What is the purpose of this feedback topology - TDA2050 amplifier

[sim0n]

I buggered up my amp a long time ago (Laney TF50, 30watt combo, based on a TDA2050 chip) and have finally got around to maybe making the thing play again. I ransacked the original board for parts so basically I'm rebuilding the amplifier from ground up, keeping only the transformer and speaker. I'm going again with the same chip. I found the schematic of the original amp on the internet, it has an extra section of feedback compared to the datasheet typical application circuit from the TDA2050.

Here is it redrawn (and I removed the headphone output) with the original component values


I can't really get the funciton of R6,R7,R8 and C4. Is this some kind of high frequency roll off thing to prevent oscillations? What advantage does it offer compared to leaving that section completely out?

[Loudthud]

Those components sense the current flowing in the load and feed it back to the power opamp. This raises the apparent output impedance to the speaker and makes the amp sound more tube like. The capacitor rolls off high frequencies in the current feedback which raises the output impedance higher still.

[sim0n]

Ah, thankyou for the reply, I assume that would be why they brandished this amp as "tube fusion"

I could more or less make this selectable by adding a switch to short over the 0.47ohm resistor right?

[kato]

I think the - lead on your speaker could be switched to connect to one side of R8 or the other for "tube fusion on" or "tube fusion off." That's probably what you mean by "short over" the 470m resistor.

I've got an old 70's solid state amp that needs some reworking so I will try your schematic above for the power section.  Thanks. Let me know how it works out.

[J M Fahey]

Yes, or you may simply short the 0.47 ohm resistor.
Not *that* big sound difference anyway, it´s somewhat subtle.

[teemuk]

It's an opinion divider: Some hear enormous effects from the non-linear response the high output impedance creates when you pair it up to a reactive speaker load, some only hear a slight bass and treble boost. I'm in the latter category myself but for some people the effects of that scheme seem to make a ton of difference. In either case, the scheme is likely found from 90% of today's solid-state guitar amps so it must mean something.

[sim0n]

Thanks again for all the responses!

I've built it finally, and in 3 minutes of messing around with that part of the circuit I couldn't hear any discernable difference (running it on +-12V at low volume into a hifi monitor on my workbench just to see if it works). I'll report more when I get this thing hooked up to a proper guitar speaker.

[joecool85]

Thanks again for all the responses!

I've built it finally, and in 3 minutes of messing around with that part of the circuit I couldn't hear any discernable difference (running it on +-12V at low volume into a hifi monitor on my workbench just to see if it works). I'll report more when I get this thing hooked up to a proper guitar speaker.

Keep us posted, I'm curious to see how it turns out.  Personally I don't hear much difference with the feedback topology thing.  I'm sure it could be "seen" on a scope, but other than that it probably doesn't matter much.

[sim0n]

Finally back with the amp for the weekend, got it hooked up to its proper power supply and the 10" speaker. Its a TDA2050 chip on +-22V so it should be doing 30 watts of output power.
Whats the verdict on this heatsink? I got it out of some old computer. After an hour of testing it was hot that I could put hold hand on it for 2 or 3 seconds. The original amp had the same chip in it and it was just attached to the amp chassis via an aluminium bracket, no idea how hot it ran.



Regarding the actual circuit I did a bit of googling and found out what exactly is happening

http://sound.westhost.com/impedanc.htm
http://sound.westhost.com/z-effects.htm
http://sound.westhost.com/project56.htm <- its more a less an implementation of this

So the artificially higher output impedance basically accentuates the quirks of whatever speaker you hook it up to. When I finally put in an equivelent circuit for a loudspeaker into the SPICE model i began to see what this leads to. In the case of most guitar speakers with a resonant peak in the impedance at around 100hz and then again rising as it goes to the higher frequencies, this doesn't create bass and treble boost but it actually cuts the mids (which is where the speaker's impedance is lowest.)

Here i stepped the value of the current sense resistor from 0 to 0.47 ohms. You could change R4 in this schematic into a potentiometer (going from about 300ohms to 5kiloohms with an audio taper pot) and make a variable "amp impedance" control.

For the stock values you get the same end product if you hooked up a roughly 5 ohm resistor with the speaker on a regular amplifier (though you'd be wasting a lot of heat in that resistor)

That 10uF capacitor that blocks DC is rather unsparing to the low end but I guess thats not an issue for a guitar amp. The 22nanofarad capacitor doesn't really have any influence on the audio band.

I was first testing it with some music and the decided to just plug in my guitar to see if it worked and was pleasantly surprised (hooray for high output humbuckers). Messing with this shorting over the current sense resistor, yes there is a difference in the sound! You can hear there is a ~4dB mid boost when you remove the current feedback. As for the supposed "woolliness" you get from higher output impedance...I don't know, I may have impaired myself by reading about it beforehand and now I think I'm hearing it, or I'm just mixing it up for the midrange difference, whatever is going on is very very very subtle.

I can post the PCB artwork if anyone wants to have a go

[phatt]

I know this may came as a shock to some but no matter how tecky you wish to view all this it maybe wise to consider what all this actually achieves.

In My View,,,, it simply alters the TONE ,,,,,, and even a bad tone circuit may well deliver better results. 8|

I consider -4 Db a rather bad result for such complexity.

cheers from Phil.

[phatt]

Oh Yeah I forgot to mention ,,, Re Heatsinks ,,, BIGGER is BETTER.
Phil.

[sim0n]

I've got another cpu heatsink thats three times bigger that I plan on affixing, I understand the princuple that more cant hurt
Complexity? Its basically three extra components compared to the datasheet circuit so it doesn't bother me. I've built it for now so its staying this way

Here is the board if anyone wants to make an amp with this current feedback scheme. As I mentioned earlier, you can change the 1kohm resistor for a potentiometer for a variable "amp output impedance" control. And upping value of the 10uF electrolytic improves the bass response which is cut off under 100hz (but it keeps the speaker from farting out too quickly)



transfer : http://img576.imageshack.us/img576/884/tda2050transfer.png
schematic : http://img812.imageshack.us/img812/5958/tda2050schematic.png

[joecool85]

Personally I don't think it makes any difference as compared to an amp without it that someone simply turned down the mid range on their 3 band EQ.

[sim0n]

Personally I don't think it makes any difference as compared to an amp without it that someone simply turned down the mid range on their 3 band EQ.

I don't have a preamp for this yet so i was running direct but my verdict was that it's benificial as it was slightly on the "honky" side without it...I've got a tubescreamer if I want that sound  :cheesy: