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Author Topic: Building a TDA7267A guitar amplifier with overdrive  (Read 8042 times)

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #90 on: March 06, 2018, 06:59:50 PM »

BTW, I tested my crappy 3'' speaker response using the function generation app and anything bellow 250 Hz sounds like crap. No wonder the coupling caps mod seemed to make no difference to the listening. So I wired the output to take stereo headphones and plugged some good ones with proper bass, and man, this little monster sounds great!
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phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #91 on: March 07, 2018, 12:29:14 AM »

WTF? a 3 inch speaker has little chance, a banana might work better.  :lmao:

Re changing the response curves.
From what I understand you are building a preamp to run into a small low wattage power chip that is running on maybe 12Volts.

If so, then when you max out the low freq at the preamp then that little power chip has no chance of producing a balanced response. All that you will get is early distortion and mud bass when you crank it up loud. xP

If you want big bass you need a power amp with headroom and that means big PSU.
and you need to get a real guitar speaker of at least 8/10 inch in a cab otherwise you are getting a false impression of the real sound/tone produced.

To give example of what is going on;
Take a 1,000Watt pa setup,, It takes 700W to drive the low Fq, 200W mid band Fq, 100W for hi Fq.
It takes a lot of energy to drive low freq.
That is why a guitar player using a 30 Watt amp can sound as loud as the bass player using a 400W rig.

The classic Marshall Amps from years ago had little low Fq below 100hZ and took a dive up around 3 or 4khZ. that is called tone focusing.
All the energy is focused in that central area and gives much better clarity/Note definition.

Now most guitar crap has way too wide bandwidth and once you add all the dirt pedals it becomes too cluttered with harmonic crud. If you want great guitar tone limit the bandwidth. 8|
Phil.
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phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #92 on: March 07, 2018, 01:09:27 AM »

Here is a sim of the response curves of the Casino circuit.
note;
The blue trace is the output of U1b "mids are boosted" via the preceding network of R5,6 and C5,6,7.
If you want to hear the difference then just remove R5,6 and C5,6,7.
That will give you a flat response out of U1b.
This preamp is not high gain, it only has maybe +20dB (25dB at 3khZ) if you want to add more gain then play with higher values of R4 and R9. You can make R4 a 50k gain pot and it will distort.
Phil.
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dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #93 on: March 07, 2018, 07:05:49 AM »

WTF? a 3 inch speaker has little chance, a banana might work better.  :lmao:

Re changing the response curves.
From what I understand you are building a preamp to run into a small low wattage power chip that is running on maybe 12Volts.

If so, then when you max out the low freq at the preamp then that little power chip has no chance of producing a balanced response. All that you will get is early distortion and mud bass when you crank it up loud. xP

If you want big bass you need a power amp with headroom and that means big PSU.
and you need to get a real guitar speaker of at least 8/10 inch in a cab otherwise you are getting a false impression of the real sound/tone produced.

To give example of what is going on;
Take a 1,000Watt pa setup,, It takes 700W to drive the low Fq, 200W mid band Fq, 100W for hi Fq.
It takes a lot of energy to drive low freq.
That is why a guitar player using a 30 Watt amp can sound as loud as the bass player using a 400W rig.

The classic Marshall Amps from years ago had little low Fq below 100hZ and took a dive up around 3 or 4khZ. that is called tone focusing.
All the energy is focused in that central area and gives much better clarity/Note definition.

Now most guitar crap has way too wide bandwidth and once you add all the dirt pedals it becomes too cluttered with harmonic crud. If you want great guitar tone limit the bandwidth. 8|
Phil.

Damn, makes complete sense.
The power amp is a 15W TDA7297, 32dB of voltage gain, driven by a  12 to 15V / 5A power supply which I think should be plenty. I initially planned on using a much smaller TDA7267A, but then I found out about that other one and went with the more powerful TDA7297. Not sure if that changes anything?

Unfortunately I don't have access to a proper cabinet to test the amp at high volume, but I might be able to borrow one

Any chance you can share the Spice file for the circuit please?
« Last Edit: March 07, 2018, 09:49:54 AM by dazz »
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phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #94 on: March 08, 2018, 06:06:02 AM »

Hi Dazz I just pm'd you.
Never had to export a spice file so this will be a experiment for me.  :lmao:
Phil.
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dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #95 on: March 17, 2018, 01:31:12 PM »

Well, the amp is finished and I'm quite happy with it considering it's my first build, I mean the thing is a mess but it works which is more than what I expected  :lmao:

I ended up using that gain pot in the first opamp as Phil suggested instead of adding another stage, & another board for the overdrive, just to keep is simpler, and I like the overdriven tone, it's definitely better than the OD of that MPF-102 JFET I built before.

The tone stack still needs work, not sure what but I'm missing something. It's usable anyway. The bass control works fine, although it doesn't have too much range. The mid works great but the treble is still behaving like a plain volume control. I'll see if there's anything I can do about that.

Thanks once again to all of you who helped me pull this off, specially Phil.

Here's a pic of my brand new Ghetto Casino 12. Please don't laugh
« Last Edit: March 17, 2018, 01:33:45 PM by dazz »
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phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #96 on: March 18, 2018, 04:39:00 AM »

Who cares what it looks like,, it works Great to hear. :dbtu:

Re the tone controls; Here is a picture of the wiring.
As should be obvious if you want the mid in the middle you will have to work that out.
This way the wires just jump to the next pot.
If you notice a lot of the old valve fenders have the mid on the end easier to wire up I guess and less chance of mistakes.
Phil.
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dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #97 on: March 18, 2018, 06:56:36 AM »

Who cares what it looks like,, it works Great to hear. :dbtu:

Re the tone controls; Here is a picture of the wiring.
As should be obvious if you want the mid in the middle you will have to work that out.
This way the wires just jump to the next pot.
If you notice a lot of the old valve fenders have the mid on the end easier to wire up I guess and less chance of mistakes.
Phil.

Excellent! that should clean up some of the wiring mess. Perhaps I can try bypassing the board's tone stack and moving it after U3 so that it's output goes straight to the volume pot hence saving another wire to the board (guess I'll need a coupling cap). That should make it easier to troubleshoot too
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phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #98 on: March 18, 2018, 07:15:16 AM »

A bad move cause it's a Hi Z tone circuit and can't work into a 10k load.
If you want to hear how sad the outcome will be then just temporarily bridge R12 (1Meg) with a 10k resistor. Your tone controls will be rendered close to useless. :duh
Phil.
« Last Edit: March 18, 2018, 07:16:21 AM by phatt »
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dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #99 on: March 18, 2018, 07:44:18 AM »

A bad move cause it's a Hi Z tone circuit and can't work into a 10k load.
If you want to hear how sad the outcome will be then just temporarily bridge R12 (1Meg) with a 10k resistor. Your tone controls will be rendered close to useless. :duh
Phil.

Damn, I see. Thanks Phil.
So if I try wiring the tone stack like that, with those caps and resistors at the pots, the ones I put in the board (R11, C10, C11, C12) will be left floating. We'll see how that goes
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phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #100 on: March 18, 2018, 09:10:12 AM »

You don't have to wire it exactly like I've drawn it up, it's just a reference to help you work it out.
Try using nodes,, that is how cad systems keep track of what connects to what.

Take the junction of R10 + C10 + R11 and call it Node 1, or make each node a colour.
Make Node 2, C10 + the top of VR1.
R11 + C11 + C12 can be Node 3.
Continue in that mind set and it will make life a lot easier.
Phil.
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dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #101 on: March 18, 2018, 04:16:13 PM »

Un-friggin-believable, I figured it out! finally!
Turns out the volume pot was bad, but it seemed to be doing what it's supposed to do: control the volume  :lmao:
By just by sheer luck I accidentally poked it "the right way" as I was testing the amp for the emptienth time and bam! it worked for a split second.

Man it was driving me crazy. haha

How can a pot fail in such a misleading way? Do they have no decency? Won't somebody please think of the children?   :grr
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dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #102 on: June 26, 2018, 10:09:27 AM »

I have an issue with the amp, not even sure if it just started happening or it always did it and I only noticed it now. It's making an intermittent, high pitched hissing/squealing noise at a frequency of 5.5KHz or thereabouts. I've ruled out power supply ripple by powering it with a pack of lithium batteries. I've been googling it and ordered some ferrite beads just in case, but I believe that won't help since it's supposed to filter radio frequencies well above those 5.5KHz. I've tried bypassing each of the 3 opamp stages in the preamp and managed to rule out the second one as the source of the problem, but when I bypass the other two, the whole thing goes nuts and starts buzzing real bad (oscillation perhaps?). I'm just about to try bypassing the entire preamp to make sure the problem is there.

I should mention I added gain pots for opamps #1 & #3 with switchable clipping diodes (asymmetrical) in all three opamps, not sure that's relevant but anyway.
Also the noise is there with no guitar, just the cable plugged in the input, but goes away if I unplug the cable.

Any ideas on how to troubleshoot it please?

ETA: I just connected the output of an overdrive pedal to the power amp's input and it's still doing the same thing, so I'm guessing it's not the preamp? But then again I also built that OD pedal so it might have the same problem (both layouts and overall build quality are atrocious TBH). I'll try a couple other pedals, I'm afraid also builds of mine...

ETA2: It's still doing it with the the other two pedals as preamps, so that seems to rule out the preamp entirely.
« Last Edit: June 26, 2018, 11:14:45 AM by dazz »
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phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #103 on: June 27, 2018, 06:51:49 AM »

Well it sounds like the power amp is unstable and oscillating.
As we have no idea of how you wired it up it's anyone's guess as to what has gone wrong.
Maybe post a schematic of the power amp you built and also a pic of the actual circuit.
Phil.
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dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #104 on: June 27, 2018, 09:45:36 AM »

Well it sounds like the power amp is unstable and oscillating.
As we have no idea of how you wired it up it's anyone's guess as to what has gone wrong.
Maybe post a schematic of the power amp you built and also a pic of the actual circuit.
Phil.

I used one of these pre-built TDA7297 boards for the power amp (pic bellow)
The thing I tested a couple other small amps I have (the 1W tin can TDA67A based one and an aftermarket headphone amp) and those also do that annoying 5.5Khz intermittent squeal. I gets attenuated quite a lot if I touch the lead casing, so I'm guessing it's picking up some mains interference and maybe I need to double check everything is properly grounded. I'll see if I can test it somewhere else later today


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