The Schiit Freya+ is a new breed of chameleon-style preamp. It does whatever you want: Unbalanced to balanced conversion or vice versa, tube or transistor, buffer or amplifier with gain, relay switched volume, remote control, the Freya+ has it all.
Part 1: Tube Rolling
Of course you can roll in or roll out all existing 6SN7 variants of the world. But if you are not really an aficionado of the 6SN7, which applies to me, there are other tube types which you can use with the help of ready made noval/octal tube adaptors.
But one thing you have to keep in mind: The 6SN7 has 0,6A heaters and the heater power supply is providing unregulated DC using passive RC-filters, which I do like as an engineering approach in general. But if you select other tube types you have to take care that the sum of heater currents of all 4 tubes is 2,4A. Otherwise the heater voltage will be too high or too low. Also the tubes you select for the voltage amplifier stage (tubes to the right if you look from the front of the preamp) which run at appr. 7mA need to be medium-µ, which means 15 to 20. The cathode follower tubes run at appr. 2mA and need to have high transconductance at this relatively low anode current. µ doesn’t matter for a cathode follower.
The high voltage power supply is also based on RC-filters. But you don’t have to worry about tube currents as they are determined by constant current sources. Whatever tube type you roll in the tube will allways run at the same current.
My idea was to use the 5687 in the differential amplifier and the ECC81 as cathode followers. There are many very different sounding variants of the 5687 and I’m still not settled regarding this tube. The Tung-Sol sounds slightly too rich at the bottom and General Electrics are adding somewhat too much of emphasis on the mids. But I love what the 5687 tube does in all other areas. An alternative at this position could be the Russian 6H30 or 6H6 but I have not tried those types in the Freya+.
The 5687 consumes 0,9A heater current which leaves 0,3A heater current per channel for the catode-follower tubes. Perfect conditions for the ECC81 tubes. The ECC81 has a relatively high transconductance which leads to low output impedance and works at relatively low currents. I settled on the Telefunken ECC801S which for me is a sort of blameless ECC81. An alternative at this position could be to use the ECC88 (or 6922, 6dj8 etc.) which has even higher transconductance. I have tried that but I found the ECC88 hides details which I hear with the ECC801S.
Part 2: Modding
The WIMA MKP10 are decent audio capacitors but they are not really close to the quality summit of audio capacitors. Freya’s PCB leaves enough room for installing 4 V-Cap ODAM 2,2µF / 400V capacitors instead of the WIMAs. The ODAMs are not cheap but their biggest advantage apart from the sound is their small space requirement. I have used ODAMs in some other projects and for me they sound as they are not really there. A very close to ideal audio capacitor. The WIMAs in comparison appear to me as driving with a slightly applied handbrake.
The second modification is to replace the 8 pieces of 2W Dale metal film resistors with 2W Audio Note Tantalum types. There are 4 pieces of 22k resistors in the differential amplifier´s anode connection and 4 pieces of 47k resistor in the cathode follower´s ground connection. These tantalum resistors are also not cheap but their impact is easily heard. There are even two other even more expensive resistor types from Audio Note available. Still, they need some significant time to break-in and results right after installation are quite poor. The sound difference from the Audio Note to the Dale is not as big as the difference in capacitors from MKP10 to ODAM.
And yes opening Freya’s case can be a pain. You have to pull off the volume control knob and release the 11mm nut from the potentiometer. Then give the Freya a few mindful blows with a rubber mallet on the edge of the top plate from the back side. The top plate will come of and release the 3 push buttons to fall out. Reinserting those buttons is fun too.
Keep in mind that performing these mods on your Schiit Freya+ voids your guarantee!