Cool Gadgets - built for personal needs

[n9voc]

Howdy!
I like to use my main Solid State amplifier (posted under "outboard amp") and a tube type Fender "Champ" clone I built at the same time.  To facilitate this, I built a splitter (schematic attached).

The power transformer came out of a junked HP 310.  It was mostly from my junk box, and is indeed a bit over designed.  My idea was to have the guitar signal split without any loss o f signal to either amplifier, or loading down the guitar.  Thus the input impedance of 1.35 Megohm, and the use of the opa2134 FET input opamp.  I had some 2134's, but I am certain it would work as well with a TLO72. :tu:

The power supply is overbuilt, but again, I used parts I had on hand.  The use of the split power supply allowed the use of noninverting input, without concerns of trace impedance variations used by an "artificial ground".  The positive and negative regulated power supply is rock solid, and dead quiet.  In fact, the entire mixer is "overbuilt". (But then again, I started by opening my junkbox, and seeing what there was available for use in making my mixer.)

The upshot is that the combined sound coming out of both my amps at the same time is just simply AWSOME!   :tu:

The splitter circuit could be used for virtually any audio circuit with a maximum signal level of about 2 volts p/p.  The power supply voltages are the limitation on the maximum signal used.  Respectfully submitted for your edification!

The second item is the "Transmogrifier".  It is a passive mixer cable assembly that I and many others locally use to play our MP3 players through our amplifiers.  The resistors provide a channel isolation, preventing possible shorts from one channel to the other.  Just a combiner cable without the resistors may work fine, but with the "transmogrifier" the system is virtually "bulletproof" to enjoy my music through my existing mono amplifers.

The input impedance of most of my amplifiers ranges from 50,000ohms to over 1 million ohms.  The signal lost across 91 ohms is insignificant compared to the voltage present at the amplifier input.

Just thought I'd share and start this "gadget thread".

I'm working on a guitar mixer, allowing two guitars to be played through one amplifer -- without loading either guitar down -- more on that later!

Anybody out there got some gadgets they use that answer their little needs like these do for mine?

[J M Fahey]

Hi n9voc, congratulations !!
Excellent and very useful "gadgets".
I encourage others to build some version of these, as well to post their own creations.  :tu:

[phatt]

 Just check the data sheets on those Reg's.
I believe it is unwise to use BIG caps *After Regulation*

The data sheets I've read suggest only 10 uF Max.

2meg on the input might be prone to noise?

Might be wise to label the reg as *7815 and 7915* as the inexperienced here might try to build it using incorrect parts.

Other than that yep good ideas :tu:
Phil.

[n9voc]

To Phatt -

I've tried twice to post with an copy of the schematic attached, and get server errors --
If my other two posts make it, please forgive this third post with the same information.

The regulators are NOT LM7815 and LM7915,  they are LM7805 and LM7905 in the power supply -- not an error or a typo.  The tabs of the regulators are ground isolated, and the R8/R7 and R9/R10 ratio brings the output voltage up to +/- 15 volts as shown.  Put a LM7815 and LM7915 in the circuit, it will NOT work.

The diodes D5 and D6 prevent the possibility of there being a greater voltage at the output of the regulator than at the input of the regulator, this is the reason for the current practice specificaiton of 10 uF on the output - to prevent that possibility.  IF these "bypass diodes" are in place, capacitance at output of regulation only reduces any ripple not rejected by the regulator, and the capacitance value has NO maximum. IMHO - the less ripple the audio circuits have to reject, the better! 

From a practical standpoint, I usually build my supplies with 4700 uF on the input side of the regulator and between 330 and 470 uF at the output side of the regulator, and usually do not use the bypass diodes with these values of input and output capacitance. The ratio of 10x the capacitance on the input side vs the output side, with the "bleed current" seen through the voltage setting resistor network, is usually quite sufficient to prevent the circumstance of the voltage at the output of the regulator being higher than the input voltage to the regulator from EVER happening.    It is far more important to make certain that you have RF bypass caps (0.1uF) on the input to ground and output to ground side of the regulator if you have the regulator located ANY distance away from the input and output main filter caps, than to have a "maximum" output filter cap value of 10uF.

You need to re-check the specifications and application notes on these, and you will find both sets of information I give above therein.  I have been building regulated power supplies in this fashion since 1979, and haven't had problem one with this design yet.

The DC input resistance of the circuit is 2.7 megohm,  however the audio IMPEDANCE of the circuit is 1.35 Megohm.   I have been using this unit continously since January, and have had zero noise problems, and have used this combination of twin 2.7 megohm resistors with a DC blocking cap on the input of several devices for almost 3 years without problems of noise.

I'm sorry I didn't respond earlier, I had changed E-mail addresses and did not update the change on this forum,  I have rectified this oversight.

Cheers!

[n9voc]

The schematic of the Guitar Splitter attached

[n9voc]

 <3)
And now, the Mic/Guitar mixer circuit:  Independent control of either: two guitars, two microphones with 1/4" outputs or one guitar and one microphone into a single amplifier.

The TL072 allows either a high impedance (1.35 Megohm for guitar) or a lower impedance (10kohm) for microphones. When in "Guitar" mode, one of the gain resistors in the feedback loop of the amplifier stage is shorted,  resulting in a lower gain -- suitable for the electric guitar output levels.  Flip the switch, and this short is removed, the lower input impedance is engaged and now it is suitable for a microphone level input.

The overall gain of the circuit in "guitar" mode is a maximum of 2.5 and a minimum of zero (this has been tested, and seen on the oscilloscope).

Output impedance is only 5000 ohms, low enough to drive virtually any amplifier quite well.

I've been using the unit since March, primarily in "guitar and mic" mode - working great!



Power supply notes:

In this case,  the power supply regulators are 7815 and 7915 in the standard usage thereof (as opposed to 7805/7905 used in an "adjustable regulator" configuration.

The 1000 uF capacitors following the regulators serve as "stiffeners" to the regulated DC output.  These provide a current "sink" to buffer the regulators from rapidly changing load currents - the capacitors can dump current faster than the regulators, thus improving the overall load regulation of the supply.

Enjoy!

[n9voc]

The transmogrifier (passive mixer cable) attached

[phatt]

Well good to know it works well, Obviously you've got it covered with PSU. :tu:
Phil.

[n9voc]

 
Good day!

I have attached another version of my "Buddybox".  It utilizes a single ended power supply with a discrete rail splitter circuit (not my idea, but found elsewhere).  If R21 is carefully matched in value with R3, and likewise R22 with R21 in the splitter circuit, the voltage split will be maximum closer to even.

The overall gain of the circuit is about 2.

This mixer was built to accomodate piezo pickups, passive coil pickups and also line level signals such as put out by a keyboard.

If only using one channel, such as to interface a piezo pickup to another system, be certain and turn the volume down to zero on the other channel - to eliminate noise issues.

One can utilize a small line operated transformer for the power source, or one can hook the unit up to a "wall wart" supply as shown.  The rail splitter can supply as much as 100 milliamps of current, thus, the power supply transformer or wall wart need only provide 200 milliamps or more of current capability.

Thanks to Sijosae for the rail splitter circuit!

I found that the OPA2134 gave the most "headroom" for the signals,  the circuit will work with a TLO-72 for U1 and a NE-5532 for U2.

Hope ya'all can use it!