentioned earlier), I would not expect the capacitance of that cable to result in audibly significant effects under most circumstances (i.e., unless cable length is particularly long and capacitance per unit length is particularly high).

Also, the OP may wish to consider constructing the attenuator such that it can be connected directly to the input jacks of the amp, with no intervening cable. That is how the Rothwells and other such fixed in-line resistive attenuators are typically used. Although eliminating the cable, or at least keeping the cable short and using one having low capacitance, assumes much greater importance with something like the Rothwells, because the resistor values they employ are much higher than the 9K/1K values the OP will be using. Low values such as 9K/1K are suitable for his particular application because of the low output impedance of his DAC.

Something not mentioned or considered is the shift in impedance as the V/C is turned up and down and again Al can easily explain this better than I

In this case, though, note that the resistive attenuator would be fixed, consisting of just a pair of resistors. The digital volume control in the DAC would continue to be used.

... we don't know anything about the V/out of the source and your V/in of the amp.... Al can expound on ... the V/out V/in through the passive

But note the OP's reference to "the desired 20 db attenuation." Presumably he has determined that to be what is necessary.

Best regards,
-- Al

Wow thats a lot of responses! Thank you!

Just to clarify, I do not wish to use a variable (stepped) attenuator. Currently I am using between 20dB to 35dB of digital attenuation in the DAC. All I want is to introduce fixed passive attenuation of 20dB. This will allow me to use 0dB to 15dB digital attenuation. Theoretically this should reduce potential digital degradation (if is any), but the attenuator could introduce its own.

To be honest, I don't detect loss of SQ in my system even when the DAC attenuates 35dB. Maybe my system or ears are not resolving enough :-). Having said that I just wanted to experiment with a fixed attenuator (built right into an XLR male connector to minimize signal degradation). Should be a fairly inexpensive experiment even with high quality resistors.

@Almarg
I am a little confused as to which circuit to use with balanced cables. Some websites suggest soldering the R2 resistor (1K in my case) between the + and - terminals and a R1/2 resistors (9/2 = 4.5K in my case) in series with the + and - wires. Other online sources suggest that using two R1 -- R2 voltage divider networks. One between + and ground and another between - and ground. Not sure which one to go with. Thoughts?

Gmudunuri, I suspect that either of those approaches would work well, but I'd suggest going with the three resistor approach. Not only because it would use one fewer resistor per channel, but because of some probably very minor but perhaps significant theoretical advantages.

One being that the four resistor approach may cause or contribute to signal current flowing in the ground connection between the two components (i.e., in the shield of the balanced cable), which may to some extent counteract the potential advantages of a balanced interface. Another being that with the four resistor approach if the exact values of the two 1K resistors are not well matched the result might be a slight degradation of common mode noise rejection. In the three resistor configuration the exact value of the 1K resistor is non-critical. You would still want to purchase 4.5K's having relatively tight tolerances, though, perhaps 1%.

There are also differences of a factor of 2 in impedance between the two configurations, both looking forward from the DAC (favoring the four resistor configuration), and looking back from the amp (favoring the three resistor configuration). But I don't think either difference would be significant with your particular components.

Good luck. Regards,
-- Al

I just received IRC 0.1% 1/4 Watt resistors. Went with double the resistence: 2K and 9K. Will try to put together the attenuator over the weekend and report my findings.