Anyone want to check my BPA schematic?

[Arx]

It's at:
http://arx.ca/projects/BassAmp

Did I miss anything?

It's pretty much just a copy of the non-inverting part of the one in the app-note.

Don't worry about the lack of decoupling caps and invisible power connections. They'll be there, they're just not on the schematic.

How does the input buffer/inverter look. I have pretty limited experience with opamps, so I could have it completely wrong, or with incorrect resistors or something.

Is the TL072 a decent choice for that purpose?

-Nick

[AndrewT]

Hi,
the tl072 will do, but try to keep the traces to the chipamps short and spaced from their neighbours to keep capacitance low. TL does not like caps to ground on it's output.

This is a bass amp, but you have filtered the input with a 5Hz low pass and again filtered the NFB at the same frequency.
That will cut-off the bass to -2db @ 10Hz and the effects will be heard all the way up to 50Hz.
Aim for about 2Hz @ the input and 1 to 1.5Hz @ the NFB leg.

No RF filtering anywhere!
add an input resistor before Rin1 (1k0 would do)
add a cap to ground (330pf//47k5) giving about 0.3uS.
Add more filtering at each chipamp input.
At pin 10 add 15pF to ground (also 0.3uS).

U7b pin 5 needs 11k to ground, to balance the input resistances.

[Arx]

Ok, the input I can easily change. I just chose that size of capacitor because I have a bunch on hand. I'll swap it for something bigger or just put 2 in parallel.

The feedback loop might be trickier. Those caps are already pretty hefty, so unless I swap them out and use servos, I think I'm stuck with what's there.

Also I should clarify that this is a "Bass Amp" as in bass guitar, and may be used with a ported speaker cabinet, so I may need to put in a fillter to roll off the very bottom end so the port won't get out of phase with the speaker. The 5Hz highpass in the feedback loop probably pales in comparison.

The alternative is to build the speaker cabinet sealed, and boost the low end for a little more bass extension.

Either way I think that the 5Hz feedback loop is probably going to matter a lot less by the time I'm done.

Am I wrong?

If you could give me a quick explanation on a couple of these it would be a big help.

add an input resistor before Rin1 (1k0 would do)
ok, done. What does this do?

add a cap to ground (330pf//47k5) giving about 0.3uS.
ok, this would be in parallel with the 47k5 to ground any high frequencies right?
done.

Add more filtering at each chipamp input.
At pin 10 add 15pF to ground (also 0.3uS).
Are both of these items the same thing, or do you mean even more filtering than that?
15pF caps added.

U7b pin 5 needs 11k to ground, to balance the input resistances.
Ok, done. What does this do? Is precision necessary or would a 10k be close enough?

New schematic will be up on the page in a few minutes. My schematic program has no export except for really bad pdf, so I need to take a couple screenshots and stich them :P~~~

-Nick

[Arx]

Okay, the new schematic is up.. Anyone else have any suggestions?

-Nick

[BWRX]

Nick, you still need to connect the output of the TL072s to the input nodes of the chips. Change the value of the resistors on the inputs of the 3886s from 20k5 to 1k. You'll then need to increase the value of the 15pF caps to 330pF to keep the cutoff frequency about the same. You may also want to add a 10k (or so) pull down resistor at the output of each TL072.

[Arx]

Originally posted by BWRX
Nick, you still need to connect the output of the TL072s to the input nodes of the chips. Change the value of the resistors on the inputs of the 3886s from 20k5 to 1k. You'll then need to increase the value of the 15pF caps to 330pF to keep the cutoff frequency about the same. You may also want to add a 10k (or so) pull down resistor at the output of each TL072.

The TL072s were connected at some point. I respaced everything to make it look a little less screwed up and accidentally lost those connections. They're back now.

Added some 10k pulldown resistors. (I'm not going to update the schematic on the website yet though)

Will switching to 1k have any other effects? I assumed that it had something to do with the DC blocking cap, since on the appnote it's 1k with the servos, and 20.5k with the caps.

I've updated the schematic, BTW. It's much uglier now. I've added in the servos. I'm not planning to use them right away, but it's probably easier if I put them on the layout anyways, then I can switch easily if I want to.

Anything else needed, or should I start on the PCB layout?


-Nick

[AndrewT]

Hi Arx,
Bass amp = bass guitar amp. Oh. I thought wide band music amp but dedicated to bass duty.
5Hz on the input is probably OK. You could probably get away with raising this a half octave and still be OK. Keep the NFB at 5Hz.
Single cone drivers will probably also be OK for a bass guitar amp.

Do you intend wiring your multiple drivers to 4ohm or to 8ohm or some other value?

Play safe with heatsink dissipation capacity.

Do keep in mind that if you play hard and loud ALL six amps will be drawing the same power at the same time.
The PSU must be designed to meet this combined and potentially heavy load.
A non regulated PSU is designed for average conditions and peak current ability comes from the smoothing caps.
An SMPS with relatively low value post reg smoothing has to meet the peak current from within it's circuitry. This is a very demanding load and a significant difference in supply philosophy that is often overlooked.

[AndrewT]

Hi Bwrx,
why change the chipamp input Rs from 20k5 to 1k0?
Are you taking advantage of the low input offset/bias currents in the chipamp, or for some other reason?

[AndrewT]

Hi,
the DC servo shows a 20k5 resistor feed back into the inverting input.
Is this value a bit low?
The ratio Rs5 to Rf5 will allow half the opamp rail voltage correction the be applied to cancell output offset. Will this chipamp ever need such gross correction?
Would 100k do here?

Is there sufficient advantage in splitting this resistor in two and filtering the junction with a cap to ground to make the extra complexity worthwhile?

[BWRX]

Originally posted by AndrewT
Hi Bwrx,
why change the chipamp input Rs from 20k5 to 1k0?
Are you taking advantage of the low input offset/bias currents in the chipamp, or for some other reason?

It's not necessary to use a high resistance in series with that input. Since he's already using 1k resistors elsewhere, why not? All you need to do is increase the value of the capacitor a bit to keep the same cutoff frequency. For that matter, he could also use 20k5 resistors instead of the 22k resistors on the mute pin.