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Old 6th March 2012, 09:15 AM   #741
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by danielwritesbac View Post
I was wondering how to work out the zener setting for the regulator (minimum required voltage drop), based on predicted voltage drop from rectifier (KBPC2506 per each secondary) and documented voltage drop from transformer and the possibility of a 4 ohm speaker load (or the similar load of a rather difficult 8 ohm speaker).

My 400va transformer datasheet gives these figures.
No load: 26v (secondaries in parallel)
7a load: 25.5v (secondaries in parallel)
10a load: 25v (secondaries in parallel)
17a load: 24v (secondaries in parallel)

Some people would choose a half price transformer, like this 200va:
No load: 28v (secondaries in parallel)
6a load: 26v (secondaries in parallel)
9a load: 25.5v (secondaries in parallel)

Apparently, if you buy a transformer based on lowest voltage drop figures, that results in a very huge transformer. It is 13cm across, 6cm tall and very heavy. I've listed two transformers in order to get 2 data points for extrapolation, so that you won't have to answer the same question for every transformer variety.

I'm assuming that the answer for monobloc (running with the 200va example above) is something like the ~6a pulldown figure (26v), multiply by 1.45? for capacitor rectification, subtract 2.5 for margin and then the zener figure is (15+20) 35v? Correct?

I'm also assuming that a stereo amp (running with the 400va example above) is going to be using 33v zeners at the regulator boards?

Gratuitous transformer overdo photo:
For an audio amplifier, you do not especially need a low drop transformer.

Predicting the actual output voltage when the transformer is used in its actual circuit is pretty difficult, because of the peak rectification.

Anyway, you can be sure of one thing, the DC output will never exceed the (Vac*1.41)-1.5, and depending on the transformer, it could be significantly lower.
With your figures, under no load that would be ~=35V, falling to 30.5V or less peak under heavy load.
But you cannot use the peak voltage, the regulator must be able to regulate down to the minima of the voltage.
If you have 10% ripple, that would be 27V, and you need a regulating margin of 3V, that brings you to 24V. See why I advise against regulators?
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Old 6th March 2012, 09:27 AM   #742
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Quote:
Originally Posted by Elvee View Post
you would have to insert a volume control in the very low level circuitry, and that would be difficult to achieve cleanly and without adding noise.
This was going to be my next question, if it was such an easy operation like adding somewhere a network consisting a couple of resistors, capacitors and small transistors.

Edit: I'm building a prototype of last revision of CFP PCB at this time. I will share files after accomplishment.

Last edited by terranigma; 6th March 2012 at 09:44 AM.
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Old 6th March 2012, 02:59 PM   #743
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Originally Posted by terranigma View Post
a network consisting a couple of resistors, capacitors and small transistors.
That becomes a preamplifier then, which is the sensible solution:
Discrete design: 2-transistor RIAA preamp
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Old 7th March 2012, 06:33 PM   #744
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Originally Posted by Elvee View Post
. . . no load that would be ~=35V, falling to 30.5V or less peak under heavy load. But you cannot use the peak voltage, the regulator must be able to regulate down to the minima of the voltage. If you have 10% ripple, that would be 27V, and you need a regulating margin of 3V, that brings you to 24V. See why I advise against regulators?
Good example! Thanks!!!!

So, I probably need an unreg supply with (40+7) 47vdc available under heaviest load conditions, in order to get 40vdc regulated?
Or did I need (40+27%) 51vdc unreg available under heaviest load conditions, in order to get 40vdc regulated?
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Old 7th March 2012, 09:52 PM   #745
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Originally Posted by danielwritesbac View Post
Good example! Thanks!!!!

So, I probably need an unreg supply with (40+7) 47vdc available under heaviest load conditions, in order to get 40vdc regulated?
Or did I need (40+27%) 51vdc unreg available under heaviest load conditions, in order to get 40vdc regulated?
Daniel, did you ever mind balanced configuration? I think something like this would fit your approach better. Though if you are not planning to use balanced conf with this power level already.
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Old 8th March 2012, 03:47 AM   #746
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I was not kidding earlier, posting the picture of the battery.
Just trickle charge a stack of two or three glass mat batteries
per rail. Disconnect charger whenever you demand zero ripple.
You want the absolute quietest supply? Battery would be it.

Put a 13V zener across each battery to prevent overcharge.
AGM should not be charged 14V, like a vented car battery.
They might last a decade if you never charge past 12.6V.
Charging is rather slow at that rate, but you got the time...

Impress your friends with the sheer absurd weight of it all.

Last edited by kenpeter; 8th March 2012 at 03:57 AM.
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Old 8th March 2012, 05:45 PM   #747
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by danielwritesbac View Post
Good example! Thanks!!!!

So, I probably need an unreg supply with (40+7) 47vdc available under heaviest load conditions, in order to get 40vdc regulated?
Or did I need (40+27%) 51vdc unreg available under heaviest load conditions, in order to get 40vdc regulated?
You can compute backwards, from the target voltage.
For 40V, you need 43V min. at the input, add 10% ripple, that is 47.3V, 2V diodes drop = 49.3V, divide by 1.41, you get 35V AC nominal voltage.
If you want to factor in the mains variation, yo need to add a further 5 or 10%.
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Old 8th March 2012, 07:19 PM   #748
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Originally Posted by terranigma View Post
Daniel, did you ever mind balanced configuration?
I hadn't considered balanced input since my sources are all single rail. Are there still advantages to balanced input?
Quote:
Originally Posted by kenpeter View Post
I was not kidding earlier, posting the picture of the battery. Just trickle charge a stack of two or three glass mat batteries per rail. Disconnect charger whenever you demand zero ripple. You want the absolute quietest supply? Battery would be it. Put a 13V zener across each battery to prevent overcharge. AGM should not be charged 14V, like a vented car battery. They might last a decade if you never charge past 12.6V. Charging is rather slow at that rate, but you got the time...

Impress your friends with the sheer absurd weight of it all.
The floor standing radio plans have changed over to bi-amp.
The midbass, midrange, tweeter amplifier can run either batteries or regs--Sometimes the needed runtime is 19 hours, so probably regs.

With such a low crossover point, bass-only amplifiers run fine on unregulated power. Perhaps the bass-only amplifiers can now use parallel output devices? Maybe?
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Old 8th March 2012, 08:49 PM   #749
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Quote:
Originally Posted by danielwritesbac View Post
I hadn't considered balanced input since my sources are all single rail. Are there still advantages to balanced input?
With a a driver like DRV134, you can get balanced signal with an unbalanced source. Very easy setup but requires extra small symmetrical rails from 5V to 15V for supply.

Since that you have to build 2 amps per channel, then load will be shared by each amp then quadruple total output power. In your case 4 ohms will be shared as 2 ohms per amp, then total ~400 watts per channel with 32V rails.
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Old 10th March 2012, 11:20 AM   #750
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I have just started on a set of Alex's pcb's, ready to etch now.
Any opinions on MJ15003's in this circuit? They look ok to me but I'm a rank amateur.
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