Two regulators from one transformer

[Andrew Borg]

Hi guys I've built two regulators. One with +9v/-9v with 7809/7909 and another +12v with 7812 .
The thing is when I connected them one to mixer that I built and the other to amp nothing is coming out from amplifier.
When I've used two 9v batteries sound was ok.
Any help please?
Thanks.

 

[JimB]

Can you draw a schematic of your connections to the transformer/regulators/amplifier.

There could be several reasons why this does not work.

JimB

 

[spec]

Hy Andrew,

You don't post on ETO too often do you.

Care to tell us where you are from and put it in your user window on the left of your posts?

We would need a schematic of your triple power supply to be able to diagnose your problem.

Can you also describe your mixer and amp?

spec

(crossed posts Jim)

 

[Andrew Borg]

Hi Jim and Spec thanks for response.
These are the schematics:
https://www.aaroncake.net/circuits/mixer2.asp
https://schematics.circuitdiagram.net/viewer.php?id=klz1235988630v.gif
https://www.circuitstune.com/2011/12/12v-power-supply-circuit-diagram.html
The double pole power supply is on 7809/7909 they I used.
Thanks.
Andrew

 

[ronsimpson]

Did you measure the output voltage?
Note the pin-out is different on the regulators! Pins 1 & 2 are different.
Do you have a heat sink? What does it look like?

 

[JimB]

Note that the 9v PSU circuit and the 12v PSU circuit cannot be powered from the same winding on a transformer.
If you do you will effectively short out the rectifier diodes and transformer.

JimB

 

[MikeMl]

Note that the 9v PSU circuit and the 12v PSU circuit cannot be powered from the same winding on a transformer.
If you do you will effectively short out the rectifier diodes and transformer.

JimB

Another way of saying it is that the +-9V supply requires a center-tapped secondary winding on the transformer, labeled J2 on that schematic. Note that it is tied to GND of the loads (amplifier/mixer). The only way you could power the full-wave bridge for the 12V supply (and tie the common-anode end of that full-wave bridge to GND) is by having a separate winding on the existing transformer, or by using a second transformer.

This schematic shows how to fix your problem. Note how the high-current supply (5V in this case, 12V in yours) is fed from an isolated winding. Also note that it takes a center-tapped winding to feed the +- regulators.

 

[ronsimpson]

Note that the 9v PSU circuit and the 12v PSU circuit cannot be powered from the same winding on a transformer.

Why?

 

[MikeMl]

Why?

I am assuming that TS needs a few mA at +-9V, while TS needs an Amp or more at +12V.

It is rare to find a center-tapped transformer that likes that... It would have to have a center-tapped secondary rated at ~30V at ~2 or more A.

Also, C1 will have to be much, much bigger.

 

[Andrew Borg]

Ok so on a single centre tapped trafo I cannot power both circuits. That's why when I use circuits separately all is well.
So I use another transformer ct for mixer or batteries.
Thanks

 

[MikeMl]

Ok so on a single centre tapped trafo I cannot power both circuits. ...

You can, using Ron's suggestion, but the transformer secondary voltage/current rating will have to simultaneously be adequate for both +9V and +12V.

 

[schmitt trigger]

If your negative supply requires only a few milliamps (about 10 mA or so) it is usually better to have a single transformer power both positive regulators, and then take a charge pump to generate the negative one.

The charge pump can be built with a 555, or even better with the ICL7662. These two are ubiquitous on the internet, there are dozens and dozens of examples.

If you require higher performance, you can use one of the best devices from Linear or Maxim.

 

[audioguru]

Are your microphones "dynamic" type? A dynamic type has a low impedance and is made with a coil moving over a magnet like a speaker. Most microphones today are "electret" type that need to be powered and use a different circuit.
The mixer circuit uses lousy old 741 opamps that were designed 48 years ago and are too noisy (hiss) for audio. Use modern low noise audio opamps instead.

 

[Nigel Goodwin]

Are your microphones "dynamic" type? A dynamic type has a low impedance and is made with a coil moving over a magnet like a speaker. Most microphones today are "electret" type that need to be powered and use a different circuit.

Not at all, most 'decent' microphones are dynamic or capacitor, only 'toy' mikes tend to be electret - such as used in phones etc.

Mixers are designed to use mikes that people use - low impedance dynamic ones - often with the capability of phantom powering capacitor ones as well.

 

[JimB]

Note that the 9v PSU circuit and the 12v PSU circuit cannot be powered from the same winding on a transformer.
If you do you will effectively short out the rectifier diodes and transformer.

Why?

I was adding two and two together and getting some number other than four.

The circuit proposed by ronsimpson will work correctly.

What I was thinking, because Andrew Borg had linked to two schematics found on the internet, he was simply connecting them together where the rectifier connects to the transformer. Indeed, this could be why "it works on batteries, but not on the PSU".

In the forum we have seen this done by various neophytes over the years, most of them wanting to know where the smoke came from.

JimB

 

[Andrew Borg]

If your negative supply requires only a few milliamps (about 10 mA or so) it is usually better to have a single transformer power both positive regulators, and then take a charge pump to generate the negative one.

The charge pump can be built with a 555, or even better with the ICL7662. These two are ubiquitous on the internet, there are dozens and dozens of examples.

If you require higher performance, you can use one of the best devices from Linear or Maxim.

Hi Schmitt can you post a circuit of yhis charge pump that you mentioned please?
Thanks

 

[audioguru]

Not at all, most 'decent' microphones are dynamic or capacitor, only 'toy' mikes tend to be electret - such as used in phones etc.
Mixers are designed to use mikes that people use - low impedance dynamic ones - often with the capability of phantom powering capacitor ones as well.

In my career, expensive modern Audio-Technica mics were used in handheld wireless mics, gooseneck lecturn mics, lavalier mics clipped to clothing and earphones with mics worn by performers. They are all electret type. I show the excellent frequency response from a cheap "toy" mic.
Some performers liked the shrill "presence" peak at about 6kHz and reduced low frequencies from expensive old dynamic mics. Old condenser mics needing a high voltage (48V) phantom supply were "old school" like amplifiers with vacuum tOObes (valves).

 

[Nigel Goodwin]

In my career, expensive modern Audio-Technica mics were used in handheld wireless mics, gooseneck lecturn mics, lavalier mics clipped to clothing and earphones with mics worn by performers. They are all electret type. I show the excellent frequency response from a cheap "toy" mic.

As far as I'm aware most handheld wireless mikes are dynamic, and pretty well all wired handheld mikes are.

The gooseneck and lavalier mikes tend to use electret for their small size.

Some performers liked the shrill "presence" peak at about 6kHz and reduced low frequencies from expensive old dynamic mics. Old condenser mics needing a high voltage (48V) phantom supply were "old school" like amplifiers with vacuum tOObes (valves).

Condenser mikes are still commonplace (although I've never actually used one), they are commonly used in recording studios - most mixers will power them (including mine).

Dynamic mikes, although used extensively in recording studios as well, are more common as live performance mikes - including miking instruments.

 

[MikeMl]

Hi Schmitt can you post a circuit of yhis charge pump that you mentioned please?
Thanks

Here is an alternate way of making a charge pump. I am assuming that the sum of the load currents for the +9V and +12V regulators is ~1A. I assume that you only need ~-15mA for the -9V regulator. Note that with a 14Vrms non-CT single-winding Tranny, you get sufficient voltage at V(plus) and V(minus) so that the regulators do not drop out, while keeping the power dissipation as low as possible.

 

[spec]

Hi Jim and Spec thanks for response.
These are the schematics:
https://www.aaroncake.net/circuits/mixer2.asp
https://schematics.circuitdiagram.net/viewer.php?id=klz1235988630v.gif
https://www.circuitstune.com/2011/12/12v-power-supply-circuit-diagram.html
The double pole power supply is on 7809/7909 they I used.
Thanks.
Andrew

Issue 03 of 2016_08_02

Hy Andrew,

Does the schematic below depict the essence of your triple power supply circuit? If so it will work fine, subject to a few detailed aspects which we can discuss.

You have not asked this, but my advice would be to just have two supply lines, 12V and -12V. Then you can power the mixer from 12V and -12V and the amplifier from just the 12V supply.

Running the mixer from 12V and -12V will improve its performance. Of course, we would need to check the mixer circuit in detail to ensure that it will take 12V and -12V supply lines, but I would be very surprised if not.

spec

​

ERRATA
(1) Connect a 1K2 resistor to the 12V output line and 0V
(2) Connect an 820R resistor to the 9V output supply line and 0V
(3) Connect an 820R resistor to the -9V output supply line and 0V
(4) Make C2 4,700uf (10mF)
(5) Make C9 1,000uF (1mF)
(6) B1 can be made from four 1N400x (where 'x' = any number from 1 to 7) rectifier diodes.
(7) Each transformer secondary should be 16V RMS at 1 A minimum (assuming a 1A current drain by the power amplifier)