The out put stage needs to be AC coupled. Add a large cap.

I missed that change. Since the power supply is positive only then the output from the output transistors averages 11VDC that will fry the speaker and cause its cone to be over to one side causing severe distortion.

 

The out put stage needs to be AC coupled. Add a large cap.

There are two "1/2 supply" nodes. R7,8 and R13,15. If there is a difference in voltages it will be amplified by 20 and sent to the transistors. With 5% resistors the error could be as large as 20%. I would either use the same voltage for U2(+) and U3(+) OR ac couple U3 using a cap like C4.

The high frequencies are rolled off by R6C2. You can get the same effect by putting a cap across R14.
View attachment 194185

Someone built that but looks wrong.

Okay, would it be possible for someone to build the circuit using the same specs, maybe the advantages and limitations listed between the proposed circuit and the new design cause I'll have to just try and use newer components.

 

It is your desire to make a 20W amplifier. Design it yourself:
1) Calculate the peak current and peak voltage that 20W into 8 ohms requires. 18V at 2.25A.
2) The circuit will have some voltage losses so add 20% so that half the supply will be 21.6V. Then at least a +/- 21.6V supply is needed, or +43.2V. More voltage can be used.
3) Look up modern matched NPN and PNP power transistors that are rated at 25V/3A or more. TIP41 and TIP42 will be fine.
3) The transistors' datasheet will say the minimum current gain at 2.25A which is about 25 then the peak base current is 90mA.
4) Look up modern matched NPN and PNP driver transistors with a rating of 200mA. TIP31 and TIP32 will be fine.
5) The transistors' datasheet will say the minimum current gain at 90mA which is about 60 then the peak base current is 1.5mA.
6) Calculate the values for your your R16 and R18.
7) Select a modern audio opamp with a supply voltage rating high enough. The opamp must be able to provide enough peak output current for the driver transistors plus for R16 and R18.
Simple after you calculate a suitable heatsink.

 

Why are there so many stages? Do you plan to build this? If so, are these parts you have laying around? The 741 is a terrible opamp, AG pointed this out to me a few weeks ago. I found the NE5532 is an old but still relevant opamp to use.

What I've learned; use opamps for filtering, use high current transistors for main amp, or there are many class D amp ICs from TI that are great (TPA series is very popular). And these class D amps don't require dual rail power supply, but you'll need a split power supply if you want to filter.

I understand if you want to use BJTs for the main amp, it's a little more satisfying to get the biasing correct for the Q-point.

 

Why are there so many stages? Do you plan to build this? If so, are these parts you have laying around? The 741 is a terrible opamp, AG pointed this out to me a few weeks ago. I found the NE5532 is an old but still relevant opamp to use.

What I've learned; use opamps for filtering, use high current transistors for main amp, or there are many class D amp ICs from TI that are great (TPA series is very popular). And these class D amps don't require dual rail power supply, but you'll need a split power supply if you want to filter.

I understand if you want to use BJTs for the main amp, it's a little more satisfying to get the biasing correct for the Q-point.

The transistor part is the struggle, I dont even know if the design of the transistor are correct. These are bjt npn.

What's the limiting factor of these.

Limiting factor of these op amps and any suggested alternative to improve to reduce the noise etc

 

The transistor part is the struggle, I dont even know if the design of the transistor are correct. These are bjt npn.

What's the limiting factor of these.

Limiting factor of these op amps and any suggested alternative to improve to reduce the noise etc

I had a bunch of 741s laying round from my school days many years ago. I made some filter circuits and the output from those 741s were so noisy my TPA amp went bonkers. I found the NE5532 and built the same filter circuit and the TPA had no issues.

Did you design this circuit or did you find it somewhere? I try and build some known working circuits so I can study the standards... When I learned how to play guitar, I first learned how to play my favorite songs, before writing my own; if you know what I'm trying to say here.

Good luck and don't give up.

 

I had a bunch of 741s laying round from my school days many years ago. I made some filter circuits and the output from those 741s were so noisy my TPA amp went bonkers. I found the NE5532 and built the same filter circuit and the TPA had no issues.

Did you design this circuit or did you find it somewhere? I try and build some known working circuits so I can study the standards... When I learned how to play guitar, I first learned how to play my favorite songs, before writing my own; if you know what I'm trying to say here.

Good luck and don't give up.

Its something I was told to do as it's a challenge. I've got the cascading and gain sorted but it's just that transistor output stage that's throwing me off.

 

I told you that a calculation for output transistors to produce 20W into 8 ohms need spec's of more than 43V at 2.25A. TIP41A and TIP42A are 60V and 6A. You can calculate a heatsink for them. Then I told you what is needed for the driver transistors and suggested TIP31A and TIP32A. You can calculate that they will not need a heatsink.
A good audio opamp is the OP134 single, OP2134 dual and OP4132 quad. Like most audio opamps they are low noise and are wideband with high gain when it is needed.

 

I dont know what the admins are doing, moving half of my post around. seriously?

This is the full circuit

View attachment 194117

When you started this thread and the circuit is already like that simple, no any Moderator to modify the circuit, you already have another thread and the circuit is more complicated, but you used the same title for two threads, so I renamed the title for this thread.

 

Sorry, the audio opamps are OPA134, OPA2134 and OPA4134. Their datasheets are on the broken hard drive of my old computer and my old mind didn't remember the "A" in their name yesterday, but today it did.

 

Sorry, the audio opamps are OPA134, OPA2134 and OPA4134.

A little over $5 for the 4134. I'm a happy camper now... This is my biggest problem, finding the parts that actually belong in these applications. I go to TI's website and try to drill down to audio opamps for filters, but wasn't finding what I was looking for. The "applications" section of the OPA opamp datasheet actually says 'active filter for audio'.

 

What do you need filters for? To make sounds tinny and muffled like an old telephone?

 

What do you need filters for? To make sounds tinny and muffled like an old telephone?

How’d you know? I’m actually trying to get my phonograph to sound like an old telephone. Maybe one day I can get it to sound like a new telephone, like my rotary dial phone; it has as least one filter I’m sure.

 

Old telephones did not use filters to cut high and low sound frequencies. The cheap and old mechanics were the filters.
In those days, any tinny and muffled sound was good enough because nothing was better.

An old rotary dial telephone used a horrible sounding carbon microphone and an earpiece with a thin disc of iron vibrated by an electric solenoid.