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Old 8th July 2006, 01:24 PM   #1
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Default Surround Decoder - Subtract amplifier

Hi Friends.

How I can make a subtract amplifier with tubes? I'm talking to a friend to make a surround sound decoder from stereo. But I'm not like to do this with op-amps. I like to do with tubes.

I haven't idea to how make this. But I think I need a negative input and a postive input.

I'm not talking about pre-amplifier, amplifiers or anymore. I'm talking to all to make a surround decoder for stereo sound!

In OP-AMPS this is very simple:
Click the image to open in full size.
Page: http://sound.westhost.com/project18.htm

But, the problem to make it with tubes is HOW I MAKE A SUBTRACT AMPLIFIER WITH TUBES?

This is the frist problem...

Best Regards,
Felipe Navarro
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Old 8th July 2006, 02:12 PM   #2
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Felipe,

Opamps were made with tubes before they were made with SS. The key element for building an opamp is the differential gain block at its I/P. The long tailed pair and Schmitt phase splitter circuits are variations on the differential amp theme. Look here.

Differential amps work best when a current sink is in the "tail". We used FETs for the CCS in "El Cheapo", but a pentode, like the 6AU6, is fine.

The 5965 is very linear and has good section to section balance. With its mu of 44, it also provides a fair amount of gain. IMO, 5965 based differential gain blocks are a good place to start your surround sound project.
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Old 8th July 2006, 02:20 PM   #3
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Eli,

Thanks for your reply.

With phase splinters or diferencial amplifiers only will amplifier the sound.

I need a subtract amplifier.

I'm stay reading this page: http://www.tubecad.com/march2000/page13.html and I think i need something similiar.

The subtract amplifier diminish the differeces of the channels and make the surround sound.

Are you undestanding me?

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Felipe Navarro
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Old 8th July 2006, 03:18 PM   #4
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Felipe,

When used as a phase splitter, the non-inverting I/P is either grounded or used for the application of NFB. For your surround project, you connect a channel to the inverting I/P and the 2nd channel to the non-inverting I/P. 2 out of phase signals are produced. Those signals are the difference between the 2 channels and that's exactly what you are looking for.



Edit: typos fixed
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Old 8th July 2006, 03:27 PM   #5
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Hi Eli,

Can you draw this? Because I'm not undestanding how are you speaking

Can you?

Please.

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Felipe Navarro
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Old 8th July 2006, 03:27 PM   #6
poobah is offline poobah  United States
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Felipe,

The second schematic in your link is correct and is also the "the long tailed pair" that Eli is talking about.

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Old 8th July 2006, 03:51 PM   #7
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Here.

You must use caps to remove DC levels... and you must adjust your resistors for a gain of 1.

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Old 8th July 2006, 04:03 PM   #8
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Hi Poobah and Friends.

I have simulated this processor on the circuit maker and I have a senoidal very very distorced.

But...

Without the V5, with 35v simetrical rails, I haven't distortion on the senoidal.

What they think about it?
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Old 8th July 2006, 04:04 PM   #9
poobah is offline poobah  United States
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Like this...
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Old 8th July 2006, 04:06 PM   #10
JoshK is offline JoshK  Canada
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Great idea! Muito bem!

However, I am not much of a fan of 5.1. I have heard an alternative to typical SS, known as ambiophonics (google search and you'll find a website named as such and tons of info) which in my mind sounded the most realistic that I have ever heard from non-live playback. A friend of mine modded a JVC pre DD SS decoder to do this type of SS, but he still has it.

Then there is the SpreadSpectrum version of taking stereo to trinaural. I think that could be done like this as well. Anyway, good luck with your project and keep us up on the progress.

Josh
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