Adding Variable Impedance to Power Amp

[mige0]

Hi

Thinking about a circuit to add to solid state amplifiers.

*Main* reason is to adjust output impedance of a standard (low impedance) amp to shift effective Qts (in the sense of electric dampening) of speakers at my desire.

(I'm well aware that there are additional effects - like that thermal distortion of speaker is lowered, FR in the lower department is flattened out (for low Qts speakers) when eff-Qts is adjusted to ~0.7 and FR in the upper department is rolled off depending to impedance rise there - but those aren't my targets for now)

What you think about?

Basically I do not want to insert a big resistor as I want to save the amp's power to drive the speaker and not to heat my room.
The second restriction is that I don't want to build a new power amp nor do I like to mod the ones I have.

So - this brings me to a relatively (?) simple active circuit built around a small current sensing resistor with feeds the power amp's input after summing with the incoming signal.
Low passing the current feedback loop would reduce impact of speaker impedance rise and allow operation even for passive XO speakers without a lot of XO re-design

Anybody who did before or has any comments?

Michael

[tiefbassuebertr]

Please read follow article:
"Synthesis of Loudspeaker Mechanical Parameters by Electrical Means"
from Audio Engineering Society (AES)
Patent US5625698
have a look also about
Simple MFB woofer project
and
STAHL - 'Acebass' Synthesis of Loudspeaker Mechanical Parameters by Electrical Means
and to page 197 of the german loudspeaker book
"Lautsprecher - Dichtung und Wahrheit"
from G. Schwamkrug and R. Römer (Elektor Verlag).

[mige0]

Thanks for the links - but I am actually interested in *increasing* impedance of the amplifier.

Michael

[boraomega]

Try Elliott... Variable Amplifier Impedance

[tiefbassuebertr]

Originally Posted by mige0

Thanks for the links - but I am actually interested in *increasing* impedance of the amplifier.

Michael

I see. But as I know this must be possible, too - isn't it?
boraomega: thank you for this URL

[mige0]

Yea very good link thanks - tho I already knew - but also here I already decided not to mod or start from scratch, but want to add a external circuit that might be pretty versatile for other too.

Application is aimed towards bass of open baffle speakers.
With those there often is a need to *increase* Qts as basically you have free air operation and on the other hand there are so many interesting speakers available with low Qts.

It sure is possible - I already run some pretty looking spice simus
Any input is appreciated...

Michael

[tiefbassuebertr]

Originally Posted by mige0

Application is aimed towards bass of open baffle speakers.
Michael

This means dipole character.
In general - dipole baffles still problematic, but in case of the low frequencies below approx. 300 Hz dipol character no useful bass transmission is possible.
Similar effects are also to observe by back loaded horns like this:
http://www.natdistribution.com/image...er_LissaV2.jpg

The variation of the output impedances by your amplifier (e. g. for Ots arround 0,7 until 0,9 instead lower values) does no compensate the minimums/maximums in your frequency response causes through sound radiation by dipole.

[unclejed613]

one way you could do this (although it requires modding the amp) is to reduce the tail current of the diff amp in the amplifier. this reduces the open loop gain, and increases the output impedance of the amp. you could also put a 1:1 transformer between the amp and speaker. this would somewhat isolate the impedance of the speaker from the amp.

[mige0]

Originally Posted by tiefbassuebertr

The variation of the output impedances by your amplifier (e. g. for Ots arround 0,7 until 0,9 instead lower values) does no compensate the minimums/maximums in your frequency response causes through sound radiation by dipole.

Well - I know, I only mentioned to give some background - it's not so important.

Here is a first schematic and the according output impedance, I set to rise to ~ 50Ohm below 100Hz:
R8 would be our speaker (not connected here for measuring output impedance only)







So it seems it basically can be done.


Michael

[bobodioulasso]

U certainly know this one too. (from JM Plantefeve)