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Author Topic: Building a TDA7267A guitar amplifier with overdrive  (Read 13756 times)

phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #105 on: June 28, 2018, 07:20:06 AM »
Yep if touching the case effects the outcome then it's likely to be a grounding problem.
Now you have to go find out where? ;)
It's a good learning tool when these problems happen because once you work out what went wrong you will remember it forever. 8|
Phil.

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #106 on: July 26, 2018, 08:34:33 PM »
I finally got round to troubleshooting my grounding problems. I found that putting more pedals in the chain exacerbates the problem and it even starts picking up a nasty 120Hz hum from one of my switching power supplies. That happens even if that particular power supply is not feeding current to any of my pedals / amp.

Next thing I found is that the noise goes away almost completely if I connect the first pedal's input signal ground to the amp's chassis. Looks like a ground loop, right?

So I started googling and found this https://www.analogictips.com/faq-ground-rules-earth-chassis-signal-ground-faq/
where they recommend isolating the signal ground from the power ground. Right now I have all my power jacks center positive, with all grounds (signal and power) tied together and also to all every chassis (every pedal and amp) I thought that's how you implement a Faraday cage

I really hope I don't need to rewire everything to isolate signal and power grounds, not sure I would know how to do that anyway  :-\
« Last Edit: July 26, 2018, 08:36:46 PM by dazz »

phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #107 on: July 27, 2018, 04:17:00 AM »
You don't have an earth loop and you have not built a Faraday cage,, that is a different thing. ;)
More likely a Bad earth connection or a severely compromised one.
Without any details it's a guessing game from here. :-\

I can only offer some observations.
Most Pedal power sockets are normally centre common so I have no idea how you wired your circuit.  All this makes the situation even more complex.  xP

Also it seems you have more than one pedal supply which adds even more potential problems.

As to the dodgy ground path you have;
With power off go measure the resistance between common at the first pedal and amplifier common. It should read very low Ohms. 1 Ohm or less is good.
Then do it again with that wire in place, the one that reduces the noise. My bet is the meter will read lower with that wire in place. ;)

If that is the case then leave your meter connected to the Amp and keep probing each pedal common connection all the way back to the last pedal. That should tell where there is a bad common. Sockets and other connections can become dirty and and cause intermittent ground problems. Even the jumper leads can cause ground issues if they are worn out.  Even the plug itself can become mechanically weak and cause ground issues.

The input to most pedals use a stereo socket to turn on the power at the common connection of battery or external power and these can become compromised so lotsa places common or ground can become slightly lifted and cause hum.

Maybe post a pic of the whole layout.
Phil

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #108 on: July 27, 2018, 12:26:52 PM »
You don't have an earth loop and you have not built a Faraday cage,, that is a different thing. ;)
More likely a Bad earth connection or a severely compromised one.

I'm on a roll! LOL. Well, that's good news because I guess it should be easier to fix

Most Pedal power sockets are normally centre common so I have no idea how you wired your circuit.  All this makes the situation even more complex.  xP

First thing I built was a fuzz pedal, a very simple one (bazz fuss) and I wired it center positive simply because the 9V power supplies I had lying around were center +. Then I built the Casino 12 amp and finally 2 overdrive pedals with spare components (one jfet based and one opamp plus clipping diodes, a simplified TS clone) It seemed to make sense to have it wired that way, with the enclosures grounded that I thought would help keep noise at bay. All pedals are true bypass.

Also it seems you have more than one pedal supply which adds even more potential problems.

I have two 9V and two 12V power supplies, but I'm only using one of each, one 9V for all the pedals, one 12V for the amp. All four are plugged to the same power strip even though I only use two at a time, that's to test noise with different combinations of PSUs

As to the dodgy ground path you have;
With power off go measure the resistance between common at the first pedal and amplifier common. It should read very low Ohms. 1 Ohm or less is good.
Then do it again with that wire in place, the one that reduces the noise. My bet is the meter will read lower with that wire in place. ;)

So weird, it measures very low resistance all the way through: 0.9 Ohms from the guitar to the amp chassis with all three pedals in the chain. Since the meter measures 0.6 Ohms with the leads shorted, that's 0.3 ohms along the ground path from guitar to amp.

I've tried different combinations of engaged/disengaged pedals and it gets even weirder: the chain is: Guitar -> OD1 -> OD2 -> Fuzz -> amp.
Well, if I engage the fuzz and disengage/bypass both OD's, I get noise from the (I guess poorly filtered) power supply.

Pretty much any other combination is fine, so looks to me like the fuzz (MPSA13 darlington transistor) with it's massive gain is amplifying noise picked up on the ground path before it. In fact, if I bypass the two OD pedals with a wire, the noise goes away for the most part even though they're disengaged (bypassed)

So yeah, I'm gonna make sure the ground connections in those OD pedals are OK.

The input to most pedals use a stereo socket to turn on the power at the common connection of battery or external power and these can become compromised so lotsa places common or ground can become slightly lifted and cause hum.

I'm not using that because I built those pedals to be used with power supplies, no batteries, so I didn't think that would be necessary.

Maybe post a pic of the whole layout.

I'm a bit embarrassed  to post pics since I built the OD pedals in tin cans and the whole thing is as ghetto as it gets, but I'll post a few of them if that helps

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #109 on: July 27, 2018, 05:51:04 PM »
OK, I put a wire across both jacks in both OD pedals and the noise is gone almost completely.  :dbtu: I still get a little bit of it if I really crank the gain, but I'm pretty sure it's the guitar cable that's picking up most of it now (EDIT: yeah, I tried a shorter cable that measures next to no resistance and the PSU noise is gone for good). I guess some noise is unavoidable sometimes, although the suspect power supply could probably do with some better filtering. I tried putting a 470uF cap across it's output but it didn't do anything to make it quieter (it already has a 1000uF). I don't think I'm gonna bother with that. It's a cheap PSU anyway and it works well enough for home practice.

BTW, my Casino 12 preamp PCB's are on their way!
« Last Edit: July 27, 2018, 06:19:28 PM by dazz »

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #110 on: July 27, 2018, 06:32:52 PM »
Damn, the noise comes back in all of its glory when I roll off my strat's volume pot... off to google this $%&t

phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #111 on: July 29, 2018, 07:36:23 AM »
If rolling the volume back on the guitar causes more buzz/hum then it's highly likely the guitar itself is at fault.
Better check that first before you blame the circuits you have built.
Continuity check the ground path inside the guitar first, don't forget the bridge is also grounded,, oh and the lead from guitar to first pedal.
Phil.

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #112 on: July 29, 2018, 08:14:03 AM »
If rolling the volume back on the guitar causes more buzz/hum then it's highly likely the guitar itself is at fault.
Better check that first before you blame the circuits you have built.
Continuity check the ground path inside the guitar first, don't forget the bridge is also grounded,, oh and the lead from guitar to first pedal.
Phil.

Continuity seems fine, but considering that it also seemed right in the pedals and they were still poorly grounded, I'll double check the wiring and solder points in my guitars and cables.
I've done some tests and found something interesting... tried unplugging the guitar, so I have guitar cable (6 foot long) -> OD1 -> OD2 -> fuzz -> amp. Then I put a wire across the guitar cable to have it act as an antenna, and as soon as I fire up the noisy power supply, the 100Hz hum starts. But, if I plug the noisy PSU in a different socket in the same room, the noise goes away. That points to common mode noise, right?

Also, I put the noisy PSU back in the same power strip as the amp, the noise came back but... I don't get this... with the guitar still unplugged, if I ground the guitar to the amp, a good chunk of the noise goes away. If I put the tip of the unplugged guitar cable next to the pups, boom! lots of hum.

I guess shielding the guitar should help... and stop getting cheap PSUs too  :lmao:
I've been googling how to filter or shield SMPS's but that's a different story
« Last Edit: July 29, 2018, 08:15:36 AM by dazz »

phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #113 on: July 29, 2018, 09:43:27 AM »
Well that sounds like you have a Smode supply that is not meant for audio, only use those intended for audio or you will have a nightmare.
The hum is likely more of a buzz than a hum.

This page may help you understand the issue.
http://ka7oei.blogspot.com.au/2014/08/completely-containing-switching-power.html

I use 2 Smode supplies in my pedal board and one had to have some extra filtering, the page above will show you how if needed.

Oh one other possible issue when using power boards, plug the amp supply into the First socket (closest to the power source) and then the other supply into the second or last, that trick worked a while back for a chap who was having noise problems. It may or may not work here but give it a try.
Phil.

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #114 on: July 29, 2018, 04:10:37 PM »
Awesome, thanks Phil.

I have some coils / chokes I cannibalized from the computer PSU I used for this amp, not sure they'll work though. The largest one measures only 6mH

I can maybe order one of these EMI filters for the mains, and see if I also need a DC output filter later. Besides, I don't think I should mess with mains considering my obvious lack of knowledge & experience.

I noticed KA7OEI mentions in his blog that he had already added bifilar chokes to both the AC and DC leads, and that took care of a significant amount of noise. How would that be done?

EDIT: I found an earlier blog entry there where he explains all that stuff http://ka7oei.blogspot.com/2012/12/reducing-switching-supply-racket-rf.html

And Here's a pic of the PSU in case it helps.

Comes with...

68uF input cap
1000uF output cap

jc817 optocoupler

tl431 adjustable shunt regulator

uc3843 PWM controller

Two (I believe) power mosfets
« Last Edit: July 29, 2018, 04:27:25 PM by dazz »

phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #115 on: July 29, 2018, 11:24:31 PM »
Hi Dazz keep at it will work out. :tu:
I just used one of these for the mains side;

https://www.conrad.com/ce/en/product/521461/Mains-filter-IEC-socket-fuse-250-Vac-1-A-37-mH-L-x-W-x-H-60-x-45-x-335-mm-Yunpen-YL01T1-1-pcs

Comes with all you need in a compact unit.
the Capacitors in the mains filter have to be mains rated so that saves you a whole lot of research finding all the right bits and it saves space. Just make sure you wire the ground to the case of the chassis and verify with your meter that earth pin is actually connected to case. Some come with built in mains fuse and even some with on/off switch.
The DC output filter can be done with standard rated parts. As I'm always picking up E waste from junked electronics I had all the parts needed. 8)

Yes I found the filters worked without the need to mount it all in an isolated metal case. I have to put the guitar directly over the Smode before I can hear switching hash. Of course Ideally the 2 Smodes I have would be even better in a fully enclosed case but this works fine for my needs.

I had to join the two Neg terminals of the DC outputs on Both Smodes to reduce grounding issues. So I have one 18VDC Smode and the other is 9VDC. If you look I've also joined the negative side of both outputs to Mains Ground as well.
I posted this a while back but pics have gone??
https://www.ssguitar.com/index.php?topic=4092.0

But not to worry here is what it looks like right now;
(Copy pics to your viewer to get the whole pic)

 






Like most of my stuff it's always ongoing research so I'll clean up the wiring when I get time but I've been using this live for about 2 years now and it's given no issues. <3)

The 9VDC is in the corner while the 18VDC drives the Phatt Box only. That one gave me no trouble but the 9V needed the output filter to shut it up.
Phil.
« Last Edit: July 29, 2018, 11:26:53 PM by phatt »

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #116 on: July 30, 2018, 04:00:41 PM »
Hi Dazz keep at it will work out. :tu:
I just used one of these for the mains side;

https://www.conrad.com/ce/en/product/521461/Mains-filter-IEC-socket-fuse-250-Vac-1-A-37-mH-L-x-W-x-H-60-x-45-x-335-mm-Yunpen-YL01T1-1-pcs


Hmm, OK, I noticed that filter of yours has larger 3.7mH inductors which should put the -3dB cutoff at some 9KHz. The cheap ones at ebay/aliexpress with 300uH coils, according to LTSpice, have a much higher cutoff at 35KHz. I'm guessing I should probably steer clear of those

What a killer pedalboard! What does that Phatt Box do if I may ask?
« Last Edit: July 30, 2018, 04:04:56 PM by dazz »

phatt

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #117 on: July 31, 2018, 08:57:53 AM »
I can't remember the exact inductance of the cores but I don't think it's over critical, the link mentioned 10~100uH. Just read the link as he mentions the exact value he used. 8|

As my pedals are all low current heat was not an issue for the cores. I do remember the inductors I had were a bit low so I simply joined 2 in series.

Re the Phatt Box;
That is 3 circuits in one box.
A compressor from member mictester (FSB forum) then a very simple diode clipper then a cab sim circuit based on a very old Nobels unit from long ago.

The PhAttBox by itself only has a subtle effect but in combination with the PhAbbTone and higher volume levels it really gives the impression of a much more expensive rig.
The aim was to be able to make a very basic clean SS amp (Namely my old Laney Keyboard amp) sound like a classic rock amp. The Compressor helps a lot as it can do a pretty good job of creating that classic triode rattle,,some call this the sweet spot on old valve amps where the amp is just starting to break up on bigger signals Or others call it the hair on the edge of a hard hit note or chord.

IME, distortion is easy to get but it's the in between edge that is much harder to get. Most compressor circuits run way too clean but this is more low fi and works well for guitar. So the more it compresses the more it distorts. Maybe not as good as the real thing but a huge improvement over a lot of compressor pedals on the market.
And simple to build. ;)
Phil.

dazz

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Re: Building a TDA7267A guitar amplifier with overdrive
« Reply #118 on: July 31, 2018, 10:05:44 AM »
I can't remember the exact inductance of the cores but I don't think it's over critical, the link mentioned 10~100uH. Just read the link as he mentions the exact value he used. 8|

Oh, but that would be for the DC output filter, right? I was referring to the mains filter, the aftermarket one.

As my pedals are all low current heat was not an issue for the cores. I do remember the inductors I had were a bit low so I simply joined 2 in series.

Actually I already have filters in my 9V pedal PSU's outputs, one of them is an RC filter that seems to work fine for low current pedals (mine are too, all three measure below 5mA). For the other one I used a 7809 because it was outputing over 12V even though it's supposed to be 9V.

The thing is that the PSU that is supposed to require filtering is one of the 12V 5A that I use to power the amp. The chipamp is limited to 2A, but I don't think I can build the filter ignoring current draw?

Re the Phatt Box;
That is 3 circuits in one box.
A compressor from member mictester (FSB forum)

That's the Really Cheap Compressor, isn't it? A great candidate for my next project  :tu:

distortion is easy to get but it's the in between edge that is much harder to get.

That explains why my crappy OD pedals sound like crap at low gain. They're OKish when cranked especially the JFET one so I ended up increasing the gain to get more and more distortion. AT first I added an extra gain stage, then put a switchable cap at the source of one of the sources... it started as a Professor Tweed OD from Run Of Groove, now its more of a nasty distortion/fuzz effect  :lmao:

I'm gonna have to try the compressor and see if I can get some of that magic here  too :dbtu:
« Last Edit: July 31, 2018, 12:48:03 PM by dazz »

 

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