How to choose an attenuator

Both your Amp2 and CAV180 have very high input impedances, perfectly suited for passive preamps (which a fixed attenuator is in a way) just that you can't adjust it.
 
Why not have it all, and have an adjustable one like the Schiit Sys, $49
http://www.schiit.com/products/sys

Cheers George

Thanks georgehifi, but that comes with RCA. My whole idea was to be able to used the balanced outputs. When I use SE outputs the Vrms is down to 2 and is more manageable anyway.

There are 10, 15, 20, 25, 30 etc dB attentuators and I would prefer to just buy one pair with the correct values and forget about them.

According to a calculator (http://aetechron.com/calculator-Volt2dB.html) a 10dB correction should be right, but then there are other criteria like impedance so I get puzzled a bit.

Why would you use attenuators if your Sonica has a variable output?

I want to bypass the volume control of the DAC, and control the volume from the app, while the information is still purely digital.

Also, when I don't use the bypass mode then there is always the danger of someone (i.e. the kids or some guests) would put the volume too high and damage the system. Especially on the app it is very easy to do so just with a slip of the hand.

You are correct that 24 db of attenuation would be way too much, and would not allow you to utilize a good deal of the amp’s power capability, even with both of the volume controls that are in the signal path set at max. You are also correct that 10 db would reduce a 4 volt signal to close to 1.4 volts. Based on the relation db = 20 x log(V1/V2), where "log" is the base-10 logarithm, 1.4 volts is 9.12 db less than 4 volts.

A pair of 10 db balanced Rothwell attenuators would do the trick, and would not result in any impedance issues given the 100K balanced input impedances of the amps you mentioned, and the unspecified but presumably fairly low output impedance of the DAC. I’ve measured the single-ended version of that attenuator as consisting of a 22K resistor in series between its input and output, and a 10K resistor in shunt between its output and ground. I’d suspect that the balanced version has a similar pair of resistors for each of the two signals in the balanced signal pair, which in combination with the 50K per leg input impedance of the amp would result in the DAC seeing a load impedance of about 30K per leg, equivalent to a balanced load impedance of 60K.

I’d avoid attenuators from other manufacturers, unless the values of the resistors they use are known and can be determined to be suitable. Based on measurements that have been reported here in the past, I can state for example that the resistor values used in Harrison Labs attenuators are too low for comfort in this and many other applications.

All of that being said, in general it is desirable to have the capability of providing a power amp with somewhat more input voltage than is required to drive it to its maximum power capability, in part because if the specs that are involved are not entirely accurate you may find yourself running out of range on the volume control (depending of course on a number of other variables such as speaker sensitivity and preferred listening volume). So you may want to consider not using an attenuator, and simply setting the volume control 10 db lower than you would if an attenuator were used. But on the other hand I understand your concern that if the amp were inadvertently driven into a severe clipping condition it could result in a destroyed tweeter or other damage.

Regards,
-- Al

Thank you very much Al, that answers my question. I think I'll get a pair of 10 dB attenuators and give it a try. Rothwell, Shure and Audio-Technica seem to have similar price levels.