Electronics > Beginners
AB-class amplifier schematic analysis & optimization
<< < (8/14) > >>
David Hess:

--- Quote from: dietert1 on May 23, 2019, 07:35:01 am ---I think the Rod Elliott circuit shown in the beginning is a little more rational. Depending on the OpAmp it will certainly outperform all those diamond circuits.
--- End quote ---

In this case, the diamond circuits are meant to be used within the feedback loop of an operational amplifier and act as a current amplifier.

My preference would be to use something based on the earlier examples I gave because full power bandwidth is increased, gain-bandwidth product is increased, and the output operates closer to the supply rails for increased available output power.  And this can be done with no significant increase in complexity compared to the class-ab output stage.

The disadvantage is that circuit operation is more complex and less applicable to learning how common audio power amplifiers work.


Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection.  Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.

Another solution which avoids using discrete parts is use several audio operational amplifiers in parallel to get the needed power dissipation.  If rail-to-rail output parts are used, then increased output power for a given supply voltage will be available also.  There are several ways to configure operational amplifiers in parallel.
Audioguru:

--- Quote from: ratatax on May 23, 2019, 08:44:49 am ---I've looked at the OPA2134 and the fact the datasheet mentions 600ohm load and +/-18V VCC is very interesting.

However it mentions the voltage output to be max +/-2.5V at 600 ohm, which is I=U/R = 4mA. 4mA @ 2.5V is about 10mW, which seems low for those purposes.
--- End quote ---
No. the datasheet says the voltage loss when driving a 600 ohm load is the negative supply plus 2.2V to the positive supply minus 2.5V. So if the supply is +/-15V the output swing into 600 ohms is 25.3V p-p which is 9V RMS which is 135mW which would be very loud. Without the positive swing clipping a little more than the negative swing then the output power into 600 ohms with extremely low distortion is 130mW. 
Alex Nikitin:

--- Quote from: David Hess on May 24, 2019, 02:54:24 am ---
Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection.  Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.

--- End quote ---

The LM7171 in a DIP8 package can output about 0.7W RMS into a 100 Ohm load continuously  with a +/-15V supply, do you even need that much for a headphone amplifier? On the other hand, driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real life (or put it differently, why measurements don't tell a complete story) and why so many different amplifier designs exist.

Cheers

Alex
Zero999:

--- Quote from: Alex Nikitin on May 24, 2019, 07:51:09 am ---
--- Quote from: David Hess on May 24, 2019, 02:54:24 am ---
Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection.  Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.

--- End quote ---

The LM7171 in a DIP8 package can output about 0.7W RMS into a 100 Ohm load continuously  with a +/-15V supply, do you even need that much for a headphone amplifier? On the other hand, driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real life (or put it differently, why measurements don't tell a complete story) and why so many different amplifier designs exist.

Cheers

Alex

--- End quote ---
Any measurements should be performed with the speaker or headphones connected to the amplifier, via a suitable length of cable. I'd say this is even more important when you're using an IC out of specification, as would be the case for the LM7171 driving headphones, rather than the purely resistive load it was designed for.

Yes, different amplifier designs exist for different purposes, which is why using a video amplifier for audio is daft, even if some people like the colourful ringing and distortion it might add to the sound.


--- Quote from: David Hess on May 24, 2019, 02:54:24 am ---Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection.  Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.
--- End quote ---
Yes, the LM7171 isn't suitable for use as a headphone amplifier. There are much better alternatives. Just because it can be done, it doesn't mean it's the best way. :horse:


--- Quote ---Another solution which avoids using discrete parts is use several audio operational amplifiers in parallel to get the needed power dissipation.  If rail-to-rail output parts are used, then increased output power for a given supply voltage will be available also.  There are several ways to configure operational amplifiers in parallel.

--- End quote ---
Yes, it also results in lower noise and distortion.
magic:

--- Quote from: Alex Nikitin on May 24, 2019, 07:51:09 am ---driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real life

--- End quote ---
True that, but after all those years of hearing about it I would like to finally see a demonstration of two amplifiers which both show no perceptible noise, 0.1dB gain flatness from 10Hz to 50kHz and some low THD figure, all measured into the actual demo speaker, and yet sound different enough that somebody (bonus: everybody) can tell them apart.

I'm not aware of any instances of such demonstration actually being done. I suppose it would be all over the Internet if it happened, given the amount of controversy.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod