How to protect AMP6 inputs.

[armstrom]

I am planning to use my AMP6-Basic as a stereo guitar amp and have been told to prevent the inputs from going above 5V to avoid damaging the tripath chip. I'm a bit of a novice at this so my question may be a bit obvious... Can I simply put a reversed zener diode (say a 4.7V 1W) between each input channel and the ground plane? In my simplistic view of things the diode would block the short to ground until the zener voltage is reached (4.7V in this case) Once this voltage is reached the diode would short the signal to ground preventing a high voltage spike from reaching the input(s) of the power amp.

Is this a suitable solution? Should I place a resistor between the diode and ground to mimic the input impedance of the AMP6 and avoid shorting preamp?

If it helps, the preamp I plan to use is this circuit: http://www.runoffgroove.com/professor.html
I will simply build a two-channel version and use a 9V regulator to drop the 12V supply I already have for the AMP6.

Any advice is greatly appreciated.
-Matt Armstrong

[armstrom]

I think I may have found an answer to my own question but I want to make sure...

If you look at the preamp circuit I linked to above you can see two 10K resistors just before the final output pot (used as low-pass filters).. If you combine that with the 22K input impedance of the AMP6-Basic then the two are acting as a voltage divider of sorts that will pretty much divide the output voltage of the preamp in half. Since the preamp is driven by +9V my theoretical maximum output voltage should be ~4.7V even with the master volume at maximum. I can get a bit more protection by simply running the preamp circuit at 8V (not to mention making use of a more common regulator in the process).

Does this sound correct?

[V-bro]

On the AMP9(-B) there are two schottky diodes (for as far as I know, they could be zeners) on each input, one is connected to 5V (in conducting direction) and one to ground (in reverse).....

My experience is however that it sometimes puts too much DC on the outputs....

------X-max. can be several meters on any driver, too bad it can be done only once...------

[armstrom]

Thanks for the information... What do you think about the assessment that the diodes may not be required based on the minimum output impedance of the preamp circut (20K) and the input impedance of the AMP6B (22K) acting as a voltage divider?? Since the rail voltage of the preamp is 9V the highest voltage swing I could possibly see on the inputs of the tripath chip is ~4.7V.

[V-bro]

That would seem the case to me as well, I think Jan would have included input clamping diodes when there would be a great hazard....

------X-max. can be several meters on any driver, too bad it can be done only once...------

[armstrom]

Yeah, he has no way to know what kind of preamp circuit will be feeding the amp. In my case it just so happens that my particular preamp stage has just the right combination of rail voltage and output impedance to ensure that the voltage will never exceed 5V. Thanks again for your help. I'll be sure to post the results of this project once everything comes together.

[armstrom]

ok, I've changed preamps again... (of course) So, I'm back to thinking of the diode option.. I'm not sure I understand why I would need a diode from the positive rail. Wouldn't that simply pipe rail voltage on to the inputs? In addition, my preamp only uses a positive rail, so it cannot produce negative output voltage.

Does anyone have any more feedback on how to protect these inputs? I really don't want to fry my amp

[V-bro]

You don't connect them to railvoltage, they are conducting from the input to 5V and non conducting from the input to ground...

I've found out they are schottky diodes...

------X-max. can be several meters on any driver, too bad it can be done only once...------