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Low Power Audio Amplifier

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Nope! Just dupe the amp circuit and its 10k voltge reference resistor. :lol:
 
audioguru said:
Hi Nigel and Indecided,
Both channels have a 10k resistor connected to the non-inverting (+) input of the opamp, and the other end of both 10k resistors connects to the junction of the two 27k resistors. :lol:

Sorry :oops:

Slight typo!, missed the 'non'!.
 
The circut has proved itself well useful and adaptative, and performs flawlessly-almost.

For the speaker preamp, there is no noise evident, even at full gain.. I am impressed, with 1% resistors and some normal grade rubycon/elna/wincaps thrown in, and of course a OPA2134 :)

However, the mic preamp stage appears to have some DC (or inteference) evident - at full gain, it picks quite a buzz.

The difference with the mic stage is that I added a 10k resistor after the 47ohm resistor to the +input before the decoupling capacitor. This is to facilliate powering of the electret capsule. I have tried an additional 1,10,100uf cap in the output stage after the 2.2uf ones but the problem is still evident. Thie capsule and wiring are of the "multimedia PC" variety.

-update-
Oops. Halfway through the testing stage, i discovered that the buzz only appeared when i held the microphone. Upon loss of bodily contact the buzz dissappeared. (mind you, the holder is plastic!) Guess the wire run is a bit too long and might need some more grounding. Any ideas? Perhaps a 100 ohm on the incoming as well?
 
Hi Indecided,
The case of a 2-wire electret mic is supposed to be connected to the circuit's ground through the shield of its connecting shielded cable. The mic's output should connect to the circuit by the shielded inner conductor of the shielded cable.

Maybe you have a 3-wire electret mic that uses a supply voltage without a 10k resistor, an output that needs a shieded cable, and a ground connection through the cable's shield to its case.
 
Ah. Problem solved, just some shielding worries.

However after case testing in the car, I realize that a lot of inteference is introduced by my PDA charger, which heart is essentially a MC34063. I believe that this is due to grounding issues, something about mixing power ground and audio ground?

My PDA has both audio ground and power ground. Currently, the audio ground is wired to the preamp(s) ground, which is also wired to the car ground (aka bare metal). Although this would constitute mixing of grounds, there is no choice. However, there is zero noise with this setup.

The moment my PDA charger is plugged in, which 5v and power ground is wired to the previous ground, ground looping comes into play.

What methods can I use to isolate the audio ground away from the power ground? i'm not sure how to do that (in the sense of wiring the input ground, which i guess is the best solution)
 
Hi Indecided,
Shielded cables stop noise pretty well, don't they?
Breaking the ground loop:
1) Have the mixer powered and grounded by the PDA during charging. But the voltage is too low for the mixer.
2) Add a 1:1 audio transformer with its primary at the output of the mixer and its secondary feeding the PDA's input. Keep the transformer away from the relay for the turn signals. :lol:
 
Hi audioguru,

For my stereo line-out preamp, I seem to have a dramatic loss of frequencies below 120Hz, which means that the bass lines have become unfocussed and dull, and pretty much missing.

Any way I can solve this? How about playing with the input/output cap values?
 
Hi Indecided,
The output impedance of the mixer needs to be low enough to drive the input impedance of the amplifier. If the mixer has an output coupling cap, increasing its value will pass lower frequencies but might just overload the output of the mixer some more.
Please find out the input impedance of the amplifier and post the schematic of the mixer. :)
 
Audioguru,

All's well that ends well. Apparently some solder off one of the traces on my stripboard had leaked - in particular, the ground plane was 'messing' with the preamp's output. Fixed it in a jiffy and vahala! I am very happy. Just have to buy better screeened cable for the microphone.. the standard one seems to pick up a substantial amount of noise in the car.

However, I am still working on figuring out how to isolate the audio ground from the signal ground, creating two grounds. Right now it's obviously mixed.. perhaps something to do with dual rails and virtual ground. Reading it up now... and planning another amplifier for my home 'cans'.

BTW, there is no mixer, it's just PDA PHONE -> Preamp(s) for line out and mic in -> hacked AUX in on stereo. :) Very happy with the performance... very happy indeed. Next going to venture into self making PCBs once i located a source for carbide bits... thanks again audioguru, you've been an invaluable help from the beginning as always!
 
Hi Indecided,
It's great to hear that you found the problem, fixed it and it works. :lol:

I bought some ordinary shielded RCA cables at Radio Shack. They picked-up all kinds of interference so I cut one and found only a couple of strands of "shield". I replaced them with excellent cables from the Dollar Store.

All your grounds need to be at a single point. But if you connect the preamp's ground to the chassis by the cigarette lighter jack, the car radio's ground is bouncing around by the current in its wire to the chassis.
A transformer is about the only way I can think of to provide isolation and break the ground loop. :lol:
 
audioguru said:
All your grounds need to be at a single point. But if you connect the preamp's ground to the chassis by the cigarette lighter jack, the car radio's ground is bouncing around by the current in its wire to the chassis.
A transformer is about the only way I can think of to provide isolation and break the ground loop. :lol:

How about disconnecting the screen at one end of the lead from preamp to power amp - a standard PA technique!.
 
Hi Nigel,
Good point and certainly worth a try. :lol:
I've done the same on inter-circuit wiring and it saves shield connections too.
 
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