Buffer/Blender help...

[MetalGod]

I'm playing round with a buffer/blender to mix some clean signal in with an OD for bass use.  I'm getting the buffer bit right first as the bass OD should be relatively straightforward as it's based around an Ampeg B15 and will be all tube with high-voltage supply (300v).

I've got a buffer (source follower) working using a supply voltage of 30v using a 2N7000, 0.1uF input cap, 2M2 resistors from drain-to-gate and gate to ground, a 4k7 resistor on the source, then a 10uF cap from the source.

I'm not very good with the design of stuff, I'm much more a 'try it and see' kind of guy, so I'm wondering what effects changing the source resistor will have as I've seen varying designs using anything from 2k up to 10k as the source resistor.

I'm looking for as unaffected a sound as possible with the highest headroom I can get (hence the 30v supply). 

[MetalGod]

nobody? - Bueller?

[MetalGod]

It's ok, I found out what I needed to know from Jack Orman's website.  In a nutshell... (paraphrased), anything from 3k3 to 10k is ok without changing your tone.  Lower values are better since this allows more drive on the negative portion of the audio cycle.

Cheers.

[MetalGod]

also (if anyone at all is following this), I'm finding that the buffer noticeably thins out the tone compared to bypass.  at present I'm using this spec...

10uF input cap, 2x 1M resistors to set the bias (from drain to gate and gate to ground), 4k7 source resistor, 10uF cap from source to output.  supply voltage is now lowered to 12.5v, oh, and I added a 100uF cap from the supply voltage to ground as there was quite a bit hum (none now).

anyone built a buffer that is 100% transparent and doesn't thin your tone?

[chicago_mike]

Is the 2n7000 a fet? Im not aware of that one. Im making a couple fet buffer/boosters with 2SK170's. but these are more for guitar than bass.

[MetalGod]

Is the 2n7000 a fet? Im not aware of that one. Im making a couple fet buffer/boosters with 2SK170's. but these are more for guitar than bass.

yep, thats what the datasheet says - 2N7000, enhancement mode field effect transistor.  the actual FET isn't too important (imo) as its being used as a buffer, though I'm willing to be proven wrong.

from the look of the datasheet, the 2SK170 is pretty much interchangeable with the 2N7000 (as long as you account for the different pinout).