2W RMS Guitar Amp with 2 Bridged LM386-4 @ 18V, Heat sink issue?

[Jasonmatthew911]

I actually made a mini amp with 2 LM386-4's in B.T.L. , with a NJM4558 Pre-amp section going into it, adding a  Volume control, Tone control, and a Gain control that goes from Clean to Overdrive and finally to Hi-Gain distortion, running it with 6-AA cells for 9V or adapter...I have Clip-on Heat sinks on all the chips and an 18V Zener Diode in the Power section, so that I can use 18V safely without going over, to avoid damage to the IC's with an 18V max limit...I'm using a %W 3" $30 speaker rated 8ohms, I but it in a small solid wood enclosure, and at 9V I get at least 1W - 1.5W of power out of it, it sounds louder than an Orange Micro Crush and Mini Fender Twin that I compared it with, but then when I ran it with 18V supply it gave me so much more headroom, it got louder and cleaner, with 18V supply I got 2W power easily, but you need to use the 386-4 that can handle 18V, it also says that this chip will give you 1W power with 18V, so with 2 Bridged I'm getting 2W...The only problem is you neee bigger heat sinks with 18V's, I'm getting more out of it with the 18V, you will get the same amount of volume as a Micro Cube, but when the Volume gets too high it starts making a resonating noise while I play, the only thing I can think of is that I need bigger heat sinks for my chips....If I can perfect this design on 18V, I will have a little 6" cube amp that sounds as loud and as good as a Micro Cube, but much smaller, with good loud enough clean, overdrive, and distortion...Does anyone here now a good heat sink part # that I could get bigger than the little clip-on wings for 8-DIP chips, maybe something I can put on with some compound?

[R.G.]

The Dual Inline Package has some sincere power dissipation problems. Most of the heat is *conducted* out the leads, not convected out the plastic.

I say this as background for what I'm about to say.

You're probably right that it's going into thermal overload and shutting down. You may get some relief from heatsinking, but what you are doing is trying to get a small device working right out at the very edge of its capability. This is not a recipe for long term reliability.

It is better to use a bigger device well inside its capabilities. If you used ONE TDA2030 or LM1875, you'd get 4-5W on 18V, and you can do the same bridged trick to get even more from the same 18V. And both of these chips are in the TO-220 package which not only dissipated about 2W in free air, but is made for attaching to an external heat sink, which are also easily available and cheap.

So my advice is to either (1) back down the power supply voltage until the thermal cutouts quit or (2) get a bigger horse; use a TO-220 power amp chip.

[Jasonmatthew911]

The Dual Inline Package has some sincere power dissipation problems. Most of the heat is *conducted* out the leads, not convected out the plastic.

I say this as background for what I'm about to say.

You're probably right that it's going into thermal overload and shutting down. You may get some relief from heatsinking, but what you are doing is trying to get a small device working right out at the very edge of its capability. This is not a recipe for long term reliability.

It is better to use a bigger device well inside its capabilities. If you used ONE TDA2030 or LM1875, you'd get 4-5W on 18V, and you can do the same bridged trick to get even more from the same 18V. And both of these chips are in the TO-220 package which not only dissipated about 2W in free air, but is made for attaching to an external heat sink, which are also easily available and cheap.

So my advice is to either (1) back down the power supply voltage until the thermal cutouts quit or (2) get a bigger horse; use a TO-220 power amp chip.

The only thing is that I wanted the option of running it on 6-AA cells or 8 max, and for it to be able to run on 9V - 18V, 500mA - 1A power supply optional...I used the 18V side of the Dunlop power brick which has 1A, it works great with it until my volume knob goes beyond 3 o'clock, thats when the background resonating sounds start...I also have all the chips in IC sockets and clip-on heat sinks...2W is all I want to accomplish with this design, from what I'm hearing I have almost accomplished it, but I think that bigger heat sinks may solve the issue at high volume level, the sound with 18V sounds great with clean and distortion, so I don't want to give up on it just yet...I also have some 14 pin LM380 chips I was gonna try for my power section, but it won't work with 9V, I know I could use it with 12V or 18V...Do you know any good chips that will give me 2W and can run well with 9V or 18V supply?...What's your take on the LM380, I also have the clip-on for 14 pin chips as well, not sure if that's enough though?

[Jasonmatthew911]

Keep in mind, what I'm trying to do is make the smallest, loudest, best sounding mini amp possible using only a 5W, 3", Full range Fostex speaker in a solid wood 6" cubicle size which already sounds very nice and warm with good low-end even, thanks to the added depth in the enclosure, this amazed me considering how small the speaker and box really are...I can't really fit more than 6-AA cells in it, but I do want to make an amp that is almost half the size of a MicroCube, but pumps out the same amount of power and quality, and with my pre-amp section, 3" speaker, 6" wood enclosure, and the bridged LM386-4's on 18V supply, I've managed to do that, except at very high volume level, thats the issue, which may just be a heat sink thing...I also figured that putting the 18V Zener diode would keep everything within it's limits....Anything else you may recommend?...Thanks again for your advice...

[Perrow]

Have a look at TDA2822

http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXsrtww.pdf

You can even find it in dip-8 package.

[Philippe]

Keep in mind, what I'm trying to do is make the smallest, loudest, best sounding mini amp possible using only a 5W, 3", Full range Fostex speaker in a solid wood 6" cubicle size which already sounds very nice and warm with good low-end even, thanks to the added depth in the enclosure...

Is it designed to outperform a 3-5W/5" speaker Pignose 7-100 in terms of tube-like tone & natural overdrive?

[Jasonmatthew911]

Keep in mind, what I'm trying to do is make the smallest, loudest, best sounding mini amp possible using only a 5W, 3", Full range Fostex speaker in a solid wood 6" cubicle size which already sounds very nice and warm with good low-end even, thanks to the added depth in the enclosure...

Is it designed to outperform a 3-5W/5" speaker Pignose 7-100 in terms of tube-like tone & natural overdrive?

I'm not really trying to make something better than the pignose, haven't heard it really, but my amp is only a 6" Size with a very high quality 3" speaker and when I run it at 18V I get quality and levels as good as a 2W Roland Micro cube, but it's only half the size, this is what matters to me, since now days it's all about how small you can make things without giving up certain features, or making it as good as something bigger...Most amps in this size range sound horrible, very bright, or like toys... I managed to make something very small and loud to look and feel like a real guitar amp with dynamics and a wide frequency response...I'm just trying to get advice to perfect it, so that it works with the 18V supply at any Volume level...Pignose should sound good for it's size, but my design is a lot smaller with a smaller speaker, and it still sounds clean, warm and loud....I'll be trying an LM-380 for my power section just to see if it's any better than the bridged LM-386-4's.......Anyone have anything to say about the LM380?

[Earthscum]

Have a look at TDA2822

http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXsrtww.pdf

You can even find it in dip-8 package.

+1... I discovered these about 2 years ago in comp speakers. 2W at 9V in bridged mode... Two Amps One Chip (sounds like a bad internet vid clip). Tayda carries them: http://www.taydaelectronics.com/servlet/the-773/Amplifiers--dsh--Audio-DUAL/Detail

[R.G.]

Keep in mind, what I'm trying to do is make the smallest, loudest, best sounding mini amp possible using only a 5W, 3", Full range Fostex speaker in a solid wood 6" cubicle size which already sounds very nice and warm with good low-end even, thanks to the added depth in the enclosure, this amazed me considering how small the speaker and box really are...I can't really fit more than 6-AA cells in it, but I do want to make an amp that is almost half the size of a MicroCube, but pumps out the same amount of power and quality, and with my pre-amp section, 3" speaker, 6" wood enclosure, and the bridged LM386-4's on 18V supply, I've managed to do that, except at very high volume level, thats the issue, which may just be a heat sink thing...

Nature seems to force us into compromises. The way this comes out in this instance is this: smallest, loudest, or best sounding, choose any two.

I think you mistake my advice. I was just making the observation that you're trying to make an LM386 do something that is just barely feasible if everything works perfectly. You're working it right at the edge of the possibilities, including its ability to dissipate heat. In any design, there comes a point where refining, tuning, tweaking, improving, polishing, etc. give diminishing returns. When a designer reaches that point, the good ones realize that they need to either quit pushing things or take a step to something bigger or more capable. You are at such a point.

You're happy with what you have - except that it's showing signs of going into thermal cutout, and you'd really prefer it to be louder. You **might** be able to glue heatsinks to an eight pin DIP and be happy with what you get. But with Mother Nature being who she is, chances are not good. Rather that pushing the edges of the LM386, you can take a device that is good up to much higher powers, but will happily work within the power supply and outputs you have now. I'm suggesting that you replace *two* LM386s with *one* TDA2030 or *one* LM1875. These devices are working at the low-power end of their range with your power supply and would be in essence loafing all the time. One TO-220 is about the same physical volume as two DIPs, and you only need one set of the resistors and capacitors to run it, so the total volume might even be smaller.

If your speaker is limited to 8 ohms, 5W, it's about perfect for a single TDA2030. At 18V supply, a single 2030 puts out about 4W into 8 ohms, no bridging. If you bridge a pair you can get to about 10W. And neither of these option would cause you to make your speaker, enclosure, or power supply bigger.

I also figured that putting the 18V Zener diode would keep everything within it's limits.

No, it won't. Putting a zener diode on your power supply would do nothing at all until the voltage exceeded the zener voltage. And at that point the power supply would start a war with the zener, trying to force the zener to be a higher voltage than the zener is willing to do. The zener would lose and burn out. You cannot use a zener in any application where you cannot demonstrate that the current will never be big enough to burn the zener out. Well, you can, but the zeners burn out like popcorn. I guess I mean that one should not use zeners this way.

[R.G.]

I forgot to mention - the TDA2822 is only good to 15V. On 18V, it commits ritual self-sacrifice, returning to the beach sand from whence it came.

[Jasonmatthew911]

Keep in mind, what I'm trying to do is make the smallest, loudest, best sounding mini amp possible using only a 5W, 3", Full range Fostex speaker in a solid wood 6" cubicle size which already sounds very nice and warm with good low-end even, thanks to the added depth in the enclosure, this amazed me considering how small the speaker and box really are...I can't really fit more than 6-AA cells in it, but I do want to make an amp that is almost half the size of a MicroCube, but pumps out the same amount of power and quality, and with my pre-amp section, 3" speaker, 6" wood enclosure, and the bridged LM386-4's on 18V supply, I've managed to do that, except at very high volume level, thats the issue, which may just be a heat sink thing...

Nature seems to force us into compromises. The way this comes out in this instance is this: smallest, loudest, or best sounding, choose any two.

I think you mistake my advice. I was just making the observation that you're trying to make an LM386 do something that is just barely feasible if everything works perfectly. You're working it right at the edge of the possibilities, including its ability to dissipate heat. In any design, there comes a point where refining, tuning, tweaking, improving, polishing, etc. give diminishing returns. When a designer reaches that point, the good ones realize that they need to either quit pushing things or take a step to something bigger or more capable. You are at such a point.

You're happy with what you have - except that it's showing signs of going into thermal cutout, and you'd really prefer it to be louder. You **might** be able to glue heatsinks to an eight pin DIP and be happy with what you get. But with Mother Nature being who she is, chances are not good. Rather that pushing the edges of the LM386, you can take a device that is good up to much higher powers, but will happily work within the power supply and outputs you have now. I'm suggesting that you replace *two* LM386s with *one* TDA2030 or *one* LM1875. These devices are working at the low-power end of their range with your power supply and would be in essence loafing all the time. One TO-220 is about the same physical volume as two DIPs, and you only need one set of the resistors and capacitors to run it, so the total volume might even be smaller.

If your speaker is limited to 8 ohms, 5W, it's about perfect for a single TDA2030. At 18V supply, a single 2030 puts out about 4W into 8 ohms, no bridging. If you bridge a pair you can get to about 10W. And neither of these option would cause you to make your speaker, enclosure, or power supply bigger.

I feel that the TDA2030 might be too much power for a 5W 3" Speaker, I think it will be cleaner and won't saturate so quickly with just 2W - 3W max pushing it....Something like the LM380 seems about right, which is 2W - 2.5W....If you know any other chip that will deliver 2W - 3W and could operate well within 9V - 18V or 12V - 18V please let me know, the packaging doesn't matter, if you know the heat sink as well, but I would really like the option that it could operate on 6-AA batteries or 18V for more headroom etc...

I also figured that putting the 18V Zener diode would keep everything within it's limits.

No, it won't. Putting a zener diode on your power supply would do nothing at all until the voltage exceeded the zener voltage. And at that point the power supply would start a war with the zener, trying to force the zener to be a higher voltage than the zener is willing to do. The zener would lose and burn out. You cannot use a zener in any application where you cannot demonstrate that the current will never be big enough to burn the zener out. Well, you can, but the zeners burn out like popcorn. I guess I mean that one should not use zeners this way.

I feel that the TDA2030 might be too much power for a 5W 3" Speaker, I think it will be cleaner and won't saturate so quickly with just 2W - 3W max pushing it....Something like the LM380 seems about right, which is 2W - 2.5W....If you know any other chip that will deliver 2W - 3W and could operate well within 9V - 18V or 12V - 18V please let me know, the packaging doesn't matter, if you know the heat sink as well, but I would really like the option that it could operate on 6-AA batteries or 18V for more headroom etc...I still want the option of running on batteries, 6-AA to 8 at the most.

[R.G.]

Good luck with it.

[Jasonmatthew911]

Good luck with it.

I'm an idiot...I was using my Dunlop 18V ECB-04 power supply which only has 150mA, So I went and found my ECB-06 18V Regulated power supply that came with the power brick and has 1A...That was the problem all along, I hadn't realized that I was only feeding it 150mA, and for this design you need at least 500mA - 1,000mA....No more resonating sounds at high volume and the amp has a nice warm but loud clean and very loud distortion, so far it's pumping 2W with no problem using the Dunlop ECB-06 18V power supply...I'll let you guys now if the chips last running them on 18V...I think they should considering that they all have heat sinks and I used an 18V/5A zener diode for extra protection....I'll probably still try the LM380 just to see which is a better option and which actually sounds better or louder....Thank you all for your advice, I appreciate it...

[Mark Hammer]

The LM380 will work fine.

The other thing to consider is that the volume difference between 2W and 5W is much smaller than you think.

[Brymus]

I actually made a mini amp with 2 LM386-4's in B.T.L. , with a NJM4558 Pre-amp section going into it, adding a  Volume control, Tone control, and a Gain control that goes from Clean to Overdrive and finally to Hi-Gain distortion, running it with 6-AA cells for 9V or adapter...I have Clip-on Heat sinks on all the chips and an 18V Zener Diode in the Power section, so that I can use 18V safely without going over, to avoid damage to the IC's with an 18V max limit...I'm using a %W 3" $30 speaker rated 8ohms, I but it in a small solid wood enclosure, and at 9V I get at least 1W - 1.5W of power out of it, it sounds louder than an Orange Micro Crush and Mini Fender Twin that I compared it with, but then when I ran it with 18V supply it gave me so much more headroom, it got louder and cleaner, with 18V supply I got 2W power easily, but you need to use the 386-4 that can handle 18V, it also says that this chip will give you 1W power with 18V, so with 2 Bridged I'm getting 2W...The only problem is you neee bigger heat sinks with 18V's, I'm getting more out of it with the 18V, you will get the same amount of volume as a Micro Cube, but when the Volume gets too high it starts making a resonating noise while I play, the only thing I can think of is that I need bigger heat sinks for my chips....If I can perfect this design on 18V, I will have a little 6" cube amp that sounds as loud and as good as a Micro Cube, but much smaller, with good loud enough clean, overdrive, and distortion...Does anyone here now a good heat sink part # that I could get bigger than the little clip-on wings for 8-DIP chips, maybe something I can put on with some compound?

I did the same thing about 1-2 years ago.
Then I went with bigger chip amps.
It is really fun seeing how far you can push the little 386.I smoked a few in the proccess.
And I am seeing you found the same thing as me - 18V PS,2x 386-4,some heat sinks and you have a screaming little amp,that runs forever off 12x AA batteries.
Do yourself a favor and ditch the 3" spkr and get a guitar spkr it will sound like a new amp.
I found the center channel spkrs made for surround sound systems,have the right frequency response and come fairly small.(its all about Spkr SPL with low wattage amps)
If you look at other manufactured designs using little chip amps you will find what R.G. is talking about,most use large copper pads in the right spots to help dissipate heat,some have the heat sink(more like wing)soldered in with the IC.
I used the kind that of heat sink that sticks to the top of the IC a little JB weld transfers the heat better than the sticky glue that comes with them.
I think the best thing about the 386 is its a gateway chip amp,makes you want to try bigger,louder ,current sucking IC's 

[Jasonmatthew911]

I actually made a mini amp with 2 LM386-4's in B.T.L. , with a NJM4558 Pre-amp section going into it, adding a  Volume control, Tone control, and a Gain control that goes from Clean to Overdrive and finally to Hi-Gain distortion, running it with 6-AA cells for 9V or adapter...I have Clip-on Heat sinks on all the chips and an 18V Zener Diode in the Power section, so that I can use 18V safely without going over, to avoid damage to the IC's with an 18V max limit...I'm using a %W 3" $30 speaker rated 8ohms, I but it in a small solid wood enclosure, and at 9V I get at least 1W - 1.5W of power out of it, it sounds louder than an Orange Micro Crush and Mini Fender Twin that I compared it with, but then when I ran it with 18V supply it gave me so much more headroom, it got louder and cleaner, with 18V supply I got 2W power easily, but you need to use the 386-4 that can handle 18V, it also says that this chip will give you 1W power with 18V, so with 2 Bridged I'm getting 2W...The only problem is you neee bigger heat sinks with 18V's, I'm getting more out of it with the 18V, you will get the same amount of volume as a Micro Cube, but when the Volume gets too high it starts making a resonating noise while I play, the only thing I can think of is that I need bigger heat sinks for my chips....If I can perfect this design on 18V, I will have a little 6" cube amp that sounds as loud and as good as a Micro Cube, but much smaller, with good loud enough clean, overdrive, and distortion...Does anyone here now a good heat sink part # that I could get bigger than the little clip-on wings for 8-DIP chips, maybe something I can put on with some compound?

I did the same thing about 1-2 years ago.
Then I went with bigger chip amps.
It is really fun seeing how far you can push the little 386.I smoked a few in the proccess.
And I am seeing you found the same thing as me - 18V PS,2x 386-4,some heat sinks and you have a screaming little amp,that runs forever off 12x AA batteries.
Do yourself a favor and ditch the 3" spkr and get a guitar spkr it will sound like a new amp.
I found the center channel spkrs made for surround sound systems,have the right frequency response and come fairly small.(its all about Spkr SPL with low wattage amps)
If you look at other manufactured designs using little chip amps you will find what R.G. is talking about,most use large copper pads in the right spots to help dissipate heat,some have the heat sink(more like wing)soldered in with the IC.
I used the kind that of heat sink that sticks to the top of the IC a little JB weld transfers the heat better than the sticky glue that comes with them.
I think the best thing about the 386 is its a gateway chip amp,makes you want to try bigger,louder ,current sucking IC's 

Cool, what SPL do you recommend?...I'm using it with a 5W, 3" Full range Fostex speaker with an SPL of 87 or 89db, mainly because I wanted to make something as small as possible that still sounds good and loud, but I added an output jack to be able to connect it to an external cab, and yes it sounds nicer through bigger speakers for sure...The other thing is that with this design I can run it from 9V-18V getting 1W - 2W and the amp is very small....Do you recommend any other chip within 2W - 3W power that could run on 9V - 18V or 12V - 18V with no problem, that isn't the LM380?...Thanks for your input...

[Brymus]

OK
I spent months obsessing over the 386 and some designs using it.
I raided the local thriftshops, my friends garages,the kids toy box.ect salvaging little spkrs.
I literally have a huge box full of tiny spkrs (4"and under)
What I found for a non musical instrument spkr was the center channel spkrs.
I found a Cerwin Vega 6" center channel spkr with a response of 70hz- 12Khz and SPL of around 90 db perfect for guitar.
( I like it better than my 5" Jensen Mod )
What you need to remember (whats been said 100s times already) is its easier to get more loudness by using a more efficient spkr than upping your supply voltage.
I would search all the usual vendors for center channel spkrs,you really only need a frequency response of 60hz - 5 Khz for normal guitar.
And anything close will work,the higher the SPL the better.
For my 18 watter I use Jensen Jets with a 98 db spl rating,and I never get to turn it up even halfway during band practice.
My Jensen Jet with a single 386 at 12 V will be ALOT louder than your 2X 386 at 18V using your 87 db spkr.

As far as different ICs any of the ones suggested are usable,I like all the TDA20** ,chips you will just have to read the data sheets for all of them and decide for yourself which to use.
One thing you will find is that the lower the supply voltage the higher the current draw to produce X amount of watts.
And there is no way around that.
Thats why its best to design from the power supply up.

[Perrow]

While we're at it, have a look at TEA2025, up to 4.7W into 8 ohm speaker at 9V 

I guess you could go for non bridged at 12V giving you 2.4W or bridged at 6V giving you 2.8W.

The example bridged circuit has nine caps and a resistor. I'd say that the whole circuit from Tadya'll come in under a dollar (had to check $0.64 with no board).

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000172.pdf

[Jasonmatthew911]

I actually made a mini amp with 2 LM386-4's in B.T.L. , with a NJM4558 Pre-amp section going into it, adding a  Volume control, Tone control, and a Gain control that goes from Clean to Overdrive and finally to Hi-Gain distortion, running it with 6-AA cells for 9V or adapter...I have Clip-on Heat sinks on all the chips and an 18V Zener Diode in the Power section, so that I can use 18V safely without going over, to avoid damage to the IC's with an 18V max limit...I'm using a %W 3" $30 speaker rated 8ohms, I but it in a small solid wood enclosure, and at 9V I get at least 1W - 1.5W of power out of it, it sounds louder than an Orange Micro Crush and Mini Fender Twin that I compared it with, but then when I ran it with 18V supply it gave me so much more headroom, it got louder and cleaner, with 18V supply I got 2W power easily, but you need to use the 386-4 that can handle 18V, it also says that this chip will give you 1W power with 18V, so with 2 Bridged I'm getting 2W...The only problem is you neee bigger heat sinks with 18V's, I'm getting more out of it with the 18V, you will get the same amount of volume as a Micro Cube, but when the Volume gets too high it starts making a resonating noise while I play, the only thing I can think of is that I need bigger heat sinks for my chips....If I can perfect this design on 18V, I will have a little 6" cube amp that sounds as loud and as good as a Micro Cube, but much smaller, with good loud enough clean, overdrive, and distortion...Does anyone here now a good heat sink part # that I could get bigger than the little clip-on wings for 8-DIP chips, maybe something I can put on with some compound?

I did the same thing about 1-2 years ago.
Then I went with bigger chip amps.
It is really fun seeing how far you can push the little 386.I smoked a few in the proccess.
And I am seeing you found the same thing as me - 18V PS,2x 386-4,some heat sinks and you have a screaming little amp,that runs forever off 12x AA batteries.
Do yourself a favor and ditch the 3" spkr and get a guitar spkr it will sound like a new amp.
I found the center channel spkrs made for surround sound systems,have the right frequency response and come fairly small.(its all about Spkr SPL with low wattage amps)
If you look at other manufactured designs using little chip amps you will find what R.G. is talking about,most use large copper pads in the right spots to help dissipate heat,some have the heat sink(more like wing)soldered in with the IC.
I used the kind that of heat sink that sticks to the top of the IC a little JB weld transfers the heat better than the sticky glue that comes with them.
I think the best thing about the 386 is its a gateway chip amp,makes you want to try bigger,louder ,current sucking IC's 

My 3" speaker is actually 89db, it's a Fostex FE87 w/ white cone...Would there be a noticeable difference between 89db & 90db SPL?....Anyways, since I'm looking to make something as small as possible, I was thinking of a 4", 15W, 93db SPL, Full range Fostex speaker at the most, so I can keep my enclosure at about 6" - 7" max, it should sound louder and have better Low-end, though the depth of my wooden enclosure helps the low-end a lot as well (It's a 6"x6"x6" Solid wooden cube, I even covered it with tolex and put steel amp corners on it)...I know it will sound better with an even bigger speaker and higher SPL, but thats why I added the output jack to connect with external cabs when I want more out of it, but I want it to be as small as possible, smaller than a Micro-cube or a Vox Mini 3, but just as loud and as good, I've already accomplished the loudness, and with a slightly bigger speaker it should be just as clean with better low-end...When I connect it to my 4x12" Cab it sounds great and very loud....Here is a pic of my Amp...I called it "Little Stella":

[Brymus]

Sounds like your on the right path.
Going from 89 to 93 db will be a noticable volume gain,and that sounds like the perfect 4" spkr for a small amp.