preamp ps problem

Briney · 16th November 2005, 02:53 AM

Greetings,
I've run into a problem building the 12v filament supply in the attached
drawing. I couldn't find 15v trans. so I got a couple of 14v hammonds.
The problem is the reg. drops the voltage from 14 to about 10.6v.
How do I bump the voltage up to 12.6v.
Thanks
Brian

cpu_bach · 16th November 2005, 03:17 AM

Try a low-drop-out regulator IC instead of the standard 7812. LM2940 family is one of the examples for the low-drop ICs.
Good luck!

kmtang · 16th November 2005, 01:02 PM

Can you check the DC voltage at the input of the 7812 regulator IC? I believe it should be somewhere around 18Vdc. It should provide sufficient volt for the 7812. If the transformer is with CT configuration, you may use 2 diodes for the full wave rectification. This will lower the volt drop across the rectifier circuit.

The total filament current draw should be 0.475A that the 7812 should able to handle.

Johnny

tubelab.com · 16th November 2005, 01:49 PM

The 7812 regulator needs at least 14 volts input to work. With the diode in the ground leg it will want 15. Check the voltage at the input to the regulator, if it is slightly low you can pick up an additional .5 to .8 volts by switching to Schottky rectifiers, they have a lower voltage drop, and are usually quieter.

If that doesn't provide enough voltage, use a low dropout voltage regulator instead of the 7812. The previously mentioned National Semiconductor part is a good option. I use the LM2941, adjustable regulator, but it has 5 pins which makes hand wiring dificult

Briney · 16th November 2005, 02:28 PM

The voltage before the regulator is about 14.6. After it drops to 10.6.

I tried removing the regulator altogether and added a 8ohm resistor
and that gave me 12.6 exactly but I am worried that I'm deviating too
far from the original design and I'll suffer noise on the phono and line
stage.

I tried a couple of 1n4004's to the ground leg but that only raised a few tenths of volts.

I'll look for LM294X at mouser.

Brian

tubelab.com · 16th November 2005, 02:48 PM

The 8 ohm resistor is not a bad idea, especially if a reasonably large capacitor is added to ground after the resistor. A resistor in series with the tube heaters limits the inrush current, and promotes longer filament life. I would leave it there, and finish the preamp. Then you can listen for noise.

Frank Berry · 16th November 2005, 03:01 PM

Quote:

Originally posted by Briney
The voltage before the regulator is about 14.6. After it drops to 10.6.

I tried removing the regulator altogether and added a 8ohm resistor
and that gave me 12.6 exactly but I am worried that I'm deviating too
far from the original design and I'll suffer noise on the phono and line
stage.

I tried a couple of 1n4004's to the ground leg but that only raised a few tenths of volts.

I'll look for LM294X at mouser.

Brian

If the voltage is 14.6 volts going into the regulator, it's possible that the 10,000uf capacitor is open or is not connected properly.
Check this capacitor. If your transformer is indeed delivering 14 volts into the full wave bridge and the input capacitor is doing its job, you should see a little over 18 volts at the input of the regulator.

amperex · 16th November 2005, 05:31 PM

Although well covered here in other posts, at least 5 volts higher than needed on the input side should be used. Line voltages can dip some during different times of the day.

Too large of an input cap will actually produce lower voltage as the transformer can not recover from the repetative peak surge current . I see this condition clearly on a power supply simulator program.

I would just purchase another transformer at 18 volts output and heatsink the regulators. I consider 15 volts AC marginal with a lower current transformer. Obviously, it is.

Frank Berry · 16th November 2005, 07:47 PM

Are your Hammond transformers capable of delivering enough current? As tubelab.com asked, when the output voltage of the regulator dips to 10.6 volts, what is the input voltage to the regulator?

I would not use a transformer that produces 18 volts. That would put 25.5 volts at the input of the 7812. Too much voltage drop across the little guy when it is delivering .425 amps.

Briney · 16th November 2005, 08:49 PM

The trannie is 1a (I'm guessing I'm at work, Ill check in two hours when I get home)
I 'll have a look at my wiring, I have plenty of the 10k caps, I'm thinking
they're backwards??
The voltage doesn't appear to change until it gets to regulator so if I'm supposed to have 18v that gives me a couple of things to check.

16th November 2005, 01:49 PM	#4
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	The 7812 regulator needs at least 14 volts input to work. With the diode in the ground leg it will want 15. Check the voltage at the input to the regulator, if it is slightly low you can pick up an additional .5 to .8 volts by switching to Schottky rectifiers, they have a lower voltage drop, and are usually quieter. If that doesn't provide enough voltage, use a low dropout voltage regulator instead of the 7812. The previously mentioned National Semiconductor part is a good option. I use the LM2941, adjustable regulator, but it has 5 pins which makes hand wiring dificult __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

16th November 2005, 02:48 PM	#6
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	The 8 ohm resistor is not a bad idea, especially if a reasonably large capacitor is added to ground after the resistor. A resistor in series with the tube heaters limits the inrush current, and promotes longer filament life. I would leave it there, and finish the preamp. Then you can listen for noise. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

16th November 2005, 07:47 PM	#9
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	Are your Hammond transformers capable of delivering enough current? As tubelab.com asked, when the output voltage of the regulator dips to 10.6 volts, what is the input voltage to the regulator? I would not use a transformer that produces 18 volts. That would put 25.5 volts at the input of the 7812. Too much voltage drop across the little guy when it is delivering .425 amps. __________________ Frank

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16th November 2005, 03:17 AM	#2
cpu_bach diyAudio Member Join Date: Jun 2004 Location: Yokohama	Try a low-drop-out regulator IC instead of the standard 7812. LM2940 family is one of the examples for the low-drop ICs. Good luck!

16th November 2005, 01:02 PM	#3
kmtang diyAudio Member Join Date: Nov 2004 Location: Vancouver	Can you check the DC voltage at the input of the 7812 regulator IC? I believe it should be somewhere around 18Vdc. It should provide sufficient volt for the 7812. If the transformer is with CT configuration, you may use 2 diodes for the full wave rectification. This will lower the volt drop across the rectifier circuit. The total filament current draw should be 0.475A that the 7812 should able to handle. Johnny

16th November 2005, 02:28 PM	#5
Briney diyAudio Member Join Date: Jan 2003 Location: chicago	The voltage before the regulator is about 14.6. After it drops to 10.6. I tried removing the regulator altogether and added a 8ohm resistor and that gave me 12.6 exactly but I am worried that I'm deviating too far from the original design and I'll suffer noise on the phono and line stage. I tried a couple of 1n4004's to the ground leg but that only raised a few tenths of volts. I'll look for LM294X at mouser. Brian

16th November 2005, 05:31 PM	#8
amperex diyAudio Member Join Date: Feb 2005 Location: MI	Although well covered here in other posts, at least 5 volts higher than needed on the input side should be used. Line voltages can dip some during different times of the day. Too large of an input cap will actually produce lower voltage as the transformer can not recover from the repetative peak surge current . I see this condition clearly on a power supply simulator program. I would just purchase another transformer at 18 volts output and heatsink the regulators. I consider 15 volts AC marginal with a lower current transformer. Obviously, it is.

16th November 2005, 08:49 PM	#10
Briney diyAudio Member Join Date: Jan 2003 Location: chicago	The trannie is 1a (I'm guessing I'm at work, Ill check in two hours when I get home) I 'll have a look at my wiring, I have plenty of the 10k caps, I'm thinking they're backwards?? The voltage doesn't appear to change until it gets to regulator so if I'm supposed to have 18v that gives me a couple of things to check.

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