6W (i guess) mini bass guitar amplifier design issues

Thread Starter

Snortuff

Joined Dec 20, 2023
6
Hello everyone, I am new to the circuit world. Now i am trying to design a mini bass guitar amplifier with my almost no knowledge

I have lots of basic components, especially the JRC4558, I know its not so good for audio but I have a lot of them so I wanted to use them.
For speakers I have 2 cheap speaker exciters (3W 4 ohm each) which I am planning to use both in the same panel.
Last but not least, if the circuit can work with a 9 volt battery it would be perfect.

I draw a simple circuit but now I can not calculate the component values for driving the exciters. I don't know how to calculate the output wattage.
I could not find a dual opamp in LTspice so I drew 2 op amps in series. Can this circuit work? and how can i calculate the component values for the exact output that i need?
 

Attachments

Ian0

Joined Aug 7, 2020
9,781
Must be one of those big 9V batteries that they don' t make any more.
A humble PP3 is only 0.5Ah. 6W/9V=0.666A, so it might last you about 45 minutes.

As for the circuit, why do you have the pickup shorted out?
The first stage looks acceptable, but I'm not sure what you were trying to achieve with the second.
Is this meant to drive a power amplifier? Because a 4558 won't drive a speaker directly.
 

dl324

Joined Mar 30, 2015
16,898
Welcome to AAC!
I have lots of basic components, especially the JRC4558, I know its not so good for audio but I have a lot of them so I wanted to use them.
You can't use just the opamp. Ordinary opamps can't sink/source much current.

You won't have much output swing with a 9V supply. If you add a class AB stage on the opamp output, the battery will limit available power.
 

Attachments

MrChips

Joined Oct 2, 2009
30,788
When designing an audio amplifier, there are a few things that you need to consider, power and impedance matching.

The pickup is a low power high impedance device. You need a proper preamplifier for this.
The loudspeaker is a high power, low impedance device. You need a power amplifier for this.
Then you have to match the preamp output to the power amplifier input. For this you need a driver stage.

In summary, you need three stages, a preamplifier, a driver, and a power output stage.
 

Thread Starter

Snortuff

Joined Dec 20, 2023
6
Fisrt of all thank you very much for quick replies

Must be one of those big 9V batteries that they don' t make any more.
A humble PP3 is only 0.5Ah. 6W/9V=0.666A, so it might last you about 45 minutes.
Ohh i see so i can use 9v power adapters and connect to grid.

As for the circuit, why do you have the pickup shorted out?
i just hoped that that grounding might lower the impadence in positive feedback

Is this meant to drive a power amplifier? Because a 4558 won't drive a speaker directly.
I had no idea about it thank you for warning/information.
 

Thread Starter

Snortuff

Joined Dec 20, 2023
6
Welcome to AAC!
You can't use just the opamp. Ordinary opamps can't sink/source much current.

You won't have much output swing with a 9V supply. If you add a class AB stage on the opamp output, the battery will limit available power.
When designing an audio amplifier, there are a few things that you need to consider, power and impedance matching.

In summary, you need three stages, a preamplifier, a driver, and a power output stage.
Thanks again for explaining patiently to me. I will research more about the stages and try to design new circuit accourdingly.

Do you know any similar circuit that i can use as referance?
 

MrChips

Joined Oct 2, 2009
30,788
Let's look at different amplifier configurations. They are categorized into classes, Class A, AB, B, etc.
Also you need to learn about transistor amplifier configurations, common base, common collector, common emitter.

For low impedance power output, you want to be considering a push-pull, class AB - B, common collector amplifier.

Here is a random circuit taken off the web to illustrate using an opamp to drive the common collector push-pull output stage.
NOTE: This circuit has not been verified as being correct and functional.

1703106157273.png
 

MrChips

Joined Oct 2, 2009
30,788
Now let's do some quick back-of-the-envelope power calculations.

Suppose you have 9V swing into an 8Ω load.
9V peak-to-peak is 4.5V peak amplitude, which is 4.5V divided by √2 = 3.2V RMS.
This will give you at most 1.3W into 8Ω, 2.6W into 4Ω, or 5W into 2Ω.

Now let us estimate the current draw from the 9V battery.
Current =
150mA @ 1.3W
300mA @ 2.6W
550mA @ 5W.

A typical PP9 9V battery is designed for no more that 50mA draw. Hence this is not your best choice of a portable bass amplifier.
 

MrChips

Joined Oct 2, 2009
30,788
If you want to design a DIY mini bass guitar amplifier, then let's begin with the power source.

If you want portability, here is a 12V 7Ah SLAB (sealed lead acid batter).
In theory, you can draw 1A for 7 hours @ 12W.

If you wish, you can substitute this with a 12V 1000mA power adapter.

1703107496665.png

A quick and simple design route is to get an off-the-shelf 12V automotive audio amp. An alternative is to buy a Class D amplifier from the many sites on the internet. For more efficient power output, make sure that the 2-channel power amp can be configured for bridged output.

1703108021724.png

If you still want to build the amplifier from scratch I can come up with a simple Class B push-pull power output circuit. However, you would have to wait awhile for me to design, build, and test it.
 

MrChips

Joined Oct 2, 2009
30,788
This is my 200W portable bass guitar amp with 12" speaker. It uses an automotive bridged 2-channel amplifier and runs on a 12V SLAB.

bass amp.jpg
 

BobTPH

Joined Jun 5, 2013
8,928
I am sorry, but you have no business trying to design any electronic circuit. It is like you think you can do brain surgery because you know how to hold a scalpel. It takes years of study before one can reasonably design their own circuits

There are amps out there for less than a dollar on AliExpress that will do what you want. Just add power supply and speaker. If you want to go to Amazon, it might cost you a few dollars.
 

MisterBill2

Joined Jan 23, 2018
18,426
I suggest visiting the site "Schematicsforfree" and select "audio" and then "circuits" and then "Amplifiers". All kinds of good circuits to see and learn from, and nothing like yoo toob fakes.
And then if you have questions you can post one of them here and get answers.
 

Thread Starter

Snortuff

Joined Dec 20, 2023
6
bass amp design.png
Thanks to your all wonderful suggestions my little project started to shape itself.

I found really nice circuits and tried to fuse them. The only thing that I added to the circuit is a low pass filter at the second stage of the preamp to cut above 16kHz.

Now I need to find out the component values to drive (4ohm 3w)x2 exciter speakers.And I have a really noob question: where can I put the output potentiometer?

I kindly ask your help once more about the reality check of this project in this current stage.

I added the circuits that I took as reference.

Thank you all...
Kind Regards
 

Attachments

Ian0

Joined Aug 7, 2020
9,781
Is that a speaker eliminator? Speakers do not like DC. That circuit will eliminate your speakers in no time. Make a note of where you found it and don’t look there for circuits in the future.

The volume control potentiometer goes between the output of the last stage of the preamp and the input of the power amp.
 

BobTPH

Joined Jun 5, 2013
8,928
Look at the capacitors above and below the two soekers. Do you see anything wrong?

If the circuit came that way, you should throw it out. If you added them, then refer to my previous post.
 

MrChips

Joined Oct 2, 2009
30,788
The circuit diagram you posted in post #17 is rather unconventional, besides being totally useless.

When I have some time today I will introduce you to the basics of transistor amplifier design.
 
Top