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Old 16th January 2010, 10:13 AM   #1
WDYSUN is offline WDYSUN  Italy
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Default ACTIVE CROSSOVER: OPAM decision!

Dear All,

a couple of years ago I could see how active biamping at 300Hz with a passive mid/tweeter network is one of the best thing to do.

I initially played around with step filters, but then II order played the best. I also tried tube xovers but it seems that this add too much complications. The best thing I could get was to use a 2-way LR II order xover.

My actual prototype has been implemented using ESP, Elliot Sound Products, PP81 boards

ttp://sound.westhost.com/project81.htm

but there is some modification, the path of the signal is the following:

1. the signal comes from my tube preamp with ECC88 where the output buffer has been removed

2. now the signal gets into the xover Input buffer with two OPA627 mounted on a board which allows to replace dual opamps with two single opamps;

3. the sgnal gets into the LP and HP filter section made with the TL072 suggested by ESP

4. and then into an output buffer with ECC88 Tubes in pure triode modes (a kind of Music Fidelity X-10 clone),

Notice that while the HP output is set so that there is 0db gain, the LP output has a stepped attenuator which provides me with fine tuning of the bass (My woofer is almost 3db higher than the midrange). This allows me to tune the frequency response just acting on the level of the woofer without any gain in the xover.

This is the best thing I got in two years of experiments. I am impressed with the result. However I have still some doubt which I would like to discuss with opamp and tube experts.

First of all I was so surprised that the introduction of a tube buffer replacing the the opamp buffer made such a big step. The improvement was so evident that I couldn't believe it! And this is in line with what people says about tube buffer in series with DAC opamp outputs. My question is now: do you thing I would gain something putting a tube buffer at the imput replacing the BB OPA627?

Second question is about the choice of the OPAMP in the two filters. What is in your opinion the best opamp to place here. Would a high quality opam make a real difference here? For instance Phil Marchand uses the OPA2134 while Elliott Sound Products suggests the use of the TL072 types. What do you thing about that?

I hope you'll find the topic interesting enough!

Cheers
Pietro
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Old 16th January 2010, 10:20 AM   #2
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I replaced the original TL072 in my xovers with OPA2134 and got a marked improvement.
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Old 16th January 2010, 12:06 PM   #3
AndrewT is offline AndrewT  Scotland
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if you do change opamps then pay careful attention to the datasheet advice on decoupling the power pins of the opamps.
Similarly go back and check you have implemented the 627s to the manufacturers advice.
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regards Andrew T.
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Old 17th January 2010, 05:19 PM   #4
WDYSUN is offline WDYSUN  Italy
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Quote:
Originally Posted by Charles Darwin View Post
I replaced the original TL072 in my xovers with OPA2134 and got a marked improvement.
Which xover are you talking about?

Pietro
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Old 18th January 2010, 08:56 AM   #5
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