Not that it matters, but I was wondering why the NFB loop is tied to the 8 ohm tap of the OT versus the 4 or 16 ohm taps?????
Just trying to understand the logic behind it....
NFB question
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Re: NFB question
That's funny, I was swapping the OT (took out Moose's and put in a Stancor A-3800) on my Liverpool last night and I was thinking the very same thing! I'm gonna wager a very uneducated guess and postulate that it's because the 8-ohm tap is in the middle of the windings? I really don't know enough about how transformers actually work to know for sure but that's gonna be my guess...
Andy
Andy
Re: NFB question
i had a nfb tab selector switch in mine.
it changes the sound of the pres. the harmonics sound different from each tab. the funny thing is that i finally ended up with the 8ohm tab for the nfb.
ralph
it changes the sound of the pres. the harmonics sound different from each tab. the funny thing is that i finally ended up with the 8ohm tab for the nfb.
ralph
Re: NFB question
probably because Ken didn't have 141k and 70k resistorsRon Worley wrote:Not that it matters, but I was wondering why the NFB loop is tied to the 8 ohm tap of the OT versus the 4 or 16 ohm taps?????
Just trying to understand the logic behind it....
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NFB
Hello!
In a hi-fi amp such as the famous Williamson circuit, they actually measure the DB of feedback. I think it used 10 DB of feedback. Some circuits use less, others more. The design is such that 10 DB of negative feedback lowers the overall volume of the amp by that amount of DB. Different taps on the OT will change the DB of feedback. There is no "law" about how much you can have, the only factor I have found is stability of the amp. Another big difference with hi-fi amps is you actually "tune" the feedback. For example, you put a .1uf cap across the speaker, then you apply a 5khz Square wave to the amp and look at the shape of the Square wave. The "ringing" and or "overshoot" (The front and rear of the wave) can be changed by adding a small cap across the feedback resistor. The .1uf cap is necessary to get the amp to have ringing in the first place. Anyway, once you have added the correcting cap across the feedback resistor, you can actually add a huge value of cap across the speaker and the wave still looks great.
In a hi-fi amp such as the famous Williamson circuit, they actually measure the DB of feedback. I think it used 10 DB of feedback. Some circuits use less, others more. The design is such that 10 DB of negative feedback lowers the overall volume of the amp by that amount of DB. Different taps on the OT will change the DB of feedback. There is no "law" about how much you can have, the only factor I have found is stability of the amp. Another big difference with hi-fi amps is you actually "tune" the feedback. For example, you put a .1uf cap across the speaker, then you apply a 5khz Square wave to the amp and look at the shape of the Square wave. The "ringing" and or "overshoot" (The front and rear of the wave) can be changed by adding a small cap across the feedback resistor. The .1uf cap is necessary to get the amp to have ringing in the first place. Anyway, once you have added the correcting cap across the feedback resistor, you can actually add a huge value of cap across the speaker and the wave still looks great.
Re: NFB question
Randall Aiken has some great info about designing NFB circuits on his website. I plugged his formulas into an Excel sheet and sure enough, Roe's values are correct. As stubbyfex said, NFB is measured as gain reduction.
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Re: NFB question
I lowered the NFB to 68k and the amp reduced in gain and sounded nice on the ohm tap.
I changed it back to 100k and the amp has increased gain on the 8 ohm tap.
Based on this chart if I switched between the 8 ohm tap and 16 ohm tap using a 100k NFB resistor I would have the same affect 6.6 reduction.
Using this same theory then I should be able to have a switch with a 68k on one end for 8.8 reduction using 8 ohm tap and a 100k on the other end of the switch to the 8 ohm tap for 6.6 reduction.
Any problems with using a switch?
Mark
I changed it back to 100k and the amp has increased gain on the 8 ohm tap.
Based on this chart if I switched between the 8 ohm tap and 16 ohm tap using a 100k NFB resistor I would have the same affect 6.6 reduction.
Using this same theory then I should be able to have a switch with a 68k on one end for 8.8 reduction using 8 ohm tap and a 100k on the other end of the switch to the 8 ohm tap for 6.6 reduction.
Any problems with using a switch?
Mark
-
Ron Worley
- Posts: 908
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- Location: Keller, TX
Re: NFB question
David Bailey (v864) sent me a pm, which makes perfect sense:
Ron,
copied this from Randall's site about NFB..in a nutshell the 16 ohm tap would give you more fb, the 4 ohm would be less..
http://www.aikenamps.com/
Quote:
Note that Marshall typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms. You can get the same attenuation from a 10K/500ohm pair as you would from a 100K/5K pair. In addition, if you were using a 100K/5K attenuator running from the 16 ohm tap, you would get roughly the same amount of feedback if you used a 47K/5K attenuator running from the 4 ohm tap. Note that the tap voltages are not linear with respect to the impedance, it varies linearly with the square root of the impedance, that is, the voltage on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the voltage on the 4 ohm tap is half the voltage on the 16 ohm tap. It helps if you think of the equation for power: P = V^2/R. If you have 100W into 16 ohms, the voltage is V = sqrt(100*16) = 40V RMS. If you have 100W into 8 ohms, the voltage is V = sqrt(100*Cool = 28.28V RMS. If you have 100W into 4 ohms, the voltage is V = sqrt(100*4) = 20V RMS.
BTW, this is information that is the property of Randall Aiken and his alone. I copied it without permission, so my bad.....Sorry Randall!!
Ron,
copied this from Randall's site about NFB..in a nutshell the 16 ohm tap would give you more fb, the 4 ohm would be less..
http://www.aikenamps.com/
Quote:
Note that Marshall typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms. You can get the same attenuation from a 10K/500ohm pair as you would from a 100K/5K pair. In addition, if you were using a 100K/5K attenuator running from the 16 ohm tap, you would get roughly the same amount of feedback if you used a 47K/5K attenuator running from the 4 ohm tap. Note that the tap voltages are not linear with respect to the impedance, it varies linearly with the square root of the impedance, that is, the voltage on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the voltage on the 4 ohm tap is half the voltage on the 16 ohm tap. It helps if you think of the equation for power: P = V^2/R. If you have 100W into 16 ohms, the voltage is V = sqrt(100*16) = 40V RMS. If you have 100W into 8 ohms, the voltage is V = sqrt(100*Cool = 28.28V RMS. If you have 100W into 4 ohms, the voltage is V = sqrt(100*4) = 20V RMS.
BTW, this is information that is the property of Randall Aiken and his alone. I copied it without permission, so my bad.....Sorry Randall!!
Last edited by Ron Worley on Tue Mar 17, 2009 10:27 pm, edited 1 time in total.
Ron
Re: NFB question
Ron,
Makes perfect sense. Thanks.
Mark
Makes perfect sense. Thanks.
Mark
Re: NFB question
Forgive me for bumping a 10 year old thread, just wanted to make sure that I got how that excel spreadsheet works right.
Let's take the AA764 circuit as example:
Following this schematic, should I put the 47Ω fixed resistor value as the "Presence Pot" in the spreadsheet and the 2700Ω as the NFB Resistor? Or the other way around?
Thanks!