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Old 29th January 2003, 11:46 AM   #1
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Default Doubling

Regulating a power amp is a tricky thing. It also is controvertible.

Even so it's still being done, particularly to attend low current demands from all stages except the output transistors.

It's also considered a good thing to have some extra volts to power those stages, usually 10-15volts more.

Some projects use an extra transformer, or add more wiring to the toroidal transformer (if you are using one), to get those extra volts.

Other designers, like Nelson Pass and Norman Thagard on some Audio Electronics projects, use a doubler to get the extra voltage, which they then regulate.

That is a very practical way to get more volts. But how compromising is doing so?

As far as I know ripple is doubled too when doubling, to 120Hz. Do we need additional filtering? There's not much around over this topic.


Carlos
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Old 29th January 2003, 11:59 AM   #2
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Default Voltgage Doublers

the amount of capacitance depends on the load impedance and maximum allowable ripple. The voltage will sag badly if you don't use large enough capacitors. Further, if building a HV supply you need to use PC mount caps (this should be obvious.)

I suggest that you download one of the SPICE freeware programs and try it out for yourself.
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Old 29th January 2003, 12:00 PM   #3
halojoy is offline halojoy  Sweden
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Arrow I prefer extra trafo

I would never do any doubling.

It is absolutely better to use extra trafo.
As the power is much lower in the first stages,
This Trafo can be small and cheap.

So I can not see any reason for introducing
voltage pumps.
5-15 more Voltage in the driver/voltage gain stage
is absolutely RIGHT.

Why on earth use same voltage as in Output Stage?
The vas transistors and driver transistors
will be working in their Sat region.

Thanks, carlmart
for pointing this out to Nelson Pass & the other Newcomers.
Will give them something Nice to Consider.

Really hurts to see bad designs.
Power Supply is VERY, VERY important in POWER of Amplifiers

/halo - thankful to carlmart - who understand these things
Others Don't
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Old 29th January 2003, 12:11 PM   #4
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Default My time to cry!



Halojoy,


This was a hopeful mail really. I was expecting people to tell me: don't worry about the doubling, it's no problem.

As long as you regulate after that (people would say) the compromises are minimum. Well I'm asking: how minimum the really are?

But maybe I was too hopeful...


Carlos

(Not intending to criticize the master Pass ).
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Old 29th January 2003, 12:21 PM   #5
halojoy is offline halojoy  Sweden
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Default Re: I prefer extra trafo

Quote:
Originally posted by halojoy
I would never do any doubling.

It is absolutely better to use extra trafo.
As the power is much lower in the first stages,
This Trafo can be small and cheap.

So I can not see any reason for introducing
voltage pumps.
5-15 more Voltage in the driver/voltage gain stage
is absolutely RIGHT.
-------------
for pointing this out to Nelson Pass & the other Newcomers.
Will give them something Nice to Consider.

Really hurts to see bad designs.
Guess Pass himself, did not come up with the idea of doubling.
Must have been one of those yuong EE, that does work for him no more ...

The Pass designs, I have seen, never use that technique
to "save money" in that foolish way.

/halo - Can recognize "a true" Pass Amp, any day.
They are not like amplifiers are most - they are Unique, beeing good/bad
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Old 29th January 2003, 12:41 PM   #6
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Default capacitor pumping

maybe we should get into the physics a little bit -- since the capacitors in the typically imagined power supply pi- filter are being charged, then "pumping" all the time.

there's nothing "wrong" with doubling if you understand what you are doing. it's like the old "dog food" problem -- linear optimization -- there are no free lunches.

again, run it through a simulation and you can readily see the constraints.
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Old 29th January 2003, 07:28 PM   #7
Jeff R is offline Jeff R  United States
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I would prefer to use a small, cheap transformer to produce the voltage you want rather than use a doubler, in which the regulator has to drop a lot of voltage. The main problem I see with a doubler, not that it is all that bad, is that you are now dealing with unnecessarily high voltages which requires higher voltage devices and much higher power dissipation in the regulator (remember, P varies as the square of the voltage). All of this makes the el cheapo transformer very attractive, IMHO.
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Old 29th January 2003, 08:02 PM   #8
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Quote:
P varies as the square of the voltage). All of this makes the el cheapo transformer very attractive, IMHO.
True for a resistor but not for an amp. Note that the current demand does not rise if you make the PSU voltage higher (at same power output).

I don’t see the advantages of a voltage doubler. It only puts higher demands on the capacitors (allowable RMS current) and causes higher ripple. Further you need more of these expensive devices and larger ones too. Using a transformer with higher voltage is much cheaper and a clever amplifier design also does help more.
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Old 29th January 2003, 08:08 PM   #9
HDTVman is offline HDTVman  United States
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I don't see any real problems using a voltage doubler in low current applications (100 ma or less). The problems start when you need high current, increased ripple, poor self regulation, very high quality caps needed because of the high ripple current.

Later
Bruce

PS on the other hand if that's the transformer you have and can't afford or want to use a different one, go ahead, just so you know what is required to make it work. Always regulate after a voltage doubler to clean things up.
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Old 30th January 2003, 12:00 AM   #10
blmn is offline blmn  Brazil
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Carlos,

I agree with HDTVman. For sure there are no problems using this approach for low currents. Correct calculation of values and good quality components will be enough for a satisfactory behavior in this case.

but, if it's easy for you to find multiple secondaries xformer, at an affordable cost, this is, in my opinion, the best way.

saudações tricolores!!!
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