Buffer noisy?

[theonefromthesky]

In a nutshell - Built Digital wireless guitar TX/RX, work perfectly with no noise etc etc. But decided I needed a buffer as the input impedance of the tx module os 20k ish. Scoured net for what would seem the best solution using an op amp  - http://www.muzique.com/news/opamp-buffer-layout/

I changed the layout and took out all the extra padding, I have also ommited the 100k resistor and used 2.7m instead of 2.2m

Built and worked straight out the box, 10khz-20khz response vastly improved.

However, I now have a constant "buzz" of 666hz through the system (do not own ossiloscope, computer spectrum analyser), inaudiable at first but as soon as DS is engauged it amplifies it exponentially.

Possible causes in my eyes;

1) Op amp too close to TX/RX unit? as it is inside the same box and almost on top of the tx module itself, yet moving rx module closer/further makes no difference.
2) Faulty Op amp, difficult to change will do as last resort
3) Op amp not suitable for this circuit, perhaps FET or Bipolar based better? Opinions please!
4) %^&* up on build.... Possible, but there are only 5 components, and it works...
5) 2.7meg resistors too much? Perhaps I need 2.2Meg

Putting a resistor accross input terminals makes noise ALMOST dissapear, but still faintly audiable, again amplified by distortion. And of course, removing the buffer output from TX module input removes sound completely, so its definatly a buffer issue.

Any help greatly apreciated!

[petemoore]

1) Op amp too close to TX/RX unit? as it is inside the same box and almost on top of the tx module itself, yet moving rx module closer/further makes no difference.
  More of a problem with high gain, buffer is 1:1.
2) Faulty Op amp, difficult to change will do as last resort
  Times I've had bad OA's is when they got hot or were about to smoke.
  3) Op amp not suitable for this circuit, perhaps FET or Bipolar based better?
   Opinions please! [you already wish it'd been socketed, probably].
  Impedance 'overmatch' might bring the noise up ? I read 20k, that isn't particularly high impedance input, the buffer probably helps and that's why it sounds better' ['cept noise]. The buffer would help with that too of course.
4) %^&* up on build.... Possible, but there are only 5 components, and it works...
5) 2.7meg resistors too much? Perhaps I need 2.2Meg
  What is the power supply like [is it easy to test/compare to battery power ?]
  I assume 2m7 refers to input pulldown resistor, IWCase that's not 'far from 2m2, 1meg may be noticable drop and is still high impedance.

[alanlan]

The circuit you refer to doesn't have any decoupling of the main supply or bias network which is bad.  I would add some.  If you add a decoupling cap to the bias network, you will need to add an extra resistor from this point to the +ve input to stop it shorting your input signal out altogether - maybe 1Meg or thereabouts.  As it stands, any pick-up or noise on the supply will couple directly to the input of the buffer.

[theonefromthesky]

Thanks guys,

Yes, I do wish that I had socketed it. Its the first chip I have ever soldered direct. But, room is an issue inside the ontainer and I I didn't want any problems. Although I may JUSt have enought room...

Just checking PDF specs, and indeed the input inpedance is 13-20k for the TX module.

Both Modules run off 9v batteries, with a 3.3v voltage regulator to power them. Just thinking about it, there is no capacitor on the input side of the voltage regulator. Didn't say it was necessary, but may help if its ocillating? Don't know... worth a quick solder....

Would the decouplaing be necessary on battery power? I have a rudementary understanding of electronics, I can read diagrams, build boards, but when it gets to the nitty gritty, I'm out of my depth, still learning!

When I have a moment, I shall upload a small Pic so you can see what I'm up to. In the mean time, thanks for the sugestions! Apreciated!

[theonefromthesky]

Ok after much  playing, I have found that its not the buffer introducing the noise, but the nature of the buffer  amplifies the noise.

The noise is actually generated by the TX module itself, but is so low its almost completely inaduable even at high gains. It turns out that the noise is generated somewhere and taken to the buffer on the positive wire, by isolating the buffer by means of another battery then it solved all but the most superficial of noise and of course having the posotive connected to input of the module by means of a resistor introduces more noise. So after much playing I found that by running the buffer from the 3.3v side of the voltage regulator, it cut out 98% of the noise completely with no loss in audio.

I will go and order some small inductors on tuesday, as I feel that in series with the power line may solve the porblem. Any sugestions as to what size to order?

thanks,

[petemoore]

  The battery has ''capacity'' to store energy, a capacitor stores energy like a battery...battery is very-pure-DC like, no ripple...just very very slowly drops voltage as it's charge is depleted, so there's nothing signal-like in the 'supply line' to filter out.
 Power supply noise is generally:
 1 what the circuit creates by 'gulping' current.
 2 ripple-remains from the AC/DC conversion.
  Ohm doesn't allow perfect DC sources of power, following the universal rule: current rise = voltage drop.

[petemoore]

  Battery itself is great supplier of DC, some 'ac' ripple can be imposed on the DC if current ramping is great enough, like what an LFO loading-ramp might introduce.
    Check out 7809 or other 3 pin regulator chip.

[familyortiz]

I would agree with alanlan... it is easy to add a bit of decoupling to try, maybe .01uF caps for the supply to the chip, keeping them very close to the chip. I've seen oscillations fixed that way.

[R.G.]

I would agree with alanlan... it is easy to add a bit of decoupling to try, maybe .01uF caps for the supply to the chip, keeping them very close to the chip. I've seen oscillations fixed that way.

... and opamp makers flatly tell you that you need to decouple the power supplies right at the power pins on every chip. The way to do a design is to first place a 0.1 and/or 0.01 across the power pins of every single chip, then take them back out selectively for economy.

But then they're promoting good design with their products. 

[amptramp]

I now have a constant "buzz" of 666hz through the system.

Possible causes in my eyes;

1) Op amp too close to TX/RX unit? as it is inside the same box and almost on top of the tx module itself, yet moving rx module closer/further makes no difference.
2) Faulty Op amp, difficult to change will do as last resort
3) Op amp not suitable for this circuit, perhaps FET or Bipolar based better? Opinions please!
4) %^&* up on build.... Possible, but there are only 5 components, and it works...
5) 2.7meg resistors too much? Perhaps I need 2.2Meg

6) The 666 Hz buzz indicates that Satan is attempting to get in touch with you or you are getting the interference from Hell.  Some Death Metal bands would pay a fortune for it.

[FiveseveN]

Some Black Metal bands would pay a fortune for it.

There, I fixed it 

[theonefromthesky]

I did wonder how long until somebody mentioned it. the only reason I investigated it is because it sounded like an ocatave high E, ran it through a PC tuner and it gave me the exact Hz reading.

Anyway, I have tried bypass capacitors to no Avail The noise still remains. Although looking at the modules the manufacturer makes sugests that the model above would be a better one as there is "No RF induced Interference".

This sugests to me that the lower models are the budget ones and you would expect a certain amount of noise. Gettign hold of these modules is.... almost impossible it would seem, as no retailer stocks them, least that I can find and I am fairly resourceful.

So anyway, most of the noise has been squished by means of running the Op amp off of the 3.3v output form the voltage regulator. Frequency response is identical to that of running from 9v, so I didn't perceive it to be of any real consiquence. But as a result, it has got me thinking that I may be able to iron out the remainder of the noise by use of small inductors on the supply lines to both circuits. I shall place an order for some of them today, hopefully my local shop is open.

I was also curious as to the 666hz scenario, is it a Harmonic of a frequency? Or something like that? I was also wondering if there was a way to invert the signal so that it would cancel itself out? I tried making an inverted signal Op amp circuit instead, but it actually made the situation worse.

Also with regard to the noise, I have noticed that If I rest my finger on certain pins on the board, the noise all but dissapears? Any Ideas?