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Old 6th June 2014, 08:02 PM   #1
Saeger is offline Saeger  United States
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Default More watts?

Generally speaking how can I replace the output stage of an amplifier with one that provides more gain? Of course I will need a separate power supply that will provide the higher voltage that the output transistors will require. How do I select components that will work well with the rest of the amp?

Ie. I have an output transistor that provides X amount of gain at 70v. And I want to replace it with a similar transistor that has 50% more gain at 105v.

Thank you!
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Old 6th June 2014, 11:50 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Generally speaking you cannot. Not without a new power supply,
and new heatsinking which obviates the standard casework.

The output stage generally has no gain * and 50% more
is meaningless, your talking 50% higher rails, which as
a modification is simply nearly always a very bad idea.

CFP output stages can be arranged for gain but I suspect
your already way out of your range of understanding
the salient points of uprating an amplifiers power.

rgds, sreten.

* Its always under 1, e.g ~ 0.97 in a good standard amplifier.

Last edited by sreten; 6th June 2014 at 11:55 PM.
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Old 7th June 2014, 12:36 AM   #3
kevinkr is offline kevinkr  United States
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As Sreten says it is not that simple, essentially the entire amplifier has to be redesigned from the ground up.

Normally we talk about power into a specified load impedance and not output voltage in a power amplifier.

The minimum meaningful power increase is about 3dB (double) and to get it you need substantially greater output voltage and current from the driver stage which requires a significant redesign. Interestingly as you ask a transistor to produce more output power its current gain decreases proportionately which means that demands on the driver stage increase commensurately.

Changing the output transistors and increasing the power supply voltages without a proper analysis of transistor SOA and heat sinking as well as those of the driver stages and not understanding how to make those changes successfully is likely to result in a lot of smoke.

I would design the amplifier from the ground up for this task.
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Old 7th June 2014, 12:50 AM   #4
Bigun is offline Bigun  Canada
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It depends. If you want more headroom, i.e. you want to be able to have a higher peak power but you don't plan to run the amplifier at full power constantly then you may get away with using the existing heatsinks although the higher power rail voltages will mean that they will run warmer at idle.

But the changes you will have to make are a lot of work - new power supply and output transistors means a fair bit of expense too and as others have said - you may as well start over because the front end of the amplifier is the lowest cost part and the bits you need to replace are the bulk of the cost.
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Old 7th June 2014, 05:17 AM   #5
Saeger is offline Saeger  United States
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Okay so clearly that was the wrong aproch. How about building an additional 100 watt amp with a gain of 2x to be drivin by the output stage of a 40 watt amp? I.e like a linear amp for ham radio but for audio.

Is that feasable?

-Thanks.
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Old 7th June 2014, 08:01 AM   #6
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Quote:
Originally Posted by Saeger View Post
... How about building an additional 100 watt amp with a gain of 2x to be drivin by the output stage of a 40 watt amp? I.e like a linear amp for ham radio but for audio....
That still requires a new, larger power supply and amplifier. It has no benefit but now takes up the space for 2 amplifers and power supplies.

Radio frequency or "linear amplifiers" provide another gain stages to obtain sufficient drive power for analog transmissions because of their inefficient operation in linear mode. So a series of matched, relatively low gain stages is a normal strategy at VHF and higher frequencies.

Audio frequencies though, present no problem for obtaining full gain of around 30 times the voltage and thousands of times the power with a handful of inexpensive parts. This is routine with inexpensive components in non-critical assemblies, as you see in almost any commercial amplifier.

Think of it as your car and if it's feasible, you'd like more power. You sure don't bolt on a second 6 cylinder motor to get more power though - you drop in a V8 and larger transmission, right? Same here - you want more power so you simply drop in a larger amplifier and power supply. That's simplest, most economical and presents an opportunity to make other improvements on possibly an ordinary design to begin with.

Others have already made it clear that you can't expand a given design to higher power levels. There are certainly ways to use matched amplifiers in parallel and in bridge circuits but they still need a larger power supply designed for that purpose too.
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Old 7th June 2014, 09:03 AM   #7
ontoaba is offline ontoaba  Indonesia
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Double it all including the loud speaker, easiest way for double watts.
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Old 7th June 2014, 07:45 PM   #8
Saeger is offline Saeger  United States
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"Think of it as your car and if it's feasible, you'd like more power. You sure don't bolt on a second 6 cylinder motor to get more power though - you drop in a V8 and larger transmission, right? Same here - you want more power so you simply drop in a larger amplifier and power supply. That's simplest, most economical and presents an opportunity to make other improvements on possibly an ordinary design to begin with. "

In the 60's Pontiac made a four cylinder motor based off the 389 v8. Some once built a dragster using two of these 1/2 389's one in front of the other. He called it 389 the hard way!

So the best way to do it would be to get a second matching amp and bridge both of them for mono and have one run the right channel and the other run the left.

Next question is how do I bridge an amp?

I have a Dynaco sca-80q that I like the sound of. Is that a suitable amp for bridging?
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Old 7th June 2014, 08:10 PM   #9
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Now talking the old stuff like dynaco, that changes things.
Nobody has mentioned yet that you can get more short term volume out of 8 ohm speakers, instead of 4 ohm speakers, because the effect is more voltage than current. Like the kind of speakers they sold when the SCA80 was made. Output transistors tend to exceed the soa with a lot of current, and many 8 ohm speakers don't need a lot of current. I've got peavey 1210's and SP2-XT's and I can get great short term peaks with a single pair of output transistors per side.
furthermore, the voltage gain is done in the input or second stage of the amp, not the output transistor stage, which just increases the current.
The sca80 was designed when output transistors couldn't put out more than about 1/3 the rail voltage. A cheap way to get more voltage is to change the output transistors to modern ones with higher gain and Ft, which I did to my dynaco ST-120. It gets about a third more peak voltage on the speaker than original due to the better output transistor, and I eliminated the regulator function that was holding the output rail voltage down.. This peak voltage is only useful for transients like the cannon shot in 1812 overture, but that is what I play, not techno rock grind em out crunch top waves. Note the ST120 has way more voltage gain than is usual for a more modern transistor amp. I dont' have a sca80 schematic to know if that practice held on into the seventies or died in 1966.
I handled the increased O.T. dissipation on my ST120, by putting PCAT power supply fans on the heat sinks. I also installed heat sinks on TO220 driver transistors, instead of the original TO5 driver transistors, some of which didn't have heat sinks.
Really, if you want more volume at 8 ohms, I'd build a honey badger kit from diyaudio supply, with a nice stiff upper range power supply. for 4 ohm speakers, the nominal voltages are fine. Or look around for a pair of SP2-XT, they are 101 db/@1 watt, which is plenty loud indoors on one pair of output transistors with an 80 v single ended supply.
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Last edited by indianajo; 7th June 2014 at 08:14 PM.
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Old 8th June 2014, 12:45 AM   #10
wadest is offline wadest  United States
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saeger, i don't know if anybody told you how to bridge two amps. all you need do is ad an invertor amp, like an ic amp in one of the amps. then run the input into both amps or the output of the first amp into the inverted amp. if each amp is designed to drive 4 ohms once bridged it will drive an 8 ohm load. if you want to know how to make the invertor amp inside one of the power amps, let me know.
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