OK. This might be long and somewhat philosophical, but in the 1970's all power amps I saw had a single supply and a huge output capacitor. They had their advantages: Simplicity and a blown output transistor would not fry the speaker.
Slowly did dual rail supplies become the norm and you could drop the output capacitor and instead add circuitry to disconnect the speaker in case of failure.
So, what are the pros and cons? Apart from getting rid of a bulky capacitor that may add some distortion an the extreme low end?
Slowly did dual rail supplies become the norm and you could drop the output capacitor and instead add circuitry to disconnect the speaker in case of failure.
So, what are the pros and cons? Apart from getting rid of a bulky capacitor that may add some distortion an the extreme low end?
I believe that most of these amplifiers back then did not have a differential input stage. The input had some kind of a bias voltage so you anyway needed an input capacitor. Due to the lacking differential input stage, it would be difficult to control the output stationary (DC-bias) voltage toward an eventual ground line had a symmetrical supply been used. Then, another capacitor would anyway be needed at the output.
In the early days, transistors were expensive and you tried to get most out of few. When in particular semiconductor integration became common, you could use more sophisticated semiconductor structures, like the differential input stage, and control of the output voltage against a mid ground-level became possible.
The discrete amplifier experts can tell you much more about this development.
Pros for the "old" amplifier designs were simplicity, a low parts count and no DC risk at the speaker as long as the output capacitor was not shorting. Cons were an output capacitor that would take quite some AC-current and sound quality (distortion).
In the early days, transistors were expensive and you tried to get most out of few. When in particular semiconductor integration became common, you could use more sophisticated semiconductor structures, like the differential input stage, and control of the output voltage against a mid ground-level became possible.
The discrete amplifier experts can tell you much more about this development.
Pros for the "old" amplifier designs were simplicity, a low parts count and no DC risk at the speaker as long as the output capacitor was not shorting. Cons were an output capacitor that would take quite some AC-current and sound quality (distortion).
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In *stereo*, cap-output means three big caps (in 2 different values), DC-coupled means two big caps (same value).
Then what FauxFrench says. When DC-accurate no-offset input stages got cheaper than the third cap, there was a rush to save a buck. (Was much more than a buck those days.)
Then what FauxFrench says. When DC-accurate no-offset input stages got cheaper than the third cap, there was a rush to save a buck. (Was much more than a buck those days.)
Speaker cap amps can get harmonic distortion down below 0.01%. See for example Apex AX6 Retro Amp 50W Single Supply - Page 19 - diyAudio
and the new design Old-fashioned amplifiers with a new twist (1) - diyAudio
Many speaker protection circuits are complete frauds, as the ones using low AC current rated relays to stop fault currents of DC hundreds of amps. I have an amp where the triac crowbar melted the trace to the speaker completely off the PCB instead of blowing the circuit breaker. AC relays would usually weld the contacts together. Some manufacturers sold competent speaker protection, but not in the budget line amps.
The advantage of .003% HD and below of some dual supply amps are in bragging rights IMHO. No speaker sold in my state will produce sound with that low a distortion level.
The big advantage of a single supply speaker cap amp to diy builders is that one solder joint popped loose will not blow ones speaker. Another advantage is that the number of wires is a lot lower for point to point construction. A third advantage is that transformers lacking a center tap can be used, as doorbell transformers, laptop wall plug supplies, higher voltage battery charger transformers etc.
The advantage of dual supply is better numbers for bragging in sales literature, cheaper components in the day of $.02 TO92 transistors installed with pick & place machines. Blown speakers are a positive advantage to manufacturers selling those also. Seven years product life is all anybody should expect, right? I use speaker I paid $600 the pair for, new price $1200, that are 21 years old. If my $50 amp project produces massive DC to the output cap, I haven't destroyed anything but $5 apiece output transistors.
and the new design Old-fashioned amplifiers with a new twist (1) - diyAudio
Many speaker protection circuits are complete frauds, as the ones using low AC current rated relays to stop fault currents of DC hundreds of amps. I have an amp where the triac crowbar melted the trace to the speaker completely off the PCB instead of blowing the circuit breaker. AC relays would usually weld the contacts together. Some manufacturers sold competent speaker protection, but not in the budget line amps.
The advantage of .003% HD and below of some dual supply amps are in bragging rights IMHO. No speaker sold in my state will produce sound with that low a distortion level.
The big advantage of a single supply speaker cap amp to diy builders is that one solder joint popped loose will not blow ones speaker. Another advantage is that the number of wires is a lot lower for point to point construction. A third advantage is that transformers lacking a center tap can be used, as doorbell transformers, laptop wall plug supplies, higher voltage battery charger transformers etc.
The advantage of dual supply is better numbers for bragging in sales literature, cheaper components in the day of $.02 TO92 transistors installed with pick & place machines. Blown speakers are a positive advantage to manufacturers selling those also. Seven years product life is all anybody should expect, right? I use speaker I paid $600 the pair for, new price $1200, that are 21 years old. If my $50 amp project produces massive DC to the output cap, I haven't destroyed anything but $5 apiece output transistors.
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And DC-coupled bridge-tied only needs a single rail, single cap as only a low-current virtual ground is needed. Of course the economics of this depend on the relative cost of a capacitor and an entire amplifier, and its mainly seen with class-D.
Absolutely true with single rail for BTL-coupling. But for BTL-coupling, I believe it is not always capacitor cost savings that are behind. Almost all ordinary (something I can afford) car amplifiers use BTL because of the voltage limitation. For higher voltages, it is a trade-off between voltage and current capability. Without at all being a semiconductor expert, I have the impression that it is easy to make FETs for high voltage and high current (simply more small FETs in parallel) while it is difficult to make bipolar transistors stand both high voltage and handle high current. Then, lower voltages and higher currents may be the best trade-off.
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BTL had to await *power* transistors cheaper than a second cap.
So a next-generation after *input* transistors cheaper than a cap.
(discounting the special case of car-sound, where straight 12V 4r doesn't cut it and 1-Ohm speakers force distressingly fat speaker lines.)
So a next-generation after *input* transistors cheaper than a cap.
(discounting the special case of car-sound, where straight 12V 4r doesn't cut it and 1-Ohm speakers force distressingly fat speaker lines.)
Unless something has changed very recently, the vast majority of even cheap small car amps have used switching supplies since the 80's to remove that limitation.
You´d be surprised 😀
Not even the simplest cheapest SMPS + "any" power amp combination can beat or even approach dirt cheap cost and simplicity of the humble BTL chipamp, fed straight 12.6V and driving a 3.2 or 4 ohm load , which is the muscle behind most generic factory supplied car radios and players, as in over 90% of them.
Only a few years ago, definitely much later than the 80´s, did classic 15/20 W per channel chipamps stop being produced for lack of massive sales,which had been previously been produced by the *millions*.
I wouldn't argue about factory units, or aftermarket head units.
Car chipamps were ghosts by 1988 or so. The vast majority of 80's-90's entry level aftermarket car amps were discrete models advertising a true power of 20-40WPC.
When FauxFrench said "ordinary (something I can afford) car amplifiers", I assumed he was talking about aftermarket stuff. There are still plenty of inexpensive aftermarket car amps out there that step the voltage up.
Car chipamps were ghosts by 1988 or so. The vast majority of 80's-90's entry level aftermarket car amps were discrete models advertising a true power of 20-40WPC.
When FauxFrench said "ordinary (something I can afford) car amplifiers", I assumed he was talking about aftermarket stuff. There are still plenty of inexpensive aftermarket car amps out there that step the voltage up.
Modern electronics tend to be more reliable so an output protect circuit is rarely needed.
Even then a protect circuit doesn't protect against everything.
Most just detect DC and turn off a relay.
If you insist on simply driving the speaker too hard the speaker will eventually die.
Even then a protect circuit doesn't protect against everything.
Most just detect DC and turn off a relay.
If you insist on simply driving the speaker too hard the speaker will eventually die.
HA HA!
Until the 1/4 phone plug is banned from speaker service, and roadies are banned from drinking alcohol, amps will continue to run into shorts. Using spade lugs I've pulled speaker wiring off the amp by tripping over the wire. Fortunately it was vacuum tube, the transformer saved it.
A local musicians exchange store is full of amps with shiny new output transistors. Not many of them are first generation kluges like the tigersaurus. Lots of speakers in there with new cheapo drivers, too.
I just replaced OT's in a $20 PV-4c that had 4 sets of initials on the heat sink. More fins on the heat sink, scotty.
Classic single supply amps had turn-on/off thumps or speaker relays, and the output caps were never large enough for ~20Hz bass. Since then we have a few solutions:
1. QSC floating supply. The amp drove "ground" and the output was the middle of two caps across a single floating voltage, one per channel.
2. Bridged (BTL) where both sides of the speaker are connected to 1/2 the supply voltage, very popular for automotive stereos.
3. Floating ground, ie the middle of two caps across the single supply, which can be shared with other channels. Ground voltage is actually set by the amp(s), which have to take their DC reference from a divider. The major drawback is that low frequency stability must be considered.
4. Local voltage inverters, not necessarily just a negative rail.
1. QSC floating supply. The amp drove "ground" and the output was the middle of two caps across a single floating voltage, one per channel.
2. Bridged (BTL) where both sides of the speaker are connected to 1/2 the supply voltage, very popular for automotive stereos.
3. Floating ground, ie the middle of two caps across the single supply, which can be shared with other channels. Ground voltage is actually set by the amp(s), which have to take their DC reference from a divider. The major drawback is that low frequency stability must be considered.
4. Local voltage inverters, not necessarily just a negative rail.
I guess another downside of a single-supply design is the thump you get on powerup. The output before the cap rising to Vcc/2 in an instant.
Hi sorry to jump in ... speaking of only solid state units like a preamp what kind of advantages can provide a dual PS ? it seems to me that all ss preamps have a dual supply. I cannot understand the reason.
The advantage of a dual power supply is that all signals will be in reference to 0 volts, ie you can skip using capacitors between stages.
Hi ! thanks for the helpful explanation. Then why some preamps have dual supply and still mantain input and output capacitors ? strange 😕
You cannot guarantee what might be connected to it, so input and output coupling caps keep things safe.
Hi thanks ! now it is all more clear. Actually all tube preamps are cap coupled and i am sure they can sound very good. I will get a battered DIY preamp soon and i would like to revise its power supply a little. I will use a single supply then
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And for power amps - bear in mind that a typical single rail solid state amp has a low output impedance to directly drive a loudspeaker and this means it flows a lot of current - the output capacitor of such an amplifier must have a large capacitance to retain a low impedance at bass frequencies and to avoid distortion; the choice of capacitor type becomes limited. In contrast, a tube amplifier usually has an output transformer instead.
This large output capacitor is typically not inside the feedback loop of the amplifier and hence it has an opportunity to impress it's own sonic signature on the sound. In a dual rail amplifier the feedback loop of the amplifier increases the amplifiers' immunity to what goes on at it's power rails so the large capacitors decoupling the power rails have a smaller impact on the sonics.
All that theory aside, an output capacitor is not something to be afraid of. Modern capacitors are waaaaay better than times of old. Back in the 70's when single rail amps were more common the capacitors they used were limited by the technology and materials of the day.
This large output capacitor is typically not inside the feedback loop of the amplifier and hence it has an opportunity to impress it's own sonic signature on the sound. In a dual rail amplifier the feedback loop of the amplifier increases the amplifiers' immunity to what goes on at it's power rails so the large capacitors decoupling the power rails have a smaller impact on the sonics.
All that theory aside, an output capacitor is not something to be afraid of. Modern capacitors are waaaaay better than times of old. Back in the 70's when single rail amps were more common the capacitors they used were limited by the technology and materials of the day.
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