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Old 4th August 2006, 09:25 PM   #31
forr is offline forr  France
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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 outpout resistance of -5 Ohm. When experimenting, a polyswitch in series with the speaker makes a good protection.
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Old 4th August 2006, 09:42 PM   #32
infinia is offline infinia  United States
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Hi Forr
Looks simple at DC, using your voltage source think about the equations for AC current using a speakers complex load impedance. Also what about back EMF from the drivers motor in terms of causing instability.
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Old 4th August 2006, 11:36 PM   #33
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Infinia,
Have a look at some JAES articles on the subject. For example, the famous paper about BR enclosures written by Thiele : there is a paragraph about negative impedance.
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Old 4th August 2006, 11:44 PM   #34
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Hi Forr
Yes I used to pour over the AES journals back at the University. Esp. concerning this subject. Have some copies of the good articles here. Mostly from the 70-80's though.
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Old 5th August 2006, 12:23 AM   #35
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Quote:
Mostly from the 70-80's though.
Yes, It was popular to build amps with negative output impedance 70 - 85 using these articles.

Still have a very good bass amp I made in 1984 for adjusting FR bandwidth and Qts.

Take a look at the schematic:

B
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Old 5th August 2006, 12:24 AM   #36
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And some numbers to consider:
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Old 5th August 2006, 12:40 AM   #37
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Quote:
Originally posted by bjorno


Yes, It was popular to build amps with negative output impedance 70 - 85 using these articles.

Still have a very good bass amp I made in 1984 for adjusting FR bandwidth and Qts.

Take a look at the schematic:

B
Hi bjorno
Thanks for posting your schematic. I like pictures instead of words. I see the output is a current source. It's a transconductance amp with multiple feedback points. In essance provides the conjugate impedance seen by the woofer.
How do you like it still?
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Old 5th August 2006, 01:40 AM   #38
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Hi Infinia,

I've been using that prototyped amp for over 20 years with great success and build more than 10 for friends.

Itís still very useful to prototype different bas cabinets with different drivers mostly below 200 Hz due to the driver Le, but when using Scan-Speak low inductance drivers much higher upper corner is achievable.

Today though, I have to change the burned on/off switch that is worn out from extensive use.

B
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Old 5th August 2006, 01:55 AM   #39
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Quote:
Originally posted by bjorno
Hi Infinia,

I've been using that prototyped amp for over 20 years with great success and build more than 10 for friends.

B
Hi bjorno
Must be around 70 Watt/8ohm or about. Can you run isobaric or push pull woofers?
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Old 5th August 2006, 11:14 AM   #40
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Thanks a lot, Bjorno, for your schematics, I collect everything I can get about negative impedance and Stahl schemes.

Just as with bridge velocity motion feedback, the problem with the Stahl scheme is Le which is not constant with frequency and within the voice coil position in the gap. Stahl and yours schematics solves this last aspect by using a push-pull configuration of the drivers.
I am happy you confirm what I thought : the Scanspeak driver wiht reduced Le were good candidates for this application.


Infinia
The current sensing circuit is not used to make a transimpedance amplifier (voltage in, current out) but to determine an amplifier with a precise output impedance.

Stahl's configuration uses are two feedback loops :
- the first one is a positive feedback loop which completely nullifies the Re of the driver. Left alone, the system would be absolutely instable.
- the second loop imposes a particular impedance to the output of the power amplifier and controls the resistance which will determine the Qe of the whole and will assure the stability.


For the driver which is the subject of this thread, I just suggest to use a little of negative resistance to get a more decent Qt by lowering the Qe.
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