[WiP] Dudson Narrowcast - stripped down Broadcast

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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dylan159
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Post by dylan159 »

I've been playing around with making the Broadcast simpler and hopefully keeping the same character as the original but with only cheap and common parts. In the end it's not far from the usual bjt overdrive/fuzz circuit already. I don't like the original that much, I don't even care about making this a complete build for now. You could say I'm doing this in pure uncalled for spite :D and fun.
narrowcast.PNG
I have tried to remove the transformer, to replace the germanium with a silicon NPN, no charge pump, and I've removed some unnecessary components from a schematic I found online. This is obviously still a work in progress because I'd like some feedback on what I've done, and because while I have boxed many many pedals, modded amps, drawn schematics and layouts, most of my breadboarding attempts end up in failure :x this one sounded pretty good while it worked though.
My guess for the transformer role is to maybe introduce some filtering, but even though i don't trust the datasheet completely because it doesn't state driving impedance, it says it's pretty much flat across the audio range, and it's not like i can't do some filtering in another way if i need to.
Plans for the future: removing the gain switch and keeping only one position. Some online schems use an on off on switch there, I guess to have a free third gain position, but I'm pretty sure the original has only two.
I hope this is the right section to post into: I was torn between this and stompbox building, but this one isn't ready to build at all sooo :hmmm:

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plush
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Post by plush »

dylan159 wrote:I've been playing around with making the Broadcast simpler and hopefully keeping the same character as the original but with only cheap and common parts. In the end it's not far from the usual bjt overdrive/fuzz circuit already. I don't like the original that much, I don't even care about making this a complete build for now. You could say I'm doing this in pure uncalled for spite :D and fun.
narrowcast.PNG
I have tried to remove the transformer, to replace the germanium with a silicon NPN, no charge pump, and I've removed some unnecessary components from a schematic I found online. This is obviously still a work in progress because I'd like some feedback on what I've done, and because while I have boxed many many pedals, modded amps, drawn schematics and layouts, most of my breadboarding attempts end up in failure :x this one sounded pretty good while it worked though.
My guess for the transformer role is to maybe introduce some filtering, but even though i don't trust the datasheet completely because it doesn't state driving impedance, it says it's pretty much flat across the audio range, and it's not like i can't do some filtering in another way if i need to.
Plans for the future: removing the gain switch and keeping only one position. Some online schems use an on off on switch there, I guess to have a free third gain position, but I'm pretty sure the original has only two.
I hope this is the right section to post into: I was torn between this and stompbox building, but this one isn't ready to build at all sooo :hmmm:
Hi!

Well, at least, your output is DC referenced, which is pretty obviously a no go.

Also, it seems that you've messed up somewhere when converting the 2nd stage from pnp to npn.
Should look for original schematic.
Image

dylan159
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Post by dylan159 »

Well, at least, your output is DC referenced, which is pretty obviously a no go.
It's just a bit of DC because i wanted to get rid of the feedback 10u cap too. if it's too bad it can be put back.
Also, it seems that you've messed up somewhere when converting the 2nd stage from pnp to npn.
Should look for original schematic.
Seems right to me. That's the schematic I used as reference.

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Some time has passed, I've picked this back up with newfound knowledge, and it's done! I've already built it and other two people are trying to and I'm very happy with it.
narrowcast.PNG

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plush
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Post by plush »

dylan159 wrote:
14 May 2020, 10:00
Some time has passed, I've picked this back up with newfound knowledge, and it's done! I've already built it and other two people are trying to and I'm very happy with it.
narrowcast.PNG
I'm not really sure why you've left your feedback path DC coupled. It's not okay to leak DC even if it's that small.

Also, removing treble bleed 1nf cap from gain pot would alter high frequency response.

I've build couple broadcasts and they had no problems with oscillations.

dylan159
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Post by dylan159 »


I'm not really sure why you've left your feedback path DC coupled. It's not okay to leak DC even if it's that small.

Also, removing treble bleed 1nf cap from gain pot would alter high frequency response.

I've build couple broadcasts and they had no problems with oscillations.
The point wasn't really to clone the narrowcast, just have fun with it and I ended up something that according to me and others sounds pretty good.
The dc leakage is the equivalent of at most a 25k (in my case 50k) in parallel with 1k if gain is cranked. Not really enough to affect bias. You're free to use another cap though.
Some things are different with silicon instead of germanium for q2, mostly because of gain probably. That's why the fully bypassed gain setting is unusable. I really like the unbypassed one and the bypassed one usually ends up with 150-200 ohms resistance.
The 1n caused oscillation no matter what, even with the feedback decoupled like the original. I really wanted it to work but it's fine for me.

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