NFB Tap???

[rmroza]

Not sure if this was discussed previously, but on the TW (Express) and Marshall designs I have gone over and built, the NFB tap always goes to the 8 Ohm tap! Why??

I think I understand what NFB does, but why would you put it only on the 8 ohm tap?? I would think when only using "that" tap, would it then be implied, but what if I'm using a 16-ohm and selected??

On other amps I've clones I've done it this way, but other amps I've done for myself, like a Plexi-style circuit, I've attaches the tap to whatever impedence is selected by using a selector switch and the center output (of what ever is selected goes to the center pin output(s) as does the NFB!

Please let me know and why it would be done the other way and one attached to ONE tap and that may not be used. Thanks in advance.

Robb

[XgamerGt03]

No matter what tap you use with the Express you will always have a signal at the 8 ohm tap. If you draw your signal off of the 8 ohm tap then your amount of feedback will always be consistent.

In the scenario you mention the amount of feedback can vary based upon which tap you are using for the speaker. If you don't also change the feedback resistors when you change the tap then the total amount of feedback applied will not be the same.

So in the schematic here the feedback is taken from where the 8 ohm tap connects to the selector switch. If you made an amp with a jack for each tap then you would just pull it from the 8 ohm jack.

[diagrammatiks]

it doesn't matter what load is on the output transformer the 8ohm tap will always provide a signal for the nfb.

if you are changing it up using a selector switch that means that whenever you change the speaker ohms you are changing the amount of negative feedback that is being sent.

it's really unnecessary.

[dorrisant]

If you switch the source for nfb from the 8 ohm tap to the 16 ohm tap, you will soften up the highs, changing your tone. This is a useful trick in some Marshall amps with "icepick" highs. It is something to play with... But I would not put it on the impedance switch for the speaker... Maybe put it on a separate switch and experiment!

Tony

[dartanion]

Changing the tap in which NFB is connected to changes the feedback voltage. Going to 16 ohm tap will give you the largest amount of feedback voltage and 4 ohm the least, and roughly half the voltage. This voltage differential is non-linear, thus the 8 ohm tap does not split the difference, but is skewed a bit above half voltage.

The reason to use the 8 ohm tap and design the NFB circuit around it makes a lot of sense. If you want/need more feedback voltage, swap to the 16 ohm tap. Need less, swap to the 4 ohm tap. Once you arrive at how much feedback voltage you require, then you can tweak the shunt/series resistors to meet the required voltage.

[rmroza]

Well, thanks for the feedback thus far. I think I may have done a slightly poor job of explaining what I did, soo here's a picture.

The NFB isn't on the selector, but the outputs (see brown wire). Light green is output from the selector and orange, yellow, and dark green OT taps.

Don't worry thou, just want general knowledge. The new Express build will be done as designed! Turrets were completed last night!

Robb

[ChrisM]

With that design of NFB the amount of NFB will be adjusted depending what load you use. You want to put it on one of the taps, either 4, 8 or 16.