I'm considering making a buffer stage, initially to go between a passive preamp and a power amp; possibly later I might add a switch and volume pot to turn it into a buffered unity gain preamp.
I have seen various ccts and now have a few questions.
First, setting the working potential. See the attached image.
I had considered cct A above, where the CF is set to a reasonably high operating potential using a pot divider Ra / Rb from B+, where these are high value resistors of the order of 500 kohms.
I have however seen cases using cct B above, ie setting the operating potential using a pot divider under the cathode which also forms the load.
Can anyone suggest why cct B does it this way?
Also, though it is very easy to configure cct A, I'm struggling a bit to work out how to set up cct B; any guidance appreciated.
Next, a question about the load.
I believe a constant current load improves a CF, so I was wondering about this. In ccts A or B, we can set whatever operating voltage we like; say (extreme case) I chose 500V B+ and set the cathode at say 450V, then I'd need a very large cathode load resistor. Now the combination of a high voltage and very large resistor is getting close to a constant current load, I think? So is this likely to give some of the benefits of constant current working?
Finally, I'm considering using a pentode such as E83F, E180F or possibly D3a; I know these work very well indeed performing the CF function in Kimmel mu stage ccts. My E180CC/E83F mu stage preamp sounds very good in the system in question but the gain is unnecessary.
I'm fully confident I can configure the perntode OK, but any comments are welcomed.
Thanks in advance.
Mike
PS: please excuse poor quality drawing; done left handed as I have a broken finger!
