Design parameters for feedback

[dobbhill]

Does anyone know which output transformer tap the feedback resistor is designed to be connected to? It does make a difference. I like the 4ohm tap myself, but the OCD bug in my brain wants to know "What is correct?"
Thanks, as always, in advance.
D

[Funkalicousgroove]

use the 4 ohm tap with 3.9-4.7k FB resistor, 8ohm tap w/ 100K +/-

[dobbhill]

Thank you, sir!
D

[Bob-I]

I like to put in a pot, adjust until I like the sound then measure. Of course I usually end up with well known values anyway

My latest build has a 4.7K from the 4 ohm tap. Sounds great.

[dobbhill]

Me too. Thanks.
D

[llemtt]

It should be 4.7k on 4ohm tap but this is for the 100w version, so I prefer 3k-3.3k on 4ohm for my 50w.

[dogears]

Hi Teo,

Are you sure you scale the resistor based on total amp output?? I am not.... Thanks!!!!

It should be 4.7k on 4ohm tap but this is for the 100w version, so I prefer 3k-3.3k on 4ohm for my 50w.

[Funkalicousgroove]

it's such a minor difference between the two I'd Imagine it has more to do with a sound/response preference, JMHO...

[dogears]

I respectfully disagree. Going from 4.7k to 3.3k is a HUGE difference. I have a switch on my 50 that goes from 3.9k to 4.7k and the tones are very different. Now imagine adding another 20% NFB! Try it and you will hear....... Of course, use what your ears want, but the difference is not minor.

it's such a minor difference between the two I'd Imagine it has more to do with a sound/response preference, JMHO...

[Normster]

I hate to ask this, but if you change the 4k7 NFB, don't you also have to change the 390R to ground? From Aiken's site:

The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer.

The actual resistor values used in the feedback attenuator aren't that important, as their ratio determines the amount of feedback. The shunt resistor value is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output.

[llemtt]

Are you sure you scale the resistor based on total amp output??

I am not completely sure about the right value to scale the 100w -> 50w, I have to redo my calculations and simulations, I'll post asap.

(This subject is somehow explained in O'Connor TUT3: how to make a 50w Plexi sound like a 100w)

Regarding 3.3k I forgot to mention that my amps include "power scaling" so it's also a compromise because at less than half power it helps to prevent the amp becoming to much loose.

Teo

[Funkalicousgroove]

Dog,

I think what YOU refer to as a HUGE difference differs GREATLY from what most of us consider even audible. Put down that Iron and Go find a band-tweaker!!!!

[llemtt]

..I am not completely sure about the right value to scale the 100w -> 50w, I have to redo my calculations and simulations, I'll post asap.
..

Here is

The 100w with 4.7k feedback resistor has 5.1dB of feedback while the 50w with 4.7k has only 3.8dB. If you want to obtain 5.1dB on the 50w the feedback resistor should be lowered to 3.3k. (see attach)

Another solution would be to increase the gain of the PI by 1.3dB but I never tried it, I fear it changes amp character too much.

Above calculations are made under assumption of standard OT values 1.900ohms primary the 100w and 4.200ohms the 50w, so this isn't exactly a 2X value. The result is a lower output impedance for the 50w at about 6.5ohms instead of 8ohms for the 100w (using 8ohms tap) and this in turn will render the 50w too much tight and controlled and with a flatter freq response.

A 3.9k feedback resistor is required to obtain the same output impedance so scaling in the 3.3k-3.9k range would be acceptable altough perfect scaling requires a 50w OT with a 2X impedance primary: 1.900ohms -> 3.800ohms not the usual value.

my .02
Teo

[dogears]

Hi Teo,

Assuming that the taps are rated correctly, I can't see how power scaling would change anything. The 50 watt amp has less gain than the 100 and therefore the db drop would scale as well... no?? You would not want the 5.1db since your y axis 100 watt graph is 4db higher.

In addition, in your sentence below, you say that the 50 watter would be tighter and more controlled when in fact, there is less NFB creating the opposite effect. Taking the voltage off of a lower rated tap causes less NFB.

I run NFB off of the 4 ohm tap btw. This also changes things since the relationship is nonlinear.

Not being argumentative, but I have various amps of various power ratings and they all exhibit roughly the same dampening feel and bandwidth with the same NFB circuit. I have tried 3.3k and 3.9k and all the amps change but then sound similar as well. I think the closed loop nature of the NFB is self regulating and scales with amp output. The actual feedback ratio is what counts and where it is tapped from. Not amp total output.

On the other hand, I am just a player and I could be really wrong. I just don't think so.

But maybe I am.... Sheesh this is confusing..... LOL

The result is a lower output impedance for the 50w at about 6.5ohms instead of 8ohms for the 100w (using 8ohms tap) and this in turn will render the 50w too much tight and controlled and with a flatter freq response.

[dobbhill]

"Sheesh this is confusing"

And that is why I started this thread........
D[/quote]