| 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? |
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| 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 |
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| 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. |
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| 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). |
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| sajti |
| quote: | 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 |
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| AKSA |
Hi P-A,
Nice post; useful, instructive.
| quote: | | 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.......:clown:
| quote: | | 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.
| quote: | | 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.
| quote: | | Are there any sonic properties involved? |
See above!
Cheers,
Hugh |
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| 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 |
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| peranders |
So Hugh, the reason for you to use bootstrap is purely for positive sonic properties?
BTW: How is the :scratch: :yikes: :yuck: :zombie: :sing: :magnify: :hypno1: :ill: :wchair: going between AKSA vs. Mirand A1 ? |
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| AKSA |
P-A,
You got it!
Hugh |
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| argo |
| quote: | 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). |
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| millwood |
bootstraps are also more stable at higher frequency.
I like boostraps due to their simplicity. I don't find it sonically superior to other arrangement. Tho one needs to be careful with it in terms of what devices it drives. |
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| peranders |
| quote: | Originally posted by millwood
bootstraps are also more stable at higher frequency. |
Can you explain? More stable than what and what kind of stability? |
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| millwood |
it mostly came from my experience.
Technically, I am not sophisitcated enough to answer that but I think most of the bootstrps use large electrolytic caps. they tend to become less cap-like at high frequencies. As such, the gain of the bootstrap goes down at high frequencies.
I am sure others can articulate a more precise answer but that's my understanding so far. |
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| AKSA |
Yes, a bootstrap is more stable at high frequencies.
The reason is one of the paradoxes of electronics, as positive feedback normally makes an amplifier less stable.......
With increasing frequency, and typically starting about 100KHz, the ESR of the bootstrap cap, being electrolytic, begins to rise. A large contribution to this is the self inductance of the foil of the cap; by Xl = 2pifL this steadily rises. As frequency rises further this higher ESR prevents ready charge flow to and from the cap, and by around 300KHz the capacitor has electrically 'shrunk' to a small portion of it's nominal capacitance.
This in turn begins to load up the collector of the VAS as the resistance to rail steadily diminishes. This pulls back gain.
In any voltage amplifier, we have poles, which define the point on the spectrum where negative feedback turns positive. In a global feedback loop we must bring OLG below unity by this pole frequency so that when feedback turns positive, gain is below one and it will NOT oscillate. This is normally implemented with a simple capacitor between collector and base of the VAS.
The bootstrap helps considerably in culling the gain to below unity by the pole frequency. This is a great strength because it means that there is no longer just the one 6dB/octave mechanism, lag compensation, for meeting the Bode-Nyquist criteria.
Cheers,
Hugh |
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| slowhands |
| quote: | 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. |
I love this debate, because we have minds from all over the earth mulling over the problem. Look at the many flags of the various posters. This cross pollination of ideas is a very good thing.
Of course the answer to "why bootstrap?" is simply to increase the voltage swing of the VAS. We've already beat this to death here. I suspect those in the bootstrap camp will stay there, and likewise those in the current source camp will stay there too.
I've tried all of these approaches to increasing the drive swing and chosen the brute force method of using higher voltages to the front end. I generally use dual voltage doublers and shunt regulators for the front end supplies, which costs 6 caps, 4 rectifiers, 2 resistors and two zeners . Sounds like a lot, but actually fits in a small area and is not expensive compared to other alternatives, such as an outboard transformer. For the usual 10-15 ma front end current this is satisfactory with 1 mv ripple. This ends up working very well, since I get sufficient drive in both directions, symmetrical clipping behavior and so on. I get every bit of output power the output devices can safely and cleanly deliver.
With a bootstrap as commonly implemented, you have asymmetrical clipping, which seems like not a good thing, but actually since I don't like to listen to an amp clipping, it's not really an issue.
If you want to partially correct that asymmetry, you can put a bootstrap on the other side too, just an R/C from output to the front-end supply (Vas emitter). It will balance the overdrive somewhat. This was depicted in the 1970s RCA Transistor Manuals, so it's nothing new. For instance, there are about 5 examples in the 1975 book, typically using 50 mf/270 ohm. I have not tried this, just reporting that it was the vogue 30 years ago. |
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| metal |
Hey guys
I wanted to know if incresaing the value of the bootstrap capacitor will yield a better bass performance and responnse.
Thanks:) |
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| phase_accurate |
| quote: | | Of course the answer to "why bootstrap?" is simply to increase the voltage swing of the VAS. |
I would say that this is only partially true. Bootstrapping is enabling the VAS to deliver enough CURRENT (it is in fact something like cheating a CCS). Without bootstrapping the collector resistor of the VAS would have to be unreasonably small, leading to increased dissipation in the VAS transistor. This in turn would call for a VAS transistor with lower beta, fT and whatsoever.
Regards
Charles |
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| mikeks |
| quote: | Originally posted by sajti
In my desings I prefer the complementary symmetric design.
sajti |
Sadly...there are no significant advantages with the so-called 'full-symmetry' approach in audio frequency power amps. |
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| markp |
| quote: | Originally posted by mikeks
Sadly...there are no significant advantages with the so-called 'full-symmetry' approach in audio frequency power amps. |
Yes, but it looks sooo good!
But seriously, I only design complimentary symmetrical drive circuits for my amps. They work well for me. I often drive them with a higher voltage than the output stage to get what I want out of them. |
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| destroyer X |
I am crazy to have my bootstrap to feel that outstanding sound some people told me.
Im my idea, can kick me with the boot, or with the strap, and i still crazy to put my hands over this wonder.
My friends is trying to corrupt me, they want to see schematics, they want to put on Ray X....and i said no!...Hugh do not want that!
- But Hugh will never know!
- But my conscience will know!
They are entirely crazy to our meeting to judgement.... we always make this as our fun.... we order to 30 years prison to some Harman Kardon.... we send to electrical chair some Pionner... we are really bad!...leach is already buried... today not more than sand!(the amplifier, long life to Mr. Leach!)
They saw your picture , and they say to me that you are in the middle of the ocean , pointing the skies, with .30 to destroy the airplane that is sending your AKSA 55 kits to damn Destroyer.
They said you are alike one old movie, one strabic (eyes convergence) German soldier trying to shot many airplanes he could see because the strabic eyes.... he said you do not put nothing in mail to me, that i loose money.. inside the pack is one stone!.... they are nervous with me , because i told them cannot see board circuit...only take a look with big distance and without turn it... and they told you made sandwiched boards with capacitors and resistor in the middle.
We are turning crazy (already i am) to see AKSA modules and to hear aksa sound.
Please, do not explode the airplane is bringing modules to me..I will not let them convince me.
No problems with customs, my wife is a graduate customs officer, and i have a lot of Federal Policeman Friends...no way to arrest my circuits, i think it is more easy that they bring here into my hands.
Have you heard about Lula?.... second degree cousin. hohohô
Carlos |
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| lubbie |
I must be missing something in the comments below. What does this have to do with the current discussion???
Carver did an interesting thing on the ouput on some of his amps where he feeded some of the output signal into the Bias Transistor CCT.... check out C7 on the following http://ns.netlynx.us/audio/phaselinear/pl400/PL400.gif
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| quote: | Originally posted by destroyer X
I am crazy to have my bootstrap to feel that outstanding sound some people told me.
Im my idea, can kick me with the boot, or with the strap, and i still crazy to put my hands over this wonder.
My friends is trying to corrupt me, they want to see schematics, they want to put on Ray X....and i said no!...Hugh do not want that!
- But Hugh will never know!
- But my conscience will know!
They are entirely crazy to our meeting to judgement.... we always make this as our fun.... we order to 30 years prison to some Harman Kardon.... we send to electrical chair some Pionner... we are really bad!...leach is already buried... today not more than sand!(the amplifier, long life to Mr. Leach!)
They saw your picture , and they say to me that you are in the middle of the ocean , pointing the skies, with .30 to destroy the airplane that is sending your AKSA 55 kits to damn Destroyer.
They said you are alike one old movie, one strabic (eyes convergence) German soldier trying to shot many airplanes he could see because the strabic eyes.... he said you do not put nothing in mail to me, that i loose money.. inside the pack is one stone!.... they are nervous with me , because i told them cannot see board circuit...only take a look with big distance and without turn it... and they told you made sandwiched boards with capacitors and resistor in the middle.
We are turning crazy (already i am) to see AKSA modules and to hear aksa sound.
Please, do not explode the airplane is bringing modules to me..I will not let them convince me.
No problems with customs, my wife is a graduate customs officer, and i have a lot of Federal Policeman Friends...no way to arrest my circuits, i think it is more easy that they bring here into my hands.
Have you heard about Lula?.... second degree cousin. hohohô
Carlos |
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| destroyer X |
Can be hear bootstrap and here not bootstraped to now really differences... can be a lot of things.
You can decide.
If you decide no conections... good too, i am happy
I will have AKSA, you discuss AKSA
Carlos |
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| thanh |
| Hi All! I want to a bootstrap symmetry circuit. Can this techique reduce distortion at high frequency more than slone' circuit? |
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| mikeks |
| quote: | Originally posted by markp
Yes, but it looks sooo good!
But seriously, I only design complimentary symmetrical drive circuits for my amps. They work well for me. I often drive them with a higher voltage than the output stage to get what I want out of them. |
I suspect subjectivists would take issue with this viewpoint on the grounds that 'sonics' are of infinitely more value than 'looks'....... |
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| jam |
Mike,
Care to explain your thoughts on bootstrapping?
Regards,
Jam |
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| Nelson Pass |
That is frequency compensation, and it is very common. |
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| Steven |
Yes, C7 is just a phase lead capacitor for frequency compensation. Actually, in that PL400 circuit I'm more surprised by the fact that the bottom of bootstrap cap C11 is not connected to the output of the amp but to the emitter of the top driver. I wonder whether it behaves any different.
Steven |
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| thanh |
| Is that the feedforward compensation? |
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| lumanauw |
I wanted to understand why Bootstrapped VAS gives more H2. Is it because the bootstrapped (R+C) or just the R?
If the VAS on one side is loaded with R only (not R+C like bootstrap), will it still produces the bootstrapped sound? |
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| lubbie |
Steven..
Initially i thought the position of the bootstrap was normal until you metioned it.. I looked around and where carver had put it is very uncommon compared to the amps i've researched. I wonder how the amp would respond if i changed it....
Maybe i see a mod on two Phase Linear 400's in my future.. LOL.. Just so everyone knows, the phase linear on the link is a very early model of 400. |
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| GEirin |
HI all.
Double bootstrap. |
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| destroyer X |
a double differential i will do a test usind double VAS, double differential and double bootstrapp.
Also will use double output.... parallel output...more pairs...one pair inside the NFB line and the one connected to the speaker out from the NFB.
It may sound interesting.... i like this Argentine design you offer to us Geirin.
regards,
Carlos |
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| GEirin |
Hola Hermano.
My idea is asymmetrical amplifier. Single diff. and Vas.
Hafler siempre hizo simétrico, ahora, el 9505 es asimétrico. Why?
Saludos.
Guillermo |
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| djk |
"Actually, in that PL400 circuit I'm more surprised by the fact that the bottom of bootstrap cap C11 is not connected to the output of the amp but to the emitter of the top driver. I wonder whether it behaves any different."
There is no bias current flowing in the output stage, so the bootstrap cap is hooked to the driver stage. |
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| GEirin |
Hi all.
What is the function of R270 + D269 // R252 + R251. |
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| djk |
| Schematic doesn't match your request. |
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| Steven |
| quote: | Originally posted by djk
"Actually, in that PL400 circuit I'm more surprised by the fact that the bottom of bootstrap cap C11 is not connected to the output of the amp but to the emitter of the top driver. I wonder whether it behaves any different."
There is no bias current flowing in the output stage, so the bootstrap cap is hooked to the driver stage. |
Hi djk,
You may be right that there is no bias current in the output stage. Dissipation in the drivers Q11 and Q12 would be quite high (5W) before significant bias current in Q13-18 starts to flow. On the other hand, the bias loop contains R41 (0.22). This makes sense if at least some bias current is flowing in the output stage.
But even if there would not be any bias current in the output stage, the minus side of C11 could have been connected at the output of the amplifier, the output impedance is still very low and able to drive the bootstrap capacitor. Also the drivers contribute to the output.
I think the main reason for connecting C11 to the emitter of the driver is that this emitter voltage follows the AC signal on the collector of Q5 much better than the output does, so the voltage across R18 is kept more constant and the collector impedance of Q5 becomes higher and more linear. I think even harmonic distortion is lower because the half wave rectified (class B) non linearity of the voltage across emitter resistors R40, 43, 45 is not fed back.
Steven
I found that the original circuit link was not accessible anymore, so I attach it again. |
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| GEirin |
Hi djk.
Sorry, component of other channal.
Bootstrapp R250+R249 // R266+ D207.
What is the function?
Thank. |
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| teemuk |
| quote: | Originally posted by GEirin
Bootstrapp R250+R249 // R266+ D207.
What is the function? |
During clipping of the negative half wave the driver transistor Q220A is cut off. When this happens the voltage potential at its base “rises higher” than what the rail voltage is. (In case of negative rail the “higher” means that the base voltage gets more negative than the rail voltage). Consequently, the diode (along with its series resistor) will limit the base voltage to a value that is only slightly higher than the collector (rail) voltage. This reduces the storage effect and allows the driver transistor to be cut off faster. This in turn reduces cross conduction and therefore protects the transistors. |
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| GEirin |
Hi Teemuk.
Thanks you very much for you reply.
Geirin |
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| GEirin |
Hi all.
What is the function of: c14,r45,r46,d11 and c15,r51,r52,d12 network. Double bootstrapp?
Thank
GEirin |
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| gaetan8888 |
| quote: | Originally posted by AKSA
Yes, a bootstrap is more stable at high frequencies.
The reason is one of the paradoxes of electronics, as positive feedback normally makes an amplifier less stable.......
With increasing frequency, and typically starting about 100KHz, the ESR of the bootstrap cap, being electrolytic, begins to rise. A large contribution to this is the self inductance of the foil of the cap; by Xl = 2pifL this steadily rises. As frequency rises further this higher ESR prevents ready charge flow to and from the cap, and by around 300KHz the capacitor has electrically 'shrunk' to a small portion of it's nominal capacitance.
This in turn begins to load up the collector of the VAS as the resistance to rail steadily diminishes. This pulls back gain.
In any voltage amplifier, we have poles, which define the point on the spectrum where negative feedback turns positive. In a global feedback loop we must bring OLG below unity by this pole frequency so that when feedback turns positive, gain is below one and it will NOT oscillate. This is normally implemented with a simple capacitor between collector and base of the VAS.
The bootstrap helps considerably in culling the gain to below unity by the pole frequency. This is a great strength because it means that there is no longer just the one 6dB/octave mechanism, lag compensation, for meeting the Bode-Nyquist criteria.
Cheers,
Hugh |
Hello
"The bootstrap helps considerably in culling the gain to below unity by the pole frequency"
So wen I'm use to put a 33 nf capacitor across the bootstrap capacitor of all amps I do, to reduce the total ESR of the bootstrap capacitor, it's not a very good thing to do ?
And maby it's even better to use a big bootstrap capacitor since it's high ESR help to use a smaller cdom capacitor ?
Thank
Bye
Gaetan |
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| destroyer X |
Or the opposite, of course...even other kind of circuit can be switched on and off..also can be replaced using switches or relays.
If you need i can do that for you and send by direct mail... the equivalent bootstrapp or equivalent CCS..the substitutor.
Them install, point to point... and a switch to enter bootstrapp or CCS and listen
This is the better way to check things and to evaluate things.
Do it real world and LISTEN.
Why to use Neuroniuns to that..also neuroniuns will never be sure without listening... save your brain for better things..this is something you can conclude using ears and only a small part of your brain, the part responsable to listen things.
regards,
Carlos |
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| GEirin |
Hi
Opinions, please.
GEirin |
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| GEirin |
Hi all.
Steven said of PL400:
I think the main reason for connecting C11 to the emitter of the driver is that this emitter voltage follows the AC signal on the collector of Q5 much better than the output does, so the voltage across R18 is kept more constant and the collector impedance of Q5 becomes higher and more linear. I think even harmonic distortion is lower because the half wave rectified (class B) non linearity of the voltage across emitter resistors R40, 43, 45 is not fed back.
My questions: the explanation is valid for NAD C-370? Where the botton of double bootstrapp (c14 c15) are connected of the emitter of the drivers - colletors of sziklai output.
And this topology (double bootstrapp in NAD) is valid for EF output?
Thanks
GEirin |
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| Steven |
| quote: | Originally posted by GEirin
Hi all.
What is the function of: c14,r45,r46,d11 and c15,r51,r52,d12 network. Double bootstrapp?
Thank
GEirin |
Yes, it is a double bootstrap. But here the bootstrap serves a different purpose than in the cases where it creates a high collector impedance for the VAS (for high loop gain). In this case the purpose is to create a constant current source for the emitter followers T15,16, that even works a bit beyond the supply lines to drive the amp closer to the supply lines.
The current through T15, 16 is set by (voltage across the TL431 minus a Vbe) / R38,43. |
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| GEirin |
Hi Steven
Thank you very much for you reply.
regards
Guillermo |
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| GEirin |
Hi
Sorry Steven. Other question.
I´m interested to know the topology the old amplifier attached below.
R9,R10,C7, is bootstrap the VAS TR3?
And R14,R15,C6, is bootstrap a constant current source?
Thank
regards
GEirin |
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| john_ellis |
Hi all
To boot or not to boot, that is the question!
The performance of a bootstrap can certainly be simulated, whatever has been said about sonic performance.
In a simple bootstrap circuit like Linsley-Hood's original Class A, the upper output stage is severely slew-rate limited by the relatively poor fT of the upper output transistor. Bootstrapping is positive feedback, and so if you have a slow output stage, the bootstrap is slow, and the output can only work at the speed of the output transistor as it would be in common emitter mode. So it is quite possible for the input transistor to suffer transient overloading effects- not by a Miller capacitor (as there wasn't one) but because the output stage is slow. The cure for this is to use fast output transistors!
(This maybe why some listeners report that the "sound" of the Class A was not as good as expected. And this would depend on individuals' set-up: what source they used, whether there were hgih frequency content or not and so on. So just because some listeners think that the Class A is fantastic, it may be that their source material is less likely to overload than anothers.)
If you add a driver transistor, the slew rate improves. Becuase the driver usually has a higher fT than the output, it can provide more drive more quickly. So, the response of a driver+output pair will be much better when bootstrapped.
In this case is there any benefit in using a current source, or mirror stage to provide the VAS signal? I suggest the answer to this is yes, because it will improve the slew rate further, making the amplifier sound crisper, perhaps. But this is not a categoric answer because then it all depends ... If you use Miller compensation, this wastes the benefits of a current drive, since it "slows down" the VAS, so although some argue that a CCS is better than a bootstrap, with Miller compensation, it is not clear why.
Of course, if you do not use a Miller, as I have often advocated, then you should be able to spot the improvements in frequency response, but stability is another matter... therefore, it is the amplifier architecture as a whole which needs to be taken into account, not just the bootstrap or not.
Finally, I propose that a balanced driver stage should give lower distortion than a single-ended VAS configuration. In a SE VAS, H2 is prevalent. In a balanced stage, whether fully symmetrical (which seems to me to be OTT - but a current mirror+differential VAS seems worthwhile) at least H2 should be all but eliminated.
So, I would say, if you use Miller, bootstrap or not is less an issue, but without a high frequency driver, bootstrap is "less good".
Cheers
John |
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| Steven |
| quote: | Originally posted by GEirin
Sorry Steven. Other question.
I´m interested to know the topology the old amplifier attached below.
R9,R10,C7, is bootstrap the VAS TR3?
And R14,R15,C6, is bootstrap a constant current source?
|
Actually, both bootstraps serve to increase the collector impedance of the VAS. For AC signals both are in parallel; it makes no difference whether the load is connected to the positive supply voltage or the negative supply voltage. The top bootstrap acts as a current source, the bottom one as a current sink. The Vbe multiplier Tr4 is an AC-short circuit between these.
In many (most) amplifiers the VAS (Tr3) is the current sink itself and the bottom bootstrap is not present, i.e. point B of the Vbe multiplier is connected to the collector of Tr3.
In this topology with the double bootstrap the maximum negative output voltage is slightly increased because point B can exceed the negative supply voltage.
Steven |
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| GEirin |
Hi Steven
Thank you very much for your detailed explanations.
You said:
In this topology with the double bootstrap the maximum negative output voltage is slightly increased because point B can exceed the negative supply voltage.
I can to use one diode + R // bootstrap network? Similar to Yamaha circuit attached below?
Guillermo |
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| Steven |
This amplifier probably uses a PNP VAS connected to the positive rail, following the red dotted line. The bootstrap is then the normal load (pseudo constant current sink) for the VAS, including the capability to drive the base of Q220A below the negative supply.
In normal operation D207 is reverse biased and does nothing. Probably it is there for switch on/off protection or overload recovery.
Steven |
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| GEirin |
Steven
Thank you very much.
Guillermo |
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