Maximum amount of negative feedback?!

[Xander8280]

Could I replace my 47k resistor with a 1k? Or 100R? I've been playing with 27k and 47k, I am curious what's the lowest resistor(or no resistor) value you've gone?

Also how far would you go until the amp is sterile/lifeless/insert four letter word.

[Structo]

What amp?

What circuit?

Is this local feedback or negative feedback like presence control?

And if it is feedback from the output, the less resistance there in most case will give a more raw sound.

But without the circuit we have no idea what you are talking about.

[Xander8280]

What amp?

What circuit?

Is this local feedback or negative feedback like presence control?

Jcm800 or marshall variants. The feedback like the presence control.

[Structo]

Sorry I was posting more info when you replied.

If it were me I would connect a 100K pot in place of the fixed resistor and play with it until I liked the sound, then measure the pot resistance and replace it with a fixed resistor.

[deiseldave]

I tried all the way down to 22k on my Plexi-6v6. It had increasing levels of sterilility (for me) down below 47K, and I really didn't like it much below 27K.
Overall I liked 100K best. However, there are some times I like that tight sound of 47K, and below.
So, like many other people, I put a pot in series with the fixed resistor. I used 250K pot & 27K fixed, and like it a lot.

[skeezbo]

Another question about negative feedback value; I hope this doesn't hijack the thread:
The increase in negative feedback should also produce a reduction in the overall volume level of the amp, right? Do you have a general idea how many DBs are lost between the optimum of 100k and the minimum acceptable value around 27K? Maybe too subtle to tell.

I haven't had much luck with master volumes; This thread has me wondering if I might experiment with a 100k-27k switch (or something similar) not so much to tweak tone but as a subtle power reduction device for playing in small rooms. Is it worth a try?
Skeeper

[Stevem]

Do you have a understanding of what feedback does overall in regards to a amps frequency response, overall gain and feel, as this will help your ears guide you on the resistor you end up with?

[Tone Lover]

And I do just the opposite 250k pot to dial up the sweet spot the lower the value the more negative feedback so i have found adding a higher value brings out the sweet spot for me my last jcm 800 style sounded best with 156k measured on the pot of course i had to take the closest match I could get with a resister . But I have found transformers,pickups and speakers add to this so do your dial up with whats going to be used. the other thing is to try it with no negative feedback at all.
Bill

[printer2]

NFB counteracts the voltage swing of the cathode developed by the signal on the grid. If you swamp the cathode with NFB it will turn into positive feedback.

NFB cleans up distortion and it also lowers the output impedance of the amp. This causes less voltage and thereby power to be delivered where the speaker impedance rises, namely at the resonant frequency and in the high end. You get tighter bass with NFB and reduced highs. That is why we have a presence control, to reduce the amount of NFB in the highs.

[roberto]

You get tighter bass with NFB and reduced highs. That is why we have a presence control, to reduce the amount of NFB in the highs.

not exactly. Increasing the NFB increases the bandwidth.

[img:545:234]http://www.turneraudio.com.au/basic-tub ... e-amp3.gif[/img]

And the frequency of the presence is way lower than the -3 dB lowpass frequency of the amp.

The reason you feel less highs is that increasing the NFB you decrease the harmonic content of the amp:


[img:520:310]https://passlabs.com/penta/images/uploa ... edback.png[/img]

[img:520:333]https://passlabs.com/penta/images/uploa ... edback.png[/img]

[EtherealWidow]

As usual, Roberto, you are a graph sensei.

[roberto]

[Roe]

And the frequency of the presence is way lower than the -3 dB lowpass frequency of the amp.

thanks for the great graphs. However, I do not understand that the above claim. With a standard bassman/marshall presence I get a lowpass starting at 318hz

[roberto]

I think you are talking about 4k7//100n.
As you have indicated, 300 something Hz is in the low-mid range of the amp.

[printer2]

You get tighter bass with NFB and reduced highs. That is why we have a presence control, to reduce the amount of NFB in the highs.

not exactly. Increasing the NFB increases the bandwidth.

So why does the treble (and upper mids) sound louder when we turn up the presence? With NFB extending the frequency response you would think more NFB would give you more highs.

[img:545:234]http://www.turneraudio.com.au/basic-tub ... e-amp3.gif[/img]

And the frequency of the presence is way lower than the -3 dB lowpass frequency of the amp.

I sure hope so.

The reason you feel less highs is that increasing the NFB you decrease the harmonic content of the amp:


[img:520:310]https://passlabs.com/penta/images/uploa ... edback.png[/img]

[img:520:333]https://passlabs.com/penta/images/uploa ... edback.png[/img]

At -40dB I don't think the little bit of harmonic content is really going to matter. Also when you reduce the NFB and you are playing clean you still get an increase in highs. Has to do with the rising impedance and the transformer swapping voltage for current to get a constant power out.