Power amps - and power supplies

[R.G.]

I'll rephrase myself here, as the forum is new.

Power amps are best thought of as a power *supply* that lets some of the power out under special, carefully controlled circumstances. The power amp itself is really just a highly specialized wart on the power supply. In most cases, the power amp circuits proper are a trivial part of the cost of the unit.

The power supply needs to be the FIRST order of business in designing a power amp. Going to get 50W into 8 ohms? You need to know how to tell what power supply you need before you dig into what power amp or chip you'll use.

After the power supply conversion stages, voltages and currents, etc. you need to consider thermal matters. Again, this is substantially independent of the power amp circuit itself. If you're putting out 50W into 8 ohms with a class AB solid state amp, the power needed from the power supply is 50/.74 = 67.6W worth of DC coming out of the power supply at peak power. But that's not where the peak dissipation on the output devices happens. That peak comes at  forty (something) percent of the max power out. The heat has to be dissipated by heatsinks so the output devices are kept below the boiling point of doped silicon. Otherwise, you have just bought a new set of output transistors - or a new chip amp. The heat generated in the power supply itself also gets released inside the box. Where does that go?

After you have a good power supply design and a good thermal design, run wild with whatever power amp circuit you like.

[junkman]

hello r.g. im new to this forum. i have been foolin around with this stuff for a while tryin to make amps out of junk i find. and i have realized recently that the power supply is the starting point for building and amp. its the part ive been overlooking and recently ive beeen trying to figure out transformers and diodes and caps  and i think i have the basics down, i am currently experimenting with a lot of tda 2030 chips i got. i think my power supply is in order and im working on the main circuit now. if i can get this thing working the only part i have paid any money for is the tda2030, the rest is from dumpster finds.

[junkman]

i have a question about filtering caps in a power supply.   if more capacitance is better, is there a point at witch you gotoo far? can you have a cap thats too large?   

[teemuk]

Sure.

When a capacitor charges its impedance is very low (its almost like a short circuit). A high filtering capacitance will therefore draw big current surges that stress the power supply components. The current surge during power up can be enormous, the surges during charging the capacitors when the amp is under loading will also be much higher than in the case when you use a smaller amount of filter capacitance.

I attached an image clarifying this: V(n002) and V(n005) are rail voltages of two circuits under similar loading. Basically the circuit consists of a power supply (having an internal resistance of 1 ohm), bridge rectifier and a 4-ohm resistor from the (rectified) supply rail to ground. There is also filter cap in parallel with the resistor (naturally). The only difference in the two circuits under comparison is that the filtering capacitance in the other one is ten times higher. You can see the effect of that in the smaller amplitude of the ripple.

I admit that this is not a circuit you would encounter in a practical amp as the loading of the supply rail drops this low only momentarily when amplifier is delivering its full power. Anyway, it nevertheless provides a fine example. For some perspective of realistic behaviour, the graphs at the bottom show the power up and “idling” with a 10-kilo-ohm rail load. This type of circuit you could actually encounter in a real amp.

I(D1) and I(D6) are currents flowing through one of the rectifier diodes in each circuit. The current flow through the other three diodes would be identical. I(D1) is the current in the circuit that has the higher filter capacitance. You can see that the initial current surge drawn to charge the filter capacitor is enormous. It’s quite high in the other circuit as well but the still the difference is drastic! Think how you would have to rate the transformer, diodes and fusing to tolerate this characteristic.

Then there are the surges that occur periodically when the capacitors are charged. The circuit with higher capacitance requires shorter charge time, thus the charging peak is shorter but also much higher in amplitude. Again, it is more stressing for the power supply. The characteristic is quite negligible in most circuits though. You can see the difference most prominently when the rail load is quite high (graph at bottom right).

Anyway, in essence you are dealing with making a compromise between high, transient-like current draw that will stress the power supply components - and a higher amount of ripple that will creep to the signal. You should calculate how much ripple you are willing to tolerate and choose the filtering capacitance accordingly. The reason why commercial designs often use less than 10 000uF per rail is actually quite understandable when you begin to consider the drawbacks of using too much capacitance. Often there isn’t even much of point in going “overkill”: A typical power amp is quite immune to ripple (which begins to occur in notable amount only during severe loading anyway) and the rails of a typical preamp are often regulated so the much more delicate preamp does not suffer from the ripple either. Having a very high filtering capacitance is basically more of an issue in single-ended circuits that also have single-ended preamps powered from rather high voltage rails that lack regulation.

[junkman]

let me make sure i have this straight,
   the caps are not just dumping out enough juice to even out the pulsing dc, they dump all of their charge and have to be completly recharged, so too much is wastefull and taxing on the transformer or would require a larger transformer with no improved filtering.   

[teemuk]

No. The caps are there just for that: Evening out the pulsing dc. But you must also remember that the filter capacitors provide current to the load when the pulsating current fed by the rectifier cannot possibly do it. (After all the current is “pulsating” which means it will periodically drop to zero). So, when the rectifier cannot supply the load current the filter caps must do it. When doing so the capacitors naturally discharge an amount that is proportional to the current draw. (Think basic RC circuit). When the rectifier can again deliver another current pulse the filter capacitors become charged again. This cycling creates the ripple in the supply rail.

In normal conditions the filter capacitors definitely do not drain out completely; that would mean that the “bottom” of the ripple waveform would be zero volts. You are not going to see that happen in conventional circuits. The capacitors may be drained enough to create a great amount of ripple but no; they won’t drain out completely if the circuit is designed properly. If they did, there was too little capacitance in the first place.

Now, if you want less ripple you increase the filter capacitance, which in turn increases the RC time constant, which means the filtering capacitors discharge slower and effectively the ripple is reduced. The downside is that the power supply will be stressed more due to increased current draw that the higher filtering capacitance causes.

If there were no current draw at all (e.g. no load at all) the capacitors wouldn’t drain at all either – neither would the rectifier supply any current, as there simply wouldn’t be a need for that. However, normally even at idle there is some quiescent current draw, thus the filter caps will drain at least a little bit during the times when the rectifier cannot feed current. During that time the filter capacitors will discharge a tiny amount and when the rectifier finally has a chance to charge them again it will do that in a short, transient pulse. The higher the filter capacitance is the higher the amplitude of this pulse will be. The more it will stress the supply.

[junkman]

so the caps are  kind of like a battery that delivers power between the pulses from the transformer rectifier?

[teemuk]

Yes.

[zabadac]

Hi, an amplifier is best thought of a s 'modulated power supply'.

The word 'amplifier' is a bit misleading because it is often thought that it makes the initial signal 'bigger', when what is happening is the signal is used to modulate the main power supply which drives the loudspeakers.

There's no way the small electronic signal produced by the interaction of guitar strings with the magnetic pickup heads has enough muscle to move the loudspeaker drivers, so a DC power supply with the capacity to produce lots of current, enough to move the loudspeaker cones, move air and hence make a lot of sound, is used.

Of course, if a DC power supply is simply connected to a loudspeaker, apart from the initial movement of the cone, nothing else happens because the power going through the loudspeaker, from the DC source, is not changing.

So, if the DC power supply, (its voltage/current) is made to change in line with the varying signal from the guitar pickups, then we get a representation, a copy, of that original 'source' electrical signal produced via the the loudspeaker.

The tubes and transistors used in various amplifier design act like 'electronic levers' - they control the changes in the voltage(es) that are supplied by the power supply.

The job to modulate that DC supply, belongs to the amplifier.

The better the quality of that power supply, the better the amplifier is.





 

[ponchojuan]

Damn this is a horrible thread.  So much misinformation and pundit opinion.  I encourage everyone to read an amplifier design book like G Randy Stones, or any of the other good design and theory books out there,  before proclaimating expertise.  Let's not create threads of misinformation.

Poncho

[joecool85]

Damn this is a horrible thread.  So much misinformation and pundit opinion.  I encourage everyone to read an amplifier design book like G Randy Stones, or any of the other good design and theory books out there,  before proclaimating expertise.  Let's not create threads of misinformation.

Poncho

Care to share which portions are misinformation?  It seems to me that all of the answers are spot on.

[R.G.]

Damn this is a horrible thread.  So much misinformation and pundit opinion.  I encourage everyone to read an amplifier design book like G Randy Stones, or any of the other good design and theory books out there,  before proclaimating expertise.  Let's not create threads of misinformation.

Tee-hee... I do love a good Sorcerer's Apprentice post!

So, Poncho - how long have YOU been designing electronics? Or even reading electronics books? And, as JC85 says, can you point out what parts are misinformation?

It turns out that I *have* read Slone's (and it's SLONE, not Stone) book, and Doug Self's book, and quite a lot of the reference books and papers they quote, from present day right back into the 1950s and some a lot older than that. And I do have degrees in EE as well as decades of practice in designing and implementing (those are NOT the same thing!) audio devices, including guitar amps. Including both custom gear for some pros and commercial production products.

So, Poncho, enlighten us poor, benighted amp hackers - what parts are misinformation? More importantly, what is the correct information for the parts that are misinformation? As you might guess, there will be some documented followup on those... 8-)

[ponchojuan]

Sorry to be hard on you guys, but as hackers I know you can take it.

After reading through much more of the site, I understand better where many of you are coming from.

Ive' been an EE for over 25 years designing HPC as well as analog sensor systems including some AA designs over the years..  We EEs often get too theoretical, and less pragmatic about construction.  I apologize for my comment;  there is a lot of practical knowledge here learned by many hard knocks.

My concerns we're due to comments like designing a power supply before the amplifier.  As an engineer this sounds silly.  Pragmatically,  ICs are a cheap date so use what PS stuff is on the shelf.  I now get it.

Randy, sorry for the spelling slip if your reading;  God Bless.

Poncho

[R.G.]

Sorry to be hard on you guys, but as hackers I know you can take it.

Indeed we can!

Ive' been an EE for over 25 years designing HPC as well as analog sensor systems including some AA designs over the years..  We EEs often get too theoretical, and less pragmatic about construction.  I apologize for my comment;  there is a lot of practical knowledge here learned by many hard knocks.

No apology needed. In my experience, there are not many of us EEs with both theoretical info and practical construction experience, even fewer left that have *analog* design experience, and even fewer with *power electronics* design experience; and of that few remaining number, remarkably few who are guitar/bass players and hackers. I designed power supplies for a living for some years, and I learned to learn from my technicians. They may not have sat through the theory, but they'd seen a lot of smoke.

My concerns we're due to comments like designing a power supply before the amplifier.  As an engineer this sounds silly.  Pragmatically,  ICs are a cheap date so use what PS stuff is on the shelf.  I now get it.

Quite a lot of the "power supplies first" comments originated with me, in other forums. I've advised audio builders/hackers on the internet since the days of usenet, before the concepts of the world wide web. I found that many builders of audio amps sweated bullets over the abstractions of the latest supersymmetrical hyper-doozie power amp schematic, then were astounded to find that the power supply cost more and was far bigger and heavier than the amps, no matter how complicated. I deliberately tried for a short, succinct, dash-of-water-in-the-face that would get junior-apprentice audio builders to pull their heads up (or out! 8-) )

That idea may sound silly perhaps; but more importantly to me, it's startling enough to get someone to think about power supplies. And actually, in today's electronics world, the idea that a power amp is primarily a power supply with a little circuitry grafted on to let some of the power out is factually correct. Back when getting a power amp to run at all and spending the equivalent of a couple hundred of today's dollars on output transistors was a challenge, there was some justification for leaving the power supply last. But today, even if you're going to do a discrete amplifier, the power amp circuitry is going to be a trivial amount of both the cost and the complexity. It's going to come out to be about 3-6 square inches of PCB coated with parts, and power transistors stuck on a heat sink. Period.

About ten minutes in the Mouser Electronics catalog will get you all the parts except the PCB. But the enclosure, the power transformer, the filter caps, the heat sink, those are going to be hard to find and expensive. Power transformers have not seen the kind of decline in prices that power transistors have!

I was extruded through formal project management training in my last job. One of the tenets there is to put the effort where the difficulty, cost and complexity are. By concentrating early on the difficulties of powering your new 50kW amplifier, you'll be less disappointed than you would be if you built the amps and found out that the power supply can't be had for love or money.

And there's the idea of maximum upgradablility. If one has decided to build a 200W amp, then the power going into the load is defined. You know ahead of time what that is. From there, one can decide what efficiency they can practically get. A little reading in Duncan, Self, Slone, or the ones they got the theory from will show you that the efficiency of the amp divided into the output power tells you immediately both what DC power you need to provide the amp and also what power has to be dissipated in the form of heat from the amplifier, clarifying a couple of big, critical practical problems. If you don't get solutions to those problems - DC power, heat, and efficiency - you cannot and will not have a working power amplifier no matter what goes on the PCBs. But if you have a suitable power supply for a say, 200W class AB amplifier, then no matter what amplifier circuit you tag onto it, you'll have a 200w amplifier. You can upgrade the power amp circuitry and outputs much more easily than you can upgrade the power supply. Chip amps, discretes, hyper-customized solid-platinum and germanium amps, anything will go in there if you have the power supply, enclosure and heat sinking right and use the same output power and class.

Looked at from the standpoint of practical building, the amplifier is almost an afterthought. The amplifier almost does not matter.

It is **exactly** that line of reasoning that led me to buy that 100W Rogue guitar amp. I had no idea that it would be a good amplifier on its own. But if it put out 100W into two speakers (and likely 4 ohms load from that) then I already knew what the power supply was inside it. If the power transformer was not burned out and it had a suitable heat sink, it was worth the money to have a body to plant a better brain in. $40 was cheaper than I could buy a similar power transformer.

Anyway, it's a good idea to think of things from different viewpoints. In this case, IMHO building gets easier if you view amplifiers from a mechanics- and power-supply-centric viewpoint. It leads to some different and I think useful results.

Sorry to have been so hard on you. You're neither the troll nor the tyro that it sounded like at first read.

[teemuk]

This thread sort of reminded me about the common mod to swap TDA20xx series chips to a higher power versions in certain amps (one Vox amp comes to mind in particular). Nothing wrong with that (I've done it too when in lack of spare parts), except that this particular mod is almost without exception mistakenly quoted to increase the output power - as if it was the chip generating the power. It's a one perfect example where we need to take a look at the power supply. If it can only provide, for example, 20 watts of power then substituting a TDA2020 with a TDA2040 won't mysteriously convert the amp to a 40W amp. Even if the heatsinks were big enough to tolerate the increase, the power supply simply isn't.