Electronics > Beginners
Op amp based preamp supply. Charge pump, single vs symmetrical & ripple
b_force:
I wouldn't go for these charge pumps anymore these days. They can be noisy, regulation is poor or limited and the output current is small.
There are quite some good converters that can do split rail (TI has a few)
Also you can use pretty much every buck converter to make a negative voltage rail.
If efficiency is not the main priority there are also of the shelf converters available.
Look for something that's at least 100kHz, but the higher the better plus some post filtering.
bd139:
Another option - use an AC wall wart and just derive two rails from it. This works pretty well, gives you no compromise rails and keeps your circuits dead simple. Typical example of deriving three voltages cheaply (I would use regulators myself and be done with the zeners):
dazz:
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.
--- End quote ---
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.
--- Quote from: T3sl4co1l on November 26, 2018, 09:21:02 am ---If you must, consider heavy filtering (or investing in opamps with high PSRR at higher frequencies).
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Well, I'm not sure I can just swap op amps like that. People will complain that all the mojo in the original LM1458 or the TL072/082 is lost. I personally have no idea if the op amp of choice plays any roll in how the thing clips and sounds. The simulations seem to suggest that the clip differently, but who knows.
--- Quote from: T3sl4co1l on November 26, 2018, 09:21:02 am ---But there should be no need for any more than a resistor divider (or a boosted one, i.e. using a spare opamp section for a rail splitter) in an application like that.
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Can you please elaborate on this? What rail would the resistor divider split and what would the opamp do there please?
dazz:
--- Quote from: Hero999 on November 26, 2018, 09:31:35 am ---
--- Quote from: T3sl4co1l on November 26, 2018, 09:21:02 am ---But there should be no need for any more than a resistor divider (or a boosted one, i.e. using a spare opamp section for a rail splitter) in an application like that.
Tim
--- End quote ---
Agreed. Let's also not forget the TLE2426.
http://www.ti.com/lit/ds/symlink/tle2426.pdf
If the power supply is only powering that circuit and isn't connected to anything else, then no AC coupling capacitors are needed: the 0V from the rail splitter can be treated as the common rail for the whole circuit.
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Thanks Hero, googling that chip now. And yes, the only thing that the power supply needs to power is that dual opamp circuit.
--- Quote from: spec on November 26, 2018, 09:36:43 am ---Crossed Posts :)
Just so we can see whats involved here is the schematic for the Marshall Lead12-12W: https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Lead12-12W-3005-5005-Schematic.pdf
My first thoughts are to use better RRIO opamps, so that the supply rails could be lowered and get the same or better dynamic range and open loop gain. That would possibly enable a simpler circuit with a better performance, including a lower noise, not that noise is an issue here.
A 12V, 15V, 18V, or 24V wall wart should do nicely for the PSU.
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Thanks for posting the schematic, should have done it myself in the 1st post.
I'm still not sure how I could use lower power rails without significantly impacting the way the amp will perform. The thing has 80dB of voltage gain (with the gain pot at max) and the gain pot is part of a low pass filter so it changes the frequency response of the amp. So if the original one clips at 50% and mine clips at 10%, they won't sound the same. At least I think so.
Also, a 9V supply is pretty much a requirement, since this will be a guitar pedal and most pedal power supplies are 9V |O
b_force:
I know 9V is pretty standard, but it's a little unfortunate.
Otherwise it would be pretty easy to use something like a Meanwell MDD01N-15 or similar.
The minimum voltage for those is either 10.8V or maximum voltage for 5.5V (for the 5V version).
The second best choice would be a Meanwell DPAN02A-15 or DPAN02E-15
they also switch at 100kHz, so that can easily be filtered out.
How much opamps/current do you need?
This is the +/- 15V version, which also gives you plenty of headroom.
There are also 12V and 5V version available.
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