NFB loop mod useless??

[GERPUD]

Hi,

I have added a negative feedback loop to my amp. The stock amp does not have one. It is for experiment purpose only.
I've put a 100k pot in series with a 5k resistor (5k-105k loop).
The loop is connecting the black wire at the secondary of to OT to pin 8 to V2-B, as shown on several other amp schematic.

I also based my expectations and design on these videos.
https://www.youtube.com/watch?v=H5fFb5_9xyA
https://www.youtube.com/watch?v=PtoPma2 ... e=youtu.be

Howerver, my NFB pot does not make any difference.
I was expecting something like in the video.

What could be the reason??

Thanks.

[sluckey]

You should also remove C18.

[pdf64]

The loop is connecting the black wire at the secondary of to OT to pin 8 to V2-B...Howerver, my NFB pot does not make any difference

The Ramparte schematic shows V2 terminal 8 as being decoupled (to prevent local negative feedback) by C18; if C18 is still in circuit, the global negative feedback loop you have tried to implement will be decoupled too.

[GERPUD]

Thanks guys!
I've removed C18 and now I get something but...
The behaviour is not what I was expecting.

1- I didn't notice a significant change in the gain with that cap removed. From my understanding, this cap should increase the gain.

2- When I put "on" the NFB loop at 100k, treble and distortion increase. I was expecting the opposite. If I decrease the resistance, high frequency increase, and distortion increase. At 50k, I start having high frequency feedback.

I though NFB loop decrease gain, gives more headroom, but I get the opposite.

What's wrong with my set up??!!

[sluckey]

I though NFB loop decrease gain, gives more headroom, but I get the opposite.

Your phase is wrong. It's a 50/50 chance of getting it right. Reverse the black and green wires on your speaker jack.

[GERPUD]

Thanks! It is working now.
I can see a slight difference, but not much from no NFB and with NFB, with whatever resistance. Not as much difference that I saw on the videos.

Is there something else that as an impact on the effectiveness of the feedback loop.

I've lost some preamp gain by removing C18. The drive channel is now almost useless....Is there a way to keep C18 and having a NFB loop?

[sluckey]

Is there a way to keep C18 and having a NFB loop?

Yes. Do it exactly like the AA764 Champ...

http://el34world.com/charts/Schematics/ ... _schem.pdf

[GERPUD]

Thanks, I'll try it...as soon as I get a 47ohm resistor