Advice on custom guitar amp design.

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
I'm on a quest to build a budget guitar amp from a broken bose SoundDock. I'm not new to electronic circuits, but definitely new to this level of electronics, I just work on guitars. Help and advice would really be appreciated.

Here's a comprehensive view of what i'm working with. I care mostly about the two 36W (or are they 18W or 10W? i'm a bit confused) speakers.
http://www.whatsinside.info/bose-sounddock-series-ii-internals/
project with same speakers, useful info - https://www.diyaudio.com/forums/pa-systems/336139-diy-column-speaker-pa.html

I have two potential approaches i'm thinking of:

app.1. Use the original pre-amp/amp circuit boards of the sounddock, and somehow install a mono guitar jack and power supply. Not sure if it's possible to instal gain,tone,volume controls with this approach, and how/where to wire mono guitar plug input.
if this is more easy than i imagine, it would be cheaper and i would have an amp sooner.

app.2. use this (or simmilar) amplifier circuit (https://www.anodas.lt/en/1x35w-tda8932-digital-amplifier-board), it matches both speaker power and impedance, with clear input, output, power terminals. I could install additional simple circuits for tone,volume, gain, maybe simple effect circuits.
question 1 - do i need an additional pre-amp circuit? i assume i do, due to the guitar pickup output impedance being in the kohm range.

__________ based on this previous advice i've gotten, i need a preamp circuit with the specs listed?______
1) The electrical signal from a guitar pickup is very weak, about 100mV.
The impedance of the pickup is generally very high, about 100kΩ. The purpose of the preamp is to present a high input impedance, about 1MΩ and output a much lower impedance of about 1kΩ. Also the preamp will boost the signal to about 1V.

2) The amplifier needs to take the 1V signal and boost both the voltage and current in order to drive a speaker with 8Ω impedance. For this the amplifier must deliver power.
___________
question 2 - would i need two separate amp circuits, for each of the two speakers individually? is it easy to split the signal from one pre-amp circuit to two amp circuits?
So, my current idea is this = mono-jack plug ---- pre-amp board ---- 2x(35Wamp board + 24V 2A dc power supply ---- bose speaker)

question 3 - am i completely R-worded, or am i looking in the right direction with this approach?
 

Audioguru

Joined Dec 20, 2007
11,251
The datasheet of the stereo amplifier IC shows 18W per channel into 8 ohm speakers when the supply is the plus and minus 18V that Bose uses. You are talking about using a +24V power supply which is completely different and has a much lower total voltage.

The amplifier board is stereo so it has two channels. Why play a single guitar in stereo??
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
the amplifier board is mono, at least it says that in the description.
I missed the different power supplies.
I want the same mono guitar signal going to both speakers. They would be wired parallel, so it works out to 4ohm total load, and 17.5w per speaker. The board description states 4,6,8 ohm as all being fine, 8 being recommended.
Or do i have something fundamentaly wrong?
do you have a cheap amplifier board to recomend, that would work with this setup, or tell me how to find an appropriate one?
 

Audioguru

Joined Dec 20, 2007
11,251
The datasheet for the old TDA8922B IC shows that it is stereo with two inputs and two outputs. Since you have two speakers then you should use both amplifiers to drive them. The amplifier circuit is designed to use a Plus and Minus power supply so it must be modified to use your single 24V supply. Your 24V is much less than the 36V total used by Bose so your output power will be much less.

A "cheap" amplifier will have poor quality and might sound bad, if it works, but it might not work and might need to be replaced over and over.
An amplifier powered from 24V and driving your 4 ohm load will have a maximum output power of about only 12.5W at clipping.
A better quality amplifier will have an output power of 45W.
 

BobTPH

Joined Jun 5, 2013
2,057
There is another issue here. Bose uses active equalization to make small speakers sound better than they would with a straight amp. If you use the Bose drivers without their amp it might not sound like you expect.

Bob
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
I have wired the speakers in paralel, and pluged them into my bass amps speaker jack, and the speakers sound great. clear, crisp and clean, with some raunchiness/snarl when gain is turned up. i don't want to shread the speakers, i just wanted to see if it could work in principle. and it could.
I just want to build a small, cheap amp, so it's allright if i don't get the highest possible quality/volume possible. i could use a guitar pre-amp to compensate later.

So what type of inexpensive amp board/circuit could i use? and should i wire parallel, series, or maybe split the signal and have individual circuits for each speaker?

Obviously i can't ask for someones time do design this thing for me, and i know how to figure out complex stuff on my own, in this case i just don't know where to start.
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
the TDA8922B is the chip used in the souddock amp, which i'm not going to be using, since the souddock itself wasn't functioning, it might be broken, i just want to use the speakers. i can't figure out their specs, that's my first problem.
 

Audioguru

Joined Dec 20, 2007
11,251
The Bose speakers are tiny so they will shread easily when fed more than a few Watts.
Use a multimeter to measure their DC resistance. An 8 ohm speaker has a DC resistance of about 6 or 7 ohms.
 
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