Bi-Amping Cornscala

[Gints]

Team,

 

I am the 3rd owner of a set of Cornscala's here in Australia. Very well built, very solid, very nice.

 

I am considering bi-amping, with my 15W Tube OTL for the Mids/Tops, and a Solid State for the bass driver. This is to get additional headroom to allow me to EQ for my room which I have trialled with good results to accompany my room treatments.

 

Midrange Driver is a PD-5VH (16 Ohm) mated to a wooden horn

Tweeter is a CT125 (8Ohms)

Woofer is a CW1526C (4 Ohm i believe)

Crossover is B&K Sound Type CS 400/4500Hz 

 

My plan to achieve this was to use a MiniDSP as an active crossover, set around 400Hz.

 

Couple of questions:

How/can I modify the passive-crossover to allow for this? Do I need to?

My hope was to feed the woofer directly (active crossover), and still use the passive crossover to handle the Mid/Tweeter crossover.

Would this approach (3 way crossover with no load hanging off the Woofer) cause issues? Would the load presented to the Valve amp change?

 

What would need to be done to allow this bi-amping?

 

Any advice would be greatly appreciated.

 

 

[pzannucci]

The amplification shouldn't be an issue.  If you are going full active outside of the crossover for the tweeter, most crossovers used on the k-55 type driver does not have a low pass so you would need to get all of the components out of the way going to the mid and leave the connection to the tweeter using the caps.  If the tweeter goes through the autoformer and uses a different tap than the midrange, things will get a little harder to manage and you may want to leave the autoformer in place.  Hard to tell since "Cornscalas" are really guaranteed to be one design or another as they many times are DIY.  You'll just have to look through the wiring.   For the woofer, just remove the low pass.  That should be it other then adding the proper connections and possibly switches to allow for re-enabling the crossover should you want to.

[Gints]

Yes thanks, they are definitely a mixed bag and not standard.

 

Attached is a clearer picture of the crossover, and some notes (no time to draw full circuit yet):

 

- Single set of inputs (top-most pair)

- Second set from top is woofer, with Input + running into the 2.5mH inductor top left, then out to Woofer + 

- 3rd set from top is Squawker, with + fed by 500uH inductor bottom right, which is fed by Pin 3 of Bottom left Unit

- 4th from Top is Tweeter, fed from output of 4.0uF smaller cap, which is fed from Pin 4 on Bottom Left Unit

- The botttom left unit is wired as follows:

   -  Pin 4 to 4.0uF cap

   -  Pin 5 to 6.8uF cap, fed by Input +

   -  Pin 0 fed to Input -

   -  Pin 3 to 500uH inductor

 

 

Let me know thoughts. I believe I can feed the woofer directly, but I am unsure of the load and crossover characteristics presented to the other amplifier once the woofer is removed.

 

Plan was to separe incoming signal: 400Hz Low Pass to woofer, 400Hz High Pass into existing crossover network (pending viability)

[Gints]

Color coded all the connections if this might help decipher:

[rplace]

While I am no expert I've been learning a lot from @Chris A on my active set up. I think what you want is for your source/pre to feed the MiniDSP and have your lows of 400 and below from minidsp to your LF amp and that amp feed your bass driver directly skipping the passive crossover all together. You want the MiniDSP's HF output to go to your HF amp then to the input of your passive B&K XO to the input side as is and in turn have it hooked up to the squawker/tweeter as is. Or a shorter way of saying it is you just need to disconnect that set of wires from the passive XO to the woofer and replace it with the LF output from MiniDSP>>Amp>>Woofer.

[mboxler]

Not sure how your DIY skills are, but the resulting circuit is quite simple...

 

Autoformer

500uH inductor

4uF capacitor

 

Here's an unfinished AB-4500 (type A, bi-amp, 4500hz) crossover just to give you an idea.  I believe for your CS you would attenuate

the hi-pass output of the miniDSP by 3db.  The nice thing about this setup is you can attenuate the PD-5VH to match your CT-125, then attenuate

the high pass output from the miniDSP to match the low pass to the woofer.

 

Mike

[Gints]

DIY skills are okay. I built the amps, so the crossover should be fine.

 

I just need to be sure of the circuit etc.

[Gints]

okay.

 

@mboxler does this sound right based on your feedback?

So my understanding of what I would need is:

 

- Remove 2.5mH Inductor from circuit which is acting as low pass for Woofer. Crossover to be implemented in MiniDSP.

- Remove 6.8uF capacitor filtering the Input+ before the autoformer. This is acting as High Pass at 400Hz. Crossover to be implemented in MiniDSP.

- Feed Input+ to Pin 5 of 3619 Autoformer and out of pin 4 (-3dB), into 500uH inductor to act as low pass to Squaker+ (4500Hz)

- Feed Input+ to 4.0uF Cap to act as 4500Hz high pass 

 

This would re-arrange the autoformer, which currently has the Squawker on Pin 3 (-6dB) and Tweeter on Pin 4 (-3dB). This is because I would do the first -3dB attenuation in MiniDSP crossover, so only the incremental -3dB to the Squawker is required. Or I could leave it as-is and have the passive crossover/autoformer handle this.

 

Would this woofer removal cause any load issues? I presume the load presented to the upper amp would increase, likely a good thing for the valve amp.

Edited September 20, 2018 by Gints

[mboxler]

Sorry for the late reply @Gints.

 

Yes, that sounds correct, but if you want to modify the current crossover, I think this would be easier...

 

Leave the 2.5mh inductor.  Since the woofer will not be connected, no current will pass through it anyway.

 

You can leave the 6.8uf cap in place IF you can run a wire from INPUT to pin 5 of the autoformer.  This wire would be in parallel with the 6.8uF capacitor, causing very little current to pass thru the capacitor, like it's not even there.

 

With this setup, the 3db attenuation would not be needed in the miniDSP.  If you don't like the results, remove the wire, and you're right back to where you started.

 

There shouldn't be any load issues.  If you want a greater and more consistent load on the high pass amp, you could solder a 10 ohm resistor from pin 0 to pin 5.  But I'd take it one step at a time. 

 

I always place an 8 ohm resistor across the tweeter output, a 13 ohm resistor across the mid output, and use a cheap amplifier to pass varying frequencies to the crossover input.  Using a true RMS multimeter,  I measure the voltages across everything just to be sure I didn't screw something up, then connect to the drivers.  If you lived in the Denver area I'd be happy to help you with this.  

 

Hope all this makes sense!

 

Mike

[mboxler]

The more I look at the picture, this may be easiest (and what you were suggesting).  Is there enough of pin 5 exposed to allow a quick disconnect to push onto?

 

If so, remove the wires you marked red from the input.  Maybe wrap the spade with electrical tape to make sure it doesn't make contact with anything.

 

Crimp a spade connect to one end of the wire and a quick disconnect to the other end.  Connect to spade end of the wire to the input red.  Push the quick disconnect onto pin 5.

 

I think that should work.

 

Mike

[Gints]

Brilliant, thanks mate!

Off to the US for 2 weeks, when I return the MiniDSP should be there and I'll test it all out!

[Gints]

Well I modified the crossover to remove the top inductor and squawker capacitor, heat shrieked them off and added a new wire to pin 5. Woofers now wired direct.

 

Worked a treat. Used MiniDDP DDRC24 to put in active crossover. Bass driver driven by Solid State with 240W into 4Ohms, so I padded 12dB. 

 

Defeinitly far more drive down low. Mids and highs now clearer too. I think at 15W OTL was struggling at high volume.

 

Great success. I have managed to create a 50Hz hum, so will review all my power this weekend.

 

Thanks for the help. This thing really thumps and sings now.