Thanks, that helps to fill in another blank, but from the ‘60,000 ft’ level, I dont yet get why a single or dual power supply (I believe in the output stage - ?........) will determine whether the necessity of a capacitor in the output stage.... I know that caps help smooth things out and remove the ripple - is there ripple in the output stage - or am I confusing the output stage with the power supply cap??

Thanks to ALL

There are two connections on the loudspeaker. These are wired to the loudspeaker voice coil.

You need to connect one terminal to point-A on the amplifier and the other terminal to point-B on the amplifier. If the voltage between point-A and point-B is 0V then you don't need an output capacitor.

If the voltage between point-A and point-B is not 0V then you need an output capacitor in series with the voice coil.

It is as simple as that.

 

The output voltage of an amplifier with a single positive supply swings up to near the positive supply and down to near 0V so there is always a positive DC voltage on it. A loudspeaker needs AC with no DC on it so a series coupling capacitor (not a filter capacitor to ground) feeds audio to the speaker but blocks the DC.

The output voltage of an amplifier with a dual polarity supply is the AC audio signal without any DC on it so an output coupling capacitor is not needed.

 

Ok, so we ARE talking about the output stage - Yea!!....So the voltage betwee point A and point B is normally zero with dual power supplies (in output stage), but with only one power supply the voltages may not be the same - correct??

 

Ok, so with dual power supplies (transformers I believe...) no capacitor is reqd...

 

Simulation of a capacitor being used to strip the DC component from a signal:

 

The purpose of the series capacitor is to act as a high pass filter. It blocks DC and allows AC.

(Did someone mention this already?)

 

Capacitors have different functions in different circuits. Sometimes, in a shunt connection, it is smoothing ripple. Other times, in a series connection, it is blocking a DC component and letting the AC signal component through. This is the case with an audio power amplifier.

By definition, the DC component of a normal audio signal is 0. Electrically, audio is a strictly AC signal. As it wanders through circuits it might be riding on a DC component because that is the way the circuit is biased, but the audio always is AC only, and the average value of the AC signal always is 0 V. If you don't get that part, nothing else will make sense.

Separate from the signal is the circuit. If an amplifier has only one power supply rail, such as +12 V, and you want the maximum possible output signal, that signal is 12 V peak-to-peak. AND, its centerpoint is +6 V. So at the output transistors there is 6 V DC plus an audio AC signal that ranges from 0 V p-p to 12 V p-p. A speaker would not be happy with 6 V DC on it all the time, so in comes the coupling capacitor.

With no audio, the output is +6 V, that goes to the capacitor, the other side of the cap goes to the speaker, and the other side of the speaker goes to GND. When the amp comes on, the output charges up the cap to +6 V. Now the amp is sitting at +6 V, the speaker is sitting at 0 V, and there is no audio.

Add audio. The audio signal is centered on and symmetrical about the 6 V centerpoint. A capacitor presents an impedance to an AC signal, and the impedance increases and the frequency decreases. So the idea is that the capacitive reactance at the lowest frequency of interest (say, 20 Hz) should be much less than the speaker impedance. This is because the capacitor and speaker form a two-element voltage divider just like two resistors in series. In order for the majority of the voltage (and hence the audio power) to appear across the speaker, the speaker must be the largest impedance in the string. This is why output coupling capacitors usually are in the hundreds and thousands of microfarads. With a large enough capacitor, all of the audio sitting on 6 V at the amp output appear across the speaker with zero DC. In a way, the capacitor acts like a bidirectional zener diode in series, except that the cap value doesn't have to be precisely tuned to the amplifier output DC value the way a diode would have to be. The audio signal is translated down from whatever the amp output DC level is to whatever the speaker DC level is, and the speaker DC lever is 0 V.

With an amplifier with bipolar supplies, its output DC level already is 0 V (if properly designed). For this reason, no capacitor is needed because there is no difference between the average DC value of the output (0 V) and the average DC value of the speaker (0 V).

ak

 

Ok, so we ARE talking about the output stage - Yea!!....So the voltage betwee point A and point B is normally zero with dual power supplies (in output stage), but with only one power supply the voltages may not be the same - correct??

Didn't I mention a bridged amplifier that uses an amplifier for each speaker wire? It can use one supply voltage and since each wire has the same DC voltage then it does not need a coupling capacitor.

Ok, so with dual power supplies (transformers I believe...) no capacitor is reqd...

Two batteries or a transformer can be used in the power supply. An amplifier output transformer has not been used for about 60 years.
Yes, with a positive and negative power supply then the DC output of the amplifier is 0V so a coupling capacitor is not needed.

 

Now I’m getting even more muddled than ever, so maybe my assumptions are wrong...I’ll try again...

1) the amps main power supply has a transformer, rectifier (for dc) and ripple cap... This transformer supplies both the input stage (a voltage amplifier) and the output stage (primarily a current amp to match speaker impedance)...However, now I referring only to the output stage, as follows:

2) The output stage has been referred to as having either a single power supply OR a dual power supply...

3) where there is a single power supply (output stage), a capacitor is employed to prevent dc from damaging the voice coil

4) where there is a dual power supply (output stage), no capacitor is needed...

5) I’m assuming that this (or these - if dual) power supply is to increase the current to the speaker, since the output transformer’s job (located separately in the amps main power supply) is mainly to supply proper voltage to the input stage and output stage...

5) For ‘non-portable’ household voltage amps, I’m assuming that these power supplies (in the output stage) are actually transformers or is this assumption flawed?.... IF Flawed, what actual device constitutes a output power supply??

Sorry for me dummy - I’m trying!!

 

Oh, I think you have gone and complicate things a bit here.

Let's talk about one thing at a time.
Do you want to discuss audio power amplifiers or do you want to discuss power supplies?
Pick either one and not both at the same time.

 

You've pretty much got it. Without getting into switching and bridging circuits, "normal" audio amplifiers have a traditional transformer / rectifier / filter capacitor design, with a low power regulator for the entry circuits. ALL of the output power in an audio power amplifier comes through that power transformer. One weird way to look at an audio power amplifier is as a variable power supply regulator. It has a bulk power source, and permits only a percentage (that varies from instant to instant) of that power to go through to the load. In a lab supply you turn a knob on the front; that varies a control signal to the regulator to tell it what output voltage to make. In an audio amp the control signal is the input audio voltage and it can change very quickly.

If you start with the speaker impedance and the amount of output power you want, Joule's Law and Watt's Law tell you the peak current and peak voltage necessary to make that power with a sine wave. From there you add in the headroom needed by the amplifier circuit to deliver that to the speaker terminals. Now you know the voltage and current the power supply has to supply to the amplifier, and that tells you how big a power transformer you need.

ak

 

All I’m interested in at this juncture is with an audio amp - strictly the ‘output stage’ and its single (or dual) power supply contained within the ‘output stage’ (not the amps main power supply) and these are my rudimentary assumptions...

There is either a single OR dual power supply within the output stage....Yes?? Or No??

If it is a single power supply then there will also be a capacitor.... Yes?? Or No?? (I think this has been established...).

The power supply (or supplies) within the amps output stage are transformers (or switchers) designed to match current to the speaker.... Yes?? Or No??

 

There are a number of different amplifier designs, the output stages are quite different. I would choose one and understand it before moving to the next. Here is a nice overview:

https://www.eetimes.com/document.asp?doc_id=1280784

 

There is either a single OR dual power supply within the output stage....Yes?? Or No??

No. The power supply or supplies is/are separate from the power amplifier stage. In some high-end amps, the supplies are in a separate box, kept away from the amplifier electronics for better noise isolation and heat management.

I have two clock radios. One has an AC line cord from the wall outlet to the back of the radio. The other has a wall wart power supply, with a DC output cable that plugs into the back of the radio. Functionally, these two arrangements are exactly identical. The only difference is packaging. For both units there is a power supply that is electrically separate from the radio. Where the supply is located does not change its function or its electrical relationship to the radio.

The power supply (or supplies) within the amps output stage are transformers (or switchers) designed to match current to the speaker.... Yes?? Or No??

Close enough. Again, the supplies usually are within the same enclosure as the amplifier circuit, and maybe even on the same pc board, but they are not electrically "within" the amplifier circuit.

ak

 

I don't know if this will help, but there is an entirely different way to understand how the capacitor operates in an output stage of a single supply amplifier.

Problem: One of the the speaker wires is tied to ground or 0V. In a single supply system, the only voltage available is positive. But the other speaker wire ,must see a negative voltage for half of the cycle of an audio waveform.

So where does this negative voltage come from?

We bias the output stage so that, when idle (no input signal) it sits at 1/2 the supply voltage. Say we have a 12V supply, the output stage will be biased to 6V.

Now we connect the other speaker wire, through a large capacitor, to that output, and the capacitor charges to 6V, since the other side of the speaker is at 0V. This is just a capacitor charging through a resistor. That charging is what causes the pop you hear when you turn on a cheap amplifier that does not have additional circuitry to suppress the pop.

So the capacitor now has +6V on one side (the side connected to the output) and 0V on the side connected to the speaker. Stating it in other words, the voltage on the speaker side of the capacitor is 6V lower than the voltage on the output side of the capacitor.

Now, we put a signal into the amplifier which is a 6V peak sine wave. The output will vary from 0V to 12V, with it at 0 when the input signal is most negative.

Now consider what happens at the speaker wire when the output is at 0V. The capacitor is large, so it does not have time to change it's voltage much as the waveform goes up and down. So it is still the case the the speaker side of the capacitor is 6V lower than the output side. But the output side is at 0V, so the speaker side is at -6V! Wow, we have magically created a negative voltage on the speaker from a power supply that has no negative voltages.

This same trick can be used to generate a negative power supply from a single supply. This is called a switched capacitor DC to DC converter.

Bob

 

Thanks to all for your thoughtful hrlp, obviously I now have lots to cogitate on....

 

Update:

Since I’m really more of a hands on guy, I ordered this amp kit and am looking forward to expanding my limited knowledge!!

https://www.tubesandmore.com/products/amp-kit-monoblock-tube-amplifier

 

The expensive kit uses 60 years old vacuum tubes that wear out soon and need replacement over and over. Where will you buy the replacements?
The kit uses an output transformer that "responds" to 20Hz and 20kHz but without any deviation dB's. Barely any response at -30dB?
Distortion is not shown, is it horrible?
It will not have a very good "damping factor" like modern solid state amplifiers so will a modern speaker sound boomy with its low level of resonance damping? Where can you find an old speaker for it to drive?

 

It was designed by a reputable guy ((now RIP) named George Fathauer an old timer in electronics design...
The reviews seemed good and its not Chinese...I’m not looking for a premium amp (for that I use a Hafler 9300 for music), but just want something to help me get a grasp on the basics, so I believe it’ll offer what I need...JMO

I’m sure that Antique electronic supply (the supplier) should carry additional parts - Lots of kit amps on ebay but they are Chinese and I’ve past found Chinese instructions to be chaotic and frustrating

 

The expensive kit uses 60 years old vacuum tubes that wear out soon and need replacement over and over. Where will you buy the replacements?
The kit uses an output transformer that "responds" to 20Hz and 20kHz but without any deviation dB's. Barely any response at -30dB?
Distortion is not shown, is it horrible?
It will not have a very good "damping factor" like modern solid state amplifiers so will a modern speaker sound boomy with its low level of resonance damping? Where can you find an old speaker for it to drive?

Hey, it has a 5-start rating from two people. And look at this great comment in one of the reviews...

puts out enough current to power most speakers fairly well