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-5 volts?

Hello.. I've built a little Guitar effects circuit, Its basic configuration is as follows Input -> LM741(variable Gain) -> ADC -> PIC

-> DAC(MAX5354) -> output. I used a voltage divider to impose a 2.5 volt DC offset on the analog input of the OPAMP so I could get a clear reading as the signal swings in either direction.

It all works fine, except that the 2.5volt offset is still on the output. Due more to the DAC then the above mentioned. Any way. I wish to power the whole thing of a regular 9 or 12 volt DC wall wart. So I need -5 volts to pull the signal down again (removing the DC offset.

Are they called charge pumps.? somthing? At the moment I only have the 5v digital supply and ground.

Thanks.

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Reply to
DaveC
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"At the moment I only have the 5v digital supply and ground."

First thing.... from

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it sez the minimum supply rail is 10V, so select another opamp, preferably with good rail to rail specifications

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.

Reply to
martin griffith

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Have a look at the LMC7660...

Reply to
SCADA

"Richard Webb" wrote in news:-IWdnWaAQ snipped-for-privacy@pipex.net:

Thanks.. The cap worked. I'm only in my first year as electronic degree so forgive me :)

What is the use of positive and negative voltage on many opamp circuits I've seen then?

DaveC

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DaveC

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If I'm correct all you want to do is remove the DC offset from the output of the DAC.

A capacitor passes AC but blocks DC.

Putting a sensibly sized cap in series with the output of your circuit will only allow the AC components of your signal to pass and block the DC component.

Its hard to tell from your description but it sounds like you might be trying to impose a 2.5v DC offset onto your input directly which could be feeding back to your guitar. No idea if this will affect your guitar or not. Use a capacitor in series with the input and after the cap place a resistor to ground, this will form a high pass filter. Set the break frequency nice and low (20Hz ish).

As for the power, why not use a regular 9v wall wart and regulate it with something simple like a 7805?

Hope this helps, Rich.

Reply to
Richard Webb

:-D

LM741, I love it!

Consider a more modern general purpose OP-amp like TL081,2,4. Rail-to-rail opamps are really cool, but expensive. Avoid if not needed. Spend a lot of time reading various op-amp datasheets to understand the specifications, and read "The Art of Electronics" a bunch of times.

You should stick with higher slew rates to provide enough full power bandwidth (not the same as small signal bandwidth, which is the front page spec given in datasheets). For an interesting extra homework problem, determine the slew rate r needed from an op-amp to avoid distortion for a desired signal output of frequency f, and peak amplitude A.

For audio, there are some really cool op-amps that aren't expensive:

OPA134, OPA2134, etc. NE5532

From TI. Go to TI's web site, register, and they will send you samples overnight (yes, to students as well) You can get about 4 each of at least two part numbers per order.

Here's a neat article:

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Good day!

--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
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Reply to
Chris Carlen

This allows for DC coupling (direct to the opamp output) without a coupling cap. This is useful when cascading stages so you don't have to worry about all those AC coupled stages rolling off you low frequency gains (bass).

As a guitar guy, you might want to look into Tubes. I'm in a project with a musician friend of mine and even though he will admit that solid state get much better specifictions and cleaner sound, tuba amps are more desirable because they become part of the musical instrument. Tube distortion due to signal overdrive (hitting the string hard) is more pleasing to the ear and make the guitar sound fuller rather than raspy. I'll admit that he is correct in that as I have heard teh difference for myself. not saying that solid state cant be made to mimick the shound of a tube, but tube amps are pretty much the norm for musicians I know.

This is NOT the Hi-Fi tube amp vs Solid State discussion/arguement and please lets not open that can of worms. :)

good luck

Reply to
Mook Johnson

he said effects amp, maybe the effects he is getting from the 741 is what he wants! :)

Reply to
Jamie

It is not necessary to invert for your slight loading, a simple power supply split will suffice-use figure 10 in the MAX5354 datasheet and run the input and output 741's off this:

View in a fixed-width font such as Courier.

78L05 +-------+ +-------------+---+---|IN OUT|---+-(+)5V | | | | | | | | | | | | | | 680 | GND | === | | | +-------+ 1u | | | | | (+)---+ | | | | | +-------+-------+----+-->

| | | | 12v === 1n5231B| | --- 1u ---/ === /// | // \ 1u (-)---+ | --- | | | | | | | | | | | | | | | | | +-------------+-----------+-------+-(-)5V

Reply to
Fred Bloggs

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