Metropoulos Forum

I got a request to install an adjustable feedback control to take the amp from plexi-ish 47K to more modern 100 to 220K amounts of negative feedback.

Idea seems pretty simple, remove the NFB resistor and replace with a pot. AUDIO or LINEAR?

I was thinking along the lines of a 250K linear pot which reads usually around 225K for a full range of adjustment.

Any ideas or suggestions beyond the basics here?

Thanks!
HD

You may want to solder some resistor in series so even when you turn pot to "0" you'll still have some resistance there.

The stock NFB resistor doesn't get removed. The pot gets added inline with the stock resistor. You can drop the value of the stock resistor to alter the range of variable NFB though. But the hookup is pretty simple other than that.

flemingmras wrote:The stock NFB resistor doesn't get removed. The pot gets added inline with the stock resistor. You can drop the value of the stock resistor to alter the range of variable NFB though. But the hookup is pretty simple other than that.

I never thought about that! I like that idea better! Thanks Flemingmras!

I looks like either a Linear or Audio log pot will work in this application too, from what I can see.

I was wanting the pot only for gettting exact fixed NFB values for each pot location but I imagine I could do the same with knowing the original NFB resistor in place as well.

Thanks I'm going look into doing this!

I'm adding a pot btw the resistor and the impedance selector. I'm using 20k + 80k at 4ohms=40k +160k at 16ohms=28k + 112k at 8ohms

Roe wrote:I'm adding a pot btw the resistor and the impedance selector. I'm using 20k + 80k at 4ohms=40k +160k at 16ohms=28k + 112k at 8ohms

If you wouldn't mind sharing, I'd like to see the schematic for this.

sorry, but I just don't see the point. just add a pot between the nfb resistor on the board and the impedance selector.

Hey guys, I'm installing the Adjustable NFB using a 47K as the base resistor and a 250K audio pot to provide higher resistance after that for a vintage to modern feel on the feedback circuit.

I know I need to use the two lugs on the pot to the right as the input and output to provide + resistance in addition to the 47K NFB resistor on the board. This is looking down inside the chassis with the front of the amp facing me.

Do I need to ground the unused lug on the left side, it doesn't seem to make a difference either way with my ohmmeter? If I ground this will it short the input signal to ground when the pot is turned down all the way?

With the pot turned all the way down I want the 47K resistance to pass through the pot untouched.

harddriver wrote:Hey guys, I'm installing the Adjustable NFB using a 47K as the base resistor and a 250K audio pot to provide higher resistance after that for a vintage to modern feel on the feedback circuit.

I know I need to use the two lugs on the pot to the right as the input and output to provide + resistance in addition to the 47K NFB resistor on the board. This is looking down inside the chassis with the front of the amp facing me.

Do I need to ground the unused lug on the left side, it doesn't seem to make a difference either way with my ohmmeter? If I ground this will it short the input signal to ground when the pot is turned down all the way?

With the pot turned all the way down I want the 47K resistance to pass through the pot untouched.

don't ground it. just connect the viper and one leg

Roe wrote: don't ground it. just connect the viper and one leg

Thanks Roe, I was kind of leaning that way,as my little voice in my head was telling me. It helps hearing the confirmation on this application. It will be interesting to see how much this will change the feel of the amp being adjustable an all from 47K to 288K!

Thanks for your help! Mucho appreciated!

on a related note can someone look at this to make sure I've got this wired correctly.

I want it so that as I turn the NFB pot clockwise it will get more distorted (more resistance, right?).

Thanks
Rip

yes, more resistance means less feedback and a rawer sound

Why not change the stock to a lower value, like 27k, so you have even more adjustment.

psychodave wrote:Why not change the stock to a lower value, like 27k, so you have even more adjustment.

if this was directed to me, I've already done that. If you look feeding the resonance is only a 1K resistor and a cap in series, the cap is to block DC. I have not decided whether or not I will use it do to the instabilities in may cause. But yes I hope to have full range of the NFB loop, in an attempt to be able to tweak the feedback with the PPIMV.

Thanks for the help
Rip

Rip,

I just got done with the installation. It really makes the amp flexible. Definitely feels looser softer on the plexi 47K setting and more modern/raw s tighter feeling as you go up. I'm using the 8 ohm tap on the Output transformer.
The depth mod becomes much less effective as your NFB is reduced when using above a 100K NFB value.

Looking at your picture, you will need to use just the two lugs on the right. I used the middle wiper lug as the input and the lug to the right as the output that feeds my depth mod. You leave the far left lug unused then the pot acts like a varistor.

THANKS for your help Roe and Flemingras! Works like a CHAMP!