By-passing speaker's passive crossover for DSP crossover - need to insert capacitor/s?

[Music1969]

Hi all

I'm doing a little project to bi-amp some passive speakers and do DSP crossover. Just for fun - this is a first for me. I will use the specified passive crossover as a starting point for the digital crossover.

Is it OK to have just wire between the power amp speaker outputs and the speaker drivers?

I read somewhere (but can't find it now) that you should insert a capacitor between power amp output and drivers - to protect the drivers?

Is this protection for when the power amp is turned on?

If I need a capacitor, what specs should I be looking for?

Thanks in advance !

 

[RayDunzl]

General Concept:

A tweeter shouldn't be fed low frequency with "too much" power.

So, a correctly sized capacitor, in-line, (not across the wires) can be used for some protection.

 

[Music1969]

General Concept:

A tweeter shouldn't be fed low frequency with "too much" power.

So, a correctly sized capacitor, in-line, (not across the wires) can be used for some protection.

Thanks! But if the crossover is done by DSP, then the risk of low frequency with too much power is low?

And any advice on 'correctly sized capacitor' ?

I see in the passive crossover there is a 15uF 75Vdc capacitor - I assume that's a good starting place?

A larger capacitance should be ok? but I should stick with 75Vdc ?

 

[RayDunzl]

Thanks! But if the crossover is done by DSP, then the risk of low frequency with too much power is low?

When it is working as anticipated, yes.

And any advice on 'correctly sized capacitor' ?

Maybe start with a crossover calculator.

---

Example of "protected" tweeter on an otherwise bi-amplified and DSP crossovered two-way speaker

 

[solderdude]

Do you have a schematic of the actual crossover.
Sometimes some passive 'EQ' is implemented. This might be 'lost' when you just use a crossover.
an example:

Of course this can be implemented in DSP but you would need to know this (or measure output)

personally I would use series capacitor (oversized) at least in experimental stages.
All it takes is a little error and bye bye tweeter. Such a cap could prevent this. You can even leave it in there for protection.

 

[Hayabusa]

Thanks! But if the crossover is done by DSP, then the risk of low frequency with too much power is low?

And any advice on 'correctly sized capacitor' ?

I see in the passive crossover there is a 15uF 75Vdc capacitor - I assume that's a good starting place?

A larger capacitance should be ok? but I should stick with 75Vdc ?

The capacitor will create a high pass filter where the combination of the speaker impedance(R) and the C defines the start of the frequency where the filtering starts.
This frequency needs to be somewhat lower than the actual xover frequency set in DSP to prevent phase shifts.
So I would set it to half the xover frequency.

use an online RC filter calculator to calculate your value for C:

http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php

 

[RayDunzl]

A larger capacitance should be ok?

All things being equal a larger in-line capacitance will pass lower frequency than a smaller capacitor.

 

[Music1969]

The capacitor will create a high pass filter where the combination of the speaker impedance(R) and the C defines the start of the frequency where the filtering starts.
This frequency needs to be somewhat lower than the actual xover frequency set in DSP to prevent phase shifts.
So I would set it to half the xover frequency.

use an online RC filter calculator to calculate your value for C:

http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php

Thanks heaps! I've just calculated 37 uF based on 6 ohms and 1500 Hz (see pics below)

What DC voltage spec should I go for in looking for a capacitor?

 

[Music1969]

Do you have a schematic of the actual crossover.
Sometimes some passive 'EQ' is implemented. This might be 'lost' when you just use a crossover.
an example:

Of course this can be implemented in DSP but you would need to know this (or measure output)

personally I would use series capacitor (oversized) at least in experimental stages.
All it takes is a little error and bye bye tweeter. Such a cap could prevent this. You can even leave it in there for protection.

I'm planning this experiment with a new pair of budget Elacs.

But before I try that I'm going to try with a 50 year old pair of Fortura 10 with Heil AMT

So I don't have any schematics sadly :-(

Any advice on what and how to measure?

 

[Music1969]

 

[RayDunzl]

What DC voltage spec should I go for in looking for a capacitor?

How many volts can your amplifier output?

Note the M2 schematic above - 250V cap rating

Does the recommended amplifier output 250V? No. 1250W @ 8ohms. Maybe more like 100V (which becomes 140V peak)

 

[solderdude]

no schematics needed for this one.
Looks like a 12dB/oct tweeter XO with 4 Ohm Lpad and 6dB/oct woofer. There is also a fuse in there.
The block behind the tweeter is only a connection.

You could measure the output of the tweeter and woofer by disconnecting the other drivers.
This way you can find out the XO frequency.
The spec.plate seems to suggest 1.5kHz.

When you own an LCR meter you could desolder the parts and individually measure each component.

 

[Music1969]

You could measure the output of the tweeter and woofer by disconnecting the other drivers.
This way you can find out the XO frequency.
The spec.plate seems to suggest 1.5kHz.

For DSP crossover I was going to start with 1500Hz anyway.

How to measure output of tweeter safely without making it ‘woof’?

In REW is it easy to specify range of test signal? So for tweeter, test signal from 1.5kHz to 20kHz is a good test?

And for the woofer 50Hz to 1.5kHz is a good test signal range ?

 

[Music1969]

How many volts can your amplifier output?

100W into 8 ohms but I think that’s RMS typically

So 28Vrms

Does that help to shortlist suitable capacitor ?

 

[RayDunzl]

28Vrms = 40Vpk so at least 50 maybe 100V (or someplace in between) for the caps would be my less than fully educated guess.

More shouldn't hurt.

Less might.

 

[Hayabusa]

100W into 8 ohms but I think that’s RMS typically

So 28Vrms

Does that help to shortlist suitable capacitor ?

Also make sure its a 'real' capacitor or bipolar elco and not a dc elco.
And I would go for 63v or higher

 

[solderdude]

How to measure output of tweeter safely without making it ‘woof’?

measure it really up close (mic almost against it) at a very low power level.
This way you can easily measure a tweeter from 500Hz up.
The woofer you can sweep from 10Hz to 5kHz or so from a meter away.

 

[Hayabusa]

measure it really up close (mic almost against it) at a very low power level.

You can set the starting frequency if you do a sweep with REW.

 

[Music1969]

measure it really up close (mic almost against it) at a very low power level.

And at low power level, it’s ok to do 20Hz to 20kHz with both tweeter and woofer?

 

[Music1969]

You can set the starting frequency if you do sweep a with REW.

Ok thanks. So if passive crossover is 1.5kHz then starting frequence sweep for tweeter at 1.3 kHz sound like a good idea?

And for woofer finishing sweep 1.7kHz should be ok to start?