Why bootstrap?

[peranders]

When I read this thread I wonder about one issue.

Hugh, and Rod are two persons here which are keen on using bootstrap for driving the output transistors.

I wonder what are the pros and cons?

Pros: More power with given design, more peak drive voltage, less parts, simplier power supply for the drive circuits.

Cons: Start-up thumps, poor bass performance(?), poor clipping characteristics(?)

Is the major reason to use bootstrap to save parts (=costs)?

Are there any sonic properties involved?

[sajti]

And one more for the cons: If the output has crossover distortion, it will be feedbacked to the VAS by the bootstrap. (As I remember Mr. Self wrote it somewhere...)

I dont think, that two small transistors, and two resistors are much cheaper than one big capacitor....

sajti

[peranders]

If you have especially mosfets you must have drive voltage + 5-10 volts above the main supply voltage so if you really want to squeeze out the max power it's cheaper with one cap instead of a voltage doubler or a separate transformer.

[boholm]

Con: The stages input impedance is enhanced up to megaohms, thus eliminating (almost) the speakers impedances influence on the stage before (normally a VAS).

[sajti]

Originally posted by peranders
If you have especially mosfets you must have drive voltage + 5-10 volts above the main supply voltage so if you really want to squeeze out the max power it's cheaper with one cap instead of a voltage doubler or a separate transformer.

This is true advantage! I saw an amplifier which used bootstrap on both rails, for the input&VAS, to increase the output power....

sajti

[AKSA]

Hi P-A,

Nice post; useful, instructive.

Pros: More power with given design, more peak drive voltage, less parts, simplier power supply for the drive circuits.

More power, perhaps, but not strictly. While the VAS won't clip until several volts above rail, this is not true at the other rail, where the VAS emitter sits. Thus the 'more power' simply means clip is asymmetrical, which strictly means power at clip onset is no different to a VAS fed from a CCS.

Less parts, yes, certainly, but not many, and in any case a quality large bootstrap cap, being electrolytic and highly stressed, is quite expensive.

Simpler drive circuits: maybe; since the bootstrap is, by its nature, one sided; then clearly the VAS is single ended. This forces a single ended design, but locks out fully complementary designs. However, as the SET set will tell you, there's nothing wrong with single ended.......

Cons: Start-up thumps, poor bass performance(?), poor clipping characteristics(?)

Start-up thumps are infrasonic, that is, you don't hear them. Certainly they can move the cone of the driver slightly, but it's not audible and generally if you power the diff pair with a CCS even this thump is largely eliminated.

Poor bass performance is an interesting one. Because the low frequency performance of a bootstrap VAS generally drops off below 100Hz, the VAS starts to load up, varies its current more, and the OLG drops. This causes quite a bit of H2 artefact to appear in the VAS output, and together with the diminishing feedback factor this has two effects; the amp's output amplitude stays pretty much the same down to about 15Hz, but with increasing H2 content AND source impedance of the amp as a whole the bass wettens up. That is, it sounds richer, and very slightly looser, rather like a tube amp which suffers similar effects through the limited primary inductance of its output transformer.

When you do comparisons between a bootstrap amp and a CCS amp you notice these things clearly; I tried for years to get a CCS powered VAS to sound good, even creating a novel current source with fixed impedance to ameliorate drive variations at crossover in the output stage, but the bootstraps always sound better. This is particularly noticeable on Rock music, and interestingly chamber music, which seems to come alive with 'wetter' bass, in much the same way as this sort of music favours tubes.

Is the major reason to use bootstraps to save parts (=costs)?

Certainly not. The cost factors might introduce another $1 of parts; no more nor less. The reasons are subjective, and focus (as they should) on sound quality.

Are there any sonic properties involved?

See above!

Cheers,

Hugh

[sajti]

In my desings I prefer the complementary symmetric design. And I allways use resistors connected to the base of the drivers, to reduce, and define the open loop gain, and reduce the output impedance.
With this resistors the load has no big influence to the open loop gain.

sajti

[peranders]

So Hugh, the reason for you to use bootstrap is purely for positive sonic properties?

BTW: How is the going between AKSA vs. Mirand A1 ?

[AKSA]

P-A,

You got it!

Hugh

[argo]

Originally posted by AKSA
...... When you do comparisons between a bootstrap amp and a CCS amp you notice these things clearly; I tried for years to get a CCS powered VAS to sound good, even creating a novel current source with fixed impedance to ameliorate drive variations at crossover in the output stage, but the bootstraps always sound better. This is particularly noticeable on Rock music, and interestingly chamber music, which seems to come alive with 'wetter' bass, in much the same way as this sort of music favours tubes......

Hugh

I noticed when I swapped between bootstrap a ccs several times in JHL class A amp that the bootstrap arrangement gave always more pleasant sound not only in the bass but midrange as well. Ccs on the other hand caused more accurate, analytic sound (not so pleasant always though).