Boostrap resistor values

[Bing Yang]

I am interested in the bootstrap circuit for VAS stage to replace CCS. However, different R1 and R2 values were used in different designs. Some design have the same value. Some has a large R1 or R2. Can someone explain how to choose the R1 and R2 value for a simple VAS stage?

[Bing Yang]

Maybe I need to show some examples.

In Figure 1 of http://sound.westhost.com/project12a.htm, R10 is 1K5 and R9 is 6K8.

However, in Figure 1 of http://sound.westhost.com/project3a.htm, R10 and R9 are both 3K3.

Why the resistor values are different in one bootstrap design and the same for another one?

I have an old Pioneer SX-626 receiver which has a capacitor coupled single supply amplifier. It sounds very nice. But R9 is 4.7K and R10 is 820. Why are they so much different?

Of course, I am wondering whether I should change the values of R9 and R10 in my SX-626. Will it sound even better if I set R9 and R10 the same value (e.g. 2.8K)?

[Nelson Pass]

In a perfect world with a perfect bootstrap capacitor and an output
stage that will drive anything, you would make the resistor attached
to the supply a lot smaller than the one attached to the VAS transistor.

In this way, you would minimize the "distortion regeneration" in which
the distortions of the output stage tend to show up at the output
of the VAS. Of course they do anyway as the gain of the OS is
finite.

As a practical matter, increasing value of the supply side resistor
makes the value and character of the bootstrap cap less important,
and of course it will also dissipate less heat.

Most of the time, the values of the supply side resistor is 1/4 to 1
times the value of the other side resistor, and there is a lot of
room to play.

[AKSA]

Agreed, I use supply side 2/3 the value of the working side. Pulls back OLG at high frequencies where the cap ESR starts to hike upwards.


Hugh

[Bing Yang]

Thanks for the comments from Nelson and Hugh.

I have recapped the SX-626 with 105C low impedance Nichicon capacitors. The bootstrap capacitor is 100 uF @ 50V. With Hugh's comments, I will bypass it with a film capacitor. I have plenty of them with different makes and values.

BTW, the SX-626 surprised me a lot. I have rebuilt a few tube amps (Dyanco ST70, HH Scott 222C, HK A250), built a 2A3 amp, a Gain Clone, a low TIM. I have bypassed many different capacitors, twisted my CAT 5 cables, modded my CD players, made my own intercon cables. Yes, the tubes sound different from SS. But I can hardly tell the differences in all the mods I did on the SS amps.

After I recapped the SX-626 and increased the feedback a bit, it sounds so much better from all my other DIY amps. I do not own any high end gears and cannot compare it against them. But the SX-626 sounds fast, clean, and with a lot of details. The percussions seem to jump out at me. It may have some flaws once I listen to it for a long time. But so far I still cannot believe how low life output electrolytic capacitors can sound good.