mgm31 said:Yes, opus module is cheaper and more "modular" than the evaluation board.
The major problem is to design an output stage using tubes as the output is differential voltage.
You can see this post : http://www.diyaudio.com/forums/showthread.php?s=&threadid=116282&highlight=
I did not yet try any solution
You can use a transformer to convert differential voltage out to unbalanced output. (This would be the same issue with the Wolfson eval board as well.) Most of the more recent high quality dacs have differential outputs.
Koifarm said:Look at link below. I use his output stage with 2c51 or 6n6 tubes. They are for I/v or voltage Dac's also for symmetrical and asymmetrtical outputs.
The SRPP tubestage is optimized for a output impedance of 8K,if a other output Z is require use link to calculate.
I have done just this and I can say the result is beyond my expectations:kevinkr said:Hypothetical:
I'm thinking something like a transformer 1:5 step up loaded with about 100 ohms on the secondary, reflecting just 4 ohms differential to the dac outputs - probably center tapped primary. Depending on dac chip and resistor values chosen you could have very close to a virtual earth (1 ohm with CT and resistor value cited) on the dac outputs and 50 - 100mVrms on the secondary - a gain of 20 - 40X (26 - 32dB) in the following stage using a high transconductance, low noise triode as previously mentioned to make the required gain. Both the mentioned types have low enough RP that they will drive 10K loads quite happily without a CF. Output levels can be tweaked for standard output levels simply by adjusting the value of the load resistor on the secondary of the I/V transformer.
The SRPP circuit has several namens. Tubecad has a hole topic on it.FibCouple said:
Thanks for the links, including pictures of your mods.
I am trying to understand the srpp circuit you posted. Lukasz Fikus on his Lampizator website calls his circuit 'an anode follower with dynamic active load'. Presumably, it is same as SRPP?
On the excellent excel file you kindly posted, what is the difference between SRPP a1 and SRPP k2, which appear to have different output impedances? Obviously, on your circuit you are not using the grid resistor between pins 2 & 6 of the 6h6p, and other grid resistor (Rg) is also 47ohms rather than 10000 ohms as in the excel file.
Sorry for the newbie questions.
The output you can take from anode V1 (a1) or from kathode V2 (k2) that is the difference.
With the SRPP it is important that you now the input impedance of the amplifier you connect to. This must be the same you calculate the SRPP for. If not there will be more distorsion as wanted.
If your amp has a higher input impedance you can simple lower the impedance by parallel a resistor to match the SRPP stage.