Last week I did quite a lot of studies on my ultimate DAC + amp all-in-one design. Since I will put DIY aside for a couple of weeks (but still some minor work) I will describe my design here so that I will not forget it.
Inspiration of Retro DAC project with exception of the PSU and output stage. For PSU I will use my battery power supply Daja (maybe try some Young super-regulator later). I will bypass SLAs w/ 22000uF low ESR caps and WIMA MKPs (I can put some 5R before the caps not to have huge currents when connected to SLAs). I plan to connect Sowter 9762 transformer directly to the Iout of the DAC and then vary (volume control) voltage with the resistance on the secondary. Read this:
Yes, a transformer transforms the impedance by the square it's winding ratio. And it will transform the current or voltage by the winding ratio (voltage up current down for stepup transformers).
In other terms, if you have a DAC with an Output current of +/-1.2mA for full scale and you apply this current to the primary of a 1:10 Transformer you will give +/- 120uA on the secondary. To get a 5.65V Peak Peak signal from this current you need around 23k5 as I/V resistor. This 23k5 will be transformed into the transformers primary by the square of the 1:10 winding ratio, namely 100. So the load reflected upon the DAC's current output is around 235 Ohm.
Clearly, an output impedance of 23KOhm is impractical, so if we follow the Transformer plus I/V resistor with an active stage we can reduce the I/V resistor.
Let's use a 6922 as Valve for the analogue stage. It has a gain of around 29db with a 22k Anode load and around 3K output impedance. For 2V RMS (5.65V P-P) from the Anode we need around 200mV P-P or 100mV Peak. So we require 100mV from 120uA or 833 Ohm. Let's use 1k for arguments sake, so we require a 1:10 transformer that can be loaded with 1k and this will then reflect an impedance of 10 Ohm plus all direct current resistive losses.
While looking good on paper this is not a very practical transformer to find of the shelf, so we are back to first converting our current to Voltage with a suitable resistor (say 10 Ohm) and then stepping it up with a traditional 1:10 Stepup Transformer (for microphones and or MC Pickups) loaded with it's usual secondary load (say 10k giving a 100R Load in parallel with the 10R I/V conversion resistor) to give use 100mV Peak from the DAC's output into our Valve stage.
The idea is to connect it to the gain stage of the power amp directly, if possible. Otherwise, either Aikido 6SN7/6SL7 octal output stage or UGS V3 preamp driven via SLAs (+ huge caps)!
I will be very short here since it is very simple: UGSs V3 preamp + FirstWatt F4 output buffer. Something like this.
All is completely balanced (differential) design.
I did quite some reading on speaker protection (DC at speaker output can destroy speaker drivers). Initially I planned to use Velleman K4700, but later I found ESP P33 much more advanced.