Single Ended Active Crossovers for Open Baffle Speakers - diyAudio
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Old 1st October 2009, 06:00 AM   #1
ljfont is offline ljfont  Canada
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Default Single Ended Active Crossovers for Open Baffle Speakers

Is there such a thing?

I'm interested in experimenting with open baffles using active crossovers, with bi-amped fullrange drivers and woofers.

There seems to be Nelson Pass B4 comming out at some point, also a document that Martin King is working on that deals with active two way crossovers for open baffles, that's not yet out.

I've sketched out a proposal for my system. It involves a JFET source follower (B1 Buffer) before and after a linkwitz riley 2nd order filter. I'm not sure if it's considered to be active. It's a passive filter that is buffered.

I guess the idea is to not have any op amps, and have a system that is single ended with no -ve feedback.

4 input pots could be used to control the level for each speaker, thus providing volume/balance and effective EQ for the full range and woofers.

Does this look feasable? or will there be issues due to DC coupling with excess capacitors in the signal path?

Thanks.
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Old 1st October 2009, 06:05 AM   #2
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A buffered passive XO. Are you thinking LC or cascaded RC? If the former, do a calc on the inductor size required. If the latter there isn't a whole lot of control over the filter shape.

We've built a prtototype of the latter with opamps. Now need to try it with followers.

dave
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Old 1st October 2009, 07:14 AM   #3
Calvin is offline Calvin  Germany
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Hi,

have a look for ´unity gain Sallen Key´. These structures are usually drawn using OP-Amps. But since the OP-Amps are configured to a gain of 1 its easy to change to a simple emitter- or sourcefollower cofiguration instead.
So You´re not bound to only ´buffered passive filtering´ but you can do real active filtering (feedback structures). Passive filtering -regardless of beeing buffered or not- is restricted to low Q values. If You need Qs>1 (Bass-Lift, Equalizers, etc.) You will need active filters. The amount of active parts in Your signal path remains the same, but is of course much smaller than using OPs. I prefer JFET-CCS loaded JFET-Sourcefollowers for the job or Trioded cathode followers. Since we are not talking about measurement equipment, linearity, distortion and bandwidth of a simple follower stage are more than sufficient for the job.
To my taste the simple JFET-sourcefollower-CRs sound much better than any OP based Crossover I came across yet. Especially with bipolar OPs I get the impression of listening to ´reproduced signals´ and not music. Seem to suck out blood and emotions. The simple JFET-buffers though seem to preserve life and emotion in music.
Apart from the sreening and selection of the transistors the effort is basically the same as for an OP amp. Select pairs of JFETs and couple them thermally to get a ´6-pined OP´ Put a couple of resistors and caps around ...voila!
Small, easy and very good sounding.

jauu
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Old 1st October 2009, 09:37 AM   #4
AndrewT is offline AndrewT  Scotland
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a L-R 2pole is a cascade of two RC filters.
Each filter requires to be fed from a low source impedance and to feed into a high load impedance.
RC + RC will require a third buffer between them.
The B1 does not have a low enough output impedance to be ignored.
You must take account of the B1's real output impedance in your calculations.
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Old 8th October 2009, 07:53 AM   #5
ljfont is offline ljfont  Canada
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Quote:
Originally Posted by planet10 View Post
A buffered passive XO. Are you thinking LC or cascaded RC?

dave
Keeping the circuit simple. A LC was what I was thinking. I haven't calculated anything yet. I'm working with MJK worksheets to identify a good woofer to mate with the FE167E driver. OB may be too large in my living room. Looking at other enclosures to work with and/or smaller woofers (~ 10" ). Have you tried with source followers?
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Old 8th October 2009, 08:05 AM   #6
ljfont is offline ljfont  Canada
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Quote:
Originally Posted by AndrewT View Post
RC + RC will require a third buffer between them.
I remember doing a electronics lab in tech school where we built a circuit.

RC + (nR)(nC)

where the cutoff frequency remains the same however it doesn't effect the loading between the filters when multiplied by n. Therefore you only don't require a third buffer.

Would this work here?
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Old 8th October 2009, 03:24 PM   #7
adason is offline adason  United States
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Hi all,

I have recently finished my line level crossover, 200Hz, which splits the signal for tube amp for midrange and for solid state amp for woofers.
After many itterations, I arrived at the following: signal goes thrue volume control into buffer (IC), than passively splits with LC/CL (stolen design from marchand), than each high and low pass has pot and is buffered again (transistor).
Sounds wonderfull.
Tough part was to get those 2-4 Henry inductors. I ended up using spare 70volt transformers. You know those small transformers which are in the speakers with 70 volt input and few secondaries, like 0.2watt, 0.5watt, 1watt, 2watts, 4watts....for volume...well those are just great for this application. The main advantage over the RC crossover is no signal loss.

Ed
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Old 8th October 2009, 11:24 PM   #8
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Quote:
Originally Posted by adason View Post
Tough part was to get those 2-4 Henry inductors. I ended up using spare 70volt transformers. You know those small transformers which are in the speakers with 70 volt input and few secondaries, like 0.2watt, 0.5watt, 1watt, 2watts, 4watts....for volume...well those are just great for this application.
Thanx for the tip Ed... i have a few of those kicking around.

dave
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