A Buffer ? Why ?

[analog_sa]

the volume pot is inside the feedback loop, and feedback changes when the pot is turned

Could you guys stop repeating this drivel?

[Bricolo]

Originally posted by analog_sa


Could you guys stop repeating this drivel?

Could you stop telling us that we are always wrong, without telling why?
Prove us you know something about electronics, just one time

[tbla]

Could you guys stop repeating this drivel?

go back to your room, and play with your tube(s).....

[analog_sa]

go back to your room, and play with your tube(s).....

There are tubes in all rooms... you have to be more specific

If you consider the inverted topology as comprising of an I/V converter fed from an input resistor which merely converts the input voltage to current in the vitual ground point, you'll see that neither the input resistor, nor the preceding circuitry are part of a feedback loop - they just set the gain.

[Pjotr]

Originally posted by Bricolo


Could you stop telling us that we are always wrong, without telling why?
Prove us you know something about electronics, just one time

Ok here it goes,

If you look at the open loop and phase graphs on the datasheets of the chips, it follows that the chips are going towards instability at noise gains lower than 10. At a noise gain of 1 most of these chips are likely to oscillate. So for a stable amp keep it in the gain range above 10.

Now for the inverting GC: The gain is roughly defined by R_fb/R_in. So if you make R_in to large the noise gain drops below 10. Look what happens if there is no low source impedance (when the input is not connected to anything). Well then the noise gain becomes 1 and your GC is likely going to oscillate. Who was telling that R_in is not part of the feedback loop?

Note that an input potentiometer is in series with the input resistance. So depending on the position of the wiper the noise gain of the chip varies. For a stable inverting amp a low impedance buffer (after the input poti) is mandatory to keep noise gain high enough and the chip out of dangerous instability regeons

Cheers

[Elkaid]

Thank you guys

You do only want a parallel amp and not a bridge/parallel amp, right?

You're right
A buffered, inverted paralleled gainclone !

And since I'm only beginning to understand exactly which part of the circuit does what... I'm not very good at designing my own ones...

I'll give it a try during lunch time. Be warned : It may be the most useless schematic you'll ever see in your life.

Thanks and see ya

[analog_sa]

Who was telling that R_in is not part of the feedback loop?

What's stability got to do with anything? You want it stable - adjust the noise gain. A single resistor between the neg input and ground will suffice. Of course all resistance on the neg input has an effect on gain - it's just not apart of fb loop.

[Pjotr]

A single resistor between the neg input and ground will suffice.

Correct, but that is wasting loop gain.

[ThorstenL]

Konnichiwa,

Well, is it practical to make a paralleled inverted Gainclone? I suppose so. I can see no reason whatsoever why it should not in a technical sense would "work".

What I would question however is the SENSE of doing so. Paralelling devices (no matter of what type) always causes problems as while these devices SHOULD IN THEORY share the load equally, they very rarely if ever do.

The result is INVARIABLY a drasitic decrease in sound quality. Just build the same discrete amplifier circuit with one pair of output transistors or with a huge bank of them.

Okay, a paralleled Gainclone will be more load tollerant, but if you have such incompetently designed speakers that their impedance falls much below 3 Ohm I would question the speakers and would then also consider if it is possible to break up the parallel connections of drivers and rather give each driver it's own amplifier.

Often speakers have a so-called 2 1/2 Way configuration which places 2 more or less identical drivers in parallel, one working only at LF the other up to crossover frequency. If such a speaker uses fairly low impedance drivers it may indeed drop below 3 Ohm. In this it is usually possible to convert the speaker to bi/tri wiring so that each driver (plus related X-Over parts) get their own binding posts which then allow each driver to be given a non-paralleled "gainclone".

Just for an aside, 5A peak (limit of LM3875) into a 3 Ohm Load allow 40W to applied to the Speaker and around 30W into a nominal 4 Ohm Load. This should be achievable using a 18V+18V secondary transformer. In most cases I would expect this level of power to be fine for "normal use" and 4 Ohm speakers with an average (87db/2.83V/m) sensitivity.

Sayonara

[tbla]

Often speakers have a so-called 2 1/2 Way configuration which places 2 more or less identical drivers in parallel, one working only at LF the other up to crossover frequency. If such a speaker uses fairly low impedance drivers it may indeed drop below 3 Ohm. In this it is usually possible to convert the speaker to bi/tri wiring so that each driver (plus related X-Over parts) get their own binding posts which then allow each driver to be given a non-paralleled "gainclone".

a very good advice.....

in my case i need a lot of watts (atc sl100p) so i need both the bridging and parallelling, but your speakers most likely only need some good stable and lowpowered amps - the biamping thing is the best thing for you, soundwise that is....

elkaid, tell us about your speaker....