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Old 17th May 2012, 12:24 PM   #4041
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The (HUGE) difference is that you may need to put (perhaps) 47 Ohms in series with the speaker unit to get the effect you want. This resistor will be wasting a very large proportion of the amp power. If you synthesize the high output impedance by using a low value (<1 Ohm) resistor in series with the speaker, and then use a feedback loop to get the desired Z(out), the power wastage is minimal.
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Old 17th May 2012, 01:09 PM   #4042
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So only a small resistor is necessary (1/10 of the load?), and the actual power (and distortion figures) will be almost the same as a normal nc400 with the same load?
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Old 17th May 2012, 01:23 PM   #4043
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Theoretically yes, but of course you will use the '-' input of the amp for the current-derived feedback signal, so the NC will use the '+' input for the input signal, so you lose the balanced i/p arrangement. I have not tried this on a UCD (I don't have any NCs), so I don't know if the amplifier will be happy connected like this, though it should be ok. BUT... a bigger problem is the fact that you are not using a customised gain-block for the amp having a high O/L gain, but a gain-block with around a 26dB gain instead. You may well find that it isn't really practicable to do this and get a high enough value of transconductance for the design.
Now of course in an ideal world you would add an op-amp based input differential gain stage in front of the NC to sum the input signal with the current-derived feedback signal to restore your gain, but this brings up the spectre of instability (gain/phase Bode plot problems) in a big way.
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Old 17th May 2012, 01:32 PM   #4044
jtwrace is offline jtwrace  United States
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Quote:
Originally Posted by Jan-Peter View Post
Guys,

Currently all NC400 and SMPS600 are on stock, however we have had some unforeseen delay due to missing SMPS and audio cables....

The cables are now in transit and will arrive next week on Monday.

All open orders (and new orders...) will be shipped next week, starting on Tuesday.

My appologize for this unforeseen delay.
Fantastic news!
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Old 17th May 2012, 02:07 PM   #4045
ChrisPa is offline ChrisPa  United Kingdom
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Originally Posted by pos View Post
So to me that is quite the same as putting a resistor inline with the driver? Will not Rf still be burning much of the outputted power? I don't really understand the practical difference.

in my experiment I was using a 44 ohms resistor inline with a 16 ohms driver (Re, but lets simplify). That gives a total load of 60 ohms, and only ~1/4 of the outputted power going into the driver.
My amp puts 690W into 8 ohms, so should be able to put max 100W into 60 ohms, so around 25W going into the driver itself.
With a 200W/8 ohms ncore the same logic would go down to less than 7W...

I am sure I am missing something here, though
have a look at
Op Amp Fundamentals: The Transconductance Amplifier
When you've digested that come back and ask the next level of questions

The output impedance of the closed loop transconductance amplifier is very high because it's part of the closed loop.
Rf isn't the output impedence of the system, it's there as an I/V convertor to provide the voltage signal to the -ve input of the opamp
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Old 17th May 2012, 02:39 PM   #4046
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Will do, thanks (and sorry for the noob questions)
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Old 17th May 2012, 02:46 PM   #4047
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In stock...Hmmmmmm....(checks account balance)...Well, if my "wire" amp doesn't work (and I'm giving that about a 50% chance), I will be ordering NCores soon.
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Old 17th May 2012, 06:12 PM   #4048
tinitus is online now tinitus  Europe
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quit the 'personal' attacks

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Old 17th May 2012, 06:16 PM   #4049
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Would it make any sense to turn the NC400 with its 26 dB gain into a transconductance amp, that is without making any modifications to the NC400 itself to increase the gain?
Found out myself, after a nice cup of coffee: it makes no sense. Open loop gain must be a lot higher than 26 dB.
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Old 17th May 2012, 06:40 PM   #4050
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Quote:
Originally Posted by StigErik View Post
Found out myself, after a nice cup of coffee: it makes no sense. Open loop gain must be a lot higher than 26 dB.
I see open loop gain being specified 56 dB (typ); the 26 dB is voltage gain of the amp at closed loop.
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