# Amplifier circuit for speakers

#### Solidus

Jun 19, 2011
349
I know this is probably one of the most well-documented circuits, as before I had much interest I've seen pages upon pages of different ones and different strengths, etc. etc.

I, on the other hand, have zero knowledge of circuits other than your basic resistor, LED, battery, relay, pot, etc., and so I wanted to post the question here rather than just follow some pre-made steps, as a lot of you can help me with the questions that I will undoubtedly have. It will serve as a jumping point for me to finally learn electronics.

Go ahead, have a laugh at my state of being a newbie at this stuff.

The background is that a friend was cleaning his garage and gave me an old portable Bose speaker dock. Now, the only connection available is an iPod dock connector, so I have no way of really using it (the only iPod I have I can't find) and I wanted to use it in a DIY-audio sense. So I've stripped out the two speaker units, and they seem to be pretty good-grade power units for the size of the dock itself.

Standard red/black two-pin hooks on each.

I was wanting to build an amplifier circuit with rotary pot for volume control and a power switch to control and run these, along with a 1/4" TRS jack for signal feeding.

#### CocaCola

Apr 7, 2012
3,635
Why not use the existing amplifier circuit? Bose designs some fine amplifiers and it's already matched to the speakers... It's simply a matter of swapping out the iPod connector for a more traditional one...

#### Solidus

Jun 19, 2011
349
Why not use the existing amplifier circuit? Bose designs some fine amplifiers and it's already matched to the speakers... It's simply a matter of swapping out the iPod connector for a more traditional one...

Because of my skill level or lack thereof, I have no idea which pins would be power versus signal, much less be able to desolder it with that level of precision. and there's always the trivial reason of I could have the satisfaction of building it myself

#### CocaCola

Apr 7, 2012
3,635
Because of my skill level or lack thereof, I have no idea which pins would be power versus signal

Reverse direction since this is the slaved device...

Pin 2 Audio Ground
Pin 3 Right input
Pin 4 Left input

much less be able to desolder it with that level of precision.
With no soldering skills you are not going to get far on building your own amp, the skills to jumper three wires is no more complicated...

and there's always the trivial reason of I could have the satisfaction of building it myself
Sure, but in all likelihood it will be an inferior (especially if lack the skills to solder) amplifier...

Do you have the speaker specifications? Ohms? Wattage?

#### Solidus

Jun 19, 2011
349
I may not have much skill in the electronics sector yet, but there are definite reasons as to why I wanted to build my own and not jumper the dock connector, which I will do if I can't find a better way.

Again, this would be a learning step. If I were to jumper the connector, I may learn how to perfect soldering but I wouldn't learn anything about how electronic circuits behave, as I would if I were to build my own amplifier circuitry.

Also, the amplifier unit on the Bose itself is a fully-integrated board and not very user-intuitive for what I'd like it to be. No power switch, just a power jack in the back and two volume controls.

Instead of a variable potentiometer for volume control, I get a button with a + and a button with a -. With a rotary or line pot I could physically determine the state of the system before turning it on, rather than being at its mercy if it had inadvertently been put at the highest volume on shutdown before.

I understand what you're saying in that it's better to modify existing, better equipment than to try and crudely reinvent the wheel, but I was hoping to use the amplifier build as an excuse to finally learn something about simple circuits and electronics.

But I'm assuming you're meaning just soldering wires to pins 2,3, and 4 and wiring to a TRS jack?

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Sparkfun have a kit based on the STA540 that could be good. It's pretty grunty.
https://www.sparkfun.com/products/9612
I wouldn't use the preamplifier stage though. You won't need any extra gain and the LM358 is not good for audio IMO.
You'll also need a dual ganged log potentiometer for the volume control.
Power supply is up to 22V. I would recommend a laptop power supply at around 19V; you can get these pretty cheaply now. You will need some heavy filtering to remove the switching noise though.

#### CocaCola

Apr 7, 2012
3,635
Sparkfun have a kit based on the STA540 that could be good. It's pretty grunty.
https://www.sparkfun.com/products/9612
I wouldn't use the preamplifier stage though. You won't need any extra gain and the LM358 is not good for audio IMO.
You'll also need a dual ganged log potentiometer for the volume control.
Power supply is up to 22V. I would recommend a laptop power supply at around 19V; you can get these pretty cheaply now. You will need some heavy filtering to remove the switching noise though.

I still wonder why anyone would gut a higher end balanced speaker system and go through all that trouble (for what will likely be an inferior end product) when you can simply jumper three wires...

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
I'm surprised but I guess I shouldn't be. I would have thought that after CC's link to the pinout you'd have been thrilled to be able to not butcher a Bose design for an unquestionably inferior home brew.

IMHO, you're starting your electronics education backward. If you're seriously interested you should be starting with basic DC circuits and a thorough understanding of Ohm's Law. Then AC fundamentals and Ohm's Law, as it applies to AC. This is then followed by Solid State Theory and Design.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
I still wonder why anyone would gut a higher end balanced speaker system and go through all that trouble (for what will likely be an inferior end product) when you can simply jumper three wires...
I guess he just wants to MAKE something. Making things is fun, interesting, educational and rewarding, especially for beginners. That's why kitsets have always been popular.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
I'm surprised but I guess I shouldn't be. I would have thought that after CC's link to the pinout you'd have been thrilled to be able to not butcher a Bose design for an unquestionably inferior home brew.
I wouldn't assume a kitset based on the STA540 will be inferior. It's a good quality IC and it and similar devices are used in commercial products. I wouldn't be surprised if the Bose amplifier was based on something similar. It's a good way to get high performance for low cost.
IMHO, you're starting your electronics education backward. If you're seriously interested you should be starting with basic DC circuits and a thorough understanding of Ohm's Law. Then AC fundamentals and Ohm's Law, as it applies to AC. This is then followed by Solid State Theory and Design.
I don't think there's anything wrong with building some kitsets before you have a good understanding of how they work. It gives people a feeling of confidence and encourages them to continue and work through the less exciting theory.

Obviously you can't learn design and repair just by building kits, and the theory you list is essential. I just don't agree that there's anything wrong with building a kit or two before you start. A lot of people like immediate hands-on experience, and anyone will benefit from the satisfaction gained from building something that works and is useful.

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
I have nothing against kits. I was a big fan of Heathkit and still own many of my first kits. My statement is based on experience from teaching an industrial electronics course. Structure from the ground up provides building blocks and a strong foundation to build upon.

#### CocaCola

Apr 7, 2012
3,635
I wouldn't assume a kitset based on the STA540 will be inferior

I will beg to differ, the STA540 at low output is decent but distortion skyrockets as soon as you push the chip...

I don't know what IC or how hard Bose is pushing it's amplifier, but from my first hand experience listening to Bose systems they are supper clean, as they have built a name based on quality, I doubt they are pushing upwards of 10% distortion like the STA540...

As for building, if you are taking a kit and an over the counter laptop supply I don't believe much knowledge is gained, besides some soldering... Certianly arguable that interfacing an Aux to the existing circuit is just as educational... Although as you suggest he might very well gain knowledge trying to (pulling his hair out) filter out all the noise and make the thing no sound like a cheap boom box when done, or he might be happy that it just makes noise...

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
I will beg to differ, the STA540 at low output is decent but distortion skyrockets as soon as you push the chip...
I guess it depends on the nature of the distortion, but it's actually very hard to hear plain old harmonic distortion. The trend towards THDs of 0.01%, 0.001% and lower is mainly a marketing race IMO. There isn't much research but from what I can glean from a quick web search, THD becomes noticeable between 1% and 10%, with lower frequencies more easily noticed than high frequencies.
I don't know what IC or how hard Bose is pushing it's amplifier, but from my first hand experience listening to Bose systems they are supper clean, as they have built a name based on quality, I doubt they are pushing upwards of 10% distortion like the STA540...
"Clean" may not mean low-distortion. Personally, I would equate "cleanness" with no crossover distortion, wide frequency range, and low noise, and at high volume, wide frequency range, low harmonic distortion, and freedom from other types of distortion (intermodulation distortion, perhaps).

That said, perhaps the Bose amps don't use the STA540 or a similar part. I don't know. But I do think an STA540 or similar would be at least "adequate".
As for building, if you are taking a kit and an over the counter laptop supply I don't believe much knowledge is gained, besides some soldering... Certianly arguable that interfacing an Aux to the existing circuit is just as educational... Although as you suggest he might very well gain knowledge trying to (pulling his hair out) filter out all the noise and make the thing no sound like a cheap boom box when done, or he might be happy that it just makes noise...
I think your "cheap boom box" comment is inappropriate. With good quality speakers and a stable power supply, I would be confident that an amplifier based on the STA540 or something similar would sound a lot better than a cheap boom box. After all, it's mainly the poor quality speakers and deliberately limited frequency range and power that cause the "cheap" sound.

Regarding your "not much knowledge is gained" comment, I agree, but that is not my point. Making something useful is a reward in itself, even if little knowledge is gained. It is the experience of working with components, and succeeding, even when you know that most of the work was already done for you, that will encourage people to get involved in the field.

I do agree with you about the laptop supply though. I suggested it because 19V is conveniently close to the maximum supply voltage of that device. But you're right about the noise.

I'm currently making a "laptop dock" - a piece of wood with speakers and an amplifier, because my laptop's speakers suck. I bought a cheapo Chinese laptop power supply rated at 19V 6.3A (the 19V suits the laptop) with the intention of powering a small amplifier from it as well. The squeaking and bleeping and wittering noise from that power supply are terrible! So I should qualify my recommendation by saying a GOOD QUALITY laptop supply and lots of filtering! Or a linear supply.

I will start another thread asking for suggestions regarding my noisy laptop dock.

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#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
BTW, all I'm trying to do on this thread is defend the OP's desire to (as I see it) do something practical before getting into a lot of theory.

I absolutely agree that theory is essential, but I don't think he's damaging his chances of learning theory properly by doing something simple and practical first, so I don't see any good reason to discourage him from doing that. I also don't think that students should have to sit through six months of theory before they even find out what a resistor feels like in their fingers. Some things cannot be taught - confidence comes from seeing for yourself that you can make this stuff work, and do something useful with it.

I'm sure that the original amplifier would be better than a simple kitset - it might even have digital signal processing and be biamped or triamped! But that really is not my point, which I hope I've made clear by now.

#### CocaCola

Apr 7, 2012
3,635
I understand some of the points, I just don't see the logic it taking a high end speaker system that is working just fine to hack up and attempt to make it work again, because it has the wrong plug...

If the whole holdb ack is not wanting to solder the three wires, for $8 delivered here is a plug in adapter... http://www.ebay.com/itm/Bose-Soundd...er_MP3_Player_Accessories&hash=item3cb564bae3 IMO if the goal is to make experiment and make your own 'boom box' get a pair of car speakers from Radioshack for$20 and knock your socks off, it just seems a lot more logical vs taking apart a working high end unit for just the speakers, IMO...