3-Way Crossover Diagram (Cerwin Vega) - Opinions and Design Analysis Sought

[Ted B]

Shown here is a diagram I made for a Cerwin Vega 3-way enclosure (CLS-215) pursuant to making some parts upgrades. It looks a bit complex to me, and I welcome any knowledgeable opinions or analysis of its design that aids my understanding. From what I can tell (feel free to correct me), the HF is 3rd order, the Mid 2nd order, and the LF a 2nd order Zobel filter. Crossover points are reported to be 280Hz and 2.4kHz. The LF section seems relatively straightforward, but the HF and especially the Mid sections seem rather complex and admittedly, a bit confusing to me. Any comments or opinions as to what they were thinking or that otherwise shed some light on the design or individual components are most definitely appreciated.

 

[JaapD]

I think in general you’re pretty spot on with your description. Indeed a 3rd order highpass for the HF section and a 2nd order lowpass / highpass for the mid section. The resistors are there to match the sensitivity of the drivers and optimize the cross-over curves. Looks pretty straightforward to me so far.

What I personally don’t like is the fuse in the LF section (unwanted series resistance, might give intermittent problems over time) and the PTC in the HF section (non-linear resistance depending on the power sent to the tweeter). If it were me I’d bridge the fuse with a piece of wire. According to the spec sheet of the PTC the initial resistance is 0.25 Ohm. I would exchange it against a 0.25 Ohm resistor.


Cheers,
JaapD

 

[Ted B]

I think in general you’re pretty spot on with your description. Indeed a 3rd order highpass for the HF section and a 2nd order lowpass / highpass for the mid section. The resistors are there to match the sensitivity of the drivers and optimize the cross-over curves. Looks pretty straightforward to me so far.

What I personally don’t like is the fuse in the LF section (unwanted series resistance, might give intermittent problems over time) and the PTC in the HF section (non-linear resistance depending on the power sent to the tweeter). If it were me I’d bridge the fuse with a piece of wire. According to the spec sheet of the PTC the initial resistance is 0.25 Ohm. I would exchange it against a 0.25 Ohm resistor.


Cheers,
JaapD

Thanks for commenting and confirming what I believed concerning orders. I understand that the fuse and particularly the PTC are suspect when it comes to audio quality. I've not triggered either with regular use in the time I've owned these enclosures, so it probably wouldn't be an issue to bypass/remove them. Given that an unequalized sine sweep shows a gradual taper above 5kHz at my listening position, I'm thinking if the outright removal of that 0.25Ω PTC increases tweeter output slightly, it may be ok. I'm going correct everything anyway.

Other questions I had for example are the C3 bypass cap (polyester film) across the larger NPE. I'm thinking that may have been done perhaps to lessen the potential impact of the NPE on the midrange signal quality. And then there is the mysterious matter of R3 and R4, the purpose of which I am trying to understand. I'm thinking these may induce a bit of phase shift(?), and if so, would these be used to bring the 2nd order Mid and LF sections into better alignment with the 3rd order HF? If I can understand these components (especially R3 and R4), I think I have a reasonably good understanding of everything, which I'd like to do before I make upgrades.

 

[JaapD]

Thanks for commenting and confirming what I believed concerning orders. I understand that the fuse and particularly the PTC are suspect when it comes to audio quality. I've not triggered either with regular use in the time I've owned these enclosures, so it probably wouldn't be an issue to bypass/remove them. Given that an unequalized sine sweep shows a gradual taper above 5kHz at my listening position, I'm thinking if the outright removal of that 0.25Ω PTC increases tweeter output slightly, it may be ok. I'm going correct everything anyway.

Other questions I had for example are the C3 bypass cap (polyester film) across the larger NPE. I'm thinking that may have been done perhaps to lessen the potential impact of the NPE on the midrange signal quality. And then there is the mysterious matter of R3 and R4, the purpose of which I am trying to understand. I'm thinking these may induce a bit of phase shift(?), and if so, would these be used to bring the 2nd order Mid and LF sections into better alignment with the 3rd order HF? If I can understand these components (especially R3 and R4), I think I have a reasonably good understanding of everything, which I'd like to do before I make upgrades.

I think that C3 is only there to create the required 60 microfarad capacitance.

I see the existence of R3 and R4 to level out the slope of the frequency characteristic a bit and also to lower the Q value (broadening the bandwidth) of the LC circuit. In general: finetuning the circuit.

W.r.t. the mentioned Q-value, take as an example the LF section and look at what the woofer ‘sees’ so to speak (look into the circuit from the right side). In an ideal situation the output resistance of the connected amplifier will be around zero ohm. Then the woofer ‘sees’ L1 to be in parallel to C6 and R4. Here R4 ‘controls’ the Q-factor (in combination with the series resistance of L1).

In many crossover circuits there is no R4, leaving the control of the Q-factor from the LC circuit fully up to the series resistance of L1 (and the amplifier with its cabling). Some are trying to improve the crossover circuit through the exchange of L1 against one with a smaller series resistance. What they forget is that they are increasing the Q-factor, narrowing the curve. Sometimes the by the designer chosen series resistance is there for a reason…..

Cheers,
JaapD

 

[AnalogSteph]

People never like fuses, until the time they need them...

You can do better than the PTC, I would think. Some smart choices for tweeter protection I've seen include:
* low-voltage lightbulbs (another form of PTC but probably a lot steeper)
* polyfuses
The latter are popular in retrofitting old Grundig active speakers with a penchant for amplifiers blowing up.

You rarely see fuses in the woofer section, maybe because it's somewhat pointless. You can still shred the thing with too much excursion if you insist.

 

[Ted B]

I think that C3 is only there to create the required 60 microfarad capacitance.

I see the existence of R3 and R4 to level out the slope of the frequency characteristic a bit and also to lower the Q value (broadening the bandwidth) of the LC circuit. In general: finetuning the circuit.

W.r.t. the mentioned Q-value, take as an example the LF section and look at what the woofer ‘sees’ so to speak (look into the circuit from the right side). In an ideal situation the output resistance of the connected amplifier will be around zero ohm. Then the woofer ‘sees’ L1 to be in parallel to C6 and R4. Here R4 ‘controls’ the Q-factor (in combination with the series resistance of L1).

In many crossover circuits there is no R4, leaving the control of the Q-factor from the LC circuit fully up to the series resistance of L1 (and the amplifier with its cabling). Some are trying to improve the crossover circuit through the exchange of L1 against one with a smaller series resistance. What they forget is that they are increasing the Q-factor, narrowing the curve. Sometimes the by the designer chosen series resistance is there for a reason…..

Cheers,
JaapD

Thanks again Jaap, this is very helpful and much appreciated!

 

[Ted B]

People never like fuses, until the time they need them...

You can do better than the PTC, I would think. Some smart choices for tweeter protection I've seen include:
* low-voltage lightbulbs (another form of PTC but probably a lot steeper)
* polyfuses
The latter are popular in retrofitting old Grundig active speakers with a penchant for amplifiers blowing up.

You rarely see fuses in the woofer section, maybe because it's somewhat pointless. You can still shred the thing with too much excursion if you insist.

I've contemplated removing the fuse from the LF circuit and use that holder to replace the PTC in the HF circuit with a ceramic slow-blow fuse of comparable value. Again, I've never triggered any fuses, I agree that some measure of protection is better than none so long as it is sonically transparent, and the HF circuit would be where it is most desirable as I've seen accounts of these enclosures losing tweeters to abuse but rarely ever woofers.