Add a subwoofer output to driver stage

[gerrittube]

Hi,

I'm about to add a subwoofer output to a 6SN7 driver stage (Williamson style). U1a is the amplifier; U2b is the phase inverter.
Too keep things simple I added a STP8NM60 MOSFET (or similar) as source follower. My idea is that the MOSFET will lot add a lot of load to the first triode (U1a),

According to LTSpice this seems to work OK. With only 1.1 mA and approx. 300 VDC across the MOSFET the dissipation is only 0.33 Watt.

Note: The original 100K plate resistor for U1a was split into 2 resistors of 85K and 15K. Adding C5 creates a short for AC. The reason for this change is to lower the gain without changing the DC settings.

Any additional suggestions will be welcome.

Regards, Gerrit

 

[gerrittube]

Work in progress… Only some small deviations from the schematic.

Tests OK, sounds OK, runs from any voltage it seems (tried 6.3 VDC up to 400 VDC).

 

[artosalo]

Note: The original 100K plate resistor for U1a was split into 2 resistors of 85K and 15K. Adding C5 creates a short for AC. The reason for this change is to lower the gain without changing the DC settings.

15k is heavy load for 6SN7 and makes the linearity worse.

 

[gerrittube]

It didn’t show any additional distortion when measuring. But I’m still working on this so I can check this one more time.
At least the DC part of the circuit is not affected.

As for the subwoofer output I set the gate to half the B+ with two 1M resistors and use a capacitor for input. B+ for the MOSFET can be anything.

 

[jcalvarez]

Hi,

I'm about to add a subwoofer output to a 6SN7 driver stage (Williamson style). U1a is the amplifier; U2b is the phase inverter.
Too keep things simple I added a STP8NM60 MOSFET (or similar) as source follower. My idea is that the MOSFET will lot add a lot of load to the first triode (U1a),

According to LTSpice this seems to work OK. With only 1.1 mA and approx. 300 VDC across the MOSFET the dissipation is only 0.33 Watt.

View attachment 1437852

Note: The original 100K plate resistor for U1a was split into 2 resistors of 85K and 15K. Adding C5 creates a short for AC. The reason for this change is to lower the gain without changing the DC settings.

Any additional suggestions will be welcome.

Regards, Gerrit

I'd be careful if U1a is included in an NFB loop. The plate signal may be distorted by it to correct the output stage distortion at higher volumes.

 

[gerrittube]

I don’t use NFB.