Audio Power Amplifier Input Stage

Just Google Single Ended Amplifier.

They have been used since the dawn of electronics time, even back when tubes (valves) were the only game in town.

They work great for cost-sensitive applications where fidelity is not as critical as the material costs.

The invention of the push-pull amplifier was the leading edge for high fidelity with its lower distortion and higher output ability. The trade-off being increased material cost and greater real estate.

I hope you know what you are talking about.

An audio power amplifier with differential input has only use in symmetrical applications, which apart from special pick up coils or microphones, just have no use as input amplifier.

Most have a single or split chip as linear amplifier.

I haven't seen many with a differential input. But perhaps I'm getting old.

I think I read that as he wanted a single ended amp without a phase splitter.

On this forum everything can happen.

I read:

...Most audio power amplifiers use a differential, chip or discrete, type circuit at the input. Some even use two of them, dual differentials.

which seems to me as the input stage, or first amplifier stage. I only met differential op -amps in microphone and pick up coil audio amplifiers.

However.... as I said, that is my opinion.

Might be right. Poorly worded question and once again the person asking seems AWOL.

Excuse the interruption in the love-fest, but there is a difference in common-mode noise rejection and sensitivity especially when used in floating input mode. Differential mode rejects common mode noise better because the noise shows up on both sides and the opposite polarities of the amps adds them 180º out of phase. Differential also allows multiple ground referencing rather than having both power and signal grounds tied together. Because of the reduced noise pick up the differential can have much higher sensitivities and hence better noise figures.

Two famous Leslie boxes for Hammond organs: Leslie 122 with differential input:

and Leslie 147 with asimmetrical input:

The 122 is better protected against the hum comming from the earth-loops.

Suggest you read this

Del

I go as simple as possible and all DIY. Even the PS is 18Vdc li-ion batteries. Tremendous sound from a small amp.

http://en.wikipedia.org/wiki/Gainclone

The answer is sIMPLE,

If you will short one of the inputs line to ground the second input shall be a single ended input, it is not clear to me why you want to invent the wheel

daveross100 at gmail dat com

The input stage in the 1950s was a triode or pentode with a rather low transconductance. Particularly with regulated plate volts they were good and stable. However recently Hifiers have been chasing other valves, cheaper ones retrieved from the dumpsters. These are usually RF triodes designed for cascode amplifiers or single triode amp e.g. in TV tuners. They invariably have a high transconductance and are sensitive to variations in current and voltage. To overcome this they are used in differential amps with the grid of the second valve earthed, and the two valves having a common cathode resistor. This long tail pair arrangement provides parallel feedback, and stabilises the combo. So your choice, the old or the new.

Yes, I have been camping. Just got back; caught 22 trout.

Some of you are stuck on tubes. They have their place but most amplifiers of the last 35 years are transistor based. If you are not familiar with modern transistor amps you will not know what I mean by a differential input stage.

The differential input stage receives the input signal on one base and the other base is connected to the output stage; usually at the speaker terminal. The main purpose is to maintain near zero voltage at the speaker terminal. It also provides negative feedback for stability and gain setting. Most of the negative feedback is DC while much of the AC is filtered off to ground. This ratio gives the gain control mentioned earlier. No it is not a balanced input like a mic in preamp. By using the differential, the power amp low frequency response can be very low, about 10 hertz. A tube amp cannot reproduce these low frequencies without very expensive output transformers.

I have repaired hundreds of these amps in past years and now I design them and make guitar amps. I do not like the way commercial guitar amps sound, so I make my own since I play guitar. I also make my own tremolo circuits and reverb circuits. Also my tremolo circuits do not lose volume when the tremolo is activated like commercially built amps do. Yes I do know what I am talking about.

What type of guitar ever needs to go down to 10Hz?

Maybe 30Hz for a bass.

I am going to argue the toss with respect to a differential amp frequency response. Lower frequency limits for tubes really are not limited by the circuitry, but limited by the output transformer.

Also, frequency response really has nothing to do with whether it is a true differential amp with a balanced line input or uses the more common phase splitters with a unbalanced line in.

You are right. The input amp, differential or not, has little to do with the low end response, but the output transformer has a lot to do with low end response. The point of mentioning the frequency response is that nearly all transistor amps, including HI-FI stereo and receivers for the last 35 years use the differential input in the front end. Choosing proper impedance matching and coupling caps can give low end response that is lower than the speaker can reproduce. This is good in that you know that the amp is not limiting the low end output. The purpose for the differential input for the transistor amp is to make the output DC voltage track very close to zero and thus eliminating the need for an output transformer which can limit the low end response. Until I invented my "no diff amp" circuit, this was the ONLY way to insure that the DC at the speaker terminal was close enough to zero to connect the speaker directly to the output transistors. I have found a way to do this without a differential input stage. This circuit also eliminates another circuit that can have heat related problems, the voltage amp stage which is next and fed directly by the output of the differential stage. The voltage of most differential input stages is less than one, making the voltage amp stage do all the work. They get so hot that they must have a heat sink and they are class A and not very efficient. After the voltage amp there are the emitter followers for current gain and then the output transistors which are usually emitter followers too. The speaker connects at the junction of the emitter resistors of the output transistors. There are very few transistor amps that are different from this circuit. The big change from this pattern is the power FET amp, but that is another ball game altogether.