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I added a pair of subs to my Merlin-based system and built them with the help of Tom, who has made a number of big honkers and has great wood-working tools. One of the requirements is that the whole system be driven with one amp so that it is useful for testing amps full range. So that required the development of a suitable crossover, and that I would unload the bass from the Merlins. 12 dB slopes were used for both the hi pass and the lo pass sections. Because the the Merlin presented an impedance rise to over 20 ohms at 60 Hz, a suitable trap needed to be added to the hi-pass section, as well as electrical damping to control that and the natural resonance you get from an L-C 12 dB filter. Likewise, damping is needed to control the resonances in the lo-pass filter and woofer. I used film capacitors and my own hand-made inductors for the filters, all sized for linear operation up to 200 watts peak.

Tom talked me into this project because he felt that the biggest limitation I had in my system was the very limited air movement that could be delivered by the 6.5-inch bass driver in the Merlins. We decided on 15-inch drivers (Dayton DVC385-88). These have a high X-max of 15 mm and reasonably high sensitivity of 90 dB. 5 cu. ft. sealed-box construction was used.

This posting and the next ones(s) have some pictures of the final steps.

Here is a continuation. The order of the pictures is reversed from how I intended them to be presented for the previous posting. Here are three more, hopefully in the right order.

David

Here is the last sequence of pictures. Although I designed the X-over to handle up to 200 watts peak, as determined in part by saturation in inductors, the system plays quite loud and clean with 6 watts from my 300B SE ZOTL (output transformerless) choke-loaded amp.

I forgot to mention that the X-over point is 70 Hz. With the bass unloaded from the Merlins, I feel safe in using a higher-power amp with the Merlins than my self-imposed 30-watt limit that I previously set.

David

Hi, David, nice old Zenith in the right corner, the old push-button model, fixed one like it with rubber coated wire that had just fallen off the conductors, what a pain! BTW, what is the amp shown in the last pic?

I want to do this on a smaller level (10 or 12 inch subs).

I would need help on the crossover.

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I reject your music, and substitute my own.

Does the external passive crossover re-act with the speakers passive crossover causing problems?

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I reject your music, and substitute my own.

The crossovers do not interact. The passive crossover in the Merlin is between the tweeter and its woofer, and crossover point is in the couple of kHz range. Therefore, the Merlin X-over becomes simply the Merlin's woofer's characteristics at my X-over point of 70 Hz into my added sub. Now that doesn't make it all that simple because the Merlin's woofer behaves badly at 60 Hz where its impedance rises to over 20 ohms. I had to kill this impedance peak in order for my X-over to give the proper transition. I did this with an additional network placed in parallel with the Merlin consisting of a series combination of an 8.6 ohm resistor, a 160 uF capacitor and a 40 mH inductor. My inductor has a DCR of 5 ohms, so I really added 3.6 ohms to give the effective 8.6 ohms. This network also interacts with my main hi-pass X-over elements, which includes a main series capacitor of 270 uF and shunt inductor of 13 mH across the Merlin. The DCR of my 13 mH inductor is 1.3 ohms. So these values complete my hi-pass filter.

The lo-pass consists of a main series inductor of 25 mH and total series resistance (including the DCR in the inductor) of 3.6 ohms. Then there is a main shunt capacitor across the sub woofer of 330 uF. For damping I have a series LCR network across the sub with values 81 mH, 270 uF, and 11.8 ohms (again including the DCR of the inductor). I used all film capacitors rather than the cheap bipolar electrolytic types that are common as I wanted stable values. I made my inductors using E-I laminations from various power and audio transformers I had on hand. Critical to this, I made my own transformer clamp housings out of aluminum so that the air gaps would not be bridged by steel. This keeps them linear. I used rather large air gaps to keep the saturation current high.

If you were to choose to use this particular driver I used in a 5 cu ft. box, I think my lo-pass would work for you (of course crossing at 70 Hz). The hi-pass would need to be modified most likely to accommodate your speaker.

Good luck- David

In Davids situation he had to use a passive crossover because he is in the business of building amps that will be used full range on speakers with passive crossovers built in. This is the perfect case for using an active crossover and a separate amp for the subs. The cost in parts that he has for his passive crossover is higher than an active crossover and high powered sub amp. Using an active design also would eliminate the impedance problems he had with trying to crossover to the Merlin woofer. Not an option in his case, but a better way to go if you are not an amp builder.

BTW, unloading the main amp from driving the subs lowers the risk of running the main amp into clipping which would be a real possibility if you were to also use it to drive the subs and result in much more objectional audio consequences full range than if a sub amp clips only into the subs. His drivers have high sensitivity but require big boxes, Hoffman's Iron Law at work. If you had a separate amp this problem would be minimized and you could use a smaller box with resulting lower sensitivity.