FrankenAmp

[n9voc]

Good Day All!

Joe had asked about if I had tried the LM1875, and I found my schematic for one of the first successful amplifiers I built based upon an LM1875 design - I called it FrankenAmp, because I used a relatively cheap practice amp for the chassis and amplifier hardware.  It is Frankenamp 2.0, because I made a new enclosure and completely reworked the innards.

The original of the attached schematic was generated as the unit was built and evolved with a lot of "cut and try" ideas along the way.  I finally got tired of messing with it and the result is as seen below. It works good, and my son uses it regularily.

There are a few changes I will make to the below design before I create another LM1875 based amplifier, but I haven't gotten around to writing them out yet - or building a prototype.  The speakers in Frankenamp 2.0 are a 5 1/2" and a 6x9" purchased at Wal-Mart for around $10.00 each.  The tweeters were from an inexpensive "miniboombox" radio from Dollar General.

Most of the rest of the parts came from my "junk box" at the time.  I used OPA2134 op amps in the design because I had handful of them at the time.  The R19/R20 combination was an adjustment to the R20 value to get a good sound level out.

Enjoy the schematic and pics that follow!

[n9voc]

Pictures of FrankenAmp!

Wood was scavenged from a construction site!  Chassis, switch, fuse, power cord and case hardware from the "donor" amplifier.

[leonefranca]

Thanks man!

I just started studying electronic, but this amp is so simple and so good that I decided to build it.

This will be the first amp that I´ll build (hope that nothing explodes).

I have one question. Do I need to use heatsinks with the lm1785? Because it will work with only 15v.

Thanks again.

[joecool85]

Thanks man!

I just started studying electronic, but this amp is so simple and so good that I decided to build it.

This will be the first amp that I´ll build (hope that nothing explodes).

I have one question. Do I need to use heatsinks with the lm1785? Because it will work with only 15v.

Thanks again.

Yes, you definetely need a heatsink for a LM1875 - they get quite hot.

[n9voc]

(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.

[leonefranca]

(You are most welcome- sorry I didn't get back sooner, been out of town!)

No problem!

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.

Well, I was thinking to use an old computer power supply. It have 2 4700uf capacitors but I found 2 more capacitors on another power supply with the same value.
Question: More capacitance = no need for voltage regulators? Just 4700uf aren´t enough? Because 4 caps occupy lot of space.

The PC supply have 235W and normally it gives +12v 8A and -12v 0.8A. But if I change some components maybe I can have +15v with 2A. I don´t know if it´s limit is 12v. The -15v needs to be 2A too? Do I need a voltage regulator or isn´t necessary?

Do you suggest that I add your "High Impedance In Booster" schematic in this amp?

Do you recommend an "aux input" right before the VR2?

Sorry for so much questions!

Thanks for both.

[n9voc]

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: