help with noisy power amplifier

[Roly]

The + input to the TL072 is going to need a reference to earth, you should connect the 1M resistor between this point and earth and scrap the 47uF Cap.

I've had too much xmas ham!  I remember sitting looking at that and thinking it didn't look right ... then the siren song of xmas cheer called and ...  :duh

Thanks Tony.   :dbtu:

In fact, if a split supply is being used to the preamp IC we can throw out the DC blocking cap as well and just have a single 1Meg resistor to ground; and we shouldn't need one in the feedback loop either; however I would retain the one on the output to avoid upsetting the power amp.

[panfilero]

thanks everyone for the tips... the noise I get is hissy and whiney and seems to change a little bit in frequency when I play guitar.  It's kinda high pitched and always present, also turns up when I turn up the gain on my PreAmp.  It does completely dissappear when I unplug my preamp from the amplifier.  And I am doing most of this stuff with long leads with alligator clips to connect everything to and from my amplifier (preamp, speakers, power... all alligator clips)... I'm hoping when I put all this on a board that the noise will go away. 

The main cause of my noise was my guitar cable was hooked up backwards?! I didn't even know there was such a thing?! I basically have a small box with an input jack on one side and terminal block on the other... I break out the tip and ring (I think that's what its called) from the input jack into the terminal block, and then tie wires to the terminal block to whatever I'm doing... I swapped the wires and a ton of the noise went away...

here's some things I've been trying with the preamp

much thanks!

[Roly]

t = CR (time constant)
f = 1/t

f = 1/CR   (corner frequency, -3dB)
C = 4700 x 10^-12  (pF in Farads)
R = 1 x 10^6  (Meg)

f = 1/((4700 * (10^-12)) * (10^6)) = 212(.76595745)Hz

The effect this has however depends on the source impedance of the guitar, and even at the highest frequency it can only present a load of 1Megohm.

Another way of looking at this is that the corner frequency (and -3dB point) will occur when the reactance Xc of the capacitor is equal to the resistor;

Xc = 1/(2 Pi f C) = R

The main cause of my noise was my guitar cable was hooked up backwards?! I didn't even know there was such a thing?! I basically have a small box with an input jack on one side and terminal block on the other... I break out the tip and ring (I think that's what its called) from the input jack into the terminal block, and then tie wires to the terminal block to whatever I'm doing... I swapped the wires and a ton of the noise went away...

As you may have now discovered, 6.5mm connectors come in some different flavours.  The standard "mono" or tip and sleeve, and the "stereo" tip, ring, sleeve, or TRS (also used for balanced feeds).

You may have found one of the traps were these are not entirely compatible.  If you plug a mono plug into a stereo socket the ring connector makes (or should) to the grounded body sleeve.  It's easy to confuse the tip and ring connections, and also any switching contacts they may be carrying.  And yeah, the amp common/ground/earth must go to the body, and the signal input to the tip.  But it sounds like you have that sorted now?

Clip leads all over the place. 

Inputs in particular need to be done in screened cable if they run any distance, and your comment;

panfilero> "the noise I get is hissy and whiney and seems to change a little bit in frequency when I play guitar"

...suggests to me that this may not be signal pickup so much as oscillation and supersonic instability due to the scratch layout.  At the very least input and output leads must be kept well apart, and you need to be sure that your amp supply voltages are bypassed with suitable caps right at the amp, not back along a snaky bit of wire in the bench power supply. (your preamp IC should also have something like a 0.1uF connected from +ve supply to -ve supply right at the IC pins.

JM posted some pix in another thread (which I hope he will link to) of how he puts the high frequency bypass caps right on the chip amp.  These chip amps are now so good they will happily amplify and oscillate up into the megahertz regions of you let them, and this tends to come out as a "sizzling, swishy, watered silk" sound on your distorted audio.

So,
- Screened leads for the inputs (scrounge from dead electronics, or tightly twist signal and earth together)
- mains-rated cable gauge for the power supply and speaker leads (also twisted)
- check the bypassing caps local to the chip amp (solder joints, right position/value)


One time I had to build a low noise mic preamp for a radio station and I ended up putting it in an old sardine tin with the lid soldered back on; screened lead in and out.  Looked a bit strange behind the panel, but it was dead quiet in a noisy environment.

HTH