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Author Topic: Simple way to scale existing circuits...?  (Read 2265 times)

mckayprod

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Simple way to scale existing circuits...?
« on: May 25, 2016, 09:30:22 PM »
I am nobody's amp designer, so I'm drawn to existing amps that I can tweak into my own devious plans.  First question:  I've been looking at Rod Elliot's P27 amp, and the power amp seems pretty well-respected on this forum.  I'd like to build a lower-power version, so what's the simplest/best way to accomplish that?  If I reduce the supply voltage from +/- 35v to something like +/- 20v, would the amp still operate, but at lower power (and less stress on the transistors, less heat...)?  I don't mean to be too specific about the design; I understand this is a pretty straightforward (representative?) topology.  Does this approach work with SS power amps generally, or am I missing something important?  Second question:  how do I calculate some of the changes that would follow from this?  How much power change could I expect from a given voltage change?  How much less heatsink can I get away with?  Would different transistors be needed and how do I spec them?  Thanks for any help anyone can offer, and I'll be searching the forum to see if anyone else has looked into this.

Enzo

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Re: Simple way to scale existing circuits...?
« Reply #1 on: May 25, 2016, 11:05:00 PM »
Decide what your goals are.  "Lower power" sounds to me more like your idea of a solution to some other goal.  Like maybe you want it less loud.  Which leads me to say that power is not loudness.  Cut power in half and it is only 3 decibels less loud, not much.  Just like a car that CAN drive 100mph, it doesn't HAVE to.  Nothing makes you turn an amp all the way up.  And unlike tube amps, solid state power amps really are not good at overdriving the outputs, they just clip.  SO turning it down doesn;t do much to the tone.

You can reduce the power supply voltages, but it isn;t always that simple.

J M Fahey

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Re: Simple way to scale existing circuits...?
« Reply #2 on: May 26, 2016, 04:09:52 AM »
Agree and add: you might as well choose anohther, lower power project, there's thousands floating around.

If you want to go step by step, fine.
You might start with a TDA2030 or LM1875 or TDA205 based project, something between 15W and 25W ,,, which is what you essentially wanted to do anyway.

As of power variation with voltage, roughly halving voltage will reduce power to 25% of original, a BIG change and amp stays as complex as before, that's why I suggeststraight going for a smaller project.

Which will work VERY well anyway; a 15W amp with a reasonably good speaker covers from bedroom to garage or Church; a 25W one, again with a good speaker, is just adequate to play along a drummer in a small Club ... unless he has a gigantic drum kit and uses tree trunks as sticks ;)

mckayprod

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Re: Simple way to scale existing circuits...?
« Reply #3 on: May 27, 2016, 10:14:09 PM »
Good points.  I guess I was hoping to avoid using a different transformer than the 20v I already have.  The way Rod built that project, you end up with a pretty compact amp, lightweight and with lots of room in a cabinet for reverb tanks and other small onboard effects.  Half power equals 3db less...words to live by!

mckayprod

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Re: Simple way to scale existing circuits...?
« Reply #4 on: July 05, 2016, 12:34:16 AM »
I guess I have another reason for wanting a lower power amp circuit.  What if I have a particular speaker that I like, but it can't handle 100w?  And, what if I'd like to monkey around with discrete components rather than a chip?  So, let's say I cut the power supply in half, giving Elliott's 100w arrangement 25w into 4 ohms.  And, my single 25w speaker is 8 ohms, so the power out is further reduced to (60%?) 15w.  I think that moves the project in the right direction, but how much can you reduce supply voltage before the circuit refuses to function at all?  If scaling a circuit is just a matter of applying fixed rules-of-thumb, I'm in.  Background:  I played through a Vox AC-15 with the Celestion greenback the other day.  Wow.  Wouldn't want to risk blowing one of those in an amp that will likely never see a stage.

phatt

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Re: Simple way to scale existing circuits...?
« Reply #5 on: July 05, 2016, 09:49:15 AM »
A 25 Watt speaker won't instantly burn out at 26 Watts,, Even less likely with guitar.
Unless you intend to play heavy metal with massive bass boost :duh in which case it will heat up and eventually fail.
You can run a 25 watt speaker all day from a 100 watt Amp,, just don't crack it up full with the bass on 10. :loco
The Bass freq is what overheats the coil.
Yes at prolonged over wattage it will burn out the coil but speakers can handle short bursts of higher wattage.

You mentioned fiddling with discrete circuits,, so if you absolutely must have a low wattage amp then find another circuit there are many schematics of low power amps if you look. 8|
Phil.

mckayprod

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Re: Simple way to scale existing circuits...?
« Reply #6 on: July 06, 2016, 11:30:13 PM »
Got it.  Back to the search engine(s).  Thanks.

phatt

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Re: Simple way to scale existing circuits...?
« Reply #7 on: July 07, 2016, 07:29:22 AM »
If you wish to experiment with discrete circuits then have a fiddle with this one;
http://sound.westhost.com/project03.htm

Use figure "1a" as a start. :tu:

I have built stuff like this and with a few tweaks they would run from +/-25Volt supply.
Maybe ~20 watts at best.
If you are not conversant with DC circuits like this then be prepared to smoke a few transistors  :'( but it's all good learning stuff. 8|
And remember the output has no protection so even a momentary dead short of the output terminals will blow the amp. :o
Phil.

J M Fahey

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Re: Simple way to scale existing circuits...?
« Reply #8 on: August 07, 2016, 07:19:41 AM »
Good points.  I guess I was hoping to avoid using a different transformer than the 20v I already have.  The way Rod built that project, you end up with a pretty compact amp, lightweight and with lots of room in a cabinet for reverb tanks and other small onboard effects.  Half power equals 3db less...words to live by!
Ok, that defines your project.
What do you exactly mean by 20V?
* 20V AC single winding ?
* 20 VAC center tapped ?
* A single 20V DC supply? (maybe a laptop one?)

Does your transformer/supply indicate current or V.A.?