chip amp and damping?...

[Johan]

..I'm trying to figure this out, but cant quite wrap my brain around it..so perhaps someone else knows..

..with most chip-amps acting like opamps ( gain set by feedback and stuff..), but with massive current abillity. will the size of the gainseting component affect the damping when used to power speakers, or is that set by other,internal parameters?...

I mean.. .like the lm3886..what goes back from the speaker to the negative input..if I vary the gain from 20 to 60, the "slow" response of the speaker should result in different responce.right?...but if I keep the gain constant at 20.   say I go from  20k feedback and 1k to ground and replace them with 200k feedback and 10k to ground..will that affect anything, or will it just be noiser, if anything at all?

..I dont know where Im going with this, but I have an idea for a simple but slight bigger practiceamp..( around 20watts), so...

any thoughts would be appreciated..

johan

[opik]

if you want build more watt, check the LM4780 it is twice and excactly 2 of LM3886 inside, get it paralel you get 120watt! you'll need big amps, say 10A dual 35V...becareful. good luck, see the datasheet from www.national.com for the lm47
cheers

[R.G.]

..with most chip-amps acting like opamps ( gain set by feedback and stuff..), but with massive current abillity. will the size of the gainseting component affect the damping when used to power speakers, or is that set by other,internal parameters?...

Not to any significant degree. Even at higher gains, the open loop gain of the circuit is sufficient to lower the output impedance to levels much, much smaller than the speaker load.

I mean.. .like the lm3886..what goes back from the speaker to the negative input..

Only the output voltage.

if I vary the gain from 20 to 60, the "slow" response of the speaker should result in different responce.right?...

No. The onl thing the "opamp" sees is the output voltage. This is essentially independent of the load for any load that the amp has enough current drive to bring up to the required voltage.

but if I keep the gain constant at 20.   say I go from  20k feedback and 1k to ground and replace them with 200k feedback and 10k to ground..will that affect anything, or will it just be noiser, if anything at all?

Just noisier, and very little of that.

[puretube]

http://cache.national.com/ds/LM/LM12CL.pdf

[A.S.P.]

power to the chippies!

[cd]

LOL I was going to suggest posting at diyaudio.com, there's billions of chipamp nuts over there, but I see you got the RG response

If you need stupidly high power, might want to check out www.41hz.com as well.

[Ardric]

Some think that increasing the output impedence, through either negative feedback or a wasteful series resistor, reduces the damping and lets the natural character of the speaker and cabinet come out, like the OT on a tube amp.  Others have said that it just adds mud, fizz and volume drop.  I'm not sure what's up with this myself.  I think the Marshall 'Valvestate' stuff is based on this principle, among others.  Maybe some of the LM386 amp designs do this because of the volume pot on the output side.

An article on the subject: http://sound.westhost.com/z-effects.htm

[R.G.]

That's a long ongoing debate there.

It is clearly possible to reduce damping from the immaculately-high damping factors of high feedback solid state stuff by diddling the way you do feedback. It's been done by Fender certainly, and possibly by others. It doesn't seem to have taken the world by storm. The ones I've listened to haven't grabbed me too much. They seem too boomy and flabby.

It's funny - the Golden Age tube amp designers would have killed to get a damping factor up to 10. That's why it's even quoted as a specification on audio equipment. Nowadays the chintziest SS amp has a damping factor too high to be worth quoting, but they quote it to have something to quote.

To Johan: your original question is still correctly answered. The size of the feedback resistors doesn't have anything substantial to do with damping factor. You have to change the WAY you do feedback to lower the damping factor, if you even want to.

[Johan]

thank you all..

I temporarily ran out of gas on this project...to many other things to explore..I do have an idea about this that I want to play around with..when I get around again, I'll let you know..

johan

[George Giblet]

>.to many other things to explore

There's not that much to explore -  it's very simple.  There's a lot of articles on damping factor, most don't get to the balls of the issue they only add confusion in peoples minds - that ESP article is one of those.  Forget all the stuff about back EMF etc.  while some of it is true that view doesn't let you see anything clearly.

With solid state amplifers you cannot easily control the output impedance with the type of amplifier or the external component values; it makes such a small different it is of no consequence.  You have to *create* an amplifier output impedance using feedback and that requires a  feedback connection which is different to the standard ones.

* What damping factor does:

Damping factor represents the output impedance of the amplifier.  High damping factor = low impedance.  Low damping = high impedance.  View the output impedance of an amplifier as an ideal amplifer with an impedance in series with the output.

The impedance of a guitar speaker varies with frequency.   If you drive that speaker with an ideal amplifier it will have a certain frequency response (as would be documented by the manufacturer).  If you now put a resistance (Rout) between the amplifier and the speaker the speaker impedance forms a divider with the added resistance.  The frequency response is modified according to:

Vspkr/Vamplifier =   Zspkr / (Zspkr + Rout)

When the speaker impedance is high you get more "boost".  A guitar speaker has a peak in the bass region and a rising impedance in the high frequency region.  Consequently  the lower the damping the more bass boost and more high frequency boost.  You can see this on the second picture of this document:

http://www.aikenamps.com/JJ_EH_tube_tests.html

That's all there is to it.   You should be aware that the resultant frequency response with low damping will depend on the speaker and also whether the speaker is in an open or closed box.

* How to change the damping factor of a solid state amplifier:

The way to do it is to modify the feedback so the output voltage depends on the output current.  A standard way to do this is represented in figure 2 of this page:

http://sound.westhost.com/project27.htm

The components R26 and R23 deviate from the normal feedback connection of a non-inverting amplifier.  R26 provides a current sensing and R23 routes the current sensing voltage to the amplifier feedbacks.  The components R26, R7, R6 and R23 are the voltage and current feedback components (one of the values is redundant, it only scales the others) .  It is possible to write down an equation which gives the output impedance of the amplifier, Rout, in terms of these part values.  There's another equation which gives the gain.

Checkout a number of modern solid state amplfiers on line:  Marshall, Fender, Hughes Kettner, Crate, Peavey, Laney (some discrete some chip).  They all use this feedback configuration.

You can do this with most amplfiiers, it's particularly straight forward with the standard discrete amplifies like the example.  It is equaly straight forward for chip amplifier that have standardard inverting/non-inverting inputs like opamps.

* Why bother with it

Tube amplifiers, especially guitar amplifiers, have low damping factors.  The low damping factor interracts with the speaker impedance (as described above) and this interraction is part of the sound/equalization of a tube amp.  Without it it sounds flat.

Mimicking the output impedance on a solid-state amplfier ensures the frequency response is more like a tube amplifer.


[Added note: when you use feedback to create the output impedance/resistance there is no additional power dissipation and no loss of voltage swing at the output terminals - unlike adding a series resistor..]

[Johan]

>.to many other things to explore

There's not that much to explore -

..sorry if I was unclear..too many things "in life" to explore....right now, a new sweetie...and new carreer...and a new band...and so on...I'll get there soon enough, but for now, other thing are occupying my time and mind.. 

johan