Lovepedal Purple Plexi / Jubilee: Caps

[Plexi]

Which would be the C1 (370p) cap function there?
High pass filter, to avoid radio noise?

Will alter anything if I place C2 (4700p/4n7) before it?

[ElectricDruid]

Yes, I'd guess that 370pF is intended to short RF to ground.

Yes, it'll change something if you swap the 4700pF to the front. Pin 3 will become grounded by 2M2. That may or may not be a problem, but it's definitely not the same. Currently pin 3 is floating, which I'm told the 386 amp is happy with - it self-biases somehow on the chip. I've not played with it, so I dunno.

T.

[Plexi]

Yes, I'd guess that 370pF is intended to short RF to ground.

Yes, it'll change something if you swap the 4700pF to the front. Pin 3 will become grounded by 2M2. That may or may not be a problem, but it's definitely not the same. Currently pin 3 is floating, which I'm told the 386 amp is happy with - it self-biases somehow on the chip. I've not played with it, so I dunno.

T.

You're right.
I uploaded the Jubilee schematic, and there's no 2M2 resistor.
Is the layout that I'm using, due the simplified gain stage: work better the 250k voltage divider at the input than the 1k pot to ground (the gain increase abruptly in the last 10% of the sweep).

The color notations in the caps are from the Purple Plexi (purple) and Black Magic (black).

I'm building a 2 channels footswitcheable one: each channel with their "pre"/gain pot.

[Plexi]

I would like to add something about this two, and why I'm focused in the caps.
The Purple Plexi sounds more "scooped", and that's why the power amp is extremely filtered from the 9v ps.. due the highs and RF that the 4n7 caps let pass (C2 and C6).

[ElectricDruid]

Is the layout that I'm using, due the simplified gain stage: work better the 250k voltage divider at the input than the 1k pot to ground (the gain increase abruptly in the last 10% of the sweep).

Well, I don't know is the short answer. The slightly longer answer is that the 470R in parallel with the 1K pot is going to heavily distort the gain response, and might well cause the effect you're talking about, but I haven't done the maths. In the case of the 250K at the front, it's much simpler, but the downside is that the chip is running at maximum gain all the time and you're only turning the input signal down, so I'd expect this arrangement to yield a worse signal-to-noise ratio, since the noise is likely to be constant and significant since it's highly boosted, and you might well turn the signal down fairly low to back the distortion off.

HTH,
Tom

[PRR]

> Pin 3 will become grounded by 2M2. That may or may not be a problem, but it's definitely not the same. Currently pin 3 is floating, which I'm told the 386 amp is happy with...

Both inputs have ~~50K to V- internally, and are expected to rest very near V- level. This makes other input parts pretty optional in most cases. Here there seems to be a deliberate 4700p+50K bass-cut at 700Hz. And that could pop(tik) if switched, so a pull-down is polite.

(Internally it is a pair of PNP Darlingtons, semi-long-tail.)

[ElectricDruid]

Thanks Paul - I knew someone around here would know the detail of how the biasing is done. As I said, it's not a chip I've played with.

T.