Reduced rails for input stages.

K-amps · 10th February 2004, 03:05 PM

I am trying to increase my rails from 80 to about 100vdc for my amp so that under load they drop to only 75vdc or so. (and not the current 62 vdc they come down to under full load.)

While I can replace the outputs and drivers with hi-volt types, I do not want to play around with the input stages and leave them at 80vdc or so.

Whats the easiest way to do it? I have considered the following:

1) Zener and resistor combo running off the main rails.
2) Connect 4-6 diodes in series to get the desired voltage drop from the main rails.
3) Dedicated power supply.

1 and 2 would have a smoothing cap at the end for some regulation.

Any thoughts would be appreciated,

Nelson Pass · 10th February 2004, 03:43 PM

Sounds like your transformer is kinda small to drop so much.

The front end could easily be driven by follower transistors
whose base is driven by Zeners (in series for the voltage you
want) in parallel with a small cap, forming reference which in
turn is biased by a resistor to the supply. Standard stuff that
works well.

K-amps · 10th February 2004, 04:46 PM

Thanks Nelson .

Your note on the transformer.... well its an Adcom 555. I think it is a 700 VA unit with 30,000 uF per rail. At max load the rails drop to 62-3 v per rail or so. Funny thing is, the AC secondaries drop only 3 volts from 60vac to 57 vac.

Does one volt drop on the AC side (pre rectification)mean more than 1.414vdc drop on the DCside or is it because of the RMS vale of the AC to DC conversion.

Or lets say I used a 1.5kVA transformer instead keeping the same caps, what voltage drop do you reckon I should be able to get under 700 watt load?

K.

Nelson Pass · 10th February 2004, 05:25 PM

The calculation is complicated because a loaded transformer
is no longer putting out a pure sine wave, and you have to
evaluate it based on peak voltages, which are what are really
charging the caps.

If it's a 555, then your loading is also a function of continuous
high power, which is not characteristic of real music - I wouldn't
worry about it. If you want, try decoupling the front end from
the main supply using simple RC filters.

K-amps · 10th February 2004, 06:41 PM

Thanks thats what I think I will do. Would you advise a zener diode for better regulation and yet keeping it simple in implementation?

thanks again!!

10th February 2004, 03:05 PM	#1
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Reduced rails for input stages. I am trying to increase my rails from 80 to about 100vdc for my amp so that under load they drop to only 75vdc or so. (and not the current 62 vdc they come down to under full load.) While I can replace the outputs and drivers with hi-volt types, I do not want to play around with the input stages and leave them at 80vdc or so. Whats the easiest way to do it? I have considered the following: 1) Zener and resistor combo running off the main rails. 2) Connect 4-6 diodes in series to get the desired voltage drop from the main rails. 3) Dedicated power supply. 1 and 2 would have a smoothing cap at the end for some regulation. Any thoughts would be appreciated,

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10th February 2004, 03:43 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Sounds like your transformer is kinda small to drop so much. The front end could easily be driven by follower transistors whose base is driven by Zeners (in series for the voltage you want) in parallel with a small cap, forming reference which in turn is biased by a resistor to the supply. Standard stuff that works well.

10th February 2004, 04:46 PM	#3
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Thanks Nelson . Your note on the transformer.... well its an Adcom 555. I think it is a 700 VA unit with 30,000 uF per rail. At max load the rails drop to 62-3 v per rail or so. Funny thing is, the AC secondaries drop only 3 volts from 60vac to 57 vac. Does one volt drop on the AC side (pre rectification)mean more than 1.414vdc drop on the DCside or is it because of the RMS vale of the AC to DC conversion. Or lets say I used a 1.5kVA transformer instead keeping the same caps, what voltage drop do you reckon I should be able to get under 700 watt load? K.

10th February 2004, 05:25 PM	#4
Nelson Pass The one and only Join Date: Mar 2001	The calculation is complicated because a loaded transformer is no longer putting out a pure sine wave, and you have to evaluate it based on peak voltages, which are what are really charging the caps. If it's a 555, then your loading is also a function of continuous high power, which is not characteristic of real music - I wouldn't worry about it. If you want, try decoupling the front end from the main supply using simple RC filters.

10th February 2004, 06:41 PM	#5
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Thanks thats what I think I will do. Would you advise a zener diode for better regulation and yet keeping it simple in implementation? thanks again!!

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