Where to add output pot between EQ and output?

[BluegrassDan]

Hey guys. Where would be the best place to incorperate an output attenuator pot in this design?

Using a cathode follower as makeup gain following EQ section, and following that with a White cathode follower in order to be able to use a 1:1. All of this tests very well and I want to keep this topology, but I'm having trouble figuring out where to incorporate an output attenuator.

If you put it after the EQ and before V3 it will screw with the EQ. Between V3 and V4 you get cathode voltages creeping in. After C35 and I worry that Miller Effect will roll off high frequencies and would counteract the benefits of the cathode follower.

I'm starting to wonder if it can be done without some detrimental effect.

Thoughts? (schematic attached)

 

[rock soderstrom]

I misread your schematic, I should go to bed 8)

 

[ruffrecords]

If this EQ is the stripped down Helios you posted about earlier then the core of it is a pot divider consisting of a 22K resistor and a 5K1 resistor. This basically means the output impedance is about 5K. I can see two solutions.

1. Replace the 5K1 resistor with a 5K pot and take the wiper to the grid of the next tube. The problem with this is pot end to end resistance is not well controlled (typically 20%) so you might get a +-2dB variation in insertion loss.
2. Leave the 5K1 where it is and take the EQ out via a 100K pot to the grid of the next tube This should at most increase the insertion loss by about 0.4dB but it should be fairly repeatable.

Cheers

Ian

 

[BluegrassDan]

If this EQ is the stripped down Helios you posted about earlier then the core of it is a pot divider consisting of a 22K resistor and a 5K1 resistor. This basically means the output impedance is about 5K. I can see two solutions.

1. Replace the 5K1 resistor with a 5K pot and take the wiper to the grid of the next tube. The problem with this is pot end to end resistance is not well controlled (typically 20%) so you might get a +-2dB variation in insertion loss.
2. Leave the 5K1 where it is and take the EQ out via a 100K pot to the grid of the next tube This should at most increase the insertion loss by about 0.4dB but it should be fairly repeatable.

Cheers

Ian

Ian, your first options sounds like a good one. Could I also use a lower value pot in series with a resistor in order to limit the amount of attenuation (6dB or 10db for example)? Maybe a 1k pot with 4k resitor?

 

[BluegrassDan]

Hmmm....

I have currently omitted the EQ in/out DPDT switch in favor of leaving it IN at all times. This means the 22k and 5.1k voltage divider is integral to the "mid" control, right? So maybe option 1 is not so good.

Will a 100k pot after the EQ mess with frequency response? I'm thinking particularly in terms of Miller effect on V3.

 

[ruffrecords]

Hmmm....

I have currently omitted the EQ in/out DPDT switch in favor of leaving it IN at all times. This means the 22k and 5.1k voltage divider is integral to the "mid" control, right? So maybe option 1 is not so good.

That's right, the 22K/5K1 divider is integral to the filter - without it it does not work

Will a 100k pot after the EQ mess with frequency response? I'm thinking particularly in terms of Miller effect on V3.

Unlikely. Worst case the 100K pot looks like 25K from grid to ground. Even with 100pF of Miller capacitance the -3dB point would be way out at 66KHz.

Cheers

Ian

 

[BluegrassDan]

Ian,

What about swapping the 100k resitor for a pot at the front of the EQ? That's R13 in this schematic.

 

[ruffrecords]

Ian,

What about swapping the 100k resitor for a pot at the front of the EQ? That's R13 in this schematic.

That 100K is mostly just to provide the caps with a dc path to 0V.  The EQ really needs a low impedance to drive it. In my tube mixers I often have the EQ following a 10K pot (fader) which make the driving impedance about 2K5 worst case and they usually work fine. The question is can your cathode follower drive a 10K load?

Cheers

Ian

 

[BluegrassDan]

Possibly. My prototype right now has a 25k pot between V1 and V2, and I’m planning to try a 10k in that position. I’ll breadboard it sometime next week and see.

So, it looks like the two best options are either a 10k ahead of the eq or 100k following. I’ll report back once I can get around to testing it.