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Old 16th April 2011, 06:16 PM   #1
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Default Gainclone as a subwoofer amp.

There have been some discussions of the use of gainclones as subwoofer amps - but nothing quite addresses my question.

I am building an OB with a field coil driver as the main component driven by a PP 6W valve amp. I want to add an active sub and have some nice 12" Woofers which will be aperiodically loaded. I want to build the Field coil supply and the Subamp in one box and it will be set to the specific frequency of the baffle design - ie no adjustment.

My question is, can I use a buffer opamp at the input and then build the power amp using an LM3875 set up as the active second order filter of the amp set at 150hz. Will the standard opamp filter design calculators produce usable results in this situation ?

Hope someone can offer some advise.

Shoog

Last edited by Shoog; 16th April 2011 at 06:28 PM.
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Old 18th April 2011, 11:17 AM   #2
AndrewT is offline AndrewT  Scotland
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the low pass filter to feed the amps driving the 12" woofers can be added inside the amp/coil drive box.
Use a summing MFB filter.

Where do the OB filters go?
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Old 18th April 2011, 11:45 AM   #3
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The OB will run fullrange, with a first order roll in of a horn tweeter to sweeten things up. This will be mounted on the tweeter.

I have found various references to a suitable opamp circuit. I am aware that LM3875's are only stable at 10x gain, but I am wondering if they will be stable at unity gain down at these frequencies.

Shoog
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Old 18th April 2011, 02:18 PM   #4
AndrewT is offline AndrewT  Scotland
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add a summing MFB filter.
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Old 19th April 2011, 06:33 PM   #5
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Quote:
Originally Posted by Shoog View Post
using an LM3875 set up as the active second order filter of the amp set at 150hz. Will the standard opamp filter design calculators produce usable results in this situation ?
It is not likely that you get such a setup to work stably.

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Originally Posted by Shoog View Post
I am aware that LM3875's are only stable at 10x gain, but I am wondering if they will be stable at unity gain down at these frequencies.
That is exactly the point. It is pretty likely that the amp will start to oscillate when you build a low-pass filter around it. If you don't want to build an active stage before the amp, but you are sure that a second order filter is enough for you, you can try a passive filter.
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Old 19th April 2011, 07:34 PM   #6
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Looking a bit closer at the theory I can see that this is almost guaranteed to induce instability.
I will look at passive, and if not practical will have to add another Opamp for an active filter.

Cheers

Shoog

Last edited by Shoog; 19th April 2011 at 08:03 PM.
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