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Old 31st August 2004, 08:10 PM   #1
PWatts is offline PWatts  South Africa
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Default Most effective use of valve pre & active xover

Hi guys,

I didn't know where to post this thread but I figured this would be the best place :-)

I've recently built a top-end improved version of the AudioNote M2 preamp. Sounds wonderful with my Leach amp and CD source. However, my new speaker project is a monitor/subwoofer combo designed to be used with an active crossover at 60Hz, with a 2nd order highpass on the main speakers. Omitting this active highpass is not an option.

However, this implies inserting an opamp stage between the preamp and power amp, and this is where the question comes in. I have nothing against opamps (my CD player's DAC has lots of them anyway), but I'm concerned that it may swamp the preamp's sound.

Basically what it comes down to, does a valve preamp sound good because of its ability to drive the load, or merely what it adds to the sound (2nd harmonic)? If it is the latter, I can insert the opamp stage and the same warm relaxed sound should prevail. If it is the former, obviously not. An alternative option is to construct a simple preamp using the opamp buffers (keeping all the opamps together, from the DAC's output to the buffer with a volume pot and highpass filter, all together). Then remove the M2's pot and use the preamp as a valve buffer between the highpass filter its power amp.

Any comments/suggestions?

Regards,
Pierre
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Old 1st September 2004, 12:38 AM   #2
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Hi,

Quote:
However, this implies inserting an opamp stage between the preamp and power amp, and this is where the question comes in.
Why are you convinced you'd need the opamp?
If the input impedance of the active X-over is too low for the preamp a tube buffer could quite probably be devised to drive it.

Quote:
Basically what it comes down to, does a valve preamp sound good because of its ability to drive the load, or merely what it adds to the sound (2nd harmonic)?
Neither IMHO.

Cheers,
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Old 1st September 2004, 01:00 AM   #3
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I'm in a similar situation, except I have a passive linestage going into an active crossover which then fans out to multiple amps. In my experience (so take it with a big grain of salt)...

The tube preamp will add its signature, the opamps in the crossover will add their signature. There's not much you can do about that. Pick good opamps, and read up on some of the basic techniques to make opamps behave. That way you will reduce the sonic impact of the opamps. Your eventual goal is to get to a point where any damage done by the opamps is less than the damage that would be done by sending bass to your main speakers. Once you get there, you're better off with the crossover than without it, and that's all you really need

P.S. You can also implement a passive high-pass filter at the input of the main amps. Then all you do is split the output of the preamp, send one to the filter+amp and the other to the sub. And since this is a line level filter, you won't need massive caps to get to 60Hz. You'll get a drop in signal level by going this route, but if that's not a problem, then this may be a better solution for you.

Hope that gives you some ideas.

Saurav
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Old 1st September 2004, 06:09 AM   #4
DougL is offline DougL  United States
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See what you think about this article on tube crossovers.

Tube cad Journal crossover article

I am convinced that lower open loop distortion and simple circuits are major advantages in tubes. Also IMHO the higher PS voltages are an asset to tubes.

Cheers;

Doug
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Old 1st September 2004, 01:18 PM   #5
PWatts is offline PWatts  South Africa
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Thanks for the replies,

I've tried using a passive highpass on the amp (which is almost purely resistive until about 2kHz), and it works fine for 1st order. With 2nd order I just could not get it to work right, even though I've desperately tried.

So, what should the preferred method be? Standard opamp highpass between pre and power, or opamp pre&filter with valve buffer between filter and power amp?

Thanks,
Pierre
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Old 1st September 2004, 06:54 PM   #6
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Quote:
Originally posted by DougL
See what you think about this article on tube crossovers.

Tube cad Journal crossover article
I love it! I hope TCJ becomes more active again, and
at least remains online. It is a fantastic resource.

I recently purchased TubCAD to support them.

Thanks Doug!

Best,

George Ferguson
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Old 1st September 2004, 10:39 PM   #7
Sjef is offline Sjef  Netherlands
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I'm using tubes and active crossovers for quite a while know. I have a tube preamp wich feeds a DEQX digital crossover (yes with op-amps and AD/DA conversion) and a couple of SE 300B power amps. A lot of people find this a very strange combination but I think it's not because it works perfectly. The system sound defenitly better with the active crossover in place and also sound defenitly better with the tube pre than with my transistor pre. So don't be afraid to try it.
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Old 3rd September 2004, 09:57 AM   #8
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Hi,

An active 2nd order high pass Sallen-Key filter is not difficult to build with tubes. You actually need a cathode follower as the filter buffer.

A low pass 2nd order Sallen-Key is not that easy with tubes because of the very low output impedance of the buffer amp needed. Have a look at this app note from TI about the Sallen–Key 2nd order filter:

http://www-s.ti.com/sc/psheets/sloa024b/sloa024b.pdf

Cheers
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Old 3rd September 2004, 04:28 PM   #9
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Examples -- Active Tube XO Maps

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