OA2 regulator question

[PeteC]

Hi ,

can anyone help me with some advice ....

I'm building a couple of preamps for my home studio that each have a current draw of c 30ma.
The design I'm following uses 2x OA2 in series to get a regulated B+ of 300v or so for a mono preamp. The point to point boards are almost complete and I'm waiting for the in/out transformers to arrive from Sowter.

http://www.sowter.co.uk/schematics/EMI-REDD47.pdf

As I'm building 2 units ( for stereo ) I want to understand whether the 2x OA2's could supply both channels ( with maybe another RC stage ) , or whether I would be better having split regulator supplies for each channel.

Max current for 0A2 is spec'd at 30ma, but I guess I could ensure that they draw less than that with series resistance - but the load itself will draw c 60ma for both channels.

I also thought about using a string of 6x51v power zeners - but again not sure whether just one string of 6x51v 5Watt zeners would be ok to supply both channels, and whether I need to have any additional circuitry - or just replace the OA2's with the zener string. ?

How should I go about it ?



Thanks in advance for any help
Cheers
Pete

[Nick]

If you are going to use zeners like that, better to add a mosfet to do the heavy lifting.

http://www.geofex.com/Article_Folders/m ... tfolly.htm

(end of that page)

[Mike H]

Yes, Zeners as a shunt is OK for just a few mA, I have done this.

You can either:
Use an 'amplified Zener' type shunt regulator, which is chain of Zeners in parallel with a power MOSFET, and where the chain is used to create a gate control Voltage (right-hand drawing of that, but have not tried this).

Or, you could use the MOSFET as a series pass regulator, using the Zener chain as the V reference.

[PeteC]

thanks both,
I'm very new to those reg circuits, so trying to get my brain around them.

the unreg voltage is probably going to be 390v at the rectifier first cap, and the reg voltage needs to be 300v

There a a few R-C filters before the shunt reg though which will drop enough voltage down to c. 320v or so i think.

The whole PS needs to supply c. 50-60ma at 300v.
Any more thoughts or suggestions very welcomed !!

Rgds
Pete

[Mike H]

Just to add that a shunt reg needs those resistors, for a series reg they probably have to be omitted.

[PeteC]

Cheers

I have 2 spare OA2s I can use but not 2 for each channel....

so , would 3 x 51v zener string sitting below a single OA2 ( with maybe a resistor across to to make sure it fires on startup ) work for each channel ?

I could then rig this up for each channel after the R-C filters.

Rgds
Pete

[Mike H]

Ah right becoming clear now, you happen to have 2 x OA2 and are trying to use them

I'll bet there's all sorts of problems with this idea.

I'd be tempted to try single pair OA2's for both channels, see how that goes. Maybe have the 1k resistors values divided by 2 to run the 2 channels.

[PeteC]

Thanks Mike,

I think I am going to try use the single PSU with a single pair of OA2s as you suggest - giving 300v at 60ma, then branch off with one further RC Filter for each channel to give some further separation - adjusting the resistors in the RC chain to suit

My original concern/query was if I could run both channels ( 2x30ma at 300v ) from the same single pair of OA2s as the reg for the whole preamp.

OR .....- whether I could dump the OA2s and use a 6x51v zener string instead. ( 5 Watters )
The amplified zener thing is a bit beyond my understanding at present - I dont know how to go about determining the component values around the
MOSFET.

as these are two separate microphone pre's in the same chassis I might be tempted to try one channel with OA2 Reg and the other channel with Zener reg.

The unit will primarily be used 2 x mono rather than as a stereo pair ( albeit I will want to do that occasionally no doubt ! )

Any more observations are very welcome !!!

And ...my nice Sowter TXs just arrived !!!! 4 little beauties.
Rgds
Pete