ruby doobie..... deux

[pinkjimiphoton]

so i was dickin' around with my ruby doobie and wondered if i could get away with bridging 2 lm386 chips... and, well.... this happened. i know the preamp part is good. i haven't built the power amp part yet, but in the spirit of a 10 toed opposable thumbed monkey with a breadboard, figured i'd draw it up and lay it out. anybody see mistakes? suggestions? redundant stuff? not sure if i still need the 220u electro on the positive speaker output. i believe everything else should be good.
the "gain" pot should be a dualie, so it can adjust gain on both chips. or ya could just say screw it, and put a cap there instead... anywhere from 47n to 4.7u apparently should work.
ideas? comedy? suggestions? criticisms? plagiarism? solipsism?anyism?
don't hold back, now.
IF ANY BRAVE SOUL DARES TO TRY THIS FOOLHARDY THING ON THE BREADBOARD, LET ME KNOW!! it may be a while before i can verify it.
too much shit on my bench!!!

[Digital Larry]

I don't see that you need 100R on either side of the gain adjust pots on the 386es, other than you have them elsewhere and just want to use 100Rs everywhere possible instead of a slightly more convenient 200R or 220R.

Regarding whether the 220 uF DC blocking cap is needed, I'd check to see if there is a DC voltage between the 2 386 output pins over the range of gain settings.  A small DC voltage here might be acceptable, or not... but if there is you want to make sure to align the polarity of the cap with that, or use a non polarized cap here.  In fact I'd recommend using a non polar if you decide to use a cap there because otherwise, at least half the time with a signal on it, the polarity will be wrong.

Your series 1N4001 connected to the battery + is adequate for polarity reversal protection, you could lose the other one.

[anotherjim]

I'd suggest a second output jack direct from the inverted 386. You know how people want to try these as overdrives into their amps and you can't use the bridge output since it would ground one amp out. Put the blocking cap in that output since it will need one (and 100k to ground so it don't pop when plugged in).

[pinkjimiphoton]

I don't see that you need 100R on either side of the gain adjust pots on the 386es, other than you have them elsewhere and just want to use 100Rs everywhere possible instead of a slightly more convenient 200R or 220R.

i've been doing that lately cuz i find some pots smoke in this kinda application. i agree, probably overkill.

Regarding whether the 220 uF DC blocking cap is needed, I'd check to see if there is a DC voltage between the 2 386 output pins over the range of gain settings.  A small DC voltage here might be acceptable, or not... but if there is you want to make sure to align the polarity of the cap with that, or use a non polarized cap here.  In fact I'd recommend using a non polar if you decide to use a cap there because otherwise, at least half the time with a signal on it, the polarity will be wrong.

larry, i can "fake" a np cap with two electros with their cathodes tied together. so bump that up to 4.7uF to get around the 2.2uF value.
(caps in series are kinda like resistors in parallel and vice versa for anybody out there unfamiliar with how to do it)
should it have blocking caps on both the positive and negative sides do you think?

Your series 1N4001 connected to the battery + is adequate for polarity reversal protection, you could lose the other one.

oft reccomended, i dunno why i keep doing that. force of habit.

[pinkjimiphoton]

I'd suggest a second output jack direct from the inverted 386. You know how people want to try these as overdrives into their amps and you can't use the bridge output since it would ground one amp out. Put the blocking cap in that output since it will need one (and 100k to ground so it don't pop when plugged in).

thanks jim, that's a great idea. i'm hoping to be able to get to my breadboard soon.
it's still got some other stuff currently on it i'm tinkering with.

do you think its gonna need a buffer between the overdrive preamp and the chips? someone suggested that on jeremy's DIYS facebook page.

should i decouple BOTH 386's with a 100u to ground, or since they are directly connected, can i get away with just the one?

thanks guys!

the plan ultimately is to mount this mess in one of these, complete with the output meter gizmo. i'm hoping to get 'em made for me with like, a champagne smoked kinda looking clear plastic instead of clear. so perhaps an led clipper on the amp will be in order, too

https://www.aliexpress.com/item/Clear-Transparent-Speaker-Box-LM386-Amplifier-Kit-With-Case-PC-Speaker-DIY-KIT/32662295472.html?spm=2114.13010208.99999999.278.lAXev6



i'm thinking a wall of about 40 of these little suckers all stacked up would sound cool

[Digital Larry]

larry, i can "fake" a np cap with two electros with their cathodes tied together. so bump that up to 4.7uF to get around the 2.2uF value.
(caps in series are kinda like resistors in parallel and vice versa for anybody out there unfamiliar with how to do it)
should it have blocking caps on both the positive and negative sides do you think?

No, I don't think that 2 are required. It is simply to block DC from going thru the speaker and 1 is adequate for that.  I've also seen bi-polars made with diodes.  I profess ignorance regarding what happens to reverse biased electro caps.

[pinkjimiphoton]

i learned about making np caps with two electros from my old soundguy/mentor bob saunders.  can come in handy sometimes!

i guess in a case with the two areses tied together, it works cuz it blocks dc both ways but bleeds ac thru. or something.

i profess total ignorance at all times on all matters except maybe fuzzes

[anotherjim]

Sometimes, apparently unnecessary parts are signatures. Anyone wise enough to delete them is probably wise enough to design their own thing from scratch anyway. A dumb cloner on the other hand, will copy them without hesitation and will have a hard time claiming the design as their own. Just a thought.

With amps, due to the high signal current, the series power protection diode can be a problem. The chip amp needs to suck and blow current from the supply - the diode makes it easy to suck, but hard to blow.

For protection, I think it's a good idea to keep that diode (better still though is to have the supply fixed in the same box so you don't need the protection at all). To this end, as much capacitance as you can on the power supply close to the chip amps is essential. If 2 chips are close, one big cap will do. If layout means they can't be that close, 2 separate caps is better.

[slashandburn]

Nice one Jimi! I might play with this sometime, I never really got round to bridging two of these and I've got plenty lying around.

Can someone enlighten me though as to why you'd drop the 220uf cap on the output?  I'm still a novice and I'm usually wrong about these things, but my understanding was that the main role of this cap was to for a high pass filter with the load of the attached speaker? As in, 220uf and 8ohm rolls of everything under 90Hz.  Have I misunderstood? Is this a drawback of the bridged arrangement, that the cap no longer performs this function?

Help me out here folks, there's something I'm not grasping!

Very nice enclosure, btw! No way of hiding anything in there, thats for sure.

[pinkjimiphoton]

thanks Jimmy and Iain

i think you only need one cap cuz .... i dunno why bro, i'm a monkey with a breadboard

jim... i wonder if the prob i had with the ruby's jfet was the diode.  i could most definitely pull it, i just thought better safe than sorry.

when i get a chance to actually breadboard it (coming soon hopefully) i'll have a play with it and see if i can ditch the diodes, and also have a play with whether it needs two big caps on the chips power supply, or just one is sufficient.

ben lyman  showed me his build of the little gem mk2 and man, it sounds most excellent. so tho i dig having the gain pots, i may just go with switchable caps for the gain instead. they sound great that way!!

also was advised on the facebook version to add another q to drive the amps with... which brings me back to ROG's original with an added preamp. lotta monkeying to be done!

rock on !!

[Digital Larry]

The cap on a power amp output performs two functions. 

First one is to block DC from going through the speaker at all times even with no signal. Without this you will dissipate power unnecessarily and could also push the speaker cone far enough off center as to get some distortion (though in this group, that's probably a good thing).

The second thing that happens is that this capacitor naturally forms a high pass filter to the load, but if you don't have any super low frequencies coming in to the amp, then there's not much benefit in that.  I think that in the case of a high power amp, things like power on transients or bzzzt ckkk glitch when you plug in your guitar might create enough low freqs to blow up your speaker if you weren't careful.

An amp with both positive and negative power supplies can keep the quiescent (no signal) output voltage very close to zero and so DC blocking is less of a concern than with a single rail power amp.  The other option shown here is a "bridged" configuration where one amp swings up and the other swings down in response to the input.  In this case, with a single supply, supposedly the outputs would both be around 4.5 and so again there's no voltage DIFFERENCE across the speaker. 

However, I think (I'm not an expert in this area) that bridged designs need extra complexity to guarantee that both outputs are at "exactly" the same voltage with no signal, so it's not unusual to see output coupling even though you are using a bridged configuration or have + and - power supplies at the output stage.

[Digital Larry]

i think you only need one cap cuz .... i dunno why bro, i'm a monkey with a breadboard

It might seem hard to believe, but if you have a bunch of R's and C's in series, the order doesn't matter.  The current is the same through all components in series.  That doesn't apply if you are taking some signal from some point in the middle of the chain.  Clearly the difference between an RC low pass and a high pass is the order of the components because your output comes from the point between them.

But if you have a speaker and two caps in series, you can rearrange them all day long and it doesn't matter.  What you'd notice is that a speaker with a cap on either side behaves the same as a speaker with those two caps together on one side, and woops those caps are in series and the effective value is half (assuming they were identical).

Finally, one capacitor blocks DC just as well as two (barring leakage resistance).