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    Building, troubleshooting and testing of these amplifiers should only be
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DC at output on startup

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Hello, I have a 12au7 balanced preamplifier and at startup, there is a DC spike of roughly 3 volts measured at the output. It eventually goes to 0 after a second or two but this is enough for the amp to have DC current at the speaker terminals. This is without any input signal connected. I did not have the 2sk373 on hand so I used the 2sk170 and I was wondering if this could be the culprit? I have attached the schematics and BOM. Any help would be greatly appreciated.
 

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Hi

I looked at your schematics and I think the total value of the output capacitance (15+15µF) is too high.

The slow change in the voltage at the cathode output of the cathode follower during start up is transmit by this high value to the right side of the capacitance.

If the load for this balanced pre amplifier is 10 k ohm, the cutoff frequncey is 0.5Hz with 30µF.

A better cutoff frequency would be 3Hz and a total capacitance ( with 10 k load ) of 5 µF.

And with a higher cut off frequency, the voltage change on the left side will not be transmitted to the right side (or will be transmitted with enough attenuation).

Jacques
 
Output caps

Thank you for the response! Just to clarify, I should replace the eight 15uF output capacitors with eight 5uF ones?

Hi

I looked at your schematics and I think the total value of the output capacitance (15+15µF) is too high.

The slow change in the voltage at the cathode output of the cathode follower during start up is transmit by this high value to the right side of the capacitance.

If the load for this balanced pre amplifier is 10 k ohm, the cutoff frequncey is 0.5Hz with 30µF.

A better cutoff frequency would be 3Hz and a total capacitance ( with 10 k load ) of 5 µF.

And with a higher cut off frequency, the voltage change on the left side will not be transmitted to the right side (or will be transmitted with enough attenuation).

Jacques
 
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Hi bvtrinh,
Most preamplifiers these days have a mute function that operates until the circuits have stabilized. The best way (in my opinion) is to short the outputs to ground while in mute mode. A power up - power off mute is a very handy function to have, then you can also mute the output when you are playing with patch cables. Don't touch the output cables while the amplifiers are on and not muted.

Yes, I know this is a tube preamplifier.

-Chris
 
Thank you

Thank you Anatech for the response! I was wondering where I could find a diy or premade power on - power off mutin module?

Hi bvtrinh,
Most preamplifiers these days have a mute function that operates until the circuits have stabilized. The best way (in my opinion) is to short the outputs to ground while in mute mode. A power up - power off mute is a very handy function to have, then you can also mute the output when you are playing with patch cables. Don't touch the output cables while the amplifiers are on and not muted.

Yes, I know this is a tube preamplifier.

-Chris
 
Administrator
Joined 2004
Paid Member
Hi bvtrinh,
I usually roll my own for the specific situation. Try and look up circuits for the 555 timer IC. It's very simple to use and can be used for all kinds of things. Also look up 555 + mute + relay, or something close to that. I'm not that good at looking things up on the web.

-Chris
 
Administrator
Joined 2004
Paid Member
Hi john_tracy,
Thank you for posting that. That kit is second from the bottom of the page.

A few have been designed by members here, but don't forget to check the store here!

This is a link to a pair of boards sold in our store. It is a combination of soft start and time delay turn on, two separate PCBs. These for the same price of $25 USD. You need to supply the parts and put them together, and that's a huge plus in my book. More fun too. Of course there are instructions with them.

I would buy from the DIYAudio store, given a choice.

-Chris
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.