Hi,
I have a single 12ax7 guitar preamp. 300V for the preamp is provided by a power supply like this: https://www.aliexpress.com/item/1005006302470816.html.
I am adding boost in front of my preamp and buffer after it. Both boost and buffer use OPA134. I want to get 18V from 300V to feed both op-amps. I have attempted to do a resistor divider with 100k and 8.2k resistors. Without load I measure 20+ volts, but with the op-amps on, I get actual voltage like 3 volts on the op-amps. And the 100k resistor gets quite hot.
What is the best way to feed two OPA134 from a 300VDC supply (of course 12ax7 doesn't go anywhere, but the supply should be able to handle it all with no problems)?
Thanks.
I have a single 12ax7 guitar preamp. 300V for the preamp is provided by a power supply like this: https://www.aliexpress.com/item/1005006302470816.html.
I am adding boost in front of my preamp and buffer after it. Both boost and buffer use OPA134. I want to get 18V from 300V to feed both op-amps. I have attempted to do a resistor divider with 100k and 8.2k resistors. Without load I measure 20+ volts, but with the op-amps on, I get actual voltage like 3 volts on the op-amps. And the 100k resistor gets quite hot.
What is the best way to feed two OPA134 from a 300VDC supply (of course 12ax7 doesn't go anywhere, but the supply should be able to handle it all with no problems)?
Thanks.
Because I already have a power supply and to keep the number of parts and footprint smaller? And audio power supply is designed to be quiet, so I just want to use the existing one. I've seen on a JFET boost schematic of Dumble ODS that the transistor is powered from B+5 (using something like 150k and 8.2k resistor and a following 10k dropping resistor). So, I don't think I want something weird, I just need to understand how to do it correctly.
How about using a simple dc-dc converter on the 6.3v rail to give +/- 15v. That Ali supplier does this:
€4.50 | 20W +- 5V 6V 9V 10V 12V 15V 24V Positive and Negative Dual Output Power Supply DC DC Step-up Boost-Buck Converter module
https://a.aliexpress.com/_EQ8tfcZ
It is very small and simple to connect and shouldn’t create excessive heat. I haven’t used this myself, but just suggesting a possible solution.
Note that if the opamps don’t draw at least 15ma from each rail, you’ll need a 1K resistor on the psu outputs to 0V to meet the minimum current requirement.
€4.50 | 20W +- 5V 6V 9V 10V 12V 15V 24V Positive and Negative Dual Output Power Supply DC DC Step-up Boost-Buck Converter module
https://a.aliexpress.com/_EQ8tfcZ
It is very small and simple to connect and shouldn’t create excessive heat. I haven’t used this myself, but just suggesting a possible solution.
Note that if the opamps don’t draw at least 15ma from each rail, you’ll need a 1K resistor on the psu outputs to 0V to meet the minimum current requirement.
The OPA134 has a quiescent current draw of 4 mA per channel.
Your divider 100 k and 8.2 k passes a current of 300V/108200Ohm = 2.8 mA
Once you connect the Op-Amp that wants to draw 8 mA of quiecent current the voltage collapses.
also, the current draw of the op-amps depends on the output and will change. I suggest kind of regulation for the op-amp supply. either a simple zener diode, a shunt- or a series regulator.
Ok, but the 8.2 k resistor will need to dissipate more than 10 W.
also keep in mind that if the 560 ohm shunt resistor is accidentally disconnected or damaged you feed 300 V DC into the op-amp with all sorts of (potentially dangerous) risks at connectors.
Your divider 100 k and 8.2 k passes a current of 300V/108200Ohm = 2.8 mA
Once you connect the Op-Amp that wants to draw 8 mA of quiecent current the voltage collapses.
also, the current draw of the op-amps depends on the output and will change. I suggest kind of regulation for the op-amp supply. either a simple zener diode, a shunt- or a series regulator.
Ok, but the 8.2 k resistor will need to dissipate more than 10 W.
also keep in mind that if the 560 ohm shunt resistor is accidentally disconnected or damaged you feed 300 V DC into the op-amp with all sorts of (potentially dangerous) risks at connectors.
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I believe this would be the best approach, you have plenty of current available on 6.3V output. a simple boost converter with a regulator IC should do nicely..
Unless you want to run parallel SMPS.
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You can buy one zenerdiode 22V and a MOSFET IRF740.
The MOSFET can take 400V and should be using a small heatsink.
Then do it like this:
The MOSFET can take 400V and should be using a small heatsink.
Then do it like this:
Thank you! 10w of heat is certainly a concern.
Will try the mosfet/zener circuit, if that doesn't work, will be trying to step-up heater voltage...
Will try the mosfet/zener circuit, if that doesn't work, will be trying to step-up heater voltage...
You can get SMPS from eBay that take 12V/24V in and produce high voltage, perhaps that's a better arrangement - then you can use a safe certified mains supply to 18V and boost from there. Mains supplies from aliexpress are a risk you should seriously consider avoiding.