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Old 5th August 2006, 01:35 PM   #41
bjorno is offline bjorno  Sweden
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forr and Indm,

I still really vote in first place for the method to lower Qts, as described in my first post.
OTOH. You are right on the knowledge- track concerning this type of bas-amps.

Infinia,

Yes, You are close when estimating the power at 8 Ohm and you can run any driver combination as long as you at first don’t hit any quality-destroying margin restricted by the power amp.

Why not change the power-amp part to a good dc stable ‘Gainclone’?

But careful while adjusting the negative resistance, preferably for beginners not less than 50 % of Rdc for system stability reasons as the internal loops are dc coupled.

Then it’s even safe to test a driver in free air if Qms is of a lower order*.

You can prove that the bas-amp only, basically, tweaks Qts (via Qes), Vas (via Cms), fs (via Cms and Mms) and the efficiency within the driver x-max limits.

The amp and speaker behaviour is so simple that you don’t need to buy any specialised program to model the whole.

The use of free modelling programs will do it satisfactorily in case of simple reflex or closed type cabinets.

Use the changed T/S parameters with your free favourite speaker-modelling program to access the new system acoustical behaviour in any cabinet of your choice.

*A good advice can be found here:
Qm value for zero NFB amps question

"You want to use a woofer with a fairly low Qms (as compared with the Qes); for instance, you'd rather have a Qms value less than 10 times the Qes value. A low Qms means that the suspension is providing a great deal of the restoring (damping) force for the woofer, while a high Qms means that the woofer is relying on the amplifier to provide the damping electrically - which your amps will not do a good job of. If you're investigating kits or DIY, take a look at the Qms and Qes values, rather than just using the Qts."

B
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Old 16th April 2008, 09:31 AM   #42
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by forr
Infinia
---pwr amp should be configured as a current source (AB w collector outputs). Small resistor samples the current at output return then applies neg feedback with trickey compensation network in feedback loop. A stability analysis would be required. IMO this technique needs a special dedicated pwr amp for woofer only.---

Negative resistance can be obtained from standard voltage amplifiers and do not use current source. I have some experience with a bootstrap circuit which was probably known before negative feedback in tube amplifiers. At these times, it was used to get a low output impedance.
Here is my circuit having a negative outout resistance of -5 Ohm. When experimenting, a polyswitch in series with the speaker makes a good protection.
re diagram in post31.
why is the output resistance -5ohms?
Is it related to the amp gain (20times) and the 0r5 resistor?
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Old 16th April 2008, 09:46 AM   #43
sreten is offline sreten  United Kingdom
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http://sound.westhost.com/project56.htm

/sreten.
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Old 16th April 2008, 10:20 AM   #44
AndrewT is offline AndrewT  Scotland
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Thanks Sreten.
Does the bootstrap version change the output impedance from -10r0 to -5r0?
(gain of 20times * 0r5=10r0)
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Old 16th April 2008, 10:41 AM   #45
sreten is offline sreten  United Kingdom
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Hi,

As far as I can tell the positive feedback leg has a gain of ~
1/2 as its a resistive divider and this does reduce -10R to -5R.

/sreten.
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