Passive EQ between Tube Stages

[systemtruck]

Hey all,

So here’s a pipe dream.

I’ve got an RCA BA-2 built, schematic below, and will be building a handful of them as channels in a mini console. I deleted the EQ, so you can just ignore/jump R11+C8.

I want to insert two shelf EQ’s between V1 and V2. It would be a shelf reduction of highs on one pot, and a shelf reduction of lows on another pot. No need for gain/boost of either range.

I believe I would be putting these on rotary switches so that you can adjust the capacitor values to select different frequencies instead of changing the resistor values for each frequency. That way the resistor values can always be stable, which in my mind sounds like a better situation for consistency of volume/loads, but I could be totally wrong of course.

It seems pretty straight forward to insert a single RC low pass shelf and a single CR high pass shelf.. choose values that work best for loads and the least insertion loss overall, and that’s that. But i would like to make each filter have more poles so that the function is a more powerful db/octave. 4 poles each would be the goal, so 24dB/octave. So this introduces some complication obviously….

I worked up something that seems to deliver, without much gain loss. A couple values are insane, and the frequencies and cap values chosen are just examples, 80Hz and 1.5k. But with these example values there is only 3.67dB of insertion loss even with both filters engaged. You can see the frequency that’s being measured at the top of the slopes, which is 360Hz, having -3.67dB. Bypassing the filters there is 0.5 dB of insertion loss still.

I changed the pot value to 1M, and inserted the low pass in the first half and high pass is second half since that made for better load relationships overall. I also multiplied each consecutive stage by 10x so that each stage doesn’t load down its preceding stage too much and cause weird performance. The original pot alone is 100K, so that’s the load that’s originally handing on the V1 output. This setup, I believe, loads V1 down to around 91K instead of 100K.

Is this just completely going to perform badly? What are the issues?

Like what about that first set of components… 10R and 10uF seems heavy or weird but i don’t know.

Is this approach just sheer madness???

Anybody else ever design a good 4-pole passive shelf situation? Let alone a DUAL one?

I would consider adding a cathode follower type tube stage into this if that is what allows me to make this happen best, but it seems like this is possible passively.

 

[ruffrecords]

You are right, many of those component values are insane which probably because you are tryiing to make the poles coincident.

I don't believe your 3.67dB of insertion loss. What source impedance are you using for the generator at the top left of your sim diagram? Even wired as a triode the 6J7 plate resistance will be several tens of kilo ohms.

I would suggest you think about whether it is necessary for the poles to coincide.

The problem with simple RC filters is they only give 6dB/octave per stage. Many passive designs use LC stages which give 12dB/octave per stage. Look up and read about constant K filters.

Cheers

Ian
.

 

[systemtruck]

Yes I was missing the output impedance which completely destroyed the whole thing when I added some, haha.

Thanks for the tips. I dove around constant K articles and landed on this…

https://en.wikipedia.org/wiki/Bartl...twork is bisected,the filter remains the same.

In Falstad I assembled the example circuit from it of a pi type low pass, shown below, and sure enough it works. The loss in the example is severe though, due to the fact that it’s going from 600 R to 50 R. (Seems like a weird example to use). So I flipped the two halves, set the output to 50R and load to 600R and it works perfectly. There is virtually no loss, and a good looking 18dB/octave slope at 15kHz. 3-pole isn’t quite the goal but perhaps it sounds good enough. Or maybe these can be cascaded.

Back to my context… I’m going to try to figure out the output impedance of V1, and will assume a 100K pot as the load and see what i can come up with for a 3 pole filter. Maybe the pot value can be increased to 250K without affecting the V1 performance too much? That would decrease the transfer loss.

I’d need to figure out the simplest and least expensive way to make it variable though. Hopefully i can dedicate either the inductors or the capacitors to be fixed, whichever is more expensive due to values, and then have the others be switched values.

 

[emrr]

I’m stuck on trying to understand the need. This doesn’t resemble any EQ i’ve experienced or longed for, regarding steepness. The theoretical plot posted most resembles what a Langevin 251 at full cut looks like, pretty sure most users are into the boost qualities with that EQ.

 

[systemtruck]

I’m stuck on trying to understand the need. This doesn’t resemble any EQ i’ve experienced or longed for, regarding steepness. The theoretical plot posted most resembles what a Langevin 251 at full cut looks like, pretty sure most users are into the boost qualities with that EQ.

This is an engineering approach of mine, for musical layering / arrangement reasons. I always want to have on hand some steep cuts of lows and highs, both for creative reasons to make extremely dark or extremely lofted via high cut, but also for somewhat surgical clutter reduction. For the latter, I like to cut upwards from the bottom in most basic sources/tracking, and I like to kill the highs as much as I can on abrasive sources.
Anyways that’s just me and how I roll. I haven’t used EQ lift / boost in over a decade. This build is just going to be for my own studio, I used to do a lot of session engineering in mid sized studios with great consoles and outboard, so I am well aware how eccentric this design is hahaha.

 

[Matador]

A regular 12AX7 can be buffered easily with a MOSFET follower to reduce the output impedance to less than a few hundred ohms without coloring the sound in a measurable way.

https://sound-au.com/project167.htm