Sonos ZP-100 Custom Robinette Box for Headphones

[ikrananka]

I'm trying to put together a custom Robinette Box that will allow me to use headphones with my Sonos ZP-100 units. Sonos made the bizarre design decision to not put headphone jacks onto these units. I'm buying my wife a decent set of headphones for Christmas but now need to build an adapter box that will allow her to use them with our ZP-100 units.

I'm basing my design on a basic Robinette Box design but with a 4PDT switch that allows the speaker output from the amplifier to be either routed to our main speakers OR the headphones. I have put together a diagram of my proposed setup below. I'm certainly no expert in these matters and so would greatly appreciate ANY constructive comments on what I'm proposing. Any pitfalls I should be aware of?

[audioguru]

Sonos did not put headphones jacks maybe because if you use their Trueplay, then the Sonos is equalized for the speakers you are using, not for headphones. Then headphones might sound awful unless you can turn off Trueplay (but of course she will forget to turn off Trueplay).

You did not say and we do not know what is a Sonos ZP-100 and we do not know what are its output signals. The Sonos website shows no details and no spec's. Their website does not list a ZP-100.

Are L- and R- outputs actually grounds, or are they the other half of differential outputs from a bridged amplifier? Looking at the attenuator resistors I think they are grounds. 

[ikrananka]

The ZP-100 is a legacy product and is a standalone amp that one connects your own speakers to.  In my case a pair of bookshelf Klipsch Reference speakers.  Trueplay only applies to Sonos's own amplified speaker lineup, i.e. PLAY:1, PLAY:3 and PLAY:5.

I did make a mistake with the grounding.  I forgot to test the ZP-100 and have confirmed that the speaker outputs are balanced and not single-ended.  My diagram above would only work if the amp was single-ended.  So, I have now added a couple of resistors to the negative outputs as suggested by Rob Robinette on his website.  New diagram below.

Edited November 30, 2015 by ikrananka

[audioguru]

Your new circuit should work fine.

[ikrananka]

Your new circuit should work fine.

Well I finally managed to get all of the parts I needed and have been testing out various configurations.  Unfortunately, I am having a major issue with audible hiss, that I have been so far unable to resolve, and is even present when there is no music playing.

My initial configuration was with the resistor network set at 12.4dB attenuation.  The hiss was clearly present and so I moved to an 18.3dB attenuation resistor network but found that if anything the hiss was now louder.  This may have been my imagination but I am certain that the volume of the music was also louder at the same output level compared to the 12.4dB attenuation.

I also completely bypassed the resistor network (but still keeping the two R5 resistors in the circuit on the common ground), and found there was little or no difference to the two attenuated configurations.

The only avenue I was thinking I might explore would be to buy some wide range potentiometers so that I can really ramp up the attenuation to see if that will make a difference.  Of note is that the Sonos amplifier is a relatively modern (2005) solid state system that can output 50W per when the connected impedance is 8 ohm but can go up to 100W when the connected impedance is 4 ohm.  So, could it be that the resistor networks I have been testing have simply been overridden by an automatic increase in output from the amplifier due to the perceived change in connected impedance?

Really appreciate any suggestions on a way forward with this.

[audioguru]

50W RMS into 8 ohms is a signal voltage of 20.0V RMS.

100W RMS into 4 ohms is a signal voltage of 20.0V RMS and the headphones will sound exactly the same as with 8 ohms.

 

Maybe the Trueplay equalization for their speakers boosts high frequencies that the headphones play too loudly.