Multi voltage power supply rail

[toptip]

I have observed that in tube electronics different B+ volages are set by resistor networks, followed by capacitors, for each voltage.

I have a solid state preamp that requires +23@135mA, +5.6V@95mA and -12V@9mV.

How bad would it be if I have a stiff 35VDC power supply with a resistor network, consuming 0.5 - 1A and tap it to get the voltages I need?

This maybe an obvious solution. I am asking because I never see it used in solid state projects.

Thank you.

 

[PRR]

> if I have a stiff

How stiff is "stiff"? 10%? 1%? How dynamic is the load? 134mA-135mA steady, or zero to 140mA every thump of the bass?

10% stiffness on 135mA load wants about 1,350mA in a resistor divider. That's 47 Watts of heat to get rid of.

And what "preamp" needs these three odd values? 1959 broadcast line driver? With an ECL kicker?

 

[toptip]

Technics RS-9900

No, it is cassette recording and playback amp. The power supply (and bias oscillator) is in the mechanical section, which I do not have. I use it with other devices that run at 1-7/8 ips.

Current demand appears to be very steady.

 

[rayma]

How would you get the negative voltage? Such RC decoupling networks are often in solid state circuits.

 

[PRR]

How would you get the negative voltage?..

His +23..,... and -12V cleverly add-up to his "stiff 35VDC".

Altho the +23V is at 11 times the current of the -12V, so not much synergy.

 

[PRR]

Take the three specified loads, the specified power source, and two "dropper" resistors, it works out fine.

The unit in question:
Technics RS-9900 Stereo Cassette Deck Manual | HiFi Engine
Tell me everything you know about the Technics 9900! - Tapeheads Tape, Audio and Music Forums

 

[toptip]

Altho the +23V is at 11 times the current of the -12V, so not much synergy.

Well, the synergy is really in having a simpler power source.

Couple of questions:

-- According to my calculations, this wastes approximately 5W total
-- I probably should add capacitors past the resistors to stabilize the voltages?

Many thanks

 

[JMFahey]

I have observed that in tube electronics different B+ volages are set by resistor networks, followed by capacitors, for each voltage.

Discrete SS preamps also do that, all the time.
Only Op Amp circuits tend to demand ubiquitous +/-15V or thereabouts,but clearly that´s not the case here.

I have a solid state preamp that requires +23@135mA, +5.6V@95mA and -12V@9mV.

OK.
Forget wasteful brute force resistor chains.
If you start with a +35V supply:

* use a 24V regulator such as 7824 for the "23V" preamp.
Heat sink it, it will dissipate some 1.5 W all the time, so a small U type one will do.

* for 5.6V use a 7805 with an 1N4002 diode between "ground/reference" leg and ground . It will dissipate 2.8W so surprisingly will need a larger heatsink than the "main" circuit, a small 30x30mm or so finned one will do.
or a larger U type.

* no way you can get negative voltages from a +35V only supply so you will need some source for that.

You give us NO details about your "35V supply" .

IF a conventional transformer+bridge+large capacitor you can kludge a -12V supply since it is low current.

IF it´s an SMPS brick, you can not.

When/if we get more details about your actual supply we might suggest a couple other solutions, but so far ....

 

[toptip]

Technics cassette amp

I think first I will try using 3 SMPS bricks.

I already have plenty 5.5V and 12V packs from various gadgets. Just found a 23V one on eBay for $20.

They maybe too noisy, I will see how it goes.

 

[v4lve lover]

This is what i would do, take your pick from the data sheet.


TL431 is 10 cents in quantities, Best thing since sliced bread.

 

[toptip]

TL431

Since I am not familiar with this device, what is its function in Fig. 30, when there is already a fixed voltage regulator?

Thanks

 

[Mark Johnson]

Here is the TL431: datasheet

It's a wonderful part, just perfect for many positive-output-voltage power supply applications.