First Neurochrome Preamp Build

I still get all giddy when I get a micro controller to turn on an LED. Then the pragmatic side of me goes, "ya know... You could just have used a switch for this..." :)

And breaking new ground on the distortion analyzer feels pretty darn good too.

Tom
 
I had a whole relay based input select system that I scrapped because it felt overly complicated. It did work but just felt like a lot of connections in the signal path so I went back to a simple switch.

I also had some pretty nasty hum from the Khozmo volume control. Did some research and found a recommendation to ground the input/output lugs to the same ground as the power supply, which only partially resolved it, so I ran a ground wire from the outside of the control to the chassis and that finally fixed it.
 
Really enjoying my preamp so far. But does anyone have suggestions on how to reduce or eliminate speaker thump when powering on/off?
 
Make sure you turn your system on from the source to the speaker and off from the speaker to the source. I.e., turn the preamp on before the power amp.

Tom
 
I use a remote that powers everything simultaneously, I haven’t had a thumping issue with other preamps.
I typically use some type of speaker protection module that keeps them disconnected until the system becomes stable after power-on, and immediately disconnects them on power off. My current favorite is the Guardian-86, available here. ;)
 
I typically use some type of speaker protection module that keeps them disconnected until the system becomes stable after power-on, and immediately disconnects them on power off.
That’s great is that in the preamp or on the power amp? I’m in the process of building a 686 and have the speaker guardians for that.

If you could point me in the direction of a module that could be added to the preamp though that would be great.
 
The Guardian-86 would go in the power amp.

Powering everything on at the same time is certainly tempting but not always the best approach. It works well if all the equipment starts up at the same time, which is not guaranteed even if the AC voltage is applied at the same time. The Preamp Power Supply (and many other SMPSes) take a second or so to start up and if your power amp starts up within that time you're guaranteed to get a plop in the speakers when the preamp turns on.

This sort of sequencing issues is why the 12 V trigger was invented and both the Preamp Power Supply and the Intelligent Soft Start support that feature.

One way to address the speaker plop that you're experiencing could be to include a mute relay. Simply take a 12 V relay and power its coil with the 12 V output of the Preamp Power Supply. The 12 V trigger output will work too. Wire the relay such that it connects the output of the preamp to ground when the power is off and to the output of the Universal Buffer when the power is on. Don't forget the flyback diode across the relay coil. The KEMET EC2-series of relays is good for this.

Tom
 
The Guardian-86 would go in the power amp.

Powering everything on at the same time is certainly tempting but not always the best approach. It works well if all the equipment starts up at the same time, which is not guaranteed even if the AC voltage is applied at the same time. The Preamp Power Supply (and many other SMPSes) take a second or so to start up and if your power amp starts up within that time you're guaranteed to get a plop in the speakers when the preamp turns on.

This sort of sequencing issues is why the 12 V trigger was invented and both the Preamp Power Supply and the Intelligent Soft Start support that feature.

One way to address the speaker plop that you're experiencing could be to include a mute relay. Simply take a 12 V relay and power its coil with the 12 V output of the Preamp Power Supply. The 12 V trigger output will work too. Wire the relay such that it connects the output of the preamp to ground when the power is off and to the output of the Universal Buffer when the power is on. Don't forget the flyback diode across the relay coil. The KEMET EC2-series of relays is good for this.

Tom
Thanks, I did try using the 12v trigger out to my power amp (Buckeye until I get the 686 built) but there was still a pop when powering down. I don’t think there was one powering up but I can check again.

Should the trigger be going from the preamp to the power amp or vice versa?
 
The ISS has a trigger INPUT. The Preamp Power Supply has a trigger OUTPUT. Connect OUTPUT to INPUT.

The trigger OUTPUT on the Preamp Power Supply follows the 12 V output, so the trigger goes low when the preamp powers down. If the power amp stays up for a while after that you'll get a plop in the speakers. That's why I designed the Guardian-86 to sense the power supply voltage and turn the speaker output off once the supply voltage has dropped to 80 % of its nominal value.

Tom
 
R
The ISS has a trigger INPUT. The Preamp Power Supply has a trigger OUTPUT. Connect OUTPUT to INPUT.

Tom
Right that’s what I thought. Maybe the buckeye amp doesn’t respond the same way as the ISS. I’ll see what happens after I get the power amp built. Thanks again
 
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