Here is my active loaded Teflon coupled phono preamp. The inspiration for this design came from Kurt S's design. I later found out that Dan had designed the original version, so this makes the 3 variation so far (I think).

Basic circuit idea. Circuit uses 6ER5 and 6GK5 VHF triodes. Both stages are CCS loaded with my battery biased CCS's. I use the low impedance output on both stages. This allows me to have a low drive impedance for the passive EQ section and low impedance to drive the output transformer. During prototyping I measured the output impedance of the MU output of the CCS on the 6ER5 triode at 2.5ma. The output Z is 450 ohms +/-20 ohms over the audio range.

At this time the output transformers are pairs of UTC PC0-25 nickel core transformers I found at a local surplus shop. The transformers have dual primaries and dual secondarys. The impedances are 2K/500 to 600/150. I use 2 transformers per channel and have all the primaries in series and  have both pair of secondarys wired for 600 ohms CT and then put the pair of secondarys in parallel. This gives a 4:1 step down ratio. The chassis has room to put in the Lundahl amorphus core transformers when I get ready to try different iron.

I'm looking into some different triodes with lower MU to try as the gain is quite high. With my Grado MM cartridge I have to turn the volume down about 10dB compared to my CD players and tuner. The little bugger is hot!

Here are some photos. The schematics further down.

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The power supply was fun! Has a whole lot of iron and I was able to fit it all inside for a nice clean look.

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All the capacitors in the signal path are the Soviet Teflon with the exception of the 4uf GE poly in oil parafeed cap. It gets bypassed with a Teflon cap just for good measure.

The output transformer is wired so the output can float, be grounded on one end for SE output or have the CT grounded for balanced output. So far I have been using it floating. This breaks the ground between the phono pre and the attenuator. Seems to work good.

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Interesting points in the power supply. The rectifier tubes are ran from the 5 volt winding. This slows down the startup even more. The damper diodes are run at such a light load it isn't an issue. When the supply is powering both the phono pre and the line stage the total load is 60ma.

The DC supply for the triodes is derived from the 5 volt winding and half of the 6.3 volt winding. With the DCR of the filament choke the voltages worked out such that I didn't have to use dropping resistors. The heater circuit is biased positive by 35 volts to cut down on noise from the heater circuit.

I find it amusing that the heater supply has less than 2mv of ripple where the B+ is just under a volt of ripple. With the CCS fed shunt regulator power supply noise is totally out of the picture.

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