Dudson Narrowcast V2

From the vendor

Dudson Narrowcast V2

Oct 6, 2022 5 min read

This is a circuit I've made a little more than a year ago, as an answer to a very popular distortion pedal based on preamplifiers of the old days (tip: that one is "broad", this one is "narrow"). While another circuit that qualifies as being derived by said desk inputs is none other than the old Fuzz Face, I want to give credit to the claim because what we are dealing with is a transistor pair with a feedback network wrapped around it, of the kind that was very popular before op-amps to get close enough to the ideal amplifier. You can learn more here.

There are many things about the original I wanted to change, starting the use of the transformer of which I strongly object the necessity:

• it's not providing isolation because the cold wires are connected;

• it's not providing impedance or voltage benefits because it's 1:1 ratio;

• it's not providing bandwidth limiting or significant distortion because it's used with a suitable source impedance and level. A worse transformer than the one used might have more of an effect.

You don't have to take my word, I took hold of a "10k" 1:1 transformer to test if there was any difference and recorded it. I won't tell you which is which both because it's a blind test and because I honestly don't remember.

Other than that, the inclusion of a charge pump, the values used and a somewhat excessive complexity asked for action. What I didn't want to change was the sound, people seem to like that part. That's how I ended up with my version:

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Actually there's two of them, one with NPN and PNP transistor like the original, and the one I actually built with two NPN transistors. Other than polarities, the other difference is that Q10's collector needs to be biased close to ground to correctly bias Q2 in the NPN version, close to Vcc in the PNP version. I leave you free to experiment with different transistors if you deem it time well spent, the 3904 is a stand-in for any generic NPN transistor and anything should work without much trouble, as explained later about bias stability.

Altough different at first glance, like the broader brother this is a series-shunt feedback amplifier (feedback is in series with the input signal, in parallel with the output). The feedback is AC-only.

• Base bias is taken straight from the collector, instead of a tap on the collector load. Since the 1k is bypassed, the collector load is made closer to the unbypassed 5.6k. This network doesn't load the amplifier significantly. The 220k matches the original bias resistor, and together with the increased feedback (straight from the collector instead of attenuated by the tap) using only two resistors here resulted in a stable bias even with different hfe and voltages. This feedback is DC-only, so there's no Miller effect, and this also means a quiet bias.

• On Q1's emitter, the parts count is reduced without changing function. The 5.6k is bypassed by the parallel capacitor and d