Designing a battery guitar amplifier

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Hello,

I'm trying to design a battery guitar amplifier for busking.

I'll be using a TPA3125.
Target power around 8W
Speaker Jensen MOD 10-35 10" 35w 8ohm 95dbm

http://www.diyaudio.com/forums/instruments-amps/292714-battery-amplifier-searching-ic.html
(thank you to everyone that already helped me there)

I tried design the preamp, this is the schematics until now. (just a general idea...)

- I measured my telecaster and give me around 100mVrms on bridge pickup (if I play really loud! it give me a peak of 150mV, but i never strum that hard)

- I'll like to have a gain stage to add some distortion without having to bring my pedals, but also that can give me a clean sound.

- I'm gonna use the TPA3125 in BTL mode, so that's why the phase splitter

It is my first time trying to design something that I'll actually build.
Also I'm learning "on the way", so I apologize for eventually enormous mistakes :)

As you probably will notice I'm struggling with filters, how to set them and mostly what to aim!
I also added a SPICE, just in case

Any idea, suggestions?

Best regards to you all

Patrizio
 

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This is exactly what I built as my personal rehearsal amp.
I can recommend Jensen Mod 6, available with 4Ohm impedance.
It is small, lightweight with mediocre efficiency about 90dB/1W.
Selecting 40hms impedance gives twice the power input of 8Ohm, so you "gain" 3dB of acoustical output compared to the same 8Ohm speaker.
 
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But if you want more acoustical power, a 10" eminence Ramrod may be an option - outperforms the jensen imho.

One of my early designs with 6.5" speaker in a 10l canister.
I call it the "trAmp" ;)
 

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I can recommend Jensen Mod 6, available with 4Ohm impedance.
It is small, lightweight with mediocre efficiency about 90dB/1W.
Selecting 40hms impedance gives twice the power input of 8Ohm, so you "gain" 3dB of acoustical output compared to the same 8Ohm speaker.

Well, correct me if I'm wrong, but using a 4Ohm 90db/1W or a 8ohm 93db/1W (Jensen 10-35 rated at 94.1) should give the same result no?
My original idea was to use a MOD 8-20 4ohm but i'm unable to find it at good price (I found it on some shops but they charge a lot for shipping)

But if you want more acoustical power, a 10" eminence Ramrod may be an option - outperforms the jensen imho.

One of my early designs with 6.5" speaker in a 10l canister.
I call it the "trAmp" ;)

I just checked the Eminence and beside my little knowledge seems also to me that is much better! Unfortunately not only the specs outperform the Jensen, but also the price :(.

Love your bluey trAmp (and the name!!!). Is that a water tank?

Just had a look at your pre-amp and yes, there are several flaws.
Anyway, I think the best way is to to start with the power stage, connecting it to power and the speaker and when this is working lets continue with the pre-amp.

I'll like that (and seems ilogical!) but I prefer to make one big order to avoid paying twice the shipping fees.
In fact I forgot to mention another parameter that is really important unfortunately, costs.

I will in fact design the power stage as soon as I have time!

Maybe one flaw in the design is the input impedance? I took it for granted that was high (input of opamp) but forgot to take in consideration R1.

Have a nice day!
 
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Well, correct me if I'm wrong, but using a 4Ohm 90db/1W or a 8ohm 93db/1W (Jensen 10-35 rated at 94.1) should give the same result no?
That is exactly what I wanted to say.

I could compare the Ramrod to the Jensen in our rehearsal room, and the difference was significant.
Indeed the blue trAmp is housed in a used 10liter water tank.
Input impedance for the standard electric guitar wiring should be >500k.
Do you plan to breadboard or do your own pcb-layouts?
It can be said that the power amp is a more less straightforward copy of the manufacturer reference design - few components and nothing special with it.
The pre-amp is another breed - much more parts and special circuitry that matured over the decades...
Most parts, including potentiometers and low ESR-caps, are available at www.reichelt.de for reasonable prices.
 
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I used to play guitar in parking lots at music and sporting events all the time, this is what I did.

I made an enclosure and mounted a free old 12" Alnico driver in it.

I mounted a recycled automotive subwoofer amp I got for free that had a switchable low pass filter, obviously the filter is set to off. The amp runs off 12v which is what you want.

I then made a very simple opamp based treble, bass and volume control preamp with spare parts from my shop. I later ditched this because my acoustic guitar has an on-board preamp built in. But if you are playing electric then you will still want a pre.

The only thing I had to buy was the RC car battery to power it.


I also had alligator clip leads and a recycled cell phone cigarette lighter adapter so I could just hook right up to a car battery if need be.
 
MockingBird, thank you for sharing your creative approach, I like it! Almost something MacGyver will do :) (was my idol when I was a kid).

Voltwide, last month I made a couple of pedal and I did the pcb design and the pcb etching with success! (the schematics was not mine...) So I'm planning on doing the same with the amp. I think I'll separate the two stages, so I can experiment with the preamp at ease and leave the power stage untouched!

Thank you for sharing that shop, I didn't knew about it.

About the preamp circuit there is something that bothers me:
why do the non inverting opamp circuit NEEDS a cap in series with Rg when used with a single supply rail? (or else is it LTSpice not working correctly?)
 
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MockingBird, thank you for sharing your creative approach, I like it! Almost something MacGyver will do :) (was my idol when I was a kid).

Voltwide, last month I made a couple of pedal and I did the pcb design and the pcb etching with success! (the schematics was not mine...) So I'm planning on doing the same with the amp. I think I'll separate the two stages, so I can experiment with the preamp at ease and leave the power stage untouched!

Thank you for sharing that shop, I didn't knew about it.

About the preamp circuit there is something that bothers me:
why do the non inverting opamp circuit NEEDS a cap in series with Rg when used with a single supply rail? (or else is it LTSpice not working correctly?)
The biasing divider is shorted to GND for audio signals.
Thus it is that series resistor that defines input resistance.

Yes, separating power stage and pre-amp makes sense for several reasons.
And it will faciliate the mechanical concept as well.
 
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About the preamp circuit there is something that bothers me:
why do the non inverting opamp circuit NEEDS a cap in series with Rg when used with a single supply rail? (or else is it LTSpice not working correctly?)
LTspice works perfectly well.
The reason for the cap is that, if it wasn't there, there would be current in Rg, and this would result in improper biasing of the opamp; its output would very likely be stuck to one of the rails.
 
LTspice works perfectly well.
The reason for the cap is that, if it wasn't there, there would be current in Rg, and this would result in improper biasing of the opamp; its output would very likely be stuck to one of the rails.

Merci Jim, that was very clear to me!

The biasing divider is shorted to GND for audio signals.
Thus it is that series resistor that defines input resistance.

Are you saying the same thing Voltwide? Are you not talking about the input impedance of the opamp right? Else I'm lost...

From the design I posted, beside the filters (the low pass after the second opamp IDK if is necessary) the concept is right?
 
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Merci Jim, that was very clear to me!



Are you saying the same thing Voltwide? Are you not talking about the input impedance of the opamp right? Else I'm lost...

From the design I posted, beside the filters (the low pass after the second opamp IDK if is necessary) the concept is right?

The input impedance of non inverting input is generally extremely high, so you can neglect it. Thus input impedance is set by R1=10k.

As you are not experienced in this kind of design, it would be less frustrating for you to start with copying some well documented and working circuits.
I know it his hard for a beginner to discern the myriads of BS found on the internet from useful information.
A reliable source of information is ESP Projects Pages - DIY Audio and Electronics. It is a pool of well documented and functional audio circuitry.
 
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Some comments on your circuit as is:
Gain of first OPA is set by 500k/3k3 = 166x, limited by antiparallel diodes.
I expect this will result in a medium to high distortion grade with guitar vol pot maxed.
C2R2=3k3*47n=150us form a highpass-filter with a low frequency rollof below 1khz - i.e. a strong bass cut.
2nd OPA is set by 10k/5k1 to 2x voltage gain.
R5C4=2k2*47n=100us is a lowpass-filter with a high frequency rollof close to 1.6kHz, cutting the treble.
There is not much to say about the following phase splitter.

All in all I expect this circuit to work at a first glance.
 
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Well, correct me if I'm wrong, but using a 4ohm 90db/1W or a 8ohm
93db/1W (Jensen 10-35 rated at 94.1) should give the same result no?

Hi,

Same maximum output SPL, but not the same battery life.

Twice as long battery life I wouldn't call the same result.

rgds, sreten.

Of course being physics something has to give, and in
theory the 6dB less efficient driver will have more bass.
 
Funny my wife and I were just now talking about McGyver...
This amp module is intriguing, the efficiency makes it a natural for a battery supply.

The data sheet mentions that low esr is of great benefit for the capacitors, which is generally true regardless, yet I wonder if perhaps some of the solid polymer type may work for power decoupling. I know these are usually not well suited for analog circuits, but given the nature of the class D module, maybe they would be of benefit.

Ok, I just read a post where a user had done that with great benefit...

Can the value of input capacitors be used to limit the low cutoff, to match the speaker and maybe help with limiting the current draw at the same time?

Cool project, Telecasters are nice too.
 
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Funny my wife and I were just now talking about McGyver...
This amp module is intriguing, the efficiency makes it a natural for a battery supply.

The data sheet mentions that low esr is of great benefit for the capacitors, which is generally true regardless, yet I wonder if perhaps some of the solid polymer type may work for power decoupling. I know these are usually not well suited for analog circuits, but given the nature of the class D module, maybe they would be of benefit.

Ok, I just read a post where a user had done that with great benefit...

Can the value of input capacitors be used to limit the low cutoff, to match the speaker and maybe help with limiting the current draw at the same time?

Cool project, Telecasters are nice too.

Which amp module are you referring to?
 
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