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!
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!
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.
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:
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.
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.
Member
Joined 2009
Paid Member
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.
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.
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.
"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?
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?
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.
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.
Last edited:
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.
I'd like to add that if making your system louder is your goal, doubling the output power only increases the output by 3dB, a just noticeable level change to the human ear. To double the apparent sound level needs about 10 times (10dB) power increase. Even going with a bridged amp setup will generally give 2 times, to maximum theoretical 4 times output power, so not really a lot of increased SPL in the end.
Mike
Mike
Well, since I was only speculating that the OP's actual goal was greater apparent volume level, I can't even comment what would be the "best way" to achieve the desired result since I don't know what that actually is.
There's more than one way to skin a cat. In many cases adding more speakers isn't an option. A very effective way to increase apparent SPL is to use more efficient speakers, it isn't always about power.
Mike
Something as old as dynaco equipment, frequently power supply issues (electroltytic capacitors) hold the output power to 1/4 to 1/10 of the advertised power.
Before doing any "upgrading" or "bridging" I'd do a voltage check on a known resistor to see if the amp needs repair. I use a simpson 260-6XLPM analog VOM on the 20 VAC (average) scale to do 35 W at 8 ohm voltage checks on a tapped 10 ohm ohmmite 200 W resistor set to 8 ohms. I used to get good results with R**** S**** 20 kohm/volt meters. What you will not get good results on is most DVM's, the ones I own produce random numbers on music, reading proper AC voltages only at 60 or 50 hz.
My ST70 tube amp was putting out 7 w/ch when I bought it. My ST120 transistor amp was incinerated when I bought it, and repair without a schematic diagram or resource, got me up to 36 watts total both channels due to having issues with the current regulation circuit. (Too much gain on the modern replacement transistor in the current limiting circuit, in fact).
Just put radio input into the amp, turn up the volume until the distortion is mildly audible, switch from speakers to resistor. Read AC volts, P=(V^2)/R. Dynacos uniformly expected 8 ohm speakers until bankrupcy, I believe. ST400 might be an exception.
I did NOT suggest "adding more speakers". I suggested buying high output 8 ohm speakers instead of modern and unsuitable 4 ohm speakers. The two models I suggested are quite loud (101 db @ 1 W 1 m) and efficient. I run my speakers at 1 Vpp base level in the living room, leaving plenty plenty of room for symphonic music peaks.
In an area where running one 6KW air conditioner in one room costs $150 a month for 10 weeks of the year, cutting the amount of amp power heating the room 18 hours a day is one of my goals. Consistent with good music reproduction, of course.
Before doing any "upgrading" or "bridging" I'd do a voltage check on a known resistor to see if the amp needs repair. I use a simpson 260-6XLPM analog VOM on the 20 VAC (average) scale to do 35 W at 8 ohm voltage checks on a tapped 10 ohm ohmmite 200 W resistor set to 8 ohms. I used to get good results with R**** S**** 20 kohm/volt meters. What you will not get good results on is most DVM's, the ones I own produce random numbers on music, reading proper AC voltages only at 60 or 50 hz.
My ST70 tube amp was putting out 7 w/ch when I bought it. My ST120 transistor amp was incinerated when I bought it, and repair without a schematic diagram or resource, got me up to 36 watts total both channels due to having issues with the current regulation circuit. (Too much gain on the modern replacement transistor in the current limiting circuit, in fact).
Just put radio input into the amp, turn up the volume until the distortion is mildly audible, switch from speakers to resistor. Read AC volts, P=(V^2)/R. Dynacos uniformly expected 8 ohm speakers until bankrupcy, I believe. ST400 might be an exception.
I did NOT suggest "adding more speakers". I suggested buying high output 8 ohm speakers instead of modern and unsuitable 4 ohm speakers. The two models I suggested are quite loud (101 db @ 1 W 1 m) and efficient. I run my speakers at 1 Vpp base level in the living room, leaving plenty plenty of room for symphonic music peaks.
In an area where running one 6KW air conditioner in one room costs $150 a month for 10 weeks of the year, cutting the amount of amp power heating the room 18 hours a day is one of my goals. Consistent with good music reproduction, of course.
Last edited:
Unless you simplify this project by bridging your Dynaco for one channel and obtain a second one for stereo, you are in for a long, hard learning curve. The principles may be simple enough but you still need to understand and follow a design - a plan and list of parts for a bridging circuit with phase reversal and with the room to accommodate them both inside the case.
As Michael Bean commented, 3 db volume increase by bridging is not much sound level increase but more efficient speakers often are. I don't know what you are currently using for speakers but it will probably help us to help you by understanding why you need more power from your amplifier in the first place.
As Michael Bean commented, 3 db volume increase by bridging is not much sound level increase but more efficient speakers often are. I don't know what you are currently using for speakers but it will probably help us to help you by understanding why you need more power from your amplifier in the first place.
Sure, that's what the spl equations tell us; 6dB loss with each doubling of distance.
Using speakers 6 dB more sensitive would effectively bring us back closer by half again.
Figuratively, sitting at 8 ft from your speakers, you would be only moving forward 2 ft
if you then switched the amplifier to bridge mode. Will this be worth it?
To make a point of the OP's request though, he wants a louder amp. so I'm not going to
suggest he just sits closer to the speakers - 'don't think he'd appreciate that too much
Using speakers 6 dB more sensitive would effectively bring us back closer by half again.
Figuratively, sitting at 8 ft from your speakers, you would be only moving forward 2 ft
if you then switched the amplifier to bridge mode. Will this be worth it?
To make a point of the OP's request though, he wants a louder amp. so I'm not going to
suggest he just sits closer to the speakers - 'don't think he'd appreciate that too much
Ian Finch, I just thought rther out in the audience.i read somewhere in all of this blogging he asked how to bridge a pair of amps. It was way back in the beginning of all of this. what I was actually talking about is how if your standing two feet from a speaker you will hardly notice a large increase in power but you will deffinatly notice it farther away.
I think you will have read that I was speaking figuratively - illustrating a proven theoretical relationship. I'm aware that live entertainment SPL is a different matter due to very high levels, often with strong cancellation and reflective sound effects from the multiple arrays used. Those effects have little to do with typical low level, home stereo listening environments, in my experience.
My point was clear enough and still is, about bridging amplifiers without changing anything else. The small increase of the 3dB SPL on offer is relative to whatever level you do hear, wherever you are and is of questionable benefit at any distance, even outside in the car park, if that's your point of reference.
Sound level distance damping decibel dB damping calculation calculator change distance versus sound level apps reduction drop dissipation SPL sound transmission loss free field loss sound and distance - decrease drop fall sound over distance versus d
Bridged and paralleled amplifiers - Wikipedia, the free encyclopedia
My point was clear enough and still is, about bridging amplifiers without changing anything else. The small increase of the 3dB SPL on offer is relative to whatever level you do hear, wherever you are and is of questionable benefit at any distance, even outside in the car park, if that's your point of reference.
Sound level distance damping decibel dB damping calculation calculator change distance versus sound level apps reduction drop dissipation SPL sound transmission loss free field loss sound and distance - decrease drop fall sound over distance versus d
Bridged and paralleled amplifiers - Wikipedia, the free encyclopedia
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.