Electronics > Beginners
Op amp based preamp supply. Charge pump, single vs symmetrical & ripple
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b_force:

--- Quote from: dazz on November 26, 2018, 03:07:52 pm ---
--- Quote from: b_force on November 26, 2018, 03:01:05 pm ---Another idea i to get a basic 9-12 or 9-15 boost converter and use a flyback or 1:1 coupled inductor to form the negative voltage.

--- End quote ---

Yeah, I already have one of those boards I got from ebay. Maybe I could add a boost converter to my board. Something based on a LM2577-adj perhaps? Ideally something that can be got for cheap off ebay or the likes

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Well, the problem is to get a good efficiency with these low currents.
Most converters are optimized for something around 100-500mA or thereabouts.
The efficiency at low currents can be as bad as 50-60%. Not really ideal for a 9V battery.
(I am personally getting rid of all my 9V pedals or rebuild them to accept two AA etc)

What I would suggest is otherwise use something like a LT1615 (or a similar TI parts) look at page 8, or LT3460, LT1316 etc (probably better because it has a constant switch Freq)
With LTSpice you can simulate this circuit and optimize it for 9V
Zero999:

--- Quote from: dazz on November 26, 2018, 01:59:29 pm ---Thanks everyone for your input. Lots of suggestions to explore.


--- Quote from: Hero999 on November 26, 2018, 08:51:36 am ---If it's low levels, then the larger possible voltage swing a +/-16V supply could give is not needed. A lower voltage op-amp could be used with a single supply.

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Sorry mate, I don't understand this. The input is low level, but the output at maximum gain is a square wave 30V peak to peak. I think I need to have that +/-16V supply to replicate the way the circuit distorts. Let me know if this is wrong, but the rationale is that the gain pot (attached schematic) changes the frequency response of the amp, at low gain it boosts higher frequencies more (around 10KHz) but as the gain is increased, the higher gain frequency shifts to about 1KHz. So if the original circuit with the +/-16V supply starts clipping at 10KHz with the gain knob at 50%, with a 9V supply it will start clipping very early at maybe 10% gain, and at all frequencies, making it almost unusable. I don't know if the circuit can be modified to behave the same way as the original one but with a single 9V supply. I can attenuate the gain slightly by increasing resistor R8 in between the op amps, but not enough to compensate for the lack of voltage swing.
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Then I misunderstood. If you need 30V peak to peak, then you need a higher total supply voltage than 30V. A +/-16V supply won't do for the TL082, which needs a couple of volts of headroom on either supply rail, to avoid clipping, unless that's desired for the distortion effect.

The OPA2196 will be able to give 30V peak to peak from a +/-16V supply. If does have a lower slew rate, than the TL082, but it will be the lower audio frequency components which generate the largest voltage swings, so this is a non-issue.
http://www.ti.com/lit/ds/sbos869/sbos869.pdf
dazz:

--- Quote from: Hero999 on November 26, 2018, 03:30:39 pm ---
Then I misunderstood. If you need 30V peak to peak, then you need a higher total supply voltage than 30V. A +/-16V supply won't do for the TL082, which needs a couple of volts of headroom on either supply rail, to avoid clipping, unless that's desired for the distortion effect

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Yes, it's built to distort, for rock'n'roll purposes!  :-DD

More precisely, it's supposed to do both clean and distorted tones. That's what the gain pot is for


--- Quote from: Hero999 on November 26, 2018, 03:30:39 pm ---
The OPA2196 will be able to give 30V peak to peak from a +/-16V supply. If does have a lower slew rate, than the TL082, but it will be the lower audio frequency components which generate the largest voltage swings, so this is a non-issue.
http://www.ti.com/lit/ds/sbos869/sbos869.pdf

--- End quote ---

I'm afraid it really needs to be an LM1458 as in the original circuit, or one of those TL082 with jfet inputs (which are supposed to clip more softly). Well, not really those 2, just one op amp with lots of mojo regardless of whether other types make more sense from an engeneering perspective. That's how guitar players roll  :-//
Zero999:

--- Quote from: dazz on November 26, 2018, 03:48:24 pm ---
--- Quote from: Hero999 on November 26, 2018, 03:30:39 pm ---
Then I misunderstood. If you need 30V peak to peak, then you need a higher total supply voltage than 30V. A +/-16V supply won't do for the TL082, which needs a couple of volts of headroom on either supply rail, to avoid clipping, unless that's desired for the distortion effect

--- End quote ---

Yes, it's built to distort, for rock'n'roll purposes!  :-DD

More precisely, it's supposed to do both clean and distorted tones. That's what the gain pot is for


--- Quote from: Hero999 on November 26, 2018, 03:30:39 pm ---
The OPA2196 will be able to give 30V peak to peak from a +/-16V supply. If does have a lower slew rate, than the TL082, but it will be the lower audio frequency components which generate the largest voltage swings, so this is a non-issue.
http://www.ti.com/lit/ds/sbos869/sbos869.pdf

--- End quote ---

I'm afraid it really needs to be an LM1458 as in the original circuit, or one of those TL082 with jfet inputs (which are supposed to clip more softly). Well, not really those 2, just one op amp with lots of mojo regardless of whether other types make more sense from an engeneering perspective. That's how guitar players roll  :-//

--- End quote ---
I admit I'm more used to avoiding distortion when building an amplifier.

Do you have a specific type of distortion i.e. transfer function in mind?

I think most op-amps will give fairly harsh clipping, as once the output goes beyond the limit, there will be no negative feedback. I suspect the clipping level may vary between op-amps with the same part number, because it's not a tightly controlled characteristic. By operating an op-amp at its clipping level, it's being driven past its deign specification.
schmitt trigger:
Another thing with using charge pumps, is that your negative output won't be completely symmetrical.

As the application circuit shows, there will be the diode's forward losses, plus any losses in the converter itself.

And indeed if not filtered correctly, the plain 7660 will generate an audible whine when used with sensitive audio circuits.
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