Look at this current mode Power amplifier!

[Shahriar]

Hi

I have designed this power amplifier. I think it is a little bit different with Normal
PA, because I have tried to use two Voltage-Current converter to drive power output
transistors to reduce zero-crossing.
I have attached the schematic to tell me anything wrong about the topology I have used.
I have not draw all things needed in details. Just about Topology.

Let me explain a little more:
the Input signal will be converted to Current around U1A and U1B. (one Sink current and one Source current)
a feedback is there around R7 & R13 to set the total gain about 40.

1- if the input voltage is greater than zero U1A and q3 sink current from the base of Q1. I(sink) = Vi/R5)
2- if the input is negative U2A and Q2 source current. Isource = Vi/R7
3- U2A veries the voltage to control the output current through speaker.

The Marked resistor values (R5 & R8) will be determined depending on desired Output power and speaker ohms.

ThanX in advance
Shahriar

CurrentPA.pdf

[audioguru]

Hi Shahriar,
1) You have an opamp that has phase-shift, a current sink circuit and a power transistor that also have phase-shift. They are inside the negative feedback loop of the first opamp that also has phase-shift. That is how to make an oscillator. It might work if the 1st opamp has a bandwidth 10 times as much as the others.

2) The opamps have input offset voltage that is amplified many times. Therefore they will either leave a huge current gap at the output creating crossover distortion or they will cause the output transisors to have too much current creating overheating.

3) The top and bottom amplifiers will need to have their gains matched to avoid even-harmonics distortion.

4) I don't think the output impedance will be low enough to make good damping of a speaker's resonances. Most audio power amplifiers have an output impedance of only 0.04 ohms or less.

[Shahriar]

ThanK you for your reply

What is the practical way to measure a Power amplifier Distortion?

ThanX
Shahriar

[audioguru]

Hi Shahriar,
To measure the harmonic distortion of an amplifier you load it then feed it a pure sine-wave and adjust the level so the output is just below clipping.
Then the distortion analyser is calibrated so that the amplifier's output is at 100%, and a notch filter is added to remove the original fundamental sine-wave leaving only harmonic distortion, mains or rectifier hum and hiss to be measured. A 'scope can be sync'd to the fundamental sine-wave to show which harmonics are measured by the distortion analyser.

I made an audio sine-wave generator that has 0.002% harmonic distortion. It uses a stepped sine-wave and an 8th-order Butterworth switched-capacitor lowpass filter. It is 10-times oversampled.
Then I made an audio notch filter with a universal switched-capacitor IC using the same digital clock as the sine-wave generator for good frequency tracking.

You should be able to make a power amplifier with 0.01% harmonic distortion at lower audio frequencies, increasing to 0.05% at 10kHz.

[Shahriar]

Dear Audioguru

what is your first choice for a 30-50W Power Amplifier If you want to build a audio System for yourself.
Are STK amplifiers good or do you know any better Power Amplifier ICs (or descret).

What do you recommend?

ThanX
Shahriar

[audioguru]

Hi Shariar,
Sanyo's STK hybrid power amplifiers are used in cheap old stereo systems. I haven't seen any new and good ones.

National Semi make the LM3875 and LM2886 power amplifier ICs that have up to 68W and are excellent. Look in Google for Gain Clone Amplifier.
Here is a project for them:
http://www.sound.westhost.com/project115.htm
That site has about 20 good power amplifier projects.