ayan wrote:The math doesn't lie: I said that in a 50W amp Rf should be decreased to 70% of the 100W value, ALL OTHER THINGS REMAINING EQUAL. But if you will be changing the TAIL resistor on the PI as well, you need to modify my statement to read: In a 50W amp, the voltage divide ratio (= Rt/(Rt+Rf))needs to be 1.4 times (reciprocal of 0.7) greater than in a 100W amp to have the same NFB loop.
I'm getting there. Now, does the "tail wag the dog?" In other words, when scaling the presence circuit from a 100W BM schematic to a 50W amp, should one try to maintain the 4k7 tail resistor, and instead of 100k Rf use a 37k, or do you go the other way and put a 12k5 tail resistor in with a 100k Rf?
I also note that the 50W Marshalls typically used the 100k/4k7 pair...
greiswig wrote:[
I'm getting there. Now, does the "tail wag the dog?" In other words, when scaling the presence circuit from a 100W BM schematic to a 50W amp, should one try to maintain the 4k7 tail resistor, and instead of 100k Rf use a 37k, or do you go the other way and put a 12k5 tail resistor in with a 100k Rf?
I also note that the 50W Marshalls typically used the 100k/4k7 pair...
To clarify, the TAIL is the last resistor on the PI chain to ground... The problem with changing IT, instead of just changing the FEEDBACK resistor, is that you will change the gain structure of the PI and the sound of the amp. A longer tail (everything else remaining the same) approximates a "current source" (which is the ideal model for this type of PI) and makes the PI more balanced. Good for some things, not so good for others, I suppose.
Note that the Marshall PI, for example, hs a much smaller overall path to ground from the cathode(s) of the PI. Dumble deliberately made a pretty balanced PI, IMHO, originally, as he was going for that very smooth tone.
greiswig wrote:[
I'm getting there. Now, does the "tail wag the dog?" In other words, when scaling the presence circuit from a 100W BM schematic to a 50W amp, should one try to maintain the 4k7 tail resistor, and instead of 100k Rf use a 37k, or do you go the other way and put a 12k5 tail resistor in with a 100k Rf?
I also note that the 50W Marshalls typically used the 100k/4k7 pair...
To clarify, the TAIL is the last resistor on the PI chain to ground... The problem with changing IT, instead of just changing the FEEDBACK resistor, is that you will change the gain structure of the PI and the sound of the amp. A longer tail (everything else remaining the same) approximates a "current source" (which is the ideal model for this type of PI) and makes the PI more balanced. Good for some things, not so good for others, I suppose.
Note that the Marshall PI, for example, hs a much smaller overall path to ground from the cathode(s) of the PI. Dumble deliberately made a pretty balanced PI, IMHO, originally, as he was going for that very smooth tone.
Gil
Okay, my mistake...I was trying too hard to make a pun. So let's ignore the tail resistor, which clearly shouldn't be changed. Now, where I previously said "tail resistor," plug in "Rl" instead. Should the Rl value be changed, or should Rf? For a 50W Bluesmaster, do you want Rf/Rl pair of 100k/4k7 (same as 100W), or 37k/4k7 (scaled Rf), or 100k/12k5 (scaled Rl)?
greiswig wrote:
Okay, my mistake...I was trying too hard to make a pun. So let's ignore the tail resistor, which clearly shouldn't be changed. Now, where I previously said "tail resistor," plug in "Rl" instead. Should the Rl value be changed, or should Rf? For a 50W Bluesmaster, do you want Rf/Rl pair of 100k/4k7 (same as 100W), or 37k/4k7 (scaled Rf), or 100k/12k5 (scaled Rl)?
Sorry to have missed the pun... If you read my posts, you will realize that the tail of the PI is the LOAD across which the feeback voltage is developed, such that Rt = Rl. I have also said that changing anything but Rf will change the sound of the PI. I am not sure I can say anything else that I haven't already said.
I think you are confusing scaling the two different PI circuits.
Keith, I wasn't addressing (confused or otherwise) any part of your post(s).... I made a statement, which I repeated in a couple of different ways, which I stand by. I was not responding to any of what you wrote.
Wow, thats funny "G" was how Greiswig signed his last post. I was trying to clarify what you (Gil) wrote for him since you already explained and clarified so much.
I "think" greiswig (G) confusion was that he is missing the fact that the BM PI is 4.99% and not 7.7%. That was what I was attempting to get across.
Gil, thanks for the posts, they definately helped me understand scaling the NFB loop. Of Course, providing my math is correct!
I think you are confusing scaling the two different PI circuits.
Keith, I wasn't addressing (confused or otherwise) any part of your post(s).... I made a statement, which I repeated in a couple of different ways, which I stand by. I was not responding to any of what you wrote.
Cheers,
Gil
Never mind, simultaneous post by Keith made my point
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keithrick wrote:Wow, thats funny "G" was how Greiswig signed his last post. I was trying to clarify what you (Gil) wrote for him since you already explained and clarified so much.
Indeed it was. I am used to being called "G" by a couple of close friends, so I thought you were talking to me!
I think you are confusing scaling the two different PI circuits.
You're absolutely right, I was. Thanks for pointing it out. I *do* want to understand the math, and the "why" behind the engineering here, but sometimes a simple yes, no or redirect is most helpful.
keithrick wrote:
See if my calculations make it clear:
=RT/(Rt+Rf)
4700/(4700+100,000) = .0449 100watt
Convert to 50watt
.0449x1.4 = .0628 50 watt
4700/(4700+70,000)= .0629
That is why Scott suggested changing RF to 68K or 75K to get the 50 watt ratio of the BM PI.
Informative thread!
I was wondering why I preferred my FB on the 8 ohm tap and now the math is catching up to what my ears already knew. my amp is 50watt but I was using the 100 watt values and wondering why it was a tad harsh in the upper mids on the 4 ohm tap. The 8 ohm tap = more voltage = more feedback. A 68k resistor change would have had a similar affect as coming off the 8 ohm tap and it is easier to wire (in my amp anyway ).
Thanks for the EE class!
"G" was how Greiswig signed his last post. I was trying to clarify what you (Gil) wrote for him since you already explained and clarified so much. 
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