I´m building a 2 stage phono preamp using a Siemens D3a (triode) as first tube, and a half russian 6H6P as the second.
Not decided yet if I shall go for passive or active load.
I have about 220-250 Volt well filtered B+ to play with. What idle current would you chose for best sound?
Not decided yet if I shall go for passive or active load.
I have about 220-250 Volt well filtered B+ to play with. What idle current would you chose for best sound?
The whole (include RIAA compensation) phono stage requiring -at least- 40dB gain (100x).
The RIAA "module" has -about- -20dB, so the all VAS stages gain must be -at least- 60dB (in reality too).
Trioded D3a has about -theoretical- 70x gain (37dB), 6N6P only 20x (26dB).
The built stages has lower gain (about 35dB, 22dB), so output lower than expected.
I suggest to build D3a, passive RIAA, D3a, 6N6P cathode follower structure.
The D3a "sweet spot" -for low anode swing- for example 140V, 20mA (-1.25V bias).
Ri is 1.9kOhm, so anode load must be about 4-5x more (for really low distortion).
On 7.5k - 10k anode load at -about- 20mA the voltage drop 150-200V.
If Ua at about 140V, and anode load voltage drop 150-200V, the estimated B+ is 290-340V.
The RIAA "module" has -about- -20dB, so the all VAS stages gain must be -at least- 60dB (in reality too).
Trioded D3a has about -theoretical- 70x gain (37dB), 6N6P only 20x (26dB).
The built stages has lower gain (about 35dB, 22dB), so output lower than expected.
I suggest to build D3a, passive RIAA, D3a, 6N6P cathode follower structure.
The D3a "sweet spot" -for low anode swing- for example 140V, 20mA (-1.25V bias).
Ri is 1.9kOhm, so anode load must be about 4-5x more (for really low distortion).
On 7.5k - 10k anode load at -about- 20mA the voltage drop 150-200V.
If Ua at about 140V, and anode load voltage drop 150-200V, the estimated B+ is 290-340V.
The best way, i think, is two D3a, first stage gain
Second stage half 6N6 in cf in dc with D3A, to drive a riaa network
the second D3A then another cf with second half of 6N6
In this way you can use a low impedance Riaa network anf the D3A always see a costant load .
One example with my Junior with different tubes
Walter
Second stage half 6N6 in cf in dc with D3A, to drive a riaa network
the second D3A then another cf with second half of 6N6
In this way you can use a low impedance Riaa network anf the D3A always see a costant load .
One example with my Junior with different tubes
Walter
In linestage duty, 15-20 mA per 6H6P has worked quite well.
I recall Gary Pimm building with mu-follower CCS-es, with their output impedance set by a series resistor( feeding the RIAA filter ). The output stage had no series resistor as there is no need for a managed output Z. I think one of them ran a pair of 6ER5's and found plenty of gain.
Douglas
I recall Gary Pimm building with mu-follower CCS-es, with their output impedance set by a series resistor( feeding the RIAA filter ). The output stage had no series resistor as there is no need for a managed output Z. I think one of them ran a pair of 6ER5's and found plenty of gain.
Douglas
