# Thread: Adding a presence control to a Deluxe AB763 type circuit that didn't have one

1. Let´s take a shortcut.
Simulating is great, online calculators are the best human invention since the hot water bottle, sadly real World parts refuse to work like their models

My approach here is:
1) start with the "original" schematic , in this case the AB763 (or any other you like), built as God/Leo intended.
Meaning with the original 820/47 ohm NFB.

2) inject enough 1 kHz tone (won´t link to my favorite tone MP3 download site for fear of being accused of trying to sell sports jerseys cheap or something ) so you´ll have to google by yourselves.

3) rise volume until you have a nice round value at the output, say, 2V RMS , call it V1 .

4) open FB loop, in this case by lifting one end of the 820r resistor.
Output will rise by a definite amount, because now you turned the power amp into an open loop one.
Measure new value, call it V2 .

5) Now calculate feedback factor, or how large a gain margin you have to use for Presence (or Resonance) duties.
You can use a calculator fo V2/V1 , throw values into a simulator or just click wedding cakes cheap .

Or do what a fascist misogynist bigot retrograde guy would do: use pencil and paper

6) you will typically get between 6dB and 12dB .

Calculate the value of a cap which will have the same impedance as the smaller resistor at 400 to 800Hz, that will give you your Presence Boost.

7) since very low resistance pots are hard to next to impossible to get, consider using a Presence **switch**.

Presence is not subtle at all, you may have it OFF (which is nothing more than the amp in its original form) or ON ; you can back down a little the Treble control as a fine tuning one.
End result is good and quite practical.

2. Juan,

Especially the second one, about cheap cake?

Justin

3. I love cake! How can I get cheap cake?

4. Originally Posted by J M Fahey
Let´s take a shortcut.
Simulating is great, online calculators are the best human invention since the hot water bottle, sadly real World parts refuse to work like their models

My approach here is:
1) start with the "original" schematic , in this case the AB763 (or any other you like), built as God/Leo intended.
Meaning with the original 820/47 ohm NFB.

2) inject enough 1 kHz tone (won´t link to my favorite tone MP3 download site for fear of being accused of trying to sell sports jerseys cheap or something ) so you´ll have to google by yourselves.

3) rise volume until you have a nice round value at the output, say, 2V RMS , call it V1 .

4) open FB loop, in this case by lifting one end of the 820r resistor.
Output will rise by a definite amount, because now you turned the power amp into an open loop one.
Measure new value, call it V2 .

5) Now calculate feedback factor, or how large a gain margin you have to use for Presence (or Resonance) duties.
You can use a calculator fo V2/V1 , throw values into a simulator or just click wedding cakes cheap .

Or do what a fascist misogynist bigot retrograde guy would do: use pencil and paper

6) you will typically get between 6dB and 12dB .

Calculate the value of a cap which will have the same impedance as the smaller resistor at 400 to 800Hz, that will give you your Presence Boost.

7) since very low resistance pots are hard to next to impossible to get, consider using a Presence **switch**.

Presence is not subtle at all, you may have it OFF (which is nothing more than the amp in its original form) or ON ; you can back down a little the Treble control as a fine tuning one.
End result is good and quite practical.
Eh.?. Low value pots aren't THAT hard to come by. I'm not talking 47R or anything. But 100R (might need to use wire wound 3W), 500R (Alpha makes them, I buy them from Mouser and use them all the time) or 1k (also Alpha/Mouser). Those values are low enough not to kludge any PI circuit operation significantly and cap values that correspond to these pot values aren't tricky either.

I'm not too hot on a presence "switch". It's one of those tonal refinements that feels better 'dialed in'. Probably because of the "feel" factor involved with NFB levels at ANY frequency.

JM2C

5. Originally Posted by J M Fahey
Let´s take a shortcut.
Simulating is great, online calculators are the best human invention since the hot water bottle, sadly real World parts refuse to work like their models
Measuring the gain of a real amplifier is cheating
I can't do that when I'm sitting here with my feet up watching TV.

The sim I posted was for a 6V6 amp driving an 8k : 8ohm OT.
LTP PI gain is about 25
The data sheet says 6V6 transconductance is 3.75mA/V. 6V6 voltage gain is therefore 3.75 * 8 = 30
OT impedance ratio is 1000 therefore voltage ratio is 1000^0.5 = 31.6
Open loop gain is- PI gain * 6V6 gain /OT ratio = 25 * 30 /31.6 = 23.7 = 27.5dB

27.5dB is nearly the same as the open loop gain in the sim. Is the calculation valid or did I just get lucky?

I repeated it for a similar EL84 amp and it also worked out to be the same as the sim.

6. Originally Posted by J M Fahey
Or do what a fascist misogynist bigot retrograde guy would do: use pencil and paper
I like pencil and paper. Does that make me a fascist misogynist bigot retrograde guy???

But seriously, both methods have their own merit.

This thread is posted in the Theory & Design sub-forum, so pencil and paper methods aren't out of line.

If your interest is limited to one particular amp then measure the open-loop gain and closed-loop gain and make changes as appropriate. that's the most efficient way to address the problem with an amp on the bench.

If your interest is in understanding the math, or gaining an understanding of a principle so that you can apply it universally across all of your future amp builds, then the most efficient way to address the problem is with pencil and paper; the pencil and paper method takes longer in the beginning but saves you bench time in the long run.

Different strokes for different folks.

7. Originally Posted by g1
So from the discussion Sarah, do you think this is a simple or complex case?
Can you recommend any of those hooking manuals?
I find it interesting that for your first post you have chosen to link to an external website.

8. Again with the vaguely related post and no links to any weirdness!?! I'm having trouble figuring this out with so little incriminating evidence.

9. Originally Posted by Sarah
Thanks Sarah. Your input is appreciated.

10. Originally Posted by Chuck H
I clicked it but didn't stick around long once I saw it was just another on line calculator.?. But the bots are becoming more strategic. They play it cool and then try to subtly slip in their wares. It never works here because:

A) No chicks post here (ok, I suspect one or two, but they never came off like a bot)
2) That calculator is only vaguely related to the subject by virtue of being electronics related. Clearly just an insertion maneuver.

11. Originally Posted by bob p
... makes me wonder why Fender went from Rk=27k in the AA763 to 22k in the AB763. Is there a signficant difference?...
But they left the same voltage notes on the AB763 schematic LoL. Theoretically, the cathode voltage on the AB763 should be a wee bit lower (not that it would much difference). The answer to your question remains elusive. (Sorry I can't be more helpful) :-)

12. Originally Posted by Leo_Gnardo
Clouseau again? "Pairhaps thees Sarahbot has confus-ed MEF with her crocheting club chatroom, eh?"

I wonder if that link leads to a special offer on sports team jerseys. Bet you half a peach it does.
Boy, you can say that again!

13. Originally Posted by SoulFetish
Boy, you can say that again!
Nope! Its not a bot, ishe's a She, and she's a electronics type, just posted on another thread.

14. Originally Posted by SoulFetish
Boy, you can say that again!
Nope! Its not a bot, she's a She, and she's a electronics type, just posted on another thread.

15. It's nice to be reminded every so often that one should not be too cynical.

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