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Old 11th April 2011, 02:44 AM   #1
Bigun is offline Bigun  Canada
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Default Bigun's Bootstrap

Seems there are two schools of thought for single ended VAS load - a bootstrap (AKSA, DX, TGM1) or a Constant Current Source made out of a transistor and LED or out of two transistors. People say the bootstrap sounds better. Hugh of AKSA fame says (on this forum) this is because the bootstrap provides some +ve feedback that helps deal with the consequences of speaker back emf on the gnf, and he also says that the capacitor rolls off gain above 100kHz allowing less Cdom. Others, particularly EVA say (on this forum) that the bootstrap is bad and is clearly inferior. Some don't like the use of a large electrolytic cap. Yet people like OS say (on this forum) that the bootstrap sounds better.

So which one ?
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Old 11th April 2011, 02:47 AM   #2
Bigun is offline Bigun  Canada
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Is the answer to combine them - Bigun's Bootstrap ?

In simulations (notoriously unreliable for some) the current variations through the VAS device with a regular bootstrap can be simulated with Bigun's bootstrap using a modulated CCS (and the FFT looks 'nice' too). The advantage - no large electrolytic and you get to 'tune' the behaviour amongst others.
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Old 11th April 2011, 02:54 AM   #3
Jay is offline Jay  Indonesia
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Originally Posted by Bigun View Post
So which one ?
Like many other similar questions, it depends. Of course I don't have sufficient knowledge to explain the physics/electronics, but I do know that if you don't have "ears" all you can do is "see" the measurements.

If you believe that simpler is better, always do it with perfection because simplicity requires attention to details.
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Old 11th April 2011, 03:01 AM   #4
Jay is offline Jay  Indonesia
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Is the answer to combine them - Bigun's Bootstrap?
My intuition is it will be worse. It will be good if we can provide the current exactly and sufficiently when we need it (just like what a ccs does) but without putting transistors there.

My approach is to pay attention to the power supply quality, short connection, capacitor quality and size.
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Old 11th April 2011, 03:05 AM   #5
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IMHO either technique is viable and it simply depends on what you want. I've used both, bootstrap in earlier designs and generally a CCS these days, but I have recently been playing again with bootstrap. Bootstrap is nice and simple with just passives and has always sounded fine. Using a CCS offers a more technical solution that likely yields better measurements most times. So, are you more about measured performance or how you percieve the sound? I find the CCS crisp sounding and bootstrap a little softer, my own untrained perception. I don't have any sophisticated measurement tools, so I set up with a DMM and check on the oscilloscope to check the basics and then just listen.
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Old 11th April 2011, 03:19 AM   #6
Jay is offline Jay  Indonesia
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My approach is to pay attention to the power supply quality, short connection, capacitor quality and size.
This doesn't mean that I'm a bootstrap guy. In a complex circuit/situation this simplicity may not give benefit at all. In a top quality system, I think ccs is a must.
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Old 11th April 2011, 06:22 AM   #7
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From my experiments with modulation / no modulation of the current source in the Zen9 schematics, I concluded:
1) If one looks for drive and speaker control - better modulation and bootstrapping.
2) If one looks for ultimate transparency, microdetails and live-like sound - then no modulation, and removing as much as possible PCB traces and wires, where alternating currents walk. Shunt PS, even local ones for separate stages, is one of the measures for achieving this, it allows to localize the region where AC present. Important also to cut out the main PS from supplying AC currents to the schematic. These measures provide drastic positive effect for the sound, although measurements could be the same or even worse.
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Old 11th April 2011, 07:14 AM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Bigun View Post
Is the answer to combine them - Bigun's Bootstrap ?

In simulations (notoriously unreliable for some) the current variations through the VAS device with a regular bootstrap can be simulated with Bigun's bootstrap using a modulated CCS (and the FFT looks 'nice' too). The advantage - no large electrolytic and you get to 'tune' the behaviour amongst others.
It works in the wrong direction: it will starve the PA stage from positive drive when it is the most needed: during the positive excursions.
This could be adressed by tying the bootstrap cap to the emitter of the CCS.
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Old 11th April 2011, 07:40 AM   #9
dadod is offline dadod  Croatia
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Default bootstrap+CCS

There is simpler way to combine bootstrap and CCS.
I am in process to test it in practice, as I was thinking for for some time about it.
dado
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Old 28th June 2011, 05:54 PM   #10
Bigun is offline Bigun  Canada
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I was wondering if anybody has tried Bigun's Bootstrap yet ? - believe it not, now that I no longer rely on spice I may actually try this out !

I'm also thinking about another type of 'CCS' that might sound better than a regular CCS and that would be the gyrator

Wavebourn Tower-III High-end hybrid amp


has anyone tried using a gyrator as a VAS load ? this is also what I'm thinking of trying. Only the resistor between base and collector on the gyrator device - move it over so that it's between base of the gyrator device and the amplifier output - and you get Bigun's Bootstrap again
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