Need analog chips to regulate +/-70vDC to +/-60vDC

[enut11]

@bdunham7
The raw 70v is direct from rectifier/filter and the capacitance can be varied.

The audio amp input caps are more difficult to change due to space on the PCB

The regulators will only feed the audio power amp.
enut11

Just one question: why are you using a regulated power supply, for an audio amplifier? It's not normally necessary, since the power supply rejection of a well designed amplifier makes voltage regulation unnecessary.

If the design you're using is for 60V, but you have 70V, on the filter capacitors, then it's better to modify the circuit for the higher voltage, than to add an unnecessary voltage regulator. It's probably as simple as increasing the voltage ratings of a few components and using larger heat-sinks.

EDIT:
I've just read the original post again and I'm confused.

I want to regulate a raw +/-70vDC supply to +/-60vDC at less than 100mA. It is a low noise metrology project so aiming to use analog chips.

I found the TL783 125v will do the positive side but have yet to find a suitable negative chip.

Any suggestions from the analog gurus?
enut11

Now you say it's an audio amplifier.

Please post a schematic or data sheet for what you're powering.

There is no schematic supplied by the vendor.
Please see reply #11 for context

[bdunham7]

The transformer is a 50VA toriodal and feeds only the amp via the regulators. 50vAC to 70vDC unloaded. Variability depends on the 240vAC mains fluctuations.
Transformer load is less than 50mA as it supplies voltage (not power) to a DMM.

OK, so then you need to know the actual quiescent and maximum current that the amplifier draws (I'd test that directly if at all possible), as well as the maximum line voltage fluctuation you want to plan for--5% ? 10% ?  B/t/w, does your audio amplifier require at least some load for a non-distorted output?

If you are drawing directly from a small, dedicated transformer then you have to account for more variation than if you were drawing from a larger regulated supply.  Some people have suggested using a much larger zener or TVS that won't blow up in case the regulator does shut down and although what I've drawn doesn't do that, it might not be a bad plan.  Those large 5W zeners are not expensive for a one-off device.  Also, your transformer isn't as grossly oversized as you might think--the peak voltage will drop quickly under any significant load and the RMS current of the transformer is multiples of the DC current, how many multiples depends on how close to the peak voltage your DC level is.  At 50mA I think you'll be fine, at 300mA I'm pretty sure you'd start to have issues.

[magic]

If found this AliExpress kit without problem

I can't find any seller showing the schematic publicly. But what I can see is two trannies marked 1815 on the silkscreen. If these are 2SC1815 and it surely does look like they are, and if they are the input pair, and if they are not cascoded, I have bad news for you.

Observe that they only rate this amp for ±56V supplies for some reason

[enut11]

The 56v rating is for use as a power amp. I am using the module as a voltage amp.
https://www.ebay.com.au/itm/124926624992

[enut11]

The transformer is a 50VA toriodal and feeds only the amp via the regulators. 50vAC to 70vDC unloaded. Variability depends on the 240vAC mains fluctuations.
Transformer load is less than 50mA as it supplies voltage (not power) to a DMM.

OK, so then you need to know the actual quiescent and maximum current that the amplifier draws (I'd test that directly if at all possible), as well as the maximum line voltage fluctuation you want to plan for--5% ? 10% ?  B/t/w, does your audio amplifier require at least some load for a non-distorted output?

If you are drawing directly from a small, dedicated transformer then you have to account for more variation than if you were drawing from a larger regulated supply.  Some people have suggested using a much larger zener or TVS that won't blow up in case the regulator does shut down and although what I've drawn doesn't do that, it might not be a bad plan.  Those large 5W zeners are not expensive for a one-off device.  Also, your transformer isn't as grossly oversized as you might think--the peak voltage will drop quickly under any significant load and the RMS current of the transformer is multiples of the DC current, how many multiples depends on how close to the peak voltage your DC level is.  At 50mA I think you'll be fine, at 300mA I'm pretty sure you'd start to have issues.

Min mains voltage is 235v, max is 245v
Audio amp quiescent current is 25mA
No minimum amp load required

[magic]

The 56v rating is for use as a power amp. I am using the module as a voltage amp.
https://www.ebay.com.au/itm/124926624992

Wouldn't matter if the input stage craps out. But it looks like I was wrong and they are the current mirror.

https://www.diyaudio.com/community/threads/ljm-audio.180625/page-28#post-4844361

[bdunham7]

Min mains voltage is 235v, max is 245v
Audio amp quiescent current is 25mA
No minimum amp load required

In that case, it will work pretty much as drawn except you have to carefully select the PTC so that its maximum trip current isn't more than the 68R input resistor can take.  If you want even lower ripple and output impedance, you would put a 10uF 100V electrolytic across each of the 11K3 voltage divider resistors.

Alternatively, if you want to make it a bit more robust, capable of more start/surge current and perhaps adjustable to a slightly lower output voltage you would change the input resistor to 33R and the zener to two 12V 5W units in series and select a PTC with a trip current under 400mA in all conditions that it is likely to see.  For a per-rail voltage adjustment feature, put a 1M trim pot in series with a 100K resistor across the 11K3 resistors, and change the value of the 11K3 resistors to more like 12k 1% to make sure you can get over 60V.

[Zero999]

The circuit I posted previously, can be used with the LM337, for negative voltages, if the polarities are swapped.
https://www.eevblog.com/forum/projects/need-analog-chips-to-regulate-70vdc-to-60vdc/msg4020262/#msg4020262

[Wolfgang]

You could try a negative floating LM723 configuration, if you want long term stability and low noise. See National datasheets.

[enut11]

Thanks @Wolfgang. Low noise is good in this application so well worth investigating a 723 solution.

[enut11]

Min mains voltage is 235v, max is 245v
Audio amp quiescent current is 25mA
No minimum amp load required

In that case, it will work pretty much as drawn except you have to carefully select the PTC so that its maximum trip current isn't more than the 68R input resistor can take.  If you want even lower ripple and output impedance, you would put a 10uF 100V electrolytic across each of the 11K3 voltage divider resistors.

Alternatively, if you want to make it a bit more robust, capable of more start/surge current and perhaps adjustable to a slightly lower output voltage you would change the input resistor to 33R and the zener to two 12V 5W units in series and select a PTC with a trip current under 400mA in all conditions that it is likely to see.  For a per-rail voltage adjustment feature, put a 1M trim pot in series with a 100K resistor across the 11K3 resistors, and change the value of the 11K3 resistors to more like 12k 1% to make sure you can get over 60V.

Thanks @bdunham7
A variable output control for the regulators is a very good idea as it allows final voltage trimming to suit both the input and output conditions.