quick info on feedback

[Ianm2]

feedback trades

gain,


ie its negative gain, or in other words subtracts it, its an 'active' process, rather like forward gain, rather than a 'passive' way of simply cutting or attenuating volume via a resistive pot.

• so gain is active, via amplification of active devices, rs.
• feedback is negative, subtractive or inverse gain
  even though its probably used via a resistive divider, a gain control I would think could be defined as active, rather than a volume control, which is on the front or at the end of a circuit, a gain control occurs in the feedback loop. both use the same method, ie resisto
• and attenuation is not the same, its volume reduction via passive components, as far as I understand it, tho its pedantic, I struggled to understand attenuation what it meant, as simply cutting. it may be attenuation can be both passive and via feedback tho

and for that you get

• lower distortion
• wider frequency
• lower noise

it can also increase stability, ie stop a circuit turning into an oscillator, which you don't want, but too much will turn it into an oscillator.

and other goodies which I can't recall from the top of my egg.

an output transformer shifts phase, due to REactive components, which are ac passive components, inductors, aka chokes, reactors and transformers, and also capacitors.

reactance is ac resistance.

so valve amps can only have less gain without feedback applied than a transistor one

and too many amplification stages give gain and increase phase shift running the risk of oscillation.

so transistor amps per se. can have less distortion as more feedback can be applied reducing distortion more, as they don't have a phase shifiting transformer in the path, nor likely phase shifting capacitors, blocking voltages from upsetting the next stage along.

quite simple when its explained like that.

there are 2 basic kinds of feedback.

local feedback around one stage, this can be done by disconnecting a capacitor
or using resistors as a voltage/current divider.

or global or loop feedback around 2 or more stages, in a kind of loop from output to input.

there is also voltage feedback and current feedback

it can be derived as a voltage or current and applied as either, so you get permutations and combinations.

for an oscillator to be formed, gain and phase shift are required.

I hope that's of some assistance, and some people aren't sucking eggs

[steve s]

Hi Ian i dont quite agree about the benifits..

when you measure the amp on its own, feedback is a a good thing.. it does all you say above....

when the speakers are connected there is an issue

speakers are an electric motor, and the electrical wave will move them backwards and forwards .. but no speaker is 100% efficient.. so the speaker does not move in time with the signal.. so theres a back voltage created by the speaker moving out of time with the signal (back emf)....
put a voltage meter on a speaker and move the cone to see what i mean..

with a zero feed back valve amp this extra signal stops at the anodes,...
an amp with feed back adds this signal to the source and amplifies that too...
no wonder the life goes out of the music with most feedback amps more so those with lots of feedback

this does not apply to all forms of feed back but it does in the general way its done

from my experence there is no better amplifcation than what is possible through zero feedback triode amps...

feed back was widely used in the 50's, many of the valves of the time had more distortion than the valves that where made 20 years before then, when outright low distortion was something to aim for, in the 30's feed back was not widely used..

no wonder modern amps are so speaker dependant for lots of reasons


steve

[Richard]

Hi Ian, Steve,

Also.... the amount of feedback is usually expressed in dB and found by the difference in dB between sensitivity of the amp with and without fb applied.

Therefore, conveniently, feedback affects sensitivity by the same amount of inversely proportional dB,

and noise by the same dB level (so +6dB of fb will lessen noise by -6dB etc),

distortion similarly (6dB will lower 1% to 0.5% etc),

and it decreases output impedance by the same, ie, 6dB fb lowers op imp by half.

(Of course the question might occur to some that to use feedback we need to start with more gain (and what's the point in making an amp twice as powerful if it needs to use half its gain to clean its act up?). If we made the amp less powerful to start with it might have less distortion and so not need to use some of its gain reducing distortion by feedback. Yes there are lots of ifs and buts in here but a lower power triode without feedback does sound good.)

Anyway, feedback seems seems jolly good til, as Steve mentions, the darned speakers are hung on the output. Then it becomes a balancing act, a bit helps an otherwise mushy amp but go too far (give it too good a connection) and the speaker kills the amp with back emf.

This will be very dependant on the speaker/room and I've had some great amps using just a bit of feedback but my room doesn't have many real nasties and the TL speakers probably control the drive unit quite well.

Also, all fb is not the same and some can be better than others, global might be worst, UL op stage only might be better, and local on a first valve without a global connection is very nice imho.

Rich

[Ianm2]

hi steve, its not a praising of feedback, nor an endorsement of tranny amps, its just a bit of info for those wanting to know a little more.

[andrew ivimey]

I don't think I do know any more.

I can't understand what you are saying, Ian. In fact for the first three lines of your original post I thought you were talking about eBay! silly old me

Though what Richard and Steve says seems to make sense.

I don't like feedback, unless I am causing it, with a fender strat and a large Marshall stack.

ho hum.

[prince of darkness]

Originally Posted by andrew ivimey

I don't like feedback, unless I am causing it, with a fender strat and a large Marshall stack.

ho hum.

Ah, but that's positive feedback!
Prefer a Guild S300 myself.

[Richard]

Originally Posted by andrew ivimey

I can't understand what you are saying, Ian. In fact for the first three lines of your original post I thought you were talking about eBay! silly old me

Now you mention it there was a change to eBay feedback about 3 months ago that had me puzzling. Overnight my feedback went up by about 20 and stayed there. Anyone else notice that?

Anyway, a couple of days ago I was perusing the "help" pages about a problem trade and noticed that feedback has been changed. I knew about the "buyers can't receive neutral or neg" change, but now you can score feedback from the same seller/buyer if the trades are a week apart. Before, once a trade had been done, no further trades with the same person counted. I can only think the system updated past trades when they changed the programming.

Rich

[Dave the bass]

Originally Posted by andrew ivimey

I don't like feedback, unless I am causing it, with a fender strat and a large Marshall stack.

Metal Head!

...and I thought you was sooo suphistikated n all that...

DTB

[steve s]

Originally Posted by Ianm2

hi steve, its not a praising of feedback, nor an endorsement of tranny amps, its just a bit of info for those wanting to know a little more.

I appreciate that Ian.. but it is easy to give the the wrong impression to those wanting to know more...

... to be fair I am talking about achieving levels higher than one can achieve with even very good bought kit
good hifi is an expensive business.. even the diy is expensive.. a good amp is generally simple and uses a high quality of components

the hard thing for most seems to be direction,and what does what, and what to spend money on... to be honest you seem to struggle with that one too..
as over the years you seem to have changed direction a few times.. but thats what learning is about.. i made a massive (for me) change in direction going from large horns to open baffles

I started out with a wad kit.. i feel strongly that a good valve amp(wd) kit, is the ideal place to start..
soldering skills and being able to understand basic electricity circuits ie wire a plug, and use a fluke meter are all thats required.. I and many others have built kits without really understanding what your doing..
for many building a kit is enough.. for me it was the start of a long learning curve.. i'm sure i'm still a relative novice..

The most realistic sounds ..i have heard to date seem to come from the higher (higher the better in my mind) effeciency speakers( baffles, horns or whatever..) and simple ish valve amps (pref with nos valves)...
but getting a good sound does take a little working at and getting to know what your doing.., and one getting used to a more realistic sound compared to main stream hifi.. many people don't actually seem to make that jump very easily, as we all like different things.. usually what we are used to hearing..

cheers
steve

[alnewall]

many people don't actually seem to make that jump very easily, as we all like different things.. usually what we are used to hearing..

You know when you've made the right "jump". Long term satisfaction kicks in.

Instead of wondering what to change, build or buy next, you start to think about how the existing setup can be improved.