I've got some newbie questions. Here is my background in effects and electronics thusfar:
Modified a TS-5 to TS808 specs with additional bass response and clipping diode mod for sweeter distortion.
Built an Orman Mini-booster with Radio Shack parts.
Modified several guitars with different pickups, pots, tone caps, etc. etc.
So I think I have a halfway decent start, but have gotten away so far without really understanding what does what, other than simple stuff (capacitors and resistors).
Anyway, long story short, I want to convert my smoky to a Little Gem (as it has 4 of the components already and I've already overpaid for it by about $25) and was trying to figure out the smoky schem, but the LM386 pins are wired differently. What are the differences?
Is there any good/easy (you choose :D) way to use the existing pcb of the smoky and mod it to a little gem?
Also, can anyone recommend a good, cheap, small speaker readily available locally in the midwest?
Last but not least, how is the reostat wired for the Little Gem? Can I simply use a regular potentiometer here?
Welcome Javacody, :)
I don't have the schematic of the smokey so I'm not much help there, but all the 386 circuits are pretty similar.
Perhaps the best place to start is by looking at the National datasheet for the chip:
http://cache.national.com/ds/LM/LM386.pdf
That gives you a good jumping off point at least. I'm sure it will be pretty easy to reuse the PCB with only minor changes at most.
When it comes to the speaker, the most important thing for great tone is to use a real guitar speaker which is fairly efficient. The smaller units are OK for a pinch, but if you want good sound, I would recommend going with an 8 or 10 inch unit.
Weber VST makes some really nice speakers and they have a clearance special on Signature Series C10S units at $20. I don't think you can go wrong there. The smaller the speaker, the more brittle the tone. I played through a 386 amp with a 3 inch speaker for a long time before trying out a full sized, real-deal unit. The difference was night and day and then some. I know that's not the small speaker you requested, but I think it's beyond worth it and they are in Kokomo, Indiana. ;)
http://www.webervst.com/bargain.html
http://www.webervst.com/
You can also use just about any car speaker. The best way to pick one is to take your amp along with a guitar and audition the possible units at the local car stereo place. They may even have a single speaker from a damaged pair sitting around... you never know. The variety of sounds is quite large and a surprising numbr of expensive speakers will not sound very good in comparison to a speaker made for guitar amp use. Let your ears guide you if you can in this case.
For an attenuator, I've used a regular 500 ohm 1/4 watt linear pot in line with one of the speaker leads without any problems. The level is almost whisper quiet at max resistance. Works GREAT! :D
Take care,
-Peter
Thanks for the tips Pete. That one about the Weber speaker on sale for $20 is almost worth a finders fee. :) Also, thanks for the tip about the pot.
Ok, I hit the shack today and got all of the parts except for the 100 uF cap. I only had a 47 uF cap lying around so I used it. Can anyone tell me the effect this will have on the little gem?
Anyway, I slapped it together and it worked great the first time around. I ended up going with a Jensen 8" Ceramic reissue for $18. Right now the amp is in a radio shack tin project box and the speaker is in the carboard box it came in (I cut a hole in one side). Does anyone have any recomendations for a cheap speaker box?
This amp sounds great! Even better than the smoky. Honestly, it took me longer to drill the holes in the box and tighten down all of the nuts than it took me to solder the components on the board.
One last question, I want to add an LED to this little amp, how would I do that? I'm assuming it wouldn't be as complicated as the millenium bypass, as I simply need a power indicator. The power is switched on via a stereo 1/4" jack.
Cool! 8)
The smaller cap (I'm assuming that's the one in-line with the speaker) will just reduce the bass a little. When you get a 100uF, try adding it in parallel with the 47uF and see how you like the response. Play that by ear. A very common value in that spot is 220uF, but a wide range will work fine.
Since you said you had the speaker mounted in a box, is it open back or closed? If the box is closed up, that will increase your bass response. You might want to try out some other boxes for experimenting. Speaker enclosure design is quite an art. I say play with it and find what you like most.
Adding a power indicator is very easy.... just connect an LED in series with a good sized resistor like 4.7K to 10K and put that across pins 6 and 4 of the LM386. Presto, you're done. :)
I'm glad your upgrade went well. :D
Take care,
-Peter
I used the schem at RunoffGroove (http://www.runoffgroove.com/littlegem.htm, the first one with a single lm386) and the cap I'm talking about is the one that is connected to +9v and goes to ground. What is this cap for?
Thanks for the tip about the LED, my little amp is about to get a cool little upgrade! :D
Rather than upgrade my smoky, I started over. I figured I could still leave the smokey in my gig bag as a backup.[/url]
power supply filter.. is what that cap is for.
also i don't know where you are located but if you are in the us you can usually find a grocery store that has a bakery they will give you one of there cake icing buckets for nothing just so they don't have to throw it away. they have these short squatty buckets that will hold a 12" speaker oh so nice and if you put in a little insulation it makes a great little speaker cabinet.. also if you use a taller bucket it makes a great resonant bass sound. you can also use these as a deadbox or an isobox. has a great reverb sound that is really fun.
Ok, so will having a 47 uF cap there instead of a 100 uF cap cause any problems?
Also, just finished my LED upgrade. Man I love blue LEDs! Very easy. Again, it took longer for me to drill the stinkin hole than it did to solder the components. Looks sharp! Thanks Peter. :D
Ansil, that plastic bucket sounds like its worth looking into. You kind of just inspired me to head down to my local hardware store to see what kind of other stuff I could rig up. Maybe a round bucket with some pvc pipe legs? :twisted:
Yep, that's power supply cap. In this circuit, that cap acts like a reservoir that lets the 386 draw more power easily when it needs it for producing bass. The value is very non-critical. If you run the circuit from an AC adapter, that cap will filter potential hum from the supply too.
That's cool that this is a new build. 8) That makes it all the sweeter!
Take care,
-Peter
Ok, the LED dims and brigthens to my playing, is there a way to fix that?
Also, I used a 10K resistor in series with the led, what affect does lowering the resistance to the LED have? Will it cause the led to brighten?
Last but not least, what effect will raising the voltage have on this circuit? Will 12 volts get me any more volume or headroom? I've looked at the voltage doubler article at geofex, is there an easy way to get 12 volts from a 9 volt source?
If you lower the 10K, you will have a brighter LED. Try 4.7K, but remember it's all a tradeoff... More power for the LED means less power available for the amp and lower battery life. The effect of the smaller power supply cap you used is directly visible as you play harder. That 9V doesn't have the ability to source very much instantaneous current, which is the reason for the reservoir cap in. Bass takes a LOT of current to produce relative to treble. If you increase the cap size, the LED will be more constant. Try something like 470uF or 1000uF or even higher. One thing to note is that the LED is showing you the amount of "sag" in the power supply.... just like tube amps have. ;)
When it comes to higher voltages, you will get more headroom and cleaner, clearer tone, but your LM386 may not be able to handle it. The 386 has been produced in several different versions over the years, mainly because of refinements in the semiconductor process and the lower requirements of the bulk of the 386 produced. This has lead to the LM386-1, which was originally the 6 volt version of the chip. Now the NS datasheet says it's good to 12v and there is no longer (for a very long time now) an LM386-2, which was the 12 volt version. Translation: the original -1 die is gone and the -2 die was used to replace it. The -3 die has larger output transistors for more wattage. The -4 die has the larger transistors and the ability to run from voltages up to 18. The LM386-4 is the only National Semi chip I would use with more than 12V. New Japan Radio makes a chip called the JRC386BD, which is the -4 equivalent.
When it comes to voltage converters, remember that they take power themselves and they are not too efficient. You’re already having trouble running the amp and LED with a huge series resistor so adding a double will only make matters worse… much worse. For higher headroom, use two batteries in series and always change them in a pair for maximum life.
Take care,
-Peter
I'm using a handy little 9-volt adapter that plugs right into the battery connector, so I don't have to worry about battery life.
I suppose I could just leave well enough alone. Nah. :lol:
Quote from: javacodyI'm using a handy little 9-volt adapter that plugs right into the battery connector, so I don't have to worry about battery life.
I suppose I could just leave well enough alone. Nah. :lol:
If it 'aint broke, improve it! :)
Two options there... doing both is best... (1) try a larger cap, and (2) try a larger adapter with higher current. Both of these will improve the sound most likely. Better is such a subjective term, so I'll say that it will be louder and the tone will be more even.
Two more things you might want to try at some point would be to add a high impedance JFET buffer to the input, and with that in place you also have the option of adding a tone stack. ;) Options abound!
Take care,
-Peter
Peter, I want to add a buffer, but it looks like the AMZ site has exceeded its bandwidth for the month and I want to build this puppy NOW! :) Can you point me to a JFET Buffer schem?
Also, I've downloaded Duncan's tone stack calculator. Which Fender version should I use? He says that the E series approximates the older Fender tone stack, but I'm not sure what exactly this means? Older as in tweed era? Also, what is z source, the input?
Would this work?
http://www.till.com/articles/GuitarPreamp/index.html
He says that this sounds much like the first stage of a Fender preamp, which is a sound I wouldn't mind emulating.
Luckily JFET buffers are fairly generic circuits so you look around a little, you can find something to adapt. Almost any high impedance audio stage will work. You don't really need any gain, just buffering. A google search for "JFET buffer" turn up.... wow.... imagine that.... Runoffgroove. :D
http://www.runoffgroove.com/grace.html
I've never used the E series Fender tone controls so I can't tell you much about them. I think they're from REALLY old amps. I just use the "standard" stack (but I like different cap choices). This is dangerously close to a design I've been looking at producing for the least three years so I'm going to get vague from this point on. Sorry. It's funny how DIY minds think alike.
The Zsrc figure is the output impedance of the buffer stage. The JFET buffer is considerably lower impedance than a 12AX7 so if you're running simulations, try a value like 5 to 10K there, but don't worry about it too much. The cap choices are more important. Also important is the value of the slope resistor. Whenever I've made this a pot (33K in series with 100K pot) on an amp upgrade, the owner has been very happy with the versatility it gives. I would suggest trying a thumbwheel trimpot there and set it to the place your ears like the best. :)
Good luck! 8)
-Peter
I just saw your next post.... and yes, that would be an excellent choice. :D
Take care,
-Peter
Are you talking about the cap choices you mentioned here http://www.diystompboxes.com/sboxforum/viewtopic.php?t=1030?
Would the schem look something like this?
(http://www.mycgiserver.com/~javacody/images/tonestack.gif)
Also, would the slope resistor be R1 in the above schem? What does it do?
Thanks for all of your help on this Peter. I've learned a ton already. Now if I could just get a tiny reverb of some kind on this, I'd be in heaven!
Almost forgot, in the post linked above, you mention a fixed resistor for the mid, and that Fender usually sets their mid at 7 in amps with only treble and bass tone controls. Am I correct in thinking that this would be approximately a 7k resistor instead of a 10k pot? Doh! Just read you suggested 6.8k, sorry. :)
I have to admit, I'm very excited about putting all of this together. The lil gem is already a smokin amp, but if I can make it more Fenderesque, I will be very, very pleased. It's almost more fun tweaking the thing that it is playing it. Almost. :wink:
Quote from: javacodyAre you talking about the cap choices you mentioned here http://www.diystompboxes.com/sboxforum/viewtopic.php?t=1030?
Would the schem look something like this?
Good sleuthing. :) That's one of my favorite combos given my very limited range of guitars and pickups. That's also a VERY common hotrod choice. It's more Marshally with a little less "bite".
Let your ears guide you there.... You might also want to consider a toggle switch in there to select a couple different choices. There isn't one best value for all occasions. A DPDT to switch between 33K/.022uF/.022uF and 100K/.1uF/.047uF is a pretty powerful feature.
Quote from: javacodyAlso, would the slope resistor be R1 in the above schem? What does it do?
Yes R1 is slope. The slope resistor will vary the amount of signal that gets bled off by the low-cut part of the stack. This is a balance with the treble cap and the ultimate effect is to vary the frequency of the mid scoop. Try the simulation with 33K in there and with controls set to 5/0/5, hit the snapshot button, and then switch R1 to 133K. You'll see the range of frequencies available. Note what it does to increase bass too.
Another mod to the stack is the use of a different bass pot. Try 500K and 1M also. So many options. :) Only your speaker and ears will tell you the right value.
Quote from: javacodyThanks for all of your help on this Peter. I've learned a ton already. Now if I could just get a tiny reverb of some kind on this, I'd be in heaven!
Glad to be of service. I believe that passing knowledge along is a very important thing. The more we all do it, the smarter we all become.
Now on the reverb.... Take a look at the 8AB2A1B tank under "
amp parts-->reverb tanks-->Accutronics" at this place: http://www.tubesandmore.com/
If you build a second 386 amp with lower gain (no fuzz) and feed it with the wiper from a 100K pot (dwell) attached to the buffer output through a cap like.... oh say... .022uF, and then use that to drive the reverb tank, you can use another JFET to amplify the tank output and recombine the signal at the input to your main 386. Sorry that's such a general description.
Check out the "Stage Center Reverb (http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=142&op=page&SubMenu=)" at GGG for another way to do it. You'll need a different tank with that circuit, but you could always combine the 386 driver and tank I spoke of above with the recovery and mixing side of the Stage Center. That will do you great. :D
Happy building!
-Peter
Wow Peter. This just turned into a step by step guide on building a tiny, full featured amp! I stayed up until the wee hours of the morning researching all the stuff you've been talking about. I've got enough information now to keep me busy for at least a month (if I totally ignore my family :D ), probably longer.
I think I'm going to build in the order that we discussed things. I've got the basic amp done, next comes the buffer.
After I have that working, I'm going to bread board the tone stack so I can try out all of the things you talked about, and then last (probably sometime in January!) I'm going to figure out how to make that reverb work. Any idea what the dimensions are on the particular tank you pointed me to?
When I have it all working, I'm going to sit down and draw up my first ever layout and etch my first board.
Peter, would a gain of 20 be ok for the reverb? I could just leave pins 1 and 8 open so that they use the lm386's internal resistor.
Wow, that GGG link has some good links off of it, namely, this one which decodes the Accutronics reverb tank codes. http://ampwares.com/reverb_codes.htm
I also found out just about everything I'm going to need to know about building my own reverb at http://members.tripod.com/~roymal/reverb.htm
Quote from: javacodyPeter, would a gain of 20 be ok for the reverb? I could just leave pins 1 and 8 open so that they use the lm386's internal resistor.
:) Yes.
The easiest way to test is to just connect your speaker to the tank send. Adjust the dwell pot so that you only hear distortion when you really bang on the strings. That should do a decent job. The final levels will depend on how much boost you have in the JFET stage and much output your pickups have.
Quote from: javacodyAny idea what the dimensions are on the particular tank you pointed me to?
....
grabbing a tape measure.... They're
9 1/4 x 3 3/8 x 1 1/2I'm glad you found that reverb article. That's a great one. If you use an opamp to drive the reverb, keep in mind that you will need one with an isolated send jack. Also The only chip I would use in that case is the NE5532 as it's quite happy driving low loads with lots of current. It's my favorite all-around opamp too.
Once you're done, I hope you can post some pictures. That's going be a really smoking little amp. Just wait until you plug it into a 4 x 12 cabinet. :D
Take care,
-Peter
Ok, the AMZ site is back online and I have a buffer question.
In the following:
Quote(http://www.muzique.com/images/buff1.gif)Before we see the circuits let us look at a circuit fragment that may be required for some of the buffer variations. As shown to the left, the resistor/capacitor network provides a reference voltage that may be used to bias the transistor or opamp into the best operating range. The point marked "Vr" is connected to the point also notated as "Vr" on the buffer schematic. If there is a reference voltage already established in a circuit to which you are planning to add a buffered input, the existing Vr can be tapped and used for the buffer's reference.
Is this neccessary? What are the drawbacks of leaving this out? Will putting it in change the sound of the buffer at all?
If you use an opamp for buffering and you run it from a single 9V battery (a single-ended supply), you need a way to 'park' the opamp in the middle of it's operating range. That's what that circuit does.
If you run opamps from a bipolar (+9 / -9) supply, the opamp inputs that don't have signal applied to them are usually tied to ground (which sits 1/2 way between the + and - power), either directly or through a resistor. That circuit simulates the bipolar supply ground when you use a single-ended supply which doesn't have that reference point that sits 1/2 way between the other two.
If you run opamps from a single ended supply and you leave that Vref or Vbias generator off, the opamp won't pass any signal at all.
Take care,
-Peter
Thanks Pete. Picked up the rest of the parts for the buffer and the tone stack at the shack today. They didn't have any 330pf caps, so I had to buy an $8 assortment. Oh well. I'll probably never need to buy small value ceramic caps again. :D
I also am taking your advice about the powersupply cap. Hey, I sprung a buck for a 470 uF cap. :wink:
Last but not least, I found this http://www.diystompboxes.com/sboxforum/viewtopic.php?t=11576&highlight=mpf102 when searching the forum on MPF102 JFETS. Very interesting, so I picked up a 10K trimpot for my drain resistor. I also printed out a bunch of RunoffGroove's small PC Board templates so I can play with a layout. I will post it when I get done. I'm doing the buffer tonight and the tone stack tomorrow night.
I figured to keep things simple, I would use a modular layout, with one board each for buffer, tone stack, amp, and reverb. That way I can tweak any one of them without hurting the others. I need some kind of easy connectors to connect the modules. Any suggestions?
Cool. 8)
That was a good post about biasing. I would advise adding a 1K in series with the trimpot so you don't accidentially let the blue smoke out of the JFET.
Since these are circuits that you're going to have connected in a case, I would recommend just soldering them together. Modular construction is a really good idea, but the blocks really have to be tweaked to work in conjunction so they're not as modular as something like a Moog. There aren't as many easy connector options when you're talking about small spaces and many of the smaller connectors don't have convenient 0.1" pin spacing. It might be easier to just little wire nuts while you tweak the circuit. They work great.
For the tone stack, I just build it directly on the pots. It makes for a smaller and cleaner build and in high gain circuits, it makes for a cleaner signal environment since you have less wire running around. Just my $.02. :)
One more thing to test out is a diffuser over the speaker. Weber sells nice ones, but you can make something fairly effective by taking an old CD and suspending it over the center of the cone. Give it a try. (they also work well as coasters)
Take care,
-Peter
OK, I'm officially frustrated. All I'm getting out of my buffer is a constant tone, a very, very annoying constant tone. What am I doing wrong?
I used the schem here:
http://www.till.com/articles/GuitarPreamp/
I left out C2 because it says it isn't needed, and I only had a 10uF for C1, so that's what I used. Help!! :oops:
Is the schematic in the link above wrong perchance? I ask, because I switched C1 from the drain to the source, and viola, it worked. This is how Jack Orman has all of his buffers. Is the schem in the link known to be wrong?
I'll bet it's the lack of C2 in there causing the oscillation.
As the 386 draws current, the power rail gets pulled on. If you don't have a well regulated source of power for the buffer, any noise on the power rail will show up in the buffer output. If that signal is out of phase, you have just created a really fast version of the circuit that runs a Fender tube tremolo. The other name for it is a "phase shift oscillator".
If you switch the resistor back to the drain and then switch what you have applied to pins 2 and 3 of the 386, it should also go away. That will reverse the phase just like swapping the drain and source.
In either case, I would not only add C2, but you might also want to put a resistor in there between the 386 and the buffer. Try something like 470 ohms. You will want to re-bias things to make the drain voltage roughly 1/2 of the voltage you see across C2, or a hair higher. You might even want to make C2 a bit larger like 47uF or even 100uF. You need solid power in there or it's going to whine at you.
If you look at tube amp schematics, you'll see that they have several high voltage supply outputs, with the most filtered one feeding the first tube. You need to emulate that topology.
What resistor values did you finally end up using?
not too sure about any mods or anything for a smokey
but i can say the biggest improvement would be to upgrade the speaker
i have one and with the speaker it comes with it basically shounds shit, all buzzy and nasty
so i tried plugging it into the 12 inch speaker in my marshall and WOW what a difference, it turned it from a piece of crap into something pretty nice and very useable, next time i go to a rehersal place im gonna try it out with a 4x12 to see what it sounds like
David
Pete, I used a 3.3M to set up my input impedance to the gate, went with the 2.2K on the source and I'm not sure what the resistance is to the drain, but I have to turn it almost off for it not to howl (I'm guessing 1 to 2 k?)
What does altering C1 do to me? I went with a 10uF instead of 4.7uF.
So. let me spit back out the instructions you just gave me so I can see if I understand. First, I'm going to add back in C2 as a 100uF cap, next, I'm going to add a 470 ohm resistor between the buffer and the amp. Last, I'm going to tweak the trimmer on the drain to rebias the FET?
Does this sound right Peter? Once I do all of this, can I put C1 back on the drain without changing my input to pin 3 on the 386? Are there any ill effects of switching the phase like I unknowingly did?
I have to admit, even with the buffer working like it is, I have my sparkle back, which is a good thing.
You have the sequence correct. :) The best way to bias when you know you're close on the meter is to switch to ears. Just to make sure on the 470 ohm... that's between the + side of C2 and pin 6 of the 386.
With stable power, you won't get oscillation, but with the circuit you have now, it's inherently resistant to oscillation which is a good thing for sure. 8)
The best place for C1 will depend on the resistor values you end up with. One side will have more gain than the other. I would pick the side you want based on boost level, and if that means C1 on the drain, try swapping pins 2 and 3 for an inherently (more) stable circuit. No difference with phase here. Either is fine.
The larger value of C1 will allow for more bass. If you have too much bass, reduce the value by ear. Take it too far down and you have a treble booster. You could use a 3 position toggle or a rotary to switch multiple caps to taste if you wanted.... more and more options. ;) I like the 3 position toggle with a small cap for C1 and the option to select either of two larger ones. That's a great cheat if you don't want a full tone stack.
-Peter
Peter, I've thrown together a schem of what we've talked about so far. Does it look right to you?
(http://mediaservice.photoisland.com/auction/Dec/20031223619833137653689.jpg)
The 100uF cap from 9v to ground in the Tillman stage is redundant since you've already got one in the Little Gem. Also, the 10uF output cap of the buffer can be omitted.
I'm not sure how the 51k resistor will affect the circuit. The Tillman stage has a bit of boost and does sound good, but I'd use the Dr. Quack input buffer instead (with both 1M resistors increased to 2M2), to keep the level at unity and for increased headroom before the amp.
Here's the AMZ buffers article, the Dr. Quack one is the third from the top.
http://www.muzique.com/lab/buffers.htm
Or I could probably just duplicate the Little Gem MKII buffer, right?
Why omit the output cap of the buffer?
Quote from: javacodyOr I could probably just duplicate the Little Gem MKII buffer, right?
Why omit the output cap of the buffer?
Yes, the MkII buffer would work fine. The Tillman stage 10uF output cap is directly followed by the .01uF input cap of the amp. Both caps are present in their respective circuits to block DC voltage (and shape the frequency response in the Little Gem). You don't need both, so just use the .01 cap so the frequency respose is the same.
If you're using a tone stack, that will also provide DC bocking. If you're not, the 386 has in imput impedance of roughly 50K to ground. With a value that high and your other resistors being just a couple K it won't really interfere much. It will just adjust the bias voltage a tiny bit.
Two schools of thought here....
The Zen side says unnecessary component and electrolytics eat tone. I must agree.
The academic side says modular building blocks and you only want to pass AC between those blocks. Nothing wrong there either. This also gives you the ability to shape the tone at one more place. It has good and bad sides.
As Brian mentioned, you don't really need to boost what the 386 is seeing unless you want mega distortion, but adding a tone stack will eat a LOT of the signal. You may want it boosted in that case.
Yes, and if you have the 51K in there from the Tillman schematic, you can omit that for sure.
Ahhhh. now your schematic has loaded. :)
Yes.... remove that 51K and without the stack in there you should have either the .01uF or the 10uF, but not both. Play with that cap to shape the tone. I didn't know you had the .01uF in there. Also remove the connection from the 9V to the buffer's 100uF cap. You want the power to be sourced from the 386 pin 6 through the 470 ohm for decoupling it. I would still keep C2, but with the non-inverting buffer the Zen camp says unnecessary. The whine without it and with the inverting buffer shouldbe evidence enough that additional filtering is necessary.
Take care,
-Peter
The tone stack is going in next. I figured as a beginner, it was easier to get each seperate section up and working before moving on to the next piece. Once I get all the bugs worked out and have finalized my components, then I will do a layout and etch a board.
Given the fact that the tone stack IS going in, what components should I change and what would stay the same? I'm assuming I should leave the 10uF cap.
Here is the schem as it stands with Peter's latest recomendations.
buffered_gem_v1.1.gif (http://www.freewebs.com/qrp/javacody1.html)
For a booster to amp stage design, the only change I would make would make would be to eliminate one of the two coupling caps connecting the two sections.
For now, drop the 10uF and play with the value of the .01uF. Try using the amp some with .01 in there, then add that 10uF back in parallel with the .01uF and play that for a while. Also try values like 0.1uF and 1uF. That's four orders of magnitude for comparison.
When you add the tone stack, add it in place of the coupling cap. The purpose of the cap is mainly to stop DC from flowing out of the buffer and into the 386. The stack will perform the same function. Without the stack in place, it's advantageous to use a smaller value cap to limit the bass. Think of it as a bass control turned down with the treble all the way up.
At this stage it's a good idea to play with booster options too. Try out as many circuit configurations as you have parts for.
Take care,
-Peter
One more note: If you want some free and easy to use schematic drawing software, check out http://www.expresspcb.com/
They also make great boards. ;)
-Peter
QuoteAt this stage it's a good idea to play with booster options too. Try out as many circuit configurations as you have parts for.
I'm not sure what you mean by that Peter? Honestly, wired as it is, it sounds very good. It sounds very Fendery. I'm going to remove the 10uF, add the 100 uF, the 470 ohm resistor, and change the power filter Cap on the amp to 470 uF and see how it sounds.
I was talking about booster topologies mainly....
With the circuit you posted, you have the option of shorting the trimpot out and then adding bias via the gate. If you then wire a 100K pot across C2 with the wiper connected to a 10uF cap to ground as well as to the now grounded end of the grid (whooops...), er uh, gate load resistor (removing the ground connection to it), then you have essentially the Dr. Quack buffer stage.
It's all about what sounds best to you. There is no single "right" way. It's a lot like cooking.... let's see.... what do I have in the fridge.... a pinch of this and a dash of that.... It's more about the method rather than an exact recipe.
Happy cooking! :D
-Peter
Cool. Thanks Peter. By the way, I just finished making the changes you suggested. Here is what it looks like currently:
(http://www.mycgiserver.com/~javacody/images/bufferedgem_v1.2.gif)
I have to say that it sounds even better now! It reminds me a lot of my old '73 SF champ, except with a little more sparkle. Even without a tone stack, it is an awesome little amp. Will I stop here? Heck no! Well, for tonight, yes. :lol: It's time to sit back and enjoy the fruits of my labor (and your help of course) and play a little while I watch the snow fall outside. Peter, if I ever have another kid, I'll name him after you. Thanks for your help. 8)
By the way, ExpressSCH is awesome! It took me ten minutes (being brand new to the program) to throw that schem together! That program should be listed in the FAQ somewhere!
Congratulations! 8)
That looks great. I hope you can post some pics of your build.
Once you play with the tone stack in there, you should also switch back to the cap for a little and compare. You might want an a/b switch to bypass the mid-cut inherent in the stack for a different tone.
Take care,
-Peter
No pics of the build until its all the way done. I'm only half way there. :P
Here is what I think it will look like with the tonestack, please let me know if I'm correct:
(http://www.mycgiserver.com/~javacody/images/bufferedgem_tonestack_v1.1.gif)
I'm not sure if the way I have the bottom connection of the tone stack looping around into the wiper of the treble pot is correct.
Three changes....
(1) You can get rid of the 1M R(load) as it's not needed. In the TSC, that value represents the load from the volume control. You could always add one by making R(load) a pot and connecting the wiper to the 386, but it isn't needed. You already have one on the guitar. In the simulation stage I should have said something about making R(load) 50K which is the input impedance of the 386. That doesn't really change any frequencies, but it will reduce the level. You already have plenty so no worries there.
(2) Eliminate the wire from the junction of the slope resistor and treble cap to the bottom of the mid pot
(3) Connect the bottom of the mid pot to ground.
Take care,
-Peter
Does this look right? I'm having a hard time picturing how I'm going to wire the tone stack. Everthing seems so interconnected.
bufferedgem_tonestack_v1.2.gif (http://www.freewebs.com/qrp/javacody2.html)
I need some help. I'm becoming obsessed with this. :shock:
Quote from: javacodyI need some help. I'm becoming obsessed with this. :shock:
uh oh.... you hit the threshold. Just remember to resist treatment. :D
Quote from: javacodyDoes this look right? I'm having a hard time picturing how I'm going to wire the tone stack. Everything seems so interconnected.:
That looks perfect! 8) Now that you have it drawn out... just drill the holes, set the pots in place, and use the component leads to connect the dots. Each time you install a part, trace over it with a highlighter.
Start with connecting the wipers to the ends of the pots for the mid and bass, then connect the ends of the pots together, then install the two .022uF caps and twist the opposite ends, then install the pot end of the 330pF, then attack the other end of the 330pF to the 100K, then attach the other end of that to the .022s, then add the input wire and the output wire and ground wire. Done. It's really easy to leave the connections floating in air and just bend the part leads so they're firm as a 3D sculpture, but you can also add a two position terminal strip for the connections to the 100K. I think the terminal strip is more trouble than it's worth, but either way is fine.
Draw it on paper first and then dig in. :D
Take care (and don't neglect the family too much),
-Peter
PS: If you have a kid that's into guitar, consider building one of these things with them. :)
Peter,
I was thinking about classic Fender amps, and don't they have at least two or more preamp stages? Would adding more buffers simulate this more closely?
I was looking at v1 on the Sovtek Mig50 ( supposedly a Bassman clone) and noticed some similarities to this buffer.
http://heysam.com/schem/sovmig50.pdf
I noticed it has 3 preamp tubes, not sure what exactly v1 is for (inital buffering?), but v2 looks like it is driving the tone stack? And what does v3 do? Any good sites that will explain this stuff to me in fairly plain language?
OK, well taking what I've learned from this process, it looks like v1 is split between the two inputs and there is a pre gain pot attached to them? This is then routed through each half of v2, which then goes to the tone stack, I'm not sure I understand this? Would it be worthwhile for me to duplicate this in my little amp?
That's a pretty classic design; now referred to as a Marshall. ;)
Look at the different cathode resistor and cap differences between the two input stages. That's called voicing. The top one will have more bass because of the larger cap. Each one has the tube biased a little differently.
The third stage the signal hits is called a cathode follower. It drops the impedance to drive the stack. A 12AX7 has a much high impedance than your JFET setup.
The third tube is a phase inverter which makes two signals to drive the output tubes. One is in phase with the input and the other is inverted. One signal is used to push while the other pulls. This is all taken care of internally by the 386.
Sorry, I don't have any sites off the top of my head and I'm in a bit of a rush now. Try a google search, I know there's a lot of into out there.
If you want to simulate the tube amp more closely, you'll also need to loose the 386 and that gets into a whole new territory. ;)
Take care,
-Peter
Wow Peter. I could pick your brain on this stuff for days (I already have). I'll do some searching. Could you give me a little hint about losing the 386? Or is that what you were talking about having in your head for the last 3 years?
Your description is correct.
If you want monster distortion you could build the same sort of thing, adding two additional JFET stages, but there is no need. That's a very different amp all together.
All (most) of your gain is provided by the 386. The JFET is there to give you a high impedance input. In the tube amp there are two high impedance gain stages followed by an impedance dropping cathode follower and then the stack, phase inverter and output stage. Totally different animal with different parts and needs.
Take care,
-Peter
Hey, nice looking ckt. I just put it together and it sounds really good. I was wondering whether anyone has put together the MkII little gem w/ 2 lm386's. I recently tried it (to get some more power) and it sounded awfull. Anyone else try it? I might put together a lm286 ckt w/ a tda2030a (pulled it out of my old ss amp) after it to get some more power...
Quote from: javacodyWow Peter. I could pick your brain on this stuff for days (I already have). I'll do some searching. Could you give me a little hint about losing the 386? Or is that what you were talking about having in your head for the last 3 years?
The neat thing about the 386 is that it takes care of a building block that gets rather complex and painful to do with individual parts. Tubes have the inherent advantage of biasing much easier. I won't even touch discrete transistor driver stages for the most part. ;) The commercial stuff I have is based on the JRC386BD and other "single chip" power amplifiers.
If you want to build the whole thing from scratch, it starts to look like this:
(http://www.diyaudio.com/forums/attachment.php?s=ea07556a19056e658de562106626781c&postid=270693)
Full size image here: http://www.lcaudio.dk/ulcamp1.gif
More stuff in this zoo: http://www.diyaudio.com/index.php
I'm almost out of my territory at that point. I just don't have the time to re-invent the wheel when the integrated chips do a better job than I could, plus they generally add things like thermal shutdown.
There are lots and lots of power amp chips around. Unfortunately the Philips site is very poorly indexed for searching, but try this link:
http://www.semiconductors.philips.com/markets/mms/products/analog/product_catalog/signal/index.html
I hope that helps,
-Peter
Sorry, I got ahead of myself there. I'm really just trying to understand the principles behind each of the three components that I'm working on thusfar.
Also, wired up the tone stack, but I think something is wrong. Only the treble is affecting the tone at all. It seems to act as a volume/treble bleed. Any ideas what I did wrong? I'm positive I followed the schem above. Did I make a mistake with the schem? Is the signal only supposed to go to the amp from the wiper of the treble pot?
Found the solution, see http://www.diystompboxes.com/sboxforum/viewtopic.php?p=93639#93639