DIY Audio Projects Forum

High Voltage Filter/Regulator
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Author:  gofar99 [ 15 Jan 2019, 15:03 ]
Post subject:  High Voltage Filter/Regulator

Hi Folks, Tis' the month for strange thinking around here. :idea: I didn't know whether to post this here or in the Solid State section. Since it is going to be part of a tube design I figured here was better. Get out your thinking caps and sim programs. I have a design that needs to drop B+ from 240 to about 140 at 50ma and filter it really well. It would seem that a FET based series filter stage in the PS would do this as the third one in a string of CRC ones. Regulation is not all that important, but the drop and filtering are. I am considering bootstrapping a LR8 with something like and IRF820 (or 730 as I have both). It would seem that the filtering and regulating properties of the LR8 would be passed on to the FET and provide enhanced filtering. The adjustable output is a plus and regulation ought to be excellent. Any thoughts on this?

Good listening

Author:  mike567 [ 15 Jan 2019, 19:01 ]
Post subject:  Re: High Voltage Filter/Regulator

try this: ... ly-0-300v/

Author:  ILoveHiFi [ 15 Jan 2019, 19:10 ]
Post subject:  Re: High Voltage Filter/Regulator

Firstly not sure why we want to drop to 140v from 240. A drop of 10-50v seems reasonalbe but not 100v.
Thats 100v and 50mA on the pass transitor and 5w, lots of expense on cooling and very poor efficency.

If you don't care about regulation much and no short circuit protection is required then I would personally go RC filter or zener + RC filter and bias on the power mosfet. Since its easier and can be low noise.

Author:  Suncalc [ 15 Jan 2019, 19:32 ]
Post subject:  Re: High Voltage Filter/Regulator

Do you have a target noise level for the 140V supply point (i.e. -x dB) and an estimate of the ripple level on the 240v supply? I am thinking that since you don't really care about regulation, and you're starting with a DC supply, with some noise level, a simple resistor and an LC filter stage may be sufficient. Definitely far simpler than a regulator IC.

Author:  ILoveHiFi [ 15 Jan 2019, 19:55 ]
Post subject:  Re: High Voltage Filter/Regulator

Heres a series regulator with short protection and feedback, current limiting at about 70-90mA.
The zener is 36v and the resistors are 0.5W.
Ay high voltage transitor will work something like mpsa42.
The 82k or 230k can be replaced with a adjustple resistor to adjust output voltage

Low noise and good.
Be sure to click reset otherwise the simulation won't run properly.

I always design things on pen and paper but I didn't want to take photos out of my phone.

Author:  gofar99 [ 15 Jan 2019, 20:25 ]
Post subject:  Re: High Voltage Filter/Regulator

Hi, Thanks for the inputs. The reason for the drop is the PS is being used to provide 240 VDC for part of the circuit and I want to add something that needs the lower voltage. CRC filtering was not enough and there is no room for a choke etc. I can use the chassis for a heat sink so that heat is not a concern. I don't have ripple measurements at this time and may have to take some. I estimate from the breadboard hum that I need about 30 db of better filtering over a simple single stage CRC. It is the 4th in the string with the other 3 going to the rest of the cirrcuit. I'll post the project when I can get to my satisfaction. I was looking at something like what Mike567 posted, but figured it would cut the B+ down, but not clean it up enough.

Then again, maybe I am being fanatical again about hum and noise. It is there :eek: , I can hear it and I want it gone. :down:

Good listening

Author:  ILoveHiFi [ 16 Jan 2019, 03:12 ]
Post subject:  Re: High Voltage Filter/Regulator

The zener diode used can be 1/4 watt and 36v tends to be commonly avalible and avalible in small packages. 47V or higher tends to be 2-5W packages only and more noisy.
If we make this zener diode a smaller votlage and then crank up the resistor ratio then the zener diode noise and vbe noise is amplified.

The output voltage is (VZener+VBE)(1+R1/R2)
VBE of the feedback transitor, R1 = 230k R2 = 82K

VBE of TO-92 typically arround 600mV at 1mA and 650 at about 8mA
Asuming zener or 36v and vbe of 600mv
We get (36+0.6)(1+230/82) = 139.25853V at output very close to simulation results.

The zener diode sets the emitter voltage of feedback transitor to a fixed constant 36v.
The independant point collector will change its voltage untill the base voltage becomes VBE higher than its emitter votlage thus acting as a feedback regulator.

The 470uF should have a 0.1uF in parallel to help kill high frequency noise and make output low noise.
I highly recomend the potential meter to replace the 82k as theres less voltage and power disipation on this one.

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