Another LM3886 question, soundwise...

[didier]

hi folks,

i will post some soundclips of my LM3886 amp a little later...

my question is whether the amp always sounds so nasally (dont know if i spell this correctly). there seems to be much bass and mid but the treble just doesn't seem to be coming out... it's allmost a wah kinda sound.

which made me wonder if my problem could be with the inductor and the 10ohm resistor at the output...
could it be that i wound this incorrectly? does it have an effect on the sound if i did? or is my problem elsewhere...
i'm afraid to test it without this protection... or shouldn't i be?
could this protection be replaced with a capacitor/resistor zobel network? of so, how? which values?

i'll record some samples and i'll read up on chipamp.com maybe i can find out some of the answers for myself

bye

[didier]

seems like the network in the chipamp.com kit is replaced by a 2.7Ohm 2w resistor and a 0.1uF capacitor...

[R.G.]

which made me wonder if my problem could be with the inductor and the 10ohm resistor at the output...

It's probably not. If you did the normal thing of winding an inductor of perhaps 20 turns on less than a 1"/25cm diameter, then the inductance is a few micro-Henries. If you connected the feedback to the output before the inductor/resistor as these things are normally connected, such an inductor is too low in value to cause audible effect.

could it be that i wound this incorrectly? does it have an effect on the sound if i did? or is my problem elsewhere...

Your problem is likely elsewhere.

i'm afraid to test it without this protection... or shouldn't i be?

The purpose of the inductor/resistor is to cause the inductor to look like a high impedance to keep the amplifier from oscillating when connected to high-capacitance cables. If your speaker cable is not coax-shielded cable and is less than maybe three feet long, there's really little danger that the cable would make it oscillate. So try it without the inductor/resistor if you want. But I think the problem is elsewhere.

could this protection be replaced with a capacitor/resistor zobel network? of so, how? which values?

The series inductor/resistor is compensation for possible cable abuses. The zobel network to ground is protection against speaker abuses. They complement each other, the don't do the same things.

It's highly likely that you have a treble loss before it goes into the amplifier.

[George Giblet]

I doubt it's the inductor or the RC.  What pot value are you using, the 10k value in the data sheet won't help.

Solid state amps can sound dull, the good sounding solid state amplifiers always have a special low damping factor arrangement or some form of treble boost.   If you don't use a fender/marshall tone control you can get a very midrange tone.

[R.G.]

That's a good point George. The stock Fender or Marshall tone stack has a big mid scoop. A guitar played through a flat EQ sounds midrangy because we expect the mid scoop.

[didier]

either way, if i connect my guitar directly to the input of the amp it sounds very dull, then i tried it with this in front of it but no matter what i do, even with treble all the way up and bass/mid down i dont like it... :s

what if i breadboard one of ROG's preamps, like the vox top boost solid state equivalent? i'll try this one tomorrow...
i'll also omit the resistor/inductor network, not that i don't believe you guys but maybe i wound it wrong or something... i'll see...

there's just not so many components on the board, the problem has to lay with one of these right...

if a dual opamp like the TL072 is used and i only need one half, could not connecting the other pins to anything affect the sound? i'm lazy

ciao

[R.G.]

either way, if i connect my guitar directly to the input of the amp it sounds very dull, then i tried it with this in front of it but no matter what i do, even with treble all the way up and bass/mid down i dont like it...
there's just not so many components on the board, the problem has to lay with one of these right...

Actually, no, I think the problem is with another part - the input bias resistor. Those are commonly about 20K for the LM3886 chips. Guitars do NOT like low impedance loads, it makes them sound dull because they are more loaded down at treble than at bass.

What you need is some kind of circuit in front of this that presents the guitar a load of 1M or over. 

The active tone control you show has a relatively low imput impedance too. If you will use the other half of that TL072 as an input buffer to drive the active tone control, things should start getting better.

[didier]

i'll try one of these in front of everything... maybe presenting even higher impedance.

optimal would be if i knew the impedance of the guitar and i could match it right?





which one should i use? i guess not the first as i have to connect the + to Vbias right?
the third one is an inverting one does it actually matter if i invert the signal or not?
the opamp in the tone control is also set to invert the signal, so in the end i would become a signal in phase with the original.

inverting configurations are recommended to prevent oscillation right? but at unity gain i shouldn't really worry about this?

right now, i'm leaning towards the inverting config but tell me please if i should choose another one...

thanks

[R.G.]

i'll try one of these in front of everything... maybe presenting even higher impedance.

Circuit one has an input impedance of about 1M. It's a good choice. Circuits two and three are identical, and have an input impedance of about 500K.

optimal would be if i knew the impedance of the guitar and i could match it right?

No. Impedance matching is for maximum power transfer. In audio signal work, we want maximum VOLTAGE transfer, which means we want the impedance being fed to be at least 10X the impedance feeding it, if not 100X or 1000X. Impedance matching is for speakers.

which one should i use? i guess not the first as i have to connect the + to Vbias right?

Actually, the first is a good choice for reasons we haven't touched on here. It's lower noise. Make Vr from a 10K/10K resistor pair in series across +9 to ground with a cap of about 1uF to 10uF from the middle to ground. Connect your R1 to there.

the third one is an inverting one does it actually matter if i invert the signal or not?

Nope, it really doesn't matter as long as you don't parallel process signals and mix them back together. If you do that, you can get phase cancellation. With only one signal path, it does not matter because the human ear cannot detect absolute phase.

the opamp in the tone control is also set to invert the signal, so in the end i would become a signal in phase with the original.
inverting configurations are recommended to prevent oscillation right? but at unity gain i shouldn't really worry about this?

If you are being careful, yes, using an inverting buffer is a good-ish idea just for electronic hygene reasons. But inverting opamp circuits with high input impedance are more difficult, and I can't see the circuit you're referring to to tell if you've used one of these or not. If you don't do any parallel processing and mixing, it doesn't matter.

Inverting configurations are indeed less prone to oscillation. However, this is a highly NON-critical application, so it would usually not be a problem.

Unity gain is actually harder on opamps internally than higher gains are. The less feedback that's used (i.e. the higher the closed loop gain) the easier it is to stabilize a feedback amp. But again, this is a moot point, as the TL07x series is already compensated for unity gain whether you want it or not, so it does not matter. The thing is unity gain stable as you get it.

You can make TL07x (or any opamp) oscillate if you're not careful about power supply impedance and decoupling and layout. The parasitic capacitance back to the noninverting input can couple in enough signal to make it oscillate on especially a JFET input opamp like the TL07x series if you are using a noninverting configuration OR if you are using an inverting configuration and you don't connect the + input to a low impedance reference voltage. It's the high impedance at the + input plus poor layout that gets you, not whether it inverts or not.

Keep reading though. Those are good points to know and worry about. All you need to add is the background understanding of them. You're getting there.

[didier]

Keep reading though. Those are good points to know and worry about. All you need to add is the background understanding of them. You're getting there.

Yep i can feel it, although i feel like i get many things wrong or don't think about thing enough...
but hey, i spend half my day reading all sorts of things about electronics... THERE'S JUST TOO MUCH

[R.G.]

Don't let the sheer amount get you down. I read electronic texts for fun - OK, I know that's weird.

But there is a tremendous amount to learn. I always felt like it was a treasure hunt will little bits of gold and jewelry scattered about on the trail, rather than a single big trove of goodies at the end of the trail. The trick is just to stay on the trail long enough. It will keep feeding you as you go. And who knows - maybe there's a BIG lump of gold under that next rock over there...