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OK, so thanks to Kevin I'm going to be utilising a RAKK DAC passive output module, complete with volume control. I have a good quality switch so I'm thinking I'll use a switched resistor arrangement for the volume control but can I just check my understanding of the arrangement. Sorry if this is basic stuff to some of you but I'm a painter by numbers, not an artist.
The volume control resistor is in parallel with the existing I/V load resistor (C1 and 2 on the Passive output PCB) so varies the output voltage by varying the overall value of the I/V load, the lower the resistance the lower the output. As such I don't require the normal shunt attenuator arrangement, just to switch through a range of resistor values? The passive output module notes refer to using a 10K potentiometer so how does a range of resistors from 50R to 10K sound? Should the increments be linear or logarithmic? Do I have that about right?
Thanks
Ray
Follow Ups:
There's a potential problem with your plan. If you build a stepped resistor I/V control and connect it directly to the DAC outputs, you will hear a substantial "pop" sound as the resistors are switched to control the volume. However, if you connect low value I/V resistors across the DAC outputs and then follwo those with a "U type" shunt attenuator, there are no "pop" noises, presumably because the value of the volume control resistance seen by the DAC outputs (> 10K) is very high compared to the I/V resistors that load the outputs.
You are suggesting something different than the first situation described above in that you have a transformer interposed between the switchable I/V resistors and the DAC outputs. I have no personal experience with this set-up, only with the one using a shunt potentiometer, which obviously results in no abrupt changes in load on the DAC outputs as the volume is varied. The practical problem with the potentiometer application is that the value of the I/V resistor should not exceed about 5K with LL1674s set to 1+1:8 as they are on the Passive Output Stage board. 5K log pots are not plentiful...
I will take my trusty passive output equipped Mk II RAKK dac and try what you propose and see what sounds other than music occur. I'll let you know in a day to two.
Kevin Carter
K&K Audio
www.kandkaudio.com
Kevin, please don't put yourself to any trouble; I was only looking to use an Elma switch I've had hanging around for a while and I can easily use a potentiometer. The potentiometer isn't in the signal path so doesn't represent a sound quality compromise compared with the switch, maybe just a little less channel tracking accuracy. I'm a little confused by the need for a 5K potentiometer though as the documentation for the passive output module specifies a 10K pot?
One other detail though, my new amps have a sensitivity of 0.8V for full output so would there be a benefit in reducing the value of the load resistor on the passive output module.
Ray
All series and shunt resistors are "in the signal path", so you don't get a free pass there. In any case, you can use a 10K pot across the recommended 1.5K resistor. With that set-up on a Mk III dac you should get something like a 0.9VRMS output at full volume with a 0dB fulls scale digital signal, so it sounds like you are set.
My earlier comment about the 5K pot didn't involve paralleling it with a 1.5K resistor, but would result in a much greater output level than you need anyway.
Kevin Carter
K&K Audio
www.kandkaudio.com
Thanks Kevin.
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