Preamp - Page 2

Original Burnedfingers · 5th January 2005, 11:30 AM

Quote:

You should be fairly safe against almost any fault involving DC on the outputs as long as the output coupling cap doesn't go leaky though.

The key word here is fairly safe. Why chance being fairly safe? Add $1 worth of 3.3v zenier diodes and protect your investment.
Two zeniers on each channel will NOT change the sound and WILL
protect your sand amp.

Note***

The last line stage I built(1 month ago) had 600v $30 ea coupling caps but still managed to do $50 in parts(damage) to my SAE amp. I repaired the SAE and added the zeniers to the input jacks......no more worries.

audiousername · 5th January 2005, 11:37 AM

Quote:

Originally posted by burnedfingers
The last line stage I built(1 month ago) had 600v $30 ea coupling caps but still managed to do $50 in parts(damage) to my SAE amp. I repaired the SAE and added the zeniers to the input jacks......no more worries.

Eeek

Did you ever figure out what caused the fault? The zeners will protect the input of the amp, but the preamp shouldn't be out to destroy the inputs anyway. Maybe a little bit of valve-sand rivalry

I probably wouldn't have posted that if I ever had an input stage destroyed.

Coulomb · 5th January 2005, 11:42 AM

Dont' forget to load the input with low value resistor to avoid parasitic feedback that can be created with the addition of the Zener Diodes on the input. Say 220 to 250 ohms.

Regards

Anthony

anatech · 5th January 2005, 10:08 PM

When a tube preamp is turned on or off the output will swing through a large range. If the coupling cap is a high enough value, the charge / discharge curent can damage the input transistors. Really bad if the amplifier power is on.
Zeners may affect the sound and you have to suze them for peaks in the music. 1N4148 in series with the zeners may help in that respect. The best solution is a relay that shorts as soon as power is removed in the preamp and stays shorted for a while after power on. This will not affect the sound as much as diodes.
-Chris

GRD · 6th January 2005, 12:12 AM

How would the zener diodes be connected? Sounds interesting. I had a bad experience with a tube pre-amp connected to a SS amp and would like to avoid a potential problem in the future.

Coulomb · 6th January 2005, 01:22 AM

Nelson Pass used a Zener network in his SOBOZ design. If you go to passdiy.com you can download the article. He describes in some detail how the circuit works, just be sure to follow his advise about the 220R resistor on the input.

Regards

Anthony

5th January 2005, 11:42 AM	#13
Coulomb diyAudio Member Join Date: Jul 2002 Location: Ancaster, Ontario	Dont' forget to load the input with low value resistor to avoid parasitic feedback that can be created with the addition of the Zener Diodes on the input. Say 220 to 250 ohms. Regards Anthony __________________ I like to stay current!

6th January 2005, 01:22 AM	#16
Coulomb diyAudio Member Join Date: Jul 2002 Location: Ancaster, Ontario	Nelson Pass used a Zener network in his SOBOZ design. If you go to passdiy.com you can download the article. He describes in some detail how the circuit works, just be sure to follow his advise about the 220R resistor on the input. Regards Anthony __________________ I like to stay current!

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5th January 2005, 11:30 AM	#11
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Quote: You should be fairly safe against almost any fault involving DC on the outputs as long as the output coupling cap doesn't go leaky though. The key word here is fairly safe. Why chance being fairly safe? Add $1 worth of 3.3v zenier diodes and protect your investment. Two zeniers on each channel will NOT change the sound and WILL protect your sand amp. Note*** The last line stage I built(1 month ago) had 600v $30 ea coupling caps but still managed to do $50 in parts(damage) to my SAE amp. I repaired the SAE and added the zeniers to the input jacks......no more worries.

5th January 2005, 10:08 PM	#14
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	When a tube preamp is turned on or off the output will swing through a large range. If the coupling cap is a high enough value, the charge / discharge curent can damage the input transistors. Really bad if the amplifier power is on. Zeners may affect the sound and you have to suze them for peaks in the music. 1N4148 in series with the zeners may help in that respect. The best solution is a relay that shorts as soon as power is removed in the preamp and stays shorted for a while after power on. This will not affect the sound as much as diodes. -Chris

6th January 2005, 12:12 AM	#15
GRD diyAudio Member Join Date: Jul 2004 Location: CA	How would the zener diodes be connected? Sounds interesting. I had a bad experience with a tube pre-amp connected to a SS amp and would like to avoid a potential problem in the future.

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