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Old 19th February 2007, 01:01 PM   #1
h_a is offline h_a  Europe
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Default Pearl: replace zener D1 with voltage divider?

Hi!

When thinking about using a 9V battery instead of the Zener to improve noise I stumpled about the idea to use a voltage divider.

As I understand the zener is a more general solution, especially if everybody can use different transformers and thus get different supply voltages.

However, given I know my supply voltage - say 48V DC - I could also use a voltage divider consisting of 80k6 Ohms instead of R5 (originally 3k32) and one gets 9.04 V over the 18k7 resistor in place of the original 9.1V zener.

(I took a randomly high total resistance to get more easily the correct values and lower the current going straight to ground)

Further the resistors can be matched and thus give better channel equality.

Any downsides to this?

Any things I'm missing?

Would be great if somebody could comment!

Thanks a lot! Have fun! Hannes
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Old 20th February 2007, 09:36 AM   #2
cviller is offline cviller  Denmark
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I'm afraid it is a bad idea... if you use a voltage divider to control the regulation in a regulated supply, you'll get in trouble. The whole point of the regulation is to trade voltage for less ripple. If you have the resitors on the unregulated supply, you'll get all the ripple there transferred to the gate of your regulating mosfet. If you place it on the regulated side, I don't think you'll get any voltage at all.

If you really want to do this, I think you could improve the performance by adding a big (relative to the resitors) capacitor in parallel with the resistor which replaces the zener.

I think you'll get better performance from decoupling the zener with a capacitor - try search the forum on this topic.
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Old 20th February 2007, 10:06 AM   #3
MaxS is offline MaxS  France
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Talking A voltage divider give a perfect voltage without output current

Hi.

Looking at the schematic. D1 is connected to the Base of a bipolar transistor. A BJT is drived by an input current. In the worst case, the input current is 10 mA. The used zener diode provided 28 mA. So, that's good.

If you use a voltage divider, V = E - RI <=> V = 40 - ( 80,6.10^3.10.10^-3) <=> V = 40 - 806 ==> Can we do it ?
A voltage divider must be followed by a buffer. A voltage divider is a theorical thing.

Let's D1 maybe it can induce some noise but no problem BJT are not the most sensible to the noise.

--------------
http://passdiy.com/pdf/pearlphono.pdf At the end of the page 2, regulator schematic.
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Old 20th February 2007, 03:42 PM   #4
h_a is offline h_a  Europe
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Thanks for your replies!

Well also it seems that large value resistors add noise - I never thought that a Zener would have noise on about the same level.

So next idea is to use the 6.9 V LM329 precision voltage regulator that seems promising to me. The drawback is that the discrete opamp needs to be adjusted accordingly (to cope with the lower voltage reference).

I read the DIY opamp article on passdiy, but didn't find any hints on how to calculate the necessary resistor values.

Any idea what resistors need to be changed to deliver the same regulated voltage with the lower voltage reference?

Thanks a lot! Have fun! Hannes

PS: info will be added to the pearl wiki - so the first giving me the values/equations will become famous! C'mon guys!!
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Old 21st February 2007, 08:29 AM   #5
MRupp is offline MRupp  Germany
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You could bias the lower resistor of your voltage divider with a simple JFET current source, thus you would get a fixed voltage accross this resistor and low noise. It means some extra effort in tweaking the current / resistor value to get to the correct voltage drop.
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Old 21st February 2007, 08:38 AM   #6
cviller is offline cviller  Denmark
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Hannes,

If you really want to add complexity, I think a rechargeable and a fancy charging circuit with a switch on the front would give you a much better sound than all the other suggestions here...

Have you measured the ripple on your regulated voltage?
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Old 21st February 2007, 09:30 AM   #7
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Quote:
I think a rechargeable and a fancy charging circuit with a switch on the front would give you a much better sound than all the other suggestions here...
That is only assuming that a) this particular regulator can still be improved substantially just by changing the voltage reference and b) that a battery does not have any noise, which is not correct. For further improvements, if at all possible, you would need to look at the whole power supply, potentially include pre-regulators, try shunt regulators and cap multipliers, etc..
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Old 21st February 2007, 09:35 AM   #8
cviller is offline cviller  Denmark
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Exactly why I think it would be a good idea to measure ripple before spending too much time on making fancy regulators for controlling the regulator....

The battery option is standard pearl, but most objections to this is that people don't want to change batteries in their phono stage.
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Old 21st February 2007, 10:35 AM   #9
h_a is offline h_a  Europe
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Thanks for your replies!

Changing batteries is not the problem! It's going to last forever, since - after removing R5 - it only feeds the leakage current of the 3300uF cap.

After much reading it seems that batteries are also noisier as one might think - but less noisy as the zener as Wayne stated once (link is in the pearl wiki).

I thought a voltage divider would be great and extremely simple - but I didn't know that metal film resistors of some kOhms would create noise of the same magnitude of the zener.

Well what does create less noise and would perfectly fit into the pcb, would be the lm329 subsurface zener. It delivers only 6.9 V, so I thought that adjusting the resistors in the regulator should be easy and would also fit perfectly into the pcb.

I'm just looking for a simple yet powerfull solution.

Unfortunately I still don't know how to fix the gain of the voltage regulator

Maybe somebody more clever than me could help out!

Cheers, Hannes
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Old 21st February 2007, 12:35 PM   #10
cviller is offline cviller  Denmark
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Cool.

Looking at the regulator, I think you can change the voltage by adjusting R10 and R11.

Right know you have 30V*10k/(10k+22k)=9.3V
Try to change the values to get approx 7.1V...
R10 = 25k
R11 = 7.5k

Or something similar.
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