Corrected Schematic attached!

Sorry folks, I was looking at the schematic for this box and realized that R2 was NOT 2.2 ohms, but rather was 2.2 Kohms.  I also forgot to suppy the value for the LED resistor.

I have attached a corrected schematic to this post.

Joe, lets see if I'm right:

The 22 uF capacitor on the wiper of R1 allows you to vary the gain of the circuit while maintaining a stable DC bias.  R3 controls the overall volume, R1 the level of distortion.

(did I pass the quiz?)

No problem with the questions!

To address each:
With more (higher) capacitance, you get less ripple on the supply - by using the 25VCT transformer as I did, the voltage was kept at about +/-15 volts maximum.  It does drop a bit under full load, but not enough to cause problems.  The OPA2134s and the LM1875As have excellent supply ripple rejection characteristics - depending upon those for lack of "hummm" in the output :tu:

Regarding your computer power supply - you really need to have a balanced current capability (same current capability on the "+" side as on the "-" side) in your supply, or you run the very real risk of DC offset - which can be real bad on your sound - not to mention DC through speakers is never any fun. (for the speaker)

The main purpose of the voltage regulators are two fold (in audio amps).
A) maintain a constant voltage with a varying load
   To accomplish this, the DC voltage applied to the input of the regulator is considerably higher than the expected output voltage - thus when the voltage is extra high due to light load, or when the input voltage "sags" due to loading the secondary of the transformer, the circuit doing the work "sees" that same constant voltage.
    I used a transformer with an output rated at 25.2 VCT and 2A output.  Under minimal (almost no load) conditions the output voltage is approximately +/- 15VDC.  Under full load, this drops to about +/- 13 VDC.  I can live with that in this amplifier.

B) to keep HUM out of the audio output
    As noted, the chips here have a very good power supply rejection ratio.  That in combination with the large capacitance across each leg has kept hum out of this unit.  If I was using my "standard" discrete JFET input, I wouldn't be able to get away without a regulator.

Second question:
No, I don't recommend the "high impedance booster" in front.  By making the mods I suggested in my earlier post, you have PLENTY of gain.

The input impedance of this amplifier is currently approximately 2.8 megohm.  To raise it, simply change the value of R1 & R2.  For example, to make the input impedance approximately 5 Megohm, substitute 10 megohm resistors for the 5.6 megohm resistor value of R1 & R2.  If you are jacking the input impedance up, I do recommend using an input jack that shorts when the guitar cord is removed - especially if you are installing an Aux input (noise is a problem otherwise).

final note - for greater headphone volume, you could eliminate R18 - might destroy eardrums, but it WILL be louder.  You also can VERY safely substitute an NE5532 for the headphone chip.  This headphone out design is VERY clean, you could use it for a variable level out to interface this to a house system.  You need a FET chip for the main input -  Opa2134 or perhaps a TLO72 - it keeps the input impedance manageable as noted previously.

If I was going to install an Aux input, I would decidedly put it at the "top" of VR1/VR2 (which are now tied together since R19 and R20 are gone) - I'd run it through at least a 100 ohm resistor though, (on each leg if running in a stereo to mono converter) and have it NOT short when there is no Aux hooked up.

As I mentioned to Teemuk elsewhere, I am a "belt AND suspenders" type of guy when I design for myself - thus this is a pretty durable amplifier design.   Please note that maximum output power is about 10 watts per speaker in this configuration - due to supply voltages.  (be sure and check out the data sheet on the LM1875, you'll see similarities between it and this design).

Good luck, and keep us posted! :tu:

(You are most welcome- sorry I didn't get back sooner, been out of town!)
I agree with Joe, the Bigger the heatsink, the better!  The LM1875 will shut down on you if it gets too hot (had it happen in an earlier version of this amp - bigger heatsink, problem went away).

Couple of notes from the designer:
There was a lot of "Cut and Try" with this design, being one of my first.  If I were to build it again I'd make the following changes:

Eliminate C4 (really unnecessary), and remove and replace with a short the
R19/R20 combination - more gain to speakers.  I would reduce R10 to about 22 Kohms. 

In the power supply, I'd use 2 4700 uF capacitors in parallel rather than one 6800 uF capacitor on each side of the bipolar supply.

I would also replace the headphone amp section based upon the opa2134 with one based upon the NE5532 - as seen in most of the other schematics I posted - keeping headphone amp gain at 10.

Attached is an improvement I made in the B.E.M.   Moving the injection point for a tape player/CD/external audio seems to work better in the new location.

Quote from: sonate on February 16, 2008, 03:52:01 AM
For a matter of debate...I have been looking at these 'Torres kits' ...Would this be better than buying my caps ect from my local electronics store.

Looks to me like Torres Engineering has done the research, shopping and ordering for you.  It may cost more overall $$ than getting the parts individually from a shop, however, these look like exceptional quality parts as well as known to the the right parts.  I'd have no problem using one of their kits to refurb an amp - if the $$ were in my budget! :tu:

Query, Master Teemuk-
I have heard that speakers perform "best" (maximum sound pressure level) when driven in the range of 70% to 90% of their maximum rated average power levels.  I don't know much about speakers, can you address this thought?  thanks! :tu:

Quote from: teemuk on February 14, 2008, 01:35:21 PM
By the way, parts R4, R15, R14 and C15 seem pretty useless.

I agree, the circuit will probably work fine without these components, however, when I create items for myself, I typically overdesign - because I am a "belt AND suspenders" kind of a guy.    And, because I am not designing this for commercial builds, and am not that concerned with either parts count or cost of parts.

my reasoning for each:
R4 & R15 - to GUARENTEE that the current will remain limited in the FETS - I just get nervous hooking a FET or bipolar directly to a rail.
R14 - to limit the input impedance of this stage to 5 Megohm - because I like it that way.
C15 - to GUARANTEE DC isolation of the output buffer - in the event of C3 being shorted.

Thus, my friends, don't be surprised to see some components in these schematics that may not be absolutely necessary for function, but are there because they help my sense of security with the design!

(Yes sir, that is just a bit neurotic :duh, but hey - I never claimed to be fully sane!)

 :tu: Short and sweet speaker test:

1) remove speaker from circuit (desolder one side, unplug from cabinet .. whatever, just so you can "get" to both terminals electrically without having it interface with other portions of the amp)

2) Get a DMM, put it onto ohms scale of 100 ohms or less.

3) Drop leads across speaker terminals, listen for slight "pop" as you connect and disconnect dmm onto terminals - and note what ohms reading it is.

The "pop" tells you that the speaker is functional, the ohms reading gives you an idea of the speaker impedance (approx 4 ohms, 6 ohms, 8 ohms, 16 ohms ... etc).

Physically examine the speaker cone, if necessary, repair any small tears (small drop of glue with tissue seems to work well for me).  (or use the info gained above to replace with a new one).

Ive attached a zip file with the schematic and layout of the 5F and AA764 versions of the Champ.  You can find all of the Schematics and layouts for the many generations of Fender Champ from various websites. :tu:

Just wanted to report that this little box works absolutely fantastic for a piezo pickup (a DYI piezo buzzer hooked up as a pickup) on my acoustic nylon strung guitar.

The 5x boost is sufficient that I have excellent signal to noise ratio, and can even run the signal out of the booster into my chorus pedal for an incredibly rich nylon tone!
:tu:

Although not a solid state amp, the Champ is a CLASSIC!

First and foremost - "RECAP" the unit -- make note of the value and voltage rating of all the capacitors, go to a house like "Mouser" for example, and buy new ones (they will be physically smaller than what is in there, but will be fine).  You may not be able to find the exact capacitance rating, I personally would get as close as possible, erring on the high side of the rated capacitance value.  If you can't find the voltage rating on the old caps, go to the next HIGHER voltage rating cap, unless you want a bunch of explosions when you turn the amp on  :tu:   

1) Speaker choice can make a HUGE difference in the sound, from the original to a custom sound you like.  If you want the original sound, get an updated version of the original speaker, other than that - just match the impedance, and make certain the speaker can handle 10 watts (The standard champ puts out 5 watts max, a 10 watt speaker will NOT blow out!)

2) Best brand of tubes -- that's a toughie!  For example, I know folks that swear by Tung Sol, and others that swear AT Tung Sol.  I also know folks that LOVE Sovtek, and others who wouldn't use Sovtek on a bet!  I personally would recommend Tung-Sol for new, RCA or Sylvania NOS for your unit.  Tube choice is like any other "taste" issue.

Well, ya gots my opinion! (That and a few bucks will get you coffee at Starbucks!)

FYI - if you see the capacitor says (.02 MF), that means 0.02 microfarad - old nomencature.

Welcome to the wonderful world of the LM386!  I am very impressed by this little chip!

I usually use it for a headphone amplifier, low power amplifier or for driving low impedance loads. 

I did find the noise factor of early LM386 units to be a bit much for preamps, however, at the time I was dealing with microphone level signals, not electric guitar signals.  (for preamps I use either a FET circuit or an NE5532)

I got my inspirations from the wonderful folks over at runoffgroove.com.  I did check out your blog, and looks great

Take a look over in the "schematics" section of the forum for some interesting uses of the LM386 as a guitar amp.   :tu:

Hello All!

I'm looking for a schematic for a Sunn SR8520P powered mixer.  Does anyone out there have one?  I do have a pdf of the owner's manual, but still need a schematic.  The SR-8520PD is available through Fender for only $5.00, but I have a pdf of the owner's manual for that one as well, and it seems to be a different animal.  (I am quite willing to share that pdf too!)

For reference, I am attaching pictures of the front and rear of one of these boxes.

Thanks for any help!

Quote from: luke on January 15, 2008, 01:42:49 PM
Hi I'm new to here. I have a 10" dual cone subwoofer I'd like to make an amp for my guitar with. I wish to use the little gem circuit with tone, treble and reverb added. Should I just connect the little gem circuit and the reverb and treble effects circuits together or should I connect their power source wires to the SPST switch?

Looking for a little more info!

First, your subwoofer - is it a very high powered one?  If so, the 1 Watt max of the Little Gem or the Ruby may be insufficient to provide good sound level out of the speaker.

Given that it gives decent sound pressure with 1 Watt maximum input, to continue:

The little gem is an excellent little amplifier, but to add things to it I suggest you go with the "Ruby".  Then, you have a consistent impedance your guitar looks into, and can parallel your tone potentiometer (and cap) with the volume potentiometer.

Attached is a Idea from what information I can glean from your description - not certain how practical putting a reverb stompbox permanently installed.  To be aware that building a digital reverb is quite complex, you can build a spring reverb using Craig Anderton's circuit.  Check the Schematics section.

Anyway, here is my first run at an idea to help you out! 

I use a stand alone reverb in my own setup.

Good day!

I have attached a schematic and pics of a high impedance input (5Mohm) booster circuit.  If I want to play my electric and not worry about imput impedance of the amp, this does the job as a preamp.  As you can see, I have the knob marked at the x1 and x5 positions. 

Enjoy!