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Op amp based preamp supply. Charge pump, single vs symmetrical & ripple
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dazz:
I'm cloning the preamp of a Marshall Lead 12 3005 into a pedal. It's a two stage op amp job with a +16V -16V supply.
The plan is to use a standard 9V power supply to power it, so I'm considering two options:

1. A charge pump based Dual-Output Voltage Doubler (pic 1) based on an LT1054 to get a symmetrical supply of about +16V -16V
2. A charge pump voltage multiplier with two ICL7660 or LT1054 in series for a single supply of about 24V (according to my LTSpice simulation)

I'm leaning towards option 2, but I'm not sure if the reasons for that are justified. I believe that having the opamp's V- tied to ground will limit/halve the amount of ripple I'll get as opposed to having a positive and negative rails, besides, the PSRR or the V+ is much higher in the TL082 I plan to use, so that should further mitigate the effect of ripple in the power rails.

I know people are using 7660S chips with the boost pin to Vcc to increase the switching frequency from 10KHz to 30KHz, outside the hearing range with good results in symmetric supply configurations, but what if I was to use the cheaper 7660 at 10KHz in single supply mode? would the fact that there's just the positive rail ripple help make up for the lower frequency?

I tried crunching some numbers, but I have no idea what I'm doing. Here they go anyway: The PSRR of the TL802 in V+ is 80dB at 10KHz. The preamp's gain at 10KHz, according to LTSpice, is about 70dB. So I'm guessing the V+ ripple will appear at the preamp's output attenuated 10dB (assuming single supply charge pump with a 7660 at 10KHz). That's an attenuation of a factor of 3, give or take. Can I expect to have low enough ripple  in the charge pump output that it won't whine?

I've also run some simulations of the charge pump (pic 2) and with a 10mA load which I think is reasonable for a dual op amp circuit like this driving a 50K load (standard chipamp input impedance) and I get only 3-4mV ripple. That would translate to 1mV in the output. That doesn't sound like too much, does it? I mean if the voltage swing at the output is 24V, 1mV won't disturb it too much, would it?

Anything I may be missing please?
Benta:
Quite frankly, the pain of running opamps on a single supply is not worth the effort if it can be avoided.
A dual supply opamp has a wonderful signal reference: ground. In a single supply design, you need to recreate this at some DC bias point. Not easy, and usually not noise free.

You have the option of running dual supplies, and I highly recommend this.
Your reflections on PSRR are correct, but can be alleviated with appropriate filtering.

dazz:
Thanks Benta. Do you think the fact that symmetrical charge pumps usually have weaker V- rails (in my simulations I got something like +16V, -15V) could have unwanted effects? Wouldn't it cause the preamp to clip asymmetrically because of the imbalance of the rails, hence affecting its tone?

I definitely plan on testing all these things myself once the parts arrive, should be a good learning exercise
Benta:
I expect your signals are at line level (~775 mV), but even if they are a bit higher or lower, supply voltage will have zero influence. Asymmetric supply voltage will also have zero impact.
You're overthinking this in my opinion.
dazz:

--- Quote from: Benta on November 25, 2018, 11:11:31 pm ---I expect your signals are at line level (~775 mV), but even if they are a bit higher or lower, supply voltage will have zero influence. Asymmetric supply voltage will also have zero impact.
You're overthinking this in my opinion.

--- End quote ---

I should have mentioned it's a guitar preamp/effect. So the input is much lower than line level, perhaps 100mV peak to peak. Does that change anything?
And yes, I tend to overthink stuff, so it may very well be the case here, haha
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