Thank you so much J M Fahey, the aluminium extrusion seems a pretty cool idea :dbtu: I'm heading to local aluminium maker next weekend.

Thanks again...
Sajad

 :duh

Quote from: phatt on October 01, 2017, 08:53:53 AM
Agree,,, a Very good effort, thanks for the pictures. :dbtu: :dbtu:
Obviously a lot of thought went into that project.
I take it you cut and bent the metal out by hand?
I know how hard that is,, because I've done it many times. :duh

Just note, You might need a bigger heat sink if you run it at full power for a long time.
Phil.

Thank you Phil, Unfortunately that's the biggest heatsink I could find in stores here! Anyway I have a CPU heatsink laying around and I might give it a shot.Untill then I have to keep the volume down :cheesy:

Quote from: J M Fahey on October 01, 2017, 07:16:23 AM
Cool project, congratulations  :tu:

By the way, nice carpets, where are you from?

Always fill Country/City information in your user profile ... sometimes we are surprised

Thanks man, glad you liked it.
I'm actually from Iran; here carpets are pretty common and can be found on every house's ceiling... :cheesy:

Quote from: J M Fahey on September 30, 2017, 09:39:05 AM
Thanks Sajy  :dbtu:

Now we need some pictures of the completed build and the cherry on the cake would be some tasty guitar playing demo  :dbtu:

Thanks man, here are some pics:
https://www.dropbox.com/s/7y0yxbi1y7fbp2z/20170912_215828.jpg?dl=0
https://www.dropbox.com/s/7twizvnunrvuehx/20170912_215846.jpg?dl=0
https://www.dropbox.com/s/27vcaq5ql3jfjw2/20170926_212818.jpg?dl=0
https://www.dropbox.com/s/o93el7x2wxknoc8/20170926_212848.jpg?dl=0
https://www.dropbox.com/s/amqx3744svgakjp/20170926_213010.jpg?dl=0

It's the so called 3886 power amp and before that I'm using ROG Thunderbird and Rebote 2.5 delay as preamp.
I wish I was a good player so I could post a video but I will try my best soon :duh

 Hi, I've recently built a combo using 3886 in mixed feedback mode; here is the project file if anyone is interested:
https://www.dropbox.com/s/o8oe5be7xn0ycqc/LM3886%20Guitar%20Amplifer.pdf?dl=0

Quote from: J M Fahey on July 06, 2017, 07:25:00 AM

Quote from: sajy_ho on July 01, 2017, 10:56:39 AMEDIT: There is somethin I don't understand; why negative current feedback reduces the gain? PS I'm using 16 ohm guitar speaker!

To drive amp you must match NFB signal.
You have two NFB voltages there, which are in series, one coming from resistive attenuator 39k/470 , call it "X", the other from [speaker impedance]/0.2 ohms , call it "Y"
So you need to match "X" voltage , which would be 84X as calculated before, PLUS "Y" voltage.
So if you have to match a higher voltage, you need a higher signal to do it, so net gain is lower.

In a simple example, suppose conventional NFB is 10k/1k , so gain is 10X
To get 10V RMS out you need 1V RMS in.  That´s the conventional way.

Now suppose you have a 4 ohm speaker and a 0.4 ohm resistor.
NFB *there* will be again 1V RMS if amp is putting out 10V RMS

Now connect the 1k resistor, not straight to ground as usual but to the Speaker/0.4 ohm junction.

Hey !!! 0.4 ohms is almost zero ohms!!! It´s nothing compared to 1k!!!! It´s practically the same as connecting straight to ground !!!!  Nothing should change!!!!
Think again: that junction is not ground at all, since it does not have ZERO volts on it (as true ground would) but 1V RMS ... which so is in series and summed to the standard 1V RMS.

Now you need 2 V RMS to drive that amp to 10V RMS out ... so now effective gain was reduced to 5X

That´s why I said earlier that even if going from 39k to 22k "looked" safe on its own ... in practice it might not  ... specially with Peavey sanctioned 4 ohm (nominal) speaker .

Thank you so much, I get your point quitely. What I was mistaking about was that I tought gain will reduce exactly by this proportion (Load R/R sense). In the mixed feedback mode, the current feedback is diffinitely reducing the gain but not by that simple proportion. I need to finally read Teemuk's book I guess...

Thanks again for all your help.

[quote author=J M Fahey link=topic=4264.msg33999#msg33999

Peavey uses 39k-470 ohms which if used alone would give you 84X , quite acceptable, BUT you also have gain reduction by the current sensing network: 4 ohm speaker / 0.2 ohm resistor, some 20X .... quite on the "dangerous" side.
[/quote]
I still don't get this; why the gain reduces by the factor of (Output Z/R sense) ?
Assume using a 16 ohms speaker instead of 4 ohm; so the gain must be reduces even more according to Mr. Fahey, right?
but I think using a higher impedance means less feedback from speaker and consequently it means reducing the gain even less, not more! Am I making any sense to you?

I found it, it was the chip! I swapped it with another TDA2040, still getting hot! Then I used a TDA2050 that I found around and it got hot again! Then I took my last chance and used another 2050; it worked without getting too much hot. I remember once somebody told me this toys(that's what he said!) are all fake today!

Anyway thanks for all your help...

Quote from: phatt on July 03, 2017, 08:53:44 AM
I just looked at the picture of your build,, your heat sink needs to be larger. 8|
Having said that I've opened a lot of small amps that use the same/similar power chip and they have less than ideal heat sink so yes they run hot and would likely last longer if the heat sink was larger.

With no signal after 10~15 minutes the chip should stay at room temp if not there is a problem.
As *J M Fahey* has already mentioned it could be an oscillation above your ability to hear and hence heating up even at idle.

These small amp chips will reach their limit quite fast and early distortion is common
Phil.

I checked and it gets gradually hot after 10 minutes of running with input connected to ground!

I changed R8 to 39k but it's still getting hot even at lowest volume! Could someone please give me a formula for gain of mixed feedback?
Thanks

Thank you so much. I'll change it back to 39K and see what happens. So if I change the feedback R to 39k, should I change R7(resistor from non inverting input to ground) to 39K so both inputs get the same bias current to prevent DC offset and noise?
Also here is my pcb drawing; I'll appreciate it if you take a look:
https://www.dropbox.com/s/nycdo2jf0qgauz0/TDA2040%20Guitar%20Amp.pdf?dl=0
And here is my build:https://www.dropbox.com/s/b2j2z817dz8z1gu/20170701_191114.jpg?dl=0


EDIT: There is somethin I don't understand; why negative current feedback reduces the gain? PS I'm using 16 ohm guitar speaker!

I built it yesterday according to schematic except changed R7 and R8 to 22k; now the amp sounds good but it gets so hot even with a oversized heat sink and at very low volume. Can changing R8 to 22k caused the overheating?

Thank you so much Teemuk for your excelent explanation.


I'm planning to use this schematic for my DIY combo, and I'm wondering if I could lower the closed loop gain of the amp by changing that 39k resistor in the feedback loop to around 22k, because I'm already using a hi gain preamp and with that closed loop gain in the schematic; the amp goes into PA distortion very easily.

So can I change the 39k resistor to 22k without the danger of low gain unstability?

Thanks again...

Hi guys, I have a question about TDA chip in Vupyr-15 power amp section:
http://music-electronics-forum.com/attachments/5037d1239065731-pv-vypyr-15-pa.pdf
What is the purpose of the current sense circuit in the speaker feedback loop? I read somewhere that it's there to raise amp's output impedance to mimic  tube amps; is that true? and how does it affect the sound?
Another question is how does it affect the amp's gain?

Thanks
Sajad

Thanks for all your help J M Fahey and others, now with your help I'm going to build my amp.
I was wandering around the web and I found this thread interesting: http://www.diyaudio.com/forums/chip-amps/9852-gainclone-no-gain-yes-pain.html

Is this idea going to work?