New? univibe tuning point

[Gus]

Thinking about finally building something like a Univbe circuit.  As some might note I sometimes like to use bootstrapping in circuits.  I also am having fun using LT spice.

So I noted bootstrapping can effect the lows.

The cap value and resistor divider and resistor from the node to the base can change things

Electrolytic caps are often not exact

I simmed a small section to check the sensitivity to the bootstrap cap value leaving some things out on purpose(1uf cap, 4.7k and LDR) to see what the bootstrap cap value does

R.G. Have you checked what the bootstrap caps do?

The graph is from C1, green is first .4uf, 1uf, 1.5uf and about 2uf

I do not remember any posts about this over the years or I missed them. 







Has anyone noted this before?  This is from a sim I have not built a Univibe like circuit.

[R.G.]

Thinking about finally building something like a Univbe circuit.  As some might note I sometimes like to use bootstrapping in circuits.  I also am having fun using LT spice.
So I noted bootstrapping can effect the lows.
...
R.G. Have you checked what the bootstrap caps do?
...
I do not remember any posts about this over the years or I missed them. 

They do have an effect. I messed with this a bit. Even more interestingly, they interact with the incoming capacitance and source impedances as well. I didn't post anything because I noticed the response effects were in guitar-amp no-man's-land, between 10Hz and 100Hz, where there's almost no signal to amplify. Wrong or right, I just passed it over. What I took away from it was to not make them too small to move the humps up into low bass.

I think there is an additional effect from the signal that goes through the LDRs, as this has a 1uF cap feeding through a variable resistance. This will add phase shift and signal opposing the shift and signal from the smaller phase cap, but a highly varying amount. It's been a while since I looked at this, though, so I don't remember all that well.

[Gus]

1uf looks like a 7 or so dB boost in the low 20Hz range from the graph.  A larger value is lower in frequency and more of a bump.

I am thinking maybe the somewhat random between stages low boosts cause more IM and this will be different between builds or OLD Univibes because the cap values are different from drying out or value tolerances.  Could this be an overlooked thing that causes another variation in sound from different Univbes?

Maybe with film caps one can select the values and they should not drift over time.

[R.G.]

Could be. It's worth some more experimentation.

Also, big humps in response down in the 50-60Hz region would make it massively more prone to hum.

[R.G.]

Ah. Found it. It's here:

http://www.diystompboxes.com/smfforum/index.php?topic=82717.0