Originally Posted by ModMax
Wait just a minute.... okay, I flipped the input select switch and suddenly I hear my music - a lot of it and it sounds pretty damn good.
Chain is still Foobar 2K (WASAPI: Speakers (USB Audio DAC)) --> grubDAC (internal) --> MOSFET-MAX --> Grado SR60.
I am confused though. This position of the input select toggle switch on my front panel breaks the connection between IS-1 and IS-2 on the MOSFET-MAX. I thought breaking this connection would turn on SW1 (which my RCA jacks are connected). But apparently IS-1 --> IS-2 break turns on SW2. Does this sound correct?
Also, with the front panel input select switch flipped to the position which connects IS-1 to IS-2, which turns on SW1, should I hear any sound coming from my grubDAC (connected to SW2)? I do hear some sound which as I mentioned in my post above has a humming / static noise in the foreground and very faint sound of my music in the background. And the humming / static sound is affected by touching and moving the L/R/Gnd wires coming from the RCA inputs and grubDAC L/R/G connections.
Should I not have grounded the MOSFET-MAX PCB to the case bottom via center standoff and the rear panel via RCA jacks?
Do I need to ground the volume pot to the PCB via the set screw on the top right corner of the volume pot and ground pad near the pot on the PCB?
The switch energizes both relays. However, the input connections to the pot go through the energized poles on SW1, but through the de-energized poles on SW2. IOW, the input block closest to the right side of the PCB (looking from the front or volume pot) is connected with the relays de-energized or OFF. The input block on the inside or left (when viewed from the front or volume pot) is connected when the relays are energized or ON.
If you have a signal going through the GrubDAC at the same time you are connected to signal through the other input block, yes - you will hear some bleed-through. The main reason this happens is that the volume travel/gain on the amp is really over 100% at 12 o'clock on the volume pot. There is not a source nor headphone in existence that will require a volume setting much beyond this. So, you are really into the >120dB regime if you are turning up the pot enough to hear the music bleed-through from the Grub. Still, the input relays are for convienience, only - not to fully shut off any chance of active signal between the two input blocks. So, if you switch from one to the other and then listen for extended periods, I would go ahead and shut off the other music source. This still gives you the convenience of not having to connect/disconnect RCA cables..
Note that isolation for the relay contacts is about 95dB at 1MHz and increases exponentially at frequencies below that. In the audible band, it should be well above 120dB.
I'm not sure about hearing differences when you move the wires around. That sounds like bad connections or at least, bad connections with the ground wires.
Yes, you should ground everything you mentioned, but truthfully, you shouldn't notice an ill effect just with the bare PCB - except for the signal ground not making a good connection at the input blocks. With that, you will definitely notice noise - sometimes very bad noise.
The pot ground is simply an irritant issue, IMHO, although to be perfectly correct - it should be grounded. 9 times out of 10, the pot will ground itself through the shaft and the case endplate. However, if it doesn't make this connection or you don't install the ground wire, then you will get some hum if you touch the volume knob (assuming you use a conductive knob).