Tube preamp, add duplicate outputs

[meq123]

I'm in the process of upgrading my old Croft Supermicro tube pre-amp to add relay switch/selecting of the inputs along with IR remote control of both input selection and volume control. Along with this I need to add a duplicate set of outputs to feed my subwoofer (through an external L+R summing buffer).

The Croft output is a cathode follower to a DC blocking cap and a resistor to ground - C1 & R1 in the (single channel) diagram. So my initial thought was to duplicate C1 & R1 (as C2 & R2) to the second output, but I'm now questioning that a bit. What is the purpose of R1? Is it just there as a safety-net in case nothing is connected to the output? In which case I don't need the added load of R2. Then, do I even need C2, or can I just parallel the current outputs?

So in summary, should I:

1. Add both C2 and R2
2. Add just C2
3. Don't overthink it, just parallel the outputs

Thoughts?

 

[rayma]

Parallel the outputs.

Make sure that the 1uF is large enough to not roll off the bass into your load.

 

[jan.didden]

I'd go with # 3. Can't see any benefit in duplicating, ultimately the total load still hangs off the cathode.
The R1 is there to discharge the cap in an open load situation.

Jan

 

[meq123]

OK, so after some offline discussions with @Mooly I've answered my own question and will be going with option 1 (two sets of CRs). The reason being that if I hang my amp, subwoofer and R1 all off C1, this parallels all those impedances and gives a too high bass rolloff point. However, if I split the amp and sub impedances across two sets of CRs, that lowers the rolloff point for each (though I'll still need to increase the C2 for the sub feed to 2.2uF).

Thanks all!

 

[jan.didden]

I still think it's superfluous. You can always increase the C value to the lowest load impedance. What's the advantage of doubling it up, except for a warm and fuzzy feeling?

Jan

 

[Bigun]

#3

 

[meq123]

I still think it's superfluous. You can always increase the C value to the lowest load impedance. What's the advantage of doubling it up, except for a warm and fuzzy feeling?

Jan

Actually, it's a practicality thing. One cap/resistor is already there (as I said) and being a hardwired device, that cap is a bit buried & difficult to remove. But more importantly, I have 2.2uF polypro caps in my 'parts drawer', whereas I'd have to buy 3-4uF caps for a single chain, and those are not the cheapest of devices. Also, axial caps of that type are large and would be difficult to fit in the space available.

So, I apologise for not taking your advice, but I *do* have my reasons 🙂

 

[jan.didden]

OK, yes, that's a good practical approach. You gotta be pragmatic ;-)

Jan