Negative Feedback on a Ruby ?

[Krinor]

I'm doing some mods on my 5F1 Tweed Champ (replica) and decided to put a switch in there to be able to cut the negative feedback loop (from the output and back to the second stage cathode). A thought occured to me: I see from the schematics that there is indeed a negative feedback loop on the tube-to-jfet emulation projects like Professor Tweed etc. and I wonder if it is possible to implement this very same effect on the regular Ruby schematic ? I'm aware that there is no jfet emulating a tubestage. I'm just asking if the same efffect could be achieved somehow.

[jakenold]

Just out of curiosity - what effect has that on the sound of your amplifier?

Kind regards, Jake

[Krinor]

Put simple negative feedback helps to keep the amp stable. It levels out the frequency respons and prevents some distortion by feeding some current back to the preamp. Without the negative feedback loop the amp goes into clipping earlier and this gives it a more unstable, bluesy feel.

[Krinor]

Since none of the "seniors" seem to have read this one so far, I have ventured into the unknown on my own. 

I tried to make sort of a feedback loop from pin 1 of the LM386 to pin 5. On this loop I put a R=15k and a C=1uF and a switch. When engaged this loop seems to flatten out the frequenzy respons of the amp. The gain goes down and so does the bass. When comparing it at similar volumes without the loop it sounds as if the loop makes the amp somewhat cleaner. What on earth is happening in that little black brick ? 

Hm... Guess I'll just go on experimenting untill something starts to smoke 

Edit:

5 minutes later... Well I extended the loop to the output jack. That fixes some of the bass loss and raises the voulme a bit - thus levelling out the difference between engaged and unengaged a little more. The result is a somewhat compressed tone. The sound that comes out of the speaker is more even with less peaks in the bass and treble areas. I have so far forgotten to mention that the Ruby I'm playing with is a Bassman modded one. It has no gain controll. Pin 1 and 8 are connected via a 2k2 resistor.

[anchovie]

Have a look at the LM386 datasheet, particularly the example circuit for "Amplifier with bass boost". This uses an RC filter between pins 5 and 1, similar to lowpass filtering with an opamp.

[Krinor]

That's exactely what I did. So that's supposed to boost the bass..? But it actually just levels things out. Sounds more Hi-fi. Very clean.

[anchovie]

It's a bass boost using the values on the datasheet: 10k and .033uF gives a lowpass rolloff frequency of 482.5Hz. Using your values of 15k and 1uF gives a rolloff frequency of 10.6Hz! So the loop you have set up is damping everything, which would account for your lower-gain sound.

[Krinor]

Ah.. Thanks for your insight. I meant I had done the same thing physically, not with the same values. So I'm damping EVERYTHING. I'm a genious  While we're at it; can you give me the formula for this kind of filter ? Would be a nice way to finally get it punched into my rather basic brain. 

[anchovie]

I use this: http://www.muzique.com/schem/filter.htm

[Krinor]

Thanks for the link.
Using this I now have 455,5 Hz which is the closest I could get with the components at hand. I used a 0,047uF and 8,2K resistor.
Now it sounds warmer and there's a much smaller volume drop.