Access 312 Gain pot
« on: June 29, 2009, 11:13:47 AM »
Heyo - I didn't want to put this in the white market thread as I didn't want to confuse a sales/support issue.

On the 312 BOM fro 3nity it calls for a 10k rev pot for gain control, other posts I have read call for a 22k or 25k rev (;topic=246.0 - for reference).  What should be used here?  Will there be a difference with a 10k?

Also I am going to do this with a lorlin pot and I need to confirm the resistors I should use.  Based on the thread I linked previously - for a 22k style pot in 3-4 db increments i would use:

db   resistor (ohms)
66   0
62   120
58   307
54   604
50   1075
46   1820
42   3001
38   4874
34   7842
30   12545
26   20000
22   31815

and for a 10k in 3-4 db increments (from the SDS calculator) I would use:

db   resistor (ohms)
66   0
62   37
58   58
54   93
50   150
46   230
42   370
38   585
34   925
30   1500
26   2300
22   3700

(I have rounded the numbers) - can someone please shed some light on whether it should be 10k or 22k (and what the difference will be) and if my 10k resistor count is correct?



« Last Edit: June 29, 2009, 11:16:03 AM by man-bot »

Re: Access 312 Gain pot
« Reply #1 on: June 29, 2009, 12:41:25 PM »
The (voltage) gain formula for this is (Rg+Rf)/Rg, where Rg is your pot/switch resistance and Rf is your feedback R.  Include the series resistor (usually 200R with Rg).  

3nity uses 10K pots because he uses 10K in the feedback loop.

Usually Rg = Rf approximately, so when your gain knob is at minimum gain (max resistance), the formula gives a voltage gain of 2. (6 dB). Don't know which opamp you are using, but this is the minimum stable gain for the melcor, for example. I don't know which, if any of the others, are unity stable, or if it matters much.

As for the dB scale in your post, you should be thinking from 6dB to 40dB, the rest of the gain in an API 312 is from transformers and doesn't come into play in this equation. You figure the opamp gain first, then add the trafo gains to get your total gain (usually 60+ dB).  Your resistors should be going from 0-22K or, if you use 10K, from 0-10K.

EDIT: I should maybe add, that this question has probably been answered more than once in the main 312 thread. No big deal, but if you have any other questions about this build, please post there.
« Last Edit: June 29, 2009, 12:43:11 PM by mitsos »


Re: Access 312 Gain pot
« Reply #2 on: June 29, 2009, 12:50:05 PM »
Use the search feature bro!! ;D

very well explanation Mit!

As Mitsos said too this question comes a lot!

« Last Edit: June 29, 2009, 12:51:59 PM by 3nity »
Life is a path, death the destination.


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