Hi all, I am 50% finished with my lm3886 amp build. I am designing the power supply for the preamp portion, which will be a "proffessor tweed" from ROG, and I have a question about the way I am proposing to derive the lower voltage rails for the preamp. I have done the calcualtions to get +/- 6v supplies from my +/-23.4v poweramp rails. The design uses a basic resitive divider from the +23.4v to ground to get the +6v rail, and the same thing from the -23.4v to ground. The idea is to use the -6v rail as the ground point for the preamp, effectively making it a +12v design.
My question is this: will running the premap with a ground point that is offset -6v relative to the ground of the poweramp create an awful groundloop/hum problem? Would it be better to derive a non symetrical single +12v supply rail from the +23.4v rail instead? I thought of this, but did not want to create an uneven current draw from one rail that might cause some sort of oscillation issues... Any advice would be helpful and greatly appreciated!
Just another quick thought. would an isolation transformer between the preamp output and the poweramp input solve this potential problem? Would I need another at the preamp input?
Hi Darwin, please post the schematic or link to your preamp. If it´s powered with +12V, just get them from +B (in this case around 24V ) with a resistor (1K 1W), a 12V Zener and a 100uFx16 or 25V electrolytic. Don´t worry about your preamp asymmetrically loading your supply, it uses nothing (a few mA) compared to the 3886. Bye.
JM, thanks for your reply.
Here is the link to the professor tweed schematic. http://www.runoffgroove.com/professor.html (http://www.runoffgroove.com/professor.html)
The schem has it running off of +9v, but for use as a pre most folks (I checked at DIY audio forum) use +12v for more headroom (with no ill effects).
I am glad to hear that it won't be an issue to just derive the lower voltage from the +24v rail in terms of current draw.
Some questions, however:
Why does one need to use a Zener diode? to keep current from flowing "backward"? or is it for voltage drop? Is it a big no no to just use a resistive divider to get the required lower voltage?
Zener diode is a more constant reference. It works this way: In addition to forward bias, every diode will also begin to conduct when it is exposed to high enough reverse voltage. Typically this reverse voltage is quite high but in Zener diodes it is intentionally lowered. For example, a 15V zener diode blocks current like any ordinary diode but when the reverse voltage exceeds 15V the diode turns conductive. Thus, it makes a "perfect" voltage reference. Far more perfect than a resistive voltage divider that simply varies its voltage according to voltage at the input.
(http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/diode_characteristics/iv_charac_3.jpg)
For example, feed 30V to a 50%-50% resistive voltage divider (that has its other end connected to ground) and the output voltage at the junction of the two resistors is 15V. Now, feed 40V and the output voltage is 20V. It is clear that a resistive divider cannot provide a steady voltage reference, which means that the supply rail ripple can creep to the preamp circuit. Not good. Now, replace the bottom resistor of the divider with a 15V zener diode and the voltage in the center junction is always 15V (unless you go below the zener voltage, which prevents the zener from conducting). The upper resistor of the divider separates the zener diode from a direct connection to the higher voltage supply, which effectively limits current so the zener will not be destroyed. Add a filter cap for smoothing and you have a very simple regulator, similar to ones used in countless cheap amps.
Teemu, Yet again you have been able to teach something to me that I did not know before! Thank you... I now fully understand the use of zener diodes in this application. I now wonder if three-pin voltage regualting IC's (like lm7812) work in the same way? I mean are they basically that circuit but in IC form? I suppose I could just use one of those regulators for this purpose (duh, why didn't I think of this earlier!), which would be doing the same job!
Hi Darwin. Nice preamp, should sound good. Will work better with 12V than with 9V. Please post some MP3´s and pictures when ready. Bye.