Building a TDA7267A guitar amplifier with overdrive

[dazz]

Hi guys & gals, a n00b here with some questions about my first amp build.
The amp is essentially the application example in the TDA7267A datasheet:



I'm also in the process of building a Bazz Fuss V3 pedal so I thought I could put that little circuit in the amp to have an overdrive "channel"



The idea is to put the Bazz Fuss in front of the TDA7267A and use a switch to bypass it for the  clean channel.
So I have a few noob question for you:

1. The initial plan is using a 12V power supply and a 7809 to get the Bazz Fuss the required 9V. I could use 9V for everything and leave the 7809 out, but would I be losing a significant amount of output by feeding the amp 9V instead of 12V? ETA: I should have RTFM before asking. The answer to this is right there in the chip's datasheet.

2. Do I need all the coupling capacitors from both circuits? Can just use an input cap & an output cap and connect the output of the Bazz Fuss directly to the input of the amp so that when I bypass the Fuzz I leave the input cap there to do it's job? or do I need another coupling cap in between the Fuzz and the amp?

3. For what I could gather, the input impedance of the Fuzz is 1K Ohms while the amp's is 100K. Mentioning just in case there could be an issue there

4. I want to have tone, gain and volume controls. The volume with a logarithmic 50K pot at the output of the amp, the gain with a 1K or 5K pot between the Fuzz's transistor emitter and ground. But I'm not sure what to do about the tone pot. Can I just put a, let's say, 4.7uF and a 68nF cap with a pot in between to implement the tone pot? Or would I be better off trying some of the tone stacks found here? http://monster.partyhat.co/article/amplifier-tone-stacks/

I think that's all for now, any suggestions/corrections appreciated. Thanks in advance

[dazz]

I forgot to mention what I have planned for heat-sinking:



The board is mounted on a 7x4cm aluminium heatsink I cannibalized from a blown PSU. The GND pins of the TDA7267A will be straightened and sandwiched between the board and the heatsink with one or two screws pressing it against the IC pins. Should be enough right?

[dazz]

Well, after doing some more research I'm going with a TDA7297 which is more powerful and has two channels. Still not sure about the preamp but I like the Buzz Fuss simplicity. I'll see if I can add a treble/mid/bass tonestack to it

[phatt]

Hi Dazz, Regards Tone stacks link;
Be aware that you will need a gain stage to make up for signal loss.
Most of those tone circuits are passive and suffer from insertion loss,, meaning a 1 volt signal swing at the input can be reduced by as much as 70%,,so you will have to make that up with another gain stage.
You will need to do some research and better still make a prototype FIRST. (hint)
An active tone circuit does not suffer insertion loss but they sound a bit different and the classic passive tone circuit were well suited for guitar tone, but both will work ok if done right.
Phil.

[dazz]

Hi Dazz, Regards Tone stacks link;
Be aware that you will need a gain stage to make up for signal loss.
Most of those tone circuits are passive and suffer from insertion loss,, meaning a 1 volt signal swing at the input can be reduced by as much as 70%,,so you will have to make that up with another gain stage.
You will need to do some research and better still make a prototype FIRST. (hint)
An active tone circuit does not suffer insertion loss but they sound a bit different and the classic passive tone circuit were well suited for guitar tone, but both will work ok if done right.
Phil.

Thanks for that great piece of info. I just installed Duncan's tonestack calculator and will definitely prototype it in a bread board to try different configs. I'm thinking maybe the massive amplification capabilities of the Bazz Fuss (MPSA13) might help compensate for some insertion loss here? No idea

[dazz]

Damn, I just can't seem to decide which power amp chip to use. Thought I was settled on the TDA7297 but just came across this little TDA3116/8 beast

https://www.ebay.com/itm/DC-8-26V-TPA3118-PBTL-Mono-Digital-Amplifier-Board-AMP-Module-1-60W-Arduino/162237431824?epid=822273080&hash=item25c61ac010:g:zUUAAOSwLF1X~ykc

But anyway, just read about the importance of power supplies in the stickied thread here so looks like the power amp is the least of my concerns, especially if I'm going to try a power amp that can draw up to 7.5A. This should be a fun project, unless I manage to electrocute myself that is

[phatt]

Some advice;
Bazzfuss is an effect ,,not a preamp. So IMO tiss a bad idea for an amplifier circuit.
I'd suggest you build a simple preamp with a tone control then into a power amp.
LM1875 chip will give you 10~20Watts,,depends on PSU.
If more power,, then LM3886 will deliver 30~50Watts, again depends on the PSU ability to supply the grunt to drive the power stage.
If this is your first venture into building these circuits then keep it simple otherwise you will be stuck when something goes wrong.
I learned a long time ago not to build a circuit if I did not know how it worked.
Phil.

[dazz]

Some advice;
Bazzfuss is an effect ,,not a preamp. So IMO tiss a bad idea for an amplifier circuit.
I'd suggest you build a simple preamp with a tone control then into a power amp.
LM1875 chip will give you 10~20Watts,,depends on PSU.
If more power,, then LM3886 will deliver 30~50Watts, again depends on the PSU ability to supply the grunt to drive the power stage.
If this is your first venture into building these circuits then keep it simple otherwise you will be stuck when something goes wrong.
I learned a long time ago not to build a circuit if I did not know how it worked.
Phil.

Yeah, you're right. I'll just keep it simple, modular and relatively low power (something that doesn't involve messing with mains) for my first build and go from there.
I got the idea of using the Bazz Fuss from here http://www.generalguitargadgets.com/how-to-build-it/technical-help/articles/practice-amp-designs/

A simple preamp would just be your favorite distortion effect pcb mounted in the same housing with the amp

But anyway, off to read about preamps now. This is not about the quality or power of the amp that results from this, it's about the learning process. Thanks again phatt

[dogbox]

Hey Dazz, I'm no expert but I've been mucking around with a similar project i.e chipamp and playing around with various preamps. In my findings I came across a preamp circuit called a Vibin' champ http://www.diystompboxes.com/smfforum/index.php?topic=47371.0

Gives a nice clean sound, adds a basic tone control. It's based on a fetzer valve, and has a tone control from an old fender champ. It sounds great.. Currently I'm looking at something with a bit more overdriven sound.

The chipamp that I'm using is AN7522(stereo) and AN7523(mono version). Bought these chips in an electronics hobby store in Malaysia..i think superceded, but you can still get them on ebay easy enough...originally used in TV sets. Great chip if your wanting a lot louder amp than lm386 rubyamp or similar. This thing puts out about 5w from a 12v supply, runs really cool..i don't think i will need heatsink..but will determine when i've finished everything. Tried to get it to heat up by playing it for hours, and it barely gets warm, and I live in Sumatra..where weather is hot. I just using a generic 12v 2A wallwart..so no high voltages to be concerned with.

If I plug straight into the chip power amp I get a very nice clean sound. There's no break up, or overloading..its just dull. I first tried a fuzzface directly into the amp..and it worked but it sounded dull and a little bit bass was missing when i turned one the pots (can't remember which) and the volume lowered. That experiment told me that I needed some kind of gain/recovery stage between the effect and amp..and probably some kind of buffer before the effect such as fetzer/ vibin' champ to give it a bit sparkle.

My next experiment I'm going to have a play around with more of these amp-in-a-box style circuits, with more distortion. Google "wampler black 57" or other similar from runoffgroove.com.

The other thing to remember its one thing to have everything working on a breadboard..but the complexity of the circuit for your preamp can be daunting. more parts/bells and whistles means there is gonna be a lot of work to turn this into a working amp. You will have to learn about creating a schematic, some kind of proto layout, or figure out how to layout a pcb in eagle cad, how to fabricate a pcb or how to order a pcb for fabrication. Fantastic learning excercise, .... but there's a learning curve involved in every decision for us newbs.

I'm going to try my hand at making a pcb for mine. There's trick they use here in Indonesia for doing cold toner transfer using (wait for it) ..mosquito repellent https://www.youtube.com/watch?v=4QB1PvV0CXA&t=502s. I think theres some ingredient in the lotion that breaks down the toner (maybe alcohol or acetone or both...or it could even be the DEET they use in these products??) that allows the toner to be rubbed off the paper and deposited directly onto the blank copper board...little bit of a knack but looks pretty easy to do. Theres a few of these vids on youtube..you won't understand what the guys are saying..but you can follow pretty easily. Indonesians are ingenious little buggers, and electronics is still hugely popular hobby/occupation and basically people are poor so still repair most stuff..they can't afford the luxury of throwing away electronics when they break (like they do in my home country in Ozzie, and other places in the west like Europe and the USA).

Anyways, hope some of this helps. Give us an update of how your amp progresses.. I will do the same.
cheers
Steve

[dazz]

Thank you Steve, of course that helps, a lot
I've been googling for preamps and found this one, but that Vibin looks much simpler.
I plan to use vero-boards. We'll see if I can pull this off

Let us know how your project goes!

[dogbox]

Actually Dazz that one looks pretty good. Geussing you could put in 10k pots to adjust bias to 4.5v or use your ear..then measure and swap out for single resistor when circuit is stable. I think i might breadboard this one up. cheers for the tip

[dazz]

The section labeled "3x stages" is supposed to be replicated 3 times in series, right?

[dazz]

I have another question if that's OK

The TDA7297 is limited to 2A in it's output. It's input impedance is 30K. So I'm wondering if I can parallel the inputs & outputs to achieve 4A in the output and perhaps be able to drive 4 Ohm speakers? The input impedance would half to 15K. I've read it needs to be some 20 times higher than the preamp's output impedance. Could this reduced poweramp input impedance be an issue?

I guess that's two questions 

ETA: found this: http://www.audiocircle.com/index.php?topic=125202.0

[dogbox]

The section labeled "3x stages" is supposed to be replicated 3 times in series, right?

 Yeh.. http://www.diystompboxes.com/analogalchemy/   theres a link to the original vulcan pedal...it's basically 3x stages of that which he inserted. The cool thing is you could try 1/2/or 3 stages..see whats needed. you could also insert a a jfet version of the vulcan in there, or pretty much whatever you want, for a distortion channel.

[dazz]

Thanks again Steve, much appreciated.

Found this thread about the JFET Vulcan and it sounds great http://www.diystompboxes.com/smfforum/index.php?topic=67677.0