Tube equaliser design-how to add a gain control?

[valvusmusicus]

Found this the other day-any idea what the control in the middle, between the 2 valve stages does? And how would you add gain (cut and boost) to the circuit please?

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[jordroid]

Cool little circuit, i'm not going to be tons of help (as i don't fully understand how it works), but i'll chime in.

I believe the control in between stages will act as a volume control, when it's all the way down the grid of the second triode will be grounded. The cathodes are both tied together, but since they are bypassed by the 100uf cap i don't think any AC signal from the first triode will be present at the cathodes, so turning down that middle control should silence the thing.

I'm not sure how much gain that circuit has, but if it's more than 1 then you already have a cut and boost control with the volume control in the middle. If that is not enough gain, you could add another triode gain stage, or an opamp/fet/bjt gain stage after it.

It's going to be kind of a pain to find those 250k/500k dual pots, but you could buy a 250k dual pot, dissect it and swap in a 500k wafer.

I could be wrong about pretty much any of that, ymmv etc.

regards

jordan

[valvusmusicus]

Hi Jordroid
That all sounds feasable, why didnt I think of that-ahh well-Ill go back to fixin' boats!!

Cheers again

[jordroid]

Out of curiosity, where did you find that and what is it a part of?

[valvusmusicus]

I was looking at this
http://www.preservationsound.com/?p=1875

Then found this
http://www.preservationsound.com/?p=2563 in the diyaudio forum here

http://www.diyaudio.com/forums/tubes-valves/191855-fully-parametric-tube-eq-gyrators.html

Its all quite interesting

Cheers

[jordroid]

Cool stuff.

After looking at the original schematic again i think it is actually pretty straightforward. Both triodes look like normal old common cathode gain stages, but with local negative feedback that is first sent though a tone shaping network.

For the "bass" triode the amplified and inverted signal at the anode is sent through a three stage high pass filter and then injected back into the grid, so we have negative feedback of only high frequencies. The "treble" triode is pretty much the same thing except with a low pass filter in the feedback loop.

I just noticed that if you turn the "treble Q" control all the way down the circuit will be silenced, kind of weird, the "bass Q" control won't do that.

If any of that sounds wrong please correct me, i just have a soft spot for tubes and like an unusual (to me) circuit.

jordan

[piedwagtail]

u1a = cathode follower
u1b =



http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=opamp_prj.htm

or

http://gyraf.dk/gy_pd/pultec/pultec.htm

[valvusmusicus]

An interesting marriage. And would there be any point in a tube gyrator in the forsell opamp?-Or have I missed the point
As you can see-still a bit hazy-but I'm learning!!