# Unicab questions



## p_wats (Jul 15, 2019)

I've built and tested my Unicab board, but am now curious about adding some additional features that things like the H&K Red Box or Omnicabsim incorporate. Specifically the following: 

*A balanced output:* I'll probably tack on the Tataylino Simple DI

*Line/speaker input: *The Omnicabsim says it can't handle the signal direct from an amp head, but the H&K Red Box says it can (just remember to add a "Thru" for the speak to still receive a load, which the H&K schem says is just connecting the tips of the jacks). I'm curious about this, as the Tagboard schematic for the Red Box shows the only difference between the speaker and line inputs is that the former hits a 470K resister first and the latter, only 47K. 

Would it be as simple as having a toggle to select either input resistor (and maybe a third option for instrument level, which I assume would be the lowest if anything at all)?


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## sertanksalot (Jul 17, 2019)

Yes, in my estimation you could do it that way.  It looks like the Red Box device has separate jacks so that there is no mixup [being a consumer item].  I have seen an instr. level/line level switch for an FX loop, which is similar to your application.

Schematic reference:


			https://i.postimg.cc/TwnqbWnb/H-K-Red-Box-2-schem.png
		


For the Red Box schematic, the magic is in the fact that the speaker takes the load... the speaker out is essentially a shunt.  I.e., compare 8 ohms to the 470K+10K + coupling capacitor that is driving the cab sim.

Make sure you have the speaker out connected if you ever test this out.  

I would like to see a *headphone out* for this circuit... for silent playing/practicing.


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## p_wats (Jul 17, 2019)

sertanksalot said:


> Yes, in my estimation you could do it that way.  It looks like the Red Box device has separate jacks so that there is no mixup [being a consumer item].  I have seen an instr. level/line level switch for an FX loop, which is similar to your application.
> 
> Schematic reference:
> 
> ...



Thanks! Yeah I'm planning to use this mostly with the direct out on an amp I built, so the speaker would still be connected, but the speaker in and thru will be useful if I ever want to use other old tube amps, etc. 

A headphone out would be useful. I looked around to see if I could find a small enough layout I could cram in, but my use case doesn't really need it, so I'm not bothering for now.


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## p_wats (Jul 17, 2019)

sertanksalot said:


> For the Red Box schematic, the magic is in the fact that the speaker takes the load... the speaker out is essentially a shunt.  I.e., compare 8 ohms to the 470K+10K + coupling capacitor that is driving the cab sim.



The input of the Unicab hist the pulldown resistor to ground, then the 100n cap. Do you think I need the 10K to ground and coupling cap on top of the 470K on the input for the speaker?


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## Robert (Jul 17, 2019)

In the Red Box schematic the 470K / 10K combination is forming a voltage divider, like a fixed volume control.

You might want to use a 10K for R1, and a 470K between the actual input jack and input pad of the PCB.

One thing to note is that in the Red Box schematic C3 has a 400V voltage rating.


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## p_wats (Jul 17, 2019)

Robert said:


> In the Red Box schematic the 470K / 10K combination is forming a voltage divider, like a fixed volume control.
> 
> You might want to use a 10K for R1, and a 470K between the actual input jack and input pad of the PCB.
> 
> One thing to note is that in the Red Box schematic C3 has a 400V voltage rating.



Thanks. That makes sense. Is it useful to have that 470K on a toggle (with options like 10K for line level and nothing for instrument level? 

The layout I was looking at on Tagboard (here) is older and doesn't have the 400v capacitor. I assume they added it for a reason, but am not sure how essential it is in a personal build. Any thoughts?


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## sertanksalot (Jul 17, 2019)

If it was my project, working with transformer output from a power amp, I would go conservative and keep to the spec.  Small Bear has orange drop capacitors that should work for you... http://smallbear-electronics.mybigcommerce.com/sprague-715-1/


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## p_wats (Jul 17, 2019)

sertanksalot said:


> If it was my project, working with transformer output from a power amp, I would go conservative and keep to the spec.  Small Bear has orange drop capacitors that should work for you... http://smallbear-electronics.mybigcommerce.com/sprague-715-1/



Thanks, I'll check my amp building parts stash for higher voltage caps (ordering from Smallbear, etc. is a pain in Canada unless I build up a bigger order). I'm adding a direct out to my homemade 5e3 tube amp anyway, so the speaker input probably won't be something I use often (and the Red Box classic never had this cap, so I'm not too worried).


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## p_wats (Jul 17, 2019)

Just found 10n and 100n 630v caps in my stash. The Classic used 100n while the Red Box 2 uses 15n (only have 100v in those). I suppose I could use the 100n which would ensure more bass response anyway (or do I have that backwards?).


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## p_wats (Jul 31, 2019)

Almost have my Unicab + DI build done. In the meantime I also built the circuit for the Red Box Classic, as per Tagboardeffects layout to compare. It works well too, but I like the tone-shaping options of the Unicab better.


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## phi1 (Jul 31, 2019)

Our of curiosity, which di box circuit did you use? You mentioned taytalino early on in the thread, looks like he's got a few different circuits.


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## p_wats (Jul 31, 2019)

phi1 said:


> Our of curiosity, which di box circuit did you use? You mentioned taytalino early on in the thread, looks like he's got a few different circuits.



I linked to it in the original post, but the forum link styling is impossible to see unless you accidentally hover, so I don't blame you. This one: http://tataylino.com/simple-di-box/


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