Parallel Preamp Outputs

[Ray P]

I'm working on a preamp that I can use with my Pass SLOB speaker project, this is what I'm working on;



This will provide sufficient gain to drive the Tokin THF-51S SIT Follower I've almost finished building to drive the full-range speaker units - that's all fine, however I would really just like to check on an assumption I've been using....

I'll be using a conventional pwer amp for the bass units of the SLOBs and they don't require the additional gain from the preamp so I would like to have a buffered passive preamp output in parallel with the 2P29L section with both controlled by a single volume attenuator.

So, if my understanding is correct, I need to consider that the attenuator will see two parallel loads, the grid of the 2P29L, as per the schematic above, and the input impeadance of the buffer device (BUF-03). I have no idea how to work out the 2P29L load, however, Rin of the buffer device is very high;

Capture.JPG (15.1 KiB) Viewed 2014 times

and is likely to have only a marginal affect on the 2P29L grid impedance?

I have a 25K shunt attenuator available and I think that will be OK as the volume control with the 2P29L and buffer in parallel.

I hope that makes sense and will be appreciative of any confirmation or observations.

Cheers

[Nick]

If I understand your question, the input impedance of the 2p29l will be 47k set by R9. If you want it to be higher, just increase R9. I would consider some startup muting from the 2p29l output though to protect the following amp at power on.

[Ray P]

Thanks Nick, I wondered if it was as simple as R9 setting the input impedance but assumed it had to be more complicated than that. Anyway, I'm now working on the premise that the switched attenuator will see a load impedance of 47K in parallel with Rin of the BUF-03 device(4 x 10^11Ohms), which will be a little under 47K - so i don't have to worry about adding the parallel output to the preamp using the buffer. I might be better going for an increase in R9 or a lower resistance attenuator though.

Thanks for the tip on the output muting too