Passive attenuator impedance question

Mr G- Something that may be of interest to you: (http://www.partsconnexion.com/controls_attenuator_khozmo_series.html)

The 121 and 321, I believe are from Wadia's current offerings. I've never used either one, so I really can't comment on how they sound. But I have had many Wadia products over the years, and still have an 861SE and a 302. The owners manuals say to keep the volume above 67 for best sound quality. They also have dip switches (861's are internal, and the 302's are external) that allow you to alter the overall gain so that you can keep the volume setting as high as possible.

Just to make sure we are talking about the same thing, when I did the test, the idea was to see if we could hear a reduction in SQ at volume settings below the recommended 67. Throughout the day of testing, no one, including myself, could hear any difference whatsoever, regardless of low low we set the volume.

Taking the above into consideration, my advice to the OP is to see if you have a problem that needs to be fixed before you start. If you hear a noticeable difference at lower volumes, then it may make sense to try a passive volume control. But if you can't hear a difference, putting the volume control in the signal path, can't do anything positive.

Just a few thoughts as I'm a passive guy ... your impedance's are fine as Al has confirmed but we don't know anything about the V/out of the source and your V/in of the amp



Because you've eliminated the ... Pre which usually takes the responsibility of driving the IC to the amp ... your source is now the driving everything



I think the IC between the Passive's output and the amp's input is much more critical than the Digital vs Analog volume control debate



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 and the V/out V/in through the passive



On the digital vs analog volume control debate ... all volume control/attenuators sound best at full volume as they have little or no resistance in the circuit to degrade the sound when set to FULL



If a digital volume controls throws away bits and compromises the sound ... then I say a analogue volume controls have to many resistors or resistance in the circuit at low volume to attenuate the volume and compromises the sound equally ... 6 of one a half dozen of another



I cheat ... after my source I use a Burson class A no neg feedback buffer feeding the volume controls in my Phase Linear D500 to do the driving



The P/Linear's volume controls are a compromise but I also have a modded P/L D500 which I've bypassed the V/Controls on and will be using a Slagle auto former set up that I'm just finishing up



The Slagle A/F doesn't suffer the bit drop out of the digital V/C or the resistance issues of a typical resistor based passive V/control and it does a better job of shifting less as you turn the V/C up or down



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

@Dave- "then I say a analogue volume controls have to many resistors or resistance in the circuit at low volume to attenuate the volume and compromises the sound equally...." The stepped attenuator I noted, as most others, only has two very high quality resistors in the path at any level setting.

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