Hmm... Gain structure problems?

OK. Gain structure is the amount of amplification or attenuation applied at each stage in the signal chain, and the resulting headroom or overload.

Figure 1 shows the prototype setup I've just tested. Pretty much a possible setup...

Figure 1: Prototype Setup

From source to speaker:

• Music sources are Radio3 streamed from the Beeb, and digital music streamed from a music server (MinimServer running on an RPi)...
• Streamer is Rune installed on a Raspberry Pi, connected by USB to...
• An ODAC USB DAC, connected using RCA analogue connections to...
• miniDSP 2x4 Inputs 1 & 2, where crossover and frequency response adjustment are performed, and Outputs 1 & 2 for Woofer & Tweeter respectively on the RCA outputs, to...
• Sony amp via the Direct Inputs RCA sockets, using Front L and Surround L inputs, to...
• A 2-way test speaker which I'm using in active mode, Front L to Woofer, Surround L to Tweeter

Basically, this isn't playing very loud! I'm testing it in a small (tiny!) room, and the Sony amp is set to 0dB i.e. max. There is vast amounts of hiss from the tweeter i.e. the amp is NOISY. I've even set the Sony amp to 8Ω rather than 4, because I believe the speakers are about that. Time for some thinking, and maybe some more structured testing - bugger. Inevitably, I was hoping it would just work! Silly me.

Right. Here's the input/output specs of the various devices that count:

• ODAC
• Output: 2.1V RMS into 5KΩ or higher
• miniDSP 2x4
• Input: 2.0V RMS, 6KΩ 
• Alternative setting 0.9V RMS, could change this, but need to ensure no overload
• Output: 0.9V RMS (2.5V P-P), 560Ω impedance
• This is fixed, I can only turn it down from there! 
• 2.5V P-P => 1.25V peak => 1.25*0.707 RMS => 0.88V RMS, so that's cool
• Sony STR-DB1070
• Input: 150mV, 50KΩ impedance
• SNR is only 96dB, hmm
• Output: Allegedly 90W into 8Ω
• This is 16.4V RMS, from V=IR, I=V/R, W=VI, W=V2/R, V=⍻(WR)
• Amplifier gain, from 150mV RMS input is therefore 20*log(16.4/0.15) = 41dB
• SL reckons you need a damping factor of >20; there is no hint of this on the amp spec!

I think I need to run some tests, using a known signal level, probably generated by REW directly into the ODAC, that will show the levels in the miniDSP. Back to the hut...