What's Going on with Catalinbread's CB30?

[Bio77]

I'm working on Aion's layout of the CB30 this weekend. The circuit has some weird features that I haven't ever encountered. Does anyone know what's going on with the (C11,C9), (C19, C18), (C24, C23) pairs? They look like a BP electro workaround but having the audio coming out of the middle of the two is something I've never seen.

[jimilee]

That is interesting. I'd like to know myself. The only thing I can find that's close is biasing.


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[Bio77]

That is interesting. I'd like to know myself. The only thing I can find that's close is biasing.


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Agreed.  I think there is some deep cut pedal knowledge in this weirdness.

[aion]

This is mostly just because of the way I drew it. Look at the top one as the gate-source cap for the upper JFET (present in all mu-amp topologies) and the bottom one as the drain-gate feedback cap of the lower JFET that cuts highs. The lower one in each stage also has a series resistor to reduce its effect. Almost all of the CB mu-amp pedals have the same setup for their tube emulation stages, but usually they're lower-value film caps so it's more obvious what they're each doing.

[Zerro]

I guess it is about BIASing. Making current source for transistors working as amp stages. It gives better qualities for stable current no matter how gate - drain of that j-fet is set. Results in much lower noise and distortion at D-S pass. Something similar I encountered in electret mics current sources. You can sit here only simple resistor for power for mic. But you can put here such a current drived source, so that j-fet inside of mic can work under stable regime, which is primarly impossible for very specific setting of capacitor charge in that mic. See pict I attached.

[Bio77]

This is mostly just because of the way I drew it. Look at the top one as the gate-source cap for the upper JFET (present in all mu-amp topologies) and the bottom one as the drain-gate feedback cap of the lower JFET that cuts highs. The lower one in each stage also has a series resistor to reduce its effect. Almost all of the CB mu-amp pedals have the same setup for their tube emulation stages, but usually they're lower-value film caps so it's more obvious what they're each doing.

Ahh OK, so, the weirdness is just that it's electros. It's the orientation of the bottom caps that looks weird to me.  Thank Kevin!

Follow up question:  If I use an electro cap for  C26, should the + side connect to the volume pot?

[jimilee]

This is mostly just because of the way I drew it. Look at the top one as the gate-source cap for the upper JFET (present in all mu-amp topologies) and the bottom one as the drain-gate feedback cap of the lower JFET that cuts highs. The lower one in each stage also has a series resistor to reduce its effect. Almost all of the CB mu-amp pedals have the same setup for their tube emulation stages, but usually they're lower-value film caps so it's more obvious what they're each doing.

Ahh OK, so, the weirdness is just that it's electros. It's the orientation of the bottom caps that looks weird to me.  Thank Kevin!

Follow up question:  If I use an electro cap for  C26, should the + side connect to the volume pot?

Right right. I need to look at more muamp topologies to become more familiar.

If I understand it right, negative should go towards the volume, if not, the signal wouldn't be able to pass through. That's a big if, I've been looking at schematics for a couple of months. Think about the feedback loop in the rat as an example. Again, that's IF I understand what I'm looking at correctly.


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[Bio77]

Right right. I need to look at more muamp topologies to become more familiar.

If I understand it right, negative should go towards the volume, if not, the signal wouldn't be able to pass through. That's a big if, I've been looking at schematics for a couple of months. Think about the feedback loop in the rat as an example. Again, that's IF I understand what I'm looking at correctly.


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From what I have read, an electro needs a bias voltage to work properly, that voltage needs to be oriented correctly.   I don't think the signal cares which way it's going through the cap, as long as that bias voltage is there.  But grain of salt here, I am pretty far out past my knowledge base now.  My thinking is that C25 has a positive voltage on top, which draws a negative voltage on its bottom, so C26 should be oriented with the negative towards C25 and the pos towards the pot and ground.

Kevin had a 2n2 film in that spot, but I don't have any on hand.   I'll probably just try both ways and see if there is a problem.  Maybe neither is going to work 

[Zerro]

Maybe simulator will show values here. I use Caneda, and I can see. But not now, too busy.

[mauman]

Follow up question:  If I use an electro cap for  C26, should the + side connect to the volume pot?

Looking at C26, you have +VA on the other side of Q8, and ground on the other size of the Volume pot.  I think the north end of C26 will generally be more positive than the south end, so I'd put the positive terminal toward the north and negative toward the Volume pot.