volume drop when negative feedback is doisconnected

[Gaz]

I wanted to add a negative feedback switch to my Plexi clone, switching from a 22k to 100k resistance in the loop. I thought before doing the mod, just for fun, I would hear how it sounded with no NFB at all, but when disconnected there was a severe volume drop. The problem was cured by reconnecting the NFB, and I can't figure out why.

I've definitely heard of amps becoming unstable with too much feedback added, but never from taking it out. The only other thing I did during the course of this mod was unhook the 4 ohm tap (to put a switch in its place), but I don't see how this would affect anything... thought I'd mention it anyway. Thanks for any advice.

-Alex

[Robert M. Martinelli]

Gaz,
no disrespect intended, but...Are you sure to have applied NFB? The point I'm trying to make, of course, is that phase matters....If the signal you're sending back has the wrong phase ( that is, in phase at the summing node ), the circuit becomes a PFB network.
Have you checked the phase of the FB signal? NFB is used to add stability and linearity at the expense of overall gain, so applying it properly surely doesn't make the amp louder. I'd bet my lunch that you have a phase problem. ( hope I'm wrong though, as I need to lose some weight according to what my wife and my doctor say ). Does the amp squeal/oscillate at higher volumes?
( Amps become unstable when PFB is applied ).

JM2CW

Hope this helps

Best regards

Bob

[Enzo]

Provide the schematic you are working from.

[Gaz]

robert:

the NFB was working properly before, and is still working with the switch added, so i'm fairly confident i'm, in fact, adding NFB. the problem only arises when there is no NFB added, which is why i'm so puzzled. there is no oscillations i can hear either. the amp sounds normal at lower volumes actually, but when the Pre-phase inverter MV is turned up, the sound distorts unpleasantly and stop raising in volume at a certain point in the rotation.

enzo:

i don't have a digital copy of the schematic, but the output section is the one from AX84's "October" amplifier.

thanks,


alex

[Enzo]

I am not familiar with the AX84 site, I find a Novemeber amp, but don't see October. Can you provide a link to it?

In any case, What Bob said about stability. If your amp goes into RF oscillation without the NFB, you would not hear it, it would be far above human hearing, and above the speaker's ability to produce. But it would sap the amp.

[Gaz]

sure thing, it's the "stage" version on the bottoms of this page.

I'm a little confused because everything I've read in Merlin's new book states that adding too much feedback will lead to oscillations and stability problems. Of course, you explanation makes sense, but I seem to have a fundamental misunderstanding about feedback...

Thanks.

[Robert M. Martinelli]

Gaz,
Merlin's book is perfectly in line with what I stated in my previous post :

Quote :
Page 211 and 212 - Stability and limitations on feedback - .........we must be sure that the phase shift of the feedback signal does not deviate from the desired value, which in all the previous circuits was 180°.
End Quote :

a 180° phase shift means that the feedback signal has to be out of phase, ( this is the necessary condition for an NFB loop to behave like its name implies ) otherwise the loop becomes a PFB one, leading to oscillation, ( BTW, that's the way "true" oscillators work ) which can happen to be in the audible range ( amp "squeals" ) or not, the result doesn't change.

Hope this helps

Best regards

Bob

[Gaz]

okay, i'm just not sure why you think it might be a problem with the polarity of the feedback? in my post i said that the amp was functional with feedback applied, and only loosing volume when the loop was disconnected. sorry if i'm missing the point here.

i haven't had a chance to do much hands-on trouble shooting, but i'll be sure to post my findings after tomorrow.

thanks for the reply.

[Robert M. Martinelli]

okay, i'm just not sure why you think it might be a problem with the polarity of the feedback? in my post i said that the amp was functional with feedback applied, and only loosing volume when the loop was disconnected. sorry if i'm missing the point here.

i haven't had a chance to do much hands-on trouble shooting, but i'll be sure to post my findings after tomorrow.

thanks for the reply.

Gaz, maybe it's me missing the point, if so I beg your pardon, and, as I said, no disrespect intended.....I was just willing to help....

I thought you might have a feedback phase problem because when NFB is applied properly, it "sucks" some of the amp gain/volume, so the amp becomes less loud. You are stating that your amp is louder with the feedback applied, and less loud with the feedback loop removed - that's the exact contrary of how it should be, and that's what made me think about the feedback loop's phase being "wrong".

If you have a scope, you can leave the feedback loop open and observe the signal's phase separately at the feedback tap and at the feedback "insertion point". Apart from the different amplitudes, for the feedback loop to be an NFB one, the two waveforms should be exactly out of phase ( 180° ).

Cheers

Bob

[Enzo]

And a scope would verify mu notion of RF oscillation.


Bob, what I had in mind, and maybe a long shot, was the amp was unstable on its own, oscillating at RF, but with the NFB in place, it settles down and amplifies. WHile RF oscillating, its ability to pass audio is severely compromised. Since you cannot hear the RF, the amp would seem louder in the audio band when it stopped.

[Robert M. Martinelli]

Hi Enzo!
I had something similar in mind, I tried to highlight the oddity of an NFB ( apparently ) working "backwards", and that's the reason why I suggested Gaz to use a scope, which, as you wisely noted, would help him to reveal, together with likely phase problems, the presence of oscillatory phenomenons ( out of the audible frequencies range )

Cheers

Bob

[Gaz]

enzo,

that doesn't sound too ridiculous. this amp is a protoype, and i wouldn't be surprised if my haphazard lead dress wasn't causing an oscillation. hopefully it'll be fixed in the next build.

[Enzo]

Do you have a scope?

If you don;t have a scope, google "RF probe" and the immediate replies will be a very simple piece of test gear you can make with a diode and a cap. Wikipedia even has one.

An RF probe is a simple circuit that detects RF and turns it into a DC voltage your meter can follow. It tells you the amount of RF there. Tells you little else, but usually that is enough. Electrically it is a basic detection circuit - same as in an AM radio.

Stick that on your output and see if you have RF.

[Steve Conner]

Is it possible that your NFB phase has been wrong since the amp was built, so the feedback was always positive? Sometimes you can get away with that, the amp may not oscillate.

[Gaz]

could be. what's the best was to check the phase of the feedback? a scope? i have one, but it's not A.C. calibrated because i don't have a signal generator. don't know if calibration would be necessary for this purpose. thanks a lot.

alex