Maybe i have to tell how i came to the idea of making this circuit.
I have a presonus audio pre-amp and i compared my pre-amp circuit with it (soundwise)
I noticed on high volume dc ripple in the ac signal (sound) in my circuit but not at all in the presonus pre amp at same volume level.
I tried to find the schematics for that presonus pre-amp to maybe learn how they do it and i saw that they did use a dual-powersupply.
Altough this presonus pre-amp comes with an external 12v DC power supply. (see atachment of backside presonus 12vDC input)
Therefore i assumed that the circuit i found converted 12vDC to +12v/-12v.
And i tought, yes that's what i want. Because i then don't have to bias the op-amps anymore, and maybe that is the cause of the ripple noise.
I allready used decouple caps on the op-amp. The circuit works and can only hear this ripple noise with cranked up volume, but on the same volume level with the presonus i don't hear this ripple.
The presonus circuit looked very simple
and i ordered exactly the same components as on the schematics. (that's where the 15v/-15v voltage regulators come from) For me +10/-10 or even +5/-5 is ok. But i don't know if it would work in that circuit (with other voltage regulators of 5 or 10v)
I build it, and then had not the result I expected.
And i don't really understand this circuit, i just copied it so i tought i will ask on a forum, maybe they see immed what is wrong.
And you did help a lot, I now know the circuit needs AC in stead of DC and it needs at least 17V AC.
Still i don't understand how they do that in the presonus pre-amp, there has to be a stage before to convert 12VDC to >=17VAC that isn't documented??? (and is that possible at all?)
So you ask for my pre-amp schematics again
.. But i don't have it right now, i will make it once i have this dual power supply +12/-12v
but also +5v/-5v should do the trick. As long a I don't solve that i cant make my new op-amp circuit. Does sound logical, no?
So that was it
I don't mean to offend or harass anyone, nor to be recognised as a genius who can make +15/-15v out of a 9vAC supply
(I wich it were true hahaha)
I know and understand that's not possible, didn't really claim that, just that i measured these voltages (not knowing that the AC supply really gave 12VAC in stead of 9vAC wich was on the label)
Hi,
Things clearing up...
AC supply 9V:
For a short circuit proof AC adapter it is not unusual, that the "no load" output voltage is that high.
But expect less output voltage when loaded. Like 9V at an ohmic load that draws 1.3A.
But your bridge rectified supply circuit is no ohmic load. This makes voltage calculation a bitw difficult.
Voltage regulators:
You urgently need to read the datasheets.
They tell (TI LM79xx datasheet) you that the ouput voltage of an LM7915 may be -14.4V ... -15.6V at 0.5A load and -14.25V ... -15.75V with 5mA ... 1A load.
I assume you measure without load. Not even with the 5mA of the datasheet condition. Thus you can expect even wider variation.
Additionally it tells you you need at least -17.5V input voltage. Don´t go below that - not even for worst case conditions: Full load, lowest expectable line voltage,.. not even for a millisecond.
I´m not sure you can guarantee this. (fluctuating ouptut voltage lets me assume you are too close to the limit) Thus I recommend to use +12V and -12V regulators. They need at least 14.5V. This should be OK.
...and the preamp will be happy with +/-12V.
Maybe it´s a good idea to run a LED with 5mA at each ouput. It gives some optical feedback and ensures a minimum load.
This sounds as if you have a preamp schematic. Where is it?
Best if you could show us your preamp schematic with dual power supply. Hand made is OK.
I´m worried about the "-9V" this makes no sense. At this -9V condition, what is the voltage at the regulator input?
400 Ohms. This will cause more than 30mA for a usual LED .. this is too much for usual LEDs.
Klaus