PPA builders' thread
Oct 24, 2006 at 12:15 AM Post #631 of 646
Thank you Tangent! The amp sounds great!

I'd like to go back and add ZNR, which is now jumpered. I am using a CRD(1N5304) which is rated at 1.8mA. I am using a LED that has 4.9V forward voltage and If = 20mA. If I use 12 x AAA battery pack and I would like the LED to shut off at 12V, then will a 6.8V or 7.5V zener work, right?
 
Oct 29, 2006 at 1:20 AM Post #632 of 646
so today I had the pleasure of upping the buffer bias from 10mA to 30mA. Wow, a HUGE improvement!! I was wondering if anyone had any luck with increasing the opamp bias point from 1mA and what value would be considered reasonable.

Also, I'm running OPA637BP x 2 + OPA227PA for the ground channel. I also have three each of OPA602AP, OPA228PA, OPA227PA, OPA637AP and OPA134PA on hand and would appreciate any recommendations on what combination to try until I get a OPA627 to put in my ground channel
wink.gif


edit: oops, I almost forgot... when using the OPA627 in the ground channel, do you recommend bumping C6 up to 100pF? Currently, I have 10pF silver mica in C6. Would this be a good location to try out a polystyrene capacitor?
 
Oct 29, 2006 at 12:14 PM Post #633 of 646
Quote:

Originally Posted by luvdunhill
I'd like to go back and add ZNR, which is now jumpered. I am using a CRD(1N5304) which is rated at 1.8mA. I am using a LED that has 4.9V forward voltage and If = 20mA. If I use 12 x AAA battery pack and I would like the LED to shut off at 12V, then will a 6.8V or 7.5V zener work, right?


Yes, something like that. Getting the exact behavior you expect may require some experimentation, because the zener's knee is fairly soft. You get the most abrupt shut-off if you set it to a lower voltage, but then that gives you less warning that the battery pack is about to crater.

Oh, and so you understand, the CRD's value and the LED's If limit are irrelevant here. All that matters is the LED's forward voltage and your desired cut-off point.

Quote:

when using the OPA627 in the ground channel, do you recommend bumping C6 up to 100pF?


Can't hurt, but I wouldn't do it in advance. Do it later, if you have problems.

Quote:

Currently, I have 10pF silver mica in C6. Would this be a good location to try out a polystyrene capacitor?


Yes. The only reason I recommend micas by default is that they're a whole lot easier to find.
 
Nov 19, 2006 at 6:02 PM Post #636 of 646
Nov 19, 2006 at 10:52 PM Post #638 of 646
Yeah, that's fine. You can mix in radials and box. It won't affect functionality. The only thing that matters is if it physically looks good to you.
 
Nov 30, 2006 at 3:52 PM Post #640 of 646
Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif
I was wondering if I could get away with 4.7k instead of 4.32k for R1. I understand that R1=R3+R5 but was wondering if this was a hard and fast rule.

Thanks!


^ polite bump ^

I'd like to understand how I can change R1 to another value
 
Nov 30, 2006 at 4:17 PM Post #641 of 646
To change R1, it still must follow these rules:

Right / Left Channels:
R1 = R3 + R5

Ground Channel:
R1 = R4

If you want to change R1, feel free, but remember to change R4 on the ground to be the same. Also, remember the whole optimizing loops thing. Usually, you would only change R4 to set the gain because R1, R3 & R5 are closely linked together.
 
Mar 9, 2008 at 11:53 AM Post #643 of 646
waiting for parts ...
 
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