Help needed!

[Aleksandar]

Hi! This is not exactly a schematic question, but maybe a question related to a part of schematic, Anyway, I apologize if post the thread on a wrong place.

I need 15V regulated dual supply, and I've decided to go with Zeners instead of IC regulators. So I have two questions:

- what would be the value for the resistors in series with the Zener diodes?
- I'd also like to have switchable 7.5V dual supply; can I use some DPST switch which would switch on and off the 7.5 Zener diodes in parallel with the 15V ones, and also, what would be the resistor value for the 7.5 diodes?

The transformer would be +-22V or +-18V.

Thanks in advance!

[J M Fahey]

The transformer would be +-22V or +-18V.

First of all clarify whether you refer to transformers 22+22 or 18+18V AC or rectified voltages or +/-22 er 18V. It's not the same.
It would also help if you have some idea of what your preamp will "eat".

[rowdy_riemer]

Why not use IC regulators?

[Aleksandar]

Well yeah, I always get confused with that +- V thing   The secondary coils of the transformers are center tapped, and from one end to the center it measures 18 (22)V.

About the preamp, well, let me describe it in few words, coz I don't have the schematic on my hard drive, and I don't have a scanner. It's a preamp that have two TL 072 op amps in it (input stage with the overdrive circuit, buffer, EQ, buffer) and add to that  two more TL072's from the Condor cab sim that I'll build for the Line out section.  In the original schematic says that I should use +-12V for the preamp. However, when experimenting with zener diodes, I found that on a lowered voltage the preamp got really interesting sound, with more harmonics, and kind of touch sensitive(OK, I won't use the buzzword ). I think that that also has to be somehow related with using Zeners instead of IC regulators, since the Zeners aren't quite stable, and that maybe attributes to that compressed sort of squishy sound I guess. Yeah, and the clipping is done with assymetrical diode clipping circuit. So all in all, when I play lightly, it's almost clean, but when I hit harder, it get's overdriven and compressed.

Now, I have this preamp built together with a TDA 2030 chip for the power amp, and it sounds good to my ears, but I'd like to make it louder, a project that intended to do a long time ago, but never really had the time for it before.

About the 15V, together with that underpowered squishy preamp, I'd also like to have the option for more headroom from time to time, so that's why I'd like to go with the optimal maximum voltage according to the TL072 datasheet, since it can be supplied with +-18V tops, if I remember correctly.

[Aleksandar]

When searching for a solution for my problem, I found this online calculator something:

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html

Do you think its accurate?

[rowdy_riemer]

I don't think the zeners will have any effect on the sound other than to limit the voltage "seen" by the op amps. What voltages do you measure accross the zeners?

[Aleksandar]

Somewhere around 7.5V. Just the amp turned on.  But that was without playing the guitar. Thing happens when I play it.

Here's something that i copied from another forum:

"Zener diode "shares" current with the load; the more is drawn by a load the less goes through zener, and that is one reason for not very stabile voltage regulation with zeners. On the other hand an IC voltage regulator draws from power source a bias current which is almost constant regardless of the load and input voltage and in most cases is negligible."

I'm not that good in science, but maybe it's something about the current, I don't know...However, it's clearly audible.

[rowdy_riemer]

Well, the zener should be parallel with your circuit. It clamps the supply voltage to it's rated voltage. As your preamp draws more current, the zener draws less as less is needed to clamp the voltage. By pulling the supply voltage down to +- 7.5V, you do not have much headspace, so you get more clipping. What is the supply voltage when you use voltage regulators?

[JPHeisz]

To calculate the resistance the dropping resistor in a resistor/zener regulator, you need a rough idea of how much current the circuit will draw. Most opamp circuits, with no discrete devices will draw (much) less than 100mA. Ohms law comes to the rescue:
V=IR
V=15
I=.1
R=150 ohms (15/.1=150)

For the 7.5V, safe to say the circuit will draw half that 100mA, so:
V=7.5
I=.05
R=150 ohms.
A word about wattage (power). The resistor will get hot and need to be of a high enough wattage to avoid burning. Formula for calculatin wattage is P=IV(dropped)
I=.1
V=Vcc-15. If Vcc is 22 that's 7V
P=.7 watts - the resistor needs to be at a minimum .7 watts. Making it double that will give some margin, so 2 watts.
Hope this helps.
EDIT: Some silly mistakes I made

[J M Fahey]

As a rule of thumb, consider each single op amp needing 2mA (see datasheets ), so each TL072 or similar will need 4 mA.
You use 2 of them=8mA.
Design for a 50% higher consumption, both to be covered during brownouts (less than rated wall voltage) and to be able to "someday" add, say, an extra one for whatever you fancy and not having to recalculate everything again.
A typical ROG type preamp will need around 2 or 3mA per FET.
An LM386 used as a preamp needs 4 or 5 mA; if used as power amp needs hundreds of mA and can't be fed from Zener stabilized supplies.
A TL072 driving a reverb tank may need 30 to 50 mA.
Well, now you know.

[Aleksandar]

Thanks!