DIY Tube Equalizer

[stellavox]

As promised earlier in the discussion on the Cello Palette preamplifier, here is a link;

Berglund Tube Equalizer - My Photo Gallery

to a 1996 "Glass Audio" tube equalizer construction article by Rickard Berglund. I got the OK from the new owners of the successor publication to use it.

I did built it (a long time back) and as I remember, it sounded very nice (don't have it anymore). The maximum boost/cut amounts were around 6dB which was fine for most of the recordings I used it on. If you do build it, based on my current Palette experience, the frequencies I'd suggest starting with are 32, 125, 1K, 2K, 4K, and 16Khz. It also helps if you have some way to sweep the audio spectrum so you can observe each bandpass characteristic, and verify / tweak the center frequency and symmetry (as I recall, slight variation in the values of each frequencies' "C" was necessary to get the "best" symmetry).

ENJOY

Charles

[ThorstenL]

Hi,

Originally Posted by stellavox

As promised earlier in the discussion on the Cello Palette preamplifier, here is a link;

Berglund Tube Equalizer - My Photo Gallery

to a 1996 "Glass Audio" tube equalizer construction article by Rickard Berglund. I got the OK from the new owners of the successor publication to use it.

While not uninteresting, this basically still just generic feedback/op-amp circuitry.

I would propose instead to use a single section of a suitable tube, set up as "concertina phasesplitter" and attach the EQ section between the two outputs.

That way, taking the output from the anode via a (DC coupled Cathode Follower?) buffer, we can easily implement boost and cut without reliance on looped feedback. If we use the kind of adjustment range staggering like shown in the original Palette the load from the EQ sections does not become that difficult either, so an ECC88 or ECC82 etc should be usable.

Given that Panasonic still makes decent chokes in values suitable for an EQ's midrange and treble I would suggest using real LC circuits instead of tubed gyrators for MF and HF.

For the low frequencies values become infeasible large, however active gyrators can be used, I would probably resort to using cheap J-Fet's and PNP transistors as CFP instead of tubes as for a gyrator they offer better performance.

The result can be a rather simple circuit with unity gain, one dual tube per channel and maybe three gyrators and three LC sections. If there is enough demand, we could start on a community project, if we can find a low cost source for a suitable case, that does mirror the original Palette suficiently...



Ciao T