Tantalum cap to set gain in LM386

[FenderBender11]

Hi,

I know this isn't exactly hi-fi, but I'm building a friend a small amp for his birthday, and I'm looking for some help on parts selection.

I'm following the National schematic for setting the gain to 200. The vale to achieve a gain of 200 is a 10uF capacitor. Obviously, I could just use a 10uF electrolytic and call it a day, but what would happen if I put something like a 10uF tantalum in there...for kicks.

Would changing from a electrolytic to a tantalum have any difference in sound (in this application)? Is it worse or better? Tants are supposed to be better quality, but I'm just curious.

Thanks.

 

[enzoR]

are you sure you want a gain of 200?

anyway electrolytic or tant wont make a difference in this application.

 

[AndrewT]

I don't know the internal circuit, but I suspect the external cap is AC bypassing an internal gain setting resistor.
If that is the case then adding a series resistor with the cap should allow an intermediate gain setting.

 

[woody]

Going to a tantalium cap is probably a bad idea they sound bad. And by the way tantalium caps are electrolitic.

 

[digits]

Sound bad is a relative thing in this particular function, which is to try to add some grunge to a toy guitar amp....

 

[FenderBender11]

I have a question regarding gain..anyway. I know what the textbook definition of gain is, but I don't understand what it really means for sound/sound quality. It should be louder right? So...what would be the point of using a low gain?

Sorry if this a big quesiton.

 

[sofaspud]

Gain doesn't mean anything to sound/sound quality. If you want a 12V output, you can amplify a 1V signal 12x, or a 3V signal 4x, or an 8V signal by 1.5x. That's the reason for the question in post#2; 200x gain is likely to clip all but the smallest of your input signals. On the other hand, 200x may be what you need. It's nothing to do with loudness though. The LM386 has a specified output capability of watts into a load. The amount of gain doesn't change that. "It should be louder right?" is a big reason why there's a steady stream of "this car amp is broken" posts.

 

[nigelwright7557]

Gain should be kept as low as possible to get full output power.
The more gain the more you are amplifiying noise from components.
My amps usually have a gain for about 45.

 

[FenderBender11]

Thanks.

I'd like to remind you that I'm an idiot...

I understand now. I'm assuming that my friend will predominatley be using this with his Zune HD, but I also am including the option to use a 1/4inch input (like his guitar) and I'm using a DPDT switch to switch the mono signals. (3.5mm jack and 1/4inch jack).

So what is a "normal" voltage output from a Zune/iPod?

Thanks.

 

[Tube_Dude]

And by the way tantalium caps are electrolitic.

Hi Woody

Tantalum capacitors are solid capacitors, not electrolytic in any way...

Maybe you mean they are polarized capacitors, like electrolytics are.

 

[FenderBender11]

Going to a tantalium cap is probably a bad idea they sound bad. And by the way tantalium caps are electrolitic.

Yeah I know that. I meant, would there be any difference between using something like a Panasonic FM vs using a Kemet solid tantalum.

 

[FenderBender11]

I have a nasty reputation for making things more complicated than I originally intended to make them. I think I'm just going to screw trying to make it into half guitar amp/ half iPod amp...The option will be there to use the 1/4 inch jack, but I'm not going to guarantee it sounds great. (Good friend, eh?).

I think I'll lower the gain to around 50, which will hopefully be a decent median. I could potentially add a gain knob...but I'm not sure if anyone really wants to toy with adjusting the gain when you want to play a song with your iPod. >.>

 

[woody]

Nope they are electrolic... Near the end of the articles 2 types of electrolitic caps are listed aluminum and tantalium.



Electrolytic capacitor - Wikipedia, the free encyclopedia

 

[FenderBender11]

Using an electrolyte is just a style of making capacitors. The electrolyte makes a gap between the conductors, and thus makes capacity.

 

[!]

^ wet (electrolytic) tantalum capacitors do exist but they are very rarely used today outside of military, aerospace, etc, places where there are extreme conditions and I'm not sure if they are even used in these fields anymore.

When anyone mentions "tantalum" without "wet" or "electrolytic", almost nobody assumes they mean other than solid non-electrolytic type anymore, and I'm not aware of anyone using wet tants in modern chipamps.

The typical little yellow epoxy blob tantalums about half the size of a cotton swab tip are solid non-electrolytic, as are the chip tantalums used in surface mount electronics built over the last couple decades or longer.

 

[FenderBender11]

Also,

I know this is a niche category, but with ~1W of power and mid-fi sound quality, what kind of driver would be ideal? I want to spend $10 or less. Remember this is just a small birthday present.

Here's some drivers I've been looking at. What's the deal on impedances? For an LM386, should I be running lower or higher impedances. I'm using LM386N-3.

2" Tymphany/Peerless: Tymphany 2" Full-Range Neodymium Driver

3" Peerless: Peerless India W3-LK76-PBK 3" Paper Cone Shielded Woofer 4 O

3" Tang Band: Tang Band W3-1053SC 3" Full Range Driver

3" Hivi: HiVi B3S 3" Shielded Aluminum Driver Square Frame

I'm kind of liking the 2" peerless because of it's price right now, but if the others can be justified, that's cool too.

Is the Hivi any good? Looks interesting.

Thanks.

 

[!]

-3 is only 700mW output, ideally you would pick one with the highest SPL level you can find. All those you linked are in the 81-85db range which could be worse, but with your chip having only 700mW, and that at THD levels you probably won't want except "Maybe" with the guitar, it doesn't seem as though a long throw driver is needed and would just add cost. Does it need to be tiny? Here's one with 89.6dB SPL for $1.50:

3" x 5" Ribbed Paper Cone Shielded Woofer Speaker

As for impedances, with a 4 ohm load they're graphing 10% THD at only ~ 350mW output on the LM386 datasheet. On 8 ohm load (with only 12V power input since it's the -3 version) it jumps to about 830mW and at 16 Ohms @ 12V input it is about 1W, but also, device dissipation at 16 ohms is only half that of driving 8 ohms (0.4W vs 0.83W). I'd consider epoxying a heatsink plate of some sort on the top and using thick traces but maybe I'm way off on the volume level you want this project to be capable of.

0.83W starts to become a lot for a little chip w/o any heatsink tab, ideally I'd go with a 16 ohm speaker IF you can keep SPL high enough and stay in the budget and size you want.

The speakers you linked, they sort of seem like a mismatch, too nice for LM386. On the other hand if the speaker will be visible, all the ones you linked look *nicer* to the naked eye so if I were to pick one I'd do it based on SPL, frequency range, and impedance, making 3" Tang Band the winner,

Tang Band W3-1053SC 3" Full Range Driver

... except, we don't know what frequency range you need or how loud you want it to go, and I can't predict how loud LM386 will sound with those before it gets unpleasant to listen.

 

[FenderBender11]

The only reason I said I'd use the -3 is because I have a few of them. I'm going to order a few things off of Digikey anyway...so should I switch over to a different version? -1? -4?

Yeah I kind of thought it was a bit of a mismatch myself.

Perhaps something like this? Seems to fit the bill. 16ohm, 4", foam surround (vs paper), $2

JAMO 40250 4" Paper Cone Shielded Midrange 16 Ohm

Kind of tempting....There's no reviews for it, but how bad could it be?

Thanks for the detailed explanation.

 

[AndrewT]

82dB/W @ 1m is far too low for battery powered anything.

 

[Andy5112405]

^ wet (electrolytic) tantalum capacitors do exist but they are very rarely used today outside of military, aerospace, etc, places where there are extreme conditions and I'm not sure if they are even used in these fields anymore.

When anyone mentions "tantalum" without "wet" or "electrolytic", almost nobody assumes they mean other than solid non-electrolytic type anymore, and I'm not aware of anyone using wet tants in modern chipamps.

The typical little yellow epoxy blob tantalums about half the size of a cotton swab tip are solid non-electrolytic, as are the chip tantalums used in surface mount electronics built over the last couple decades or longer.

Speaking from the MoD point of view - Yes I'm a Chief Engineer in the Royal Navy. Wet Tants have become obsolete because they tended to explode.

In Civvy Street Tants are more expensive than Electrolytics, especially in higher values.

The only real benefit of tants is that they tend to perform better when the DC voltage across them is very low. Standard electrolytics rely upon the DC voltage to make them capacitors in the first place.