Howard Dawson Audio HRL-1 application notes (also applies to Decca/Kelly units)

Crossover Requirements and Design

Please note that the following guidelines and crossover circuits are equally applicabable to the Decca/Kelly units as well as the HRL-1.

In addition, the following is all of relevance to Decca/Kelly owners wishing to get the most from their units and to maximise ribbon life. Decca were not too hot on 'applications' in our opinion. This comment does not apply to Stan Kelly of course, who fortunately did know what he was doing when he did the initial design of these fine units.

Traditional ribbon tweeters of this kind are highly intolerant to bass signals, and if you have not used them before you are advised to study the potential pitfalls so that ribbon life will be maximised.

Use of a 1.6kHz crossover as recommended by Decca for the London model will chew through ribbons using anything but miniscule amplifiers. A far better policy is to up the x/o to 2.5 or 3kHz and add a mid-range unit if necessary using a 3-way crossover as detailed below. Not only will this make the ribbons last, it will represent a huge improvement in clarity and definition.
If London owners can get hold of the DK30 crossover type CO/1/8 (2.5kHz), then that is a good step, but these are quite rare. Be careful as there was also a CO/1/15 fifteen ohm version for some of the early Kelly-produced units.

For HRL-1, the main points to observe about the chosen crossover design are that the minimum x-over frequency should be 3kHz (there is no upper limit, the unit being ideally suited for use as a "supertweeter" in an existing two or three way system), and that due to the intolerance of the ribbon to bass frequencies, it is IMPERATIVE that 2-section L-C designs (12dB/octave) or greater are used. Single section (simple series capacitor) crossovers are NOT suitable, however high the crossover frequency. Clearly at crossover frequencies higher than 3kHz, the larger the music power handling capability of the ribbon speaker/crossover combination.

It is difficult to offer hard and fast crossover designs for the home constructor since so much depends on the partnering units and their cabinets : those keen enough to experiment will no doubt be able to tailor their system to their own requirements. Some "bare-minimum" 12dB/octave crossover designs are given in fig1 for those wishing to make their own, or as a basis for further experiment. It is assumed that the partnering unit is of similar efficiency to the ribbon tweeter such that resistive pads are not required. If the drivers are not matched within 1 or 2dB, then resistive attenuators may be used, but do ensure that an impedance of 8ohms is still "seen" by the crossover output. This normally involves the use of two resistors, one in series with the drive unit and one in parallel to trim the impedance back down to 8ohms: see fig1. Take care to use resistors of an adequate power rating.

Your HRL-1 ribbons may be used either as a full-range tweeter in a ground-up design or as a supertweeter to substantially enhance the upper-octave performance of an existing two or three-way system. In the latter case it is possible to treat the existing system as a single drive unit and then add an external crossover at 12dB/octave as detailed below. You should take into account the impedance of your existing system at the chosen crossover frequency when doing this. Efficiency mismatch, if any, may be relieved by adding suitable resistors as described above. In this way it is not necessary to "butcher" the existing system at all (unless of course it is wished to permanently mount the supertweeters into the cabinets).

If you decide to use the units stood on top of an existing system, you should try to fit them as physically close to the existing treble unit as possible. If this is not done, then there are likely to be anomalies in apparent response which will vary with listening position. This effect is due to variation in path length from the two units concerned to the listening position. Best possible system performance will of course be obtained with the arrangement of fig2, where a minimum possible separation of the two units is achieved.

To scale the 3kHz inductor and capacitor values for a higher crossover frequency, first determine the ratio of required crossover frequency to 3kHz. For example, if the required X/O is 5kHz, then the calculated ratio would be 5000 divided by 3000 or 1.67 : BOTH the capacitor and inductor values should now be DIVIDED by this ratio. ......................
Crossover Construction
Ensure when building crossover units that any axial inductors are physically separated and that they are mounted with their axes at right angles. This minimises stray coupling which would cause unexpected errors in the design crossover frequency and slope of roll-off. If closed-path inductors such as pot-cores or toroids are used, stray coupling of this kind cannot occur. This type of core does however tend to be more expensive than the "ferrite rod" type.

Good quality film capacitors should be used in any crossover unit built, at least for the treble/mid sections. It is realised that the larger values required for any bass/mid sections in three-way designs can become prohibitively expensive in film dielectric, so electrolytic types may have to be used here purely for this reason - use film dielectric if you can afford them. It is probably worth doing any experimenting using electrolytic, only changing to film when the values are finalised.

If an "electronic" crossover is used and the tweeter given its own dedicated power amplifier, ensure that the output is capacitor-coupled to the tweeter : a DC offset of just a few millivolts can cause large DC currents resulting in partial saturation of the matching transformer (since the DC resistance of the windings is very small).
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Mounting in an Enclosure
The ribbon tweeter is designed to be mounted flush on the front face of the cabinet. The recommended arrangement is shown in fig2. This gives the minimum possible separation between the acoustic centres of the drive units. Observe that the tweeter flange is quite narrow along part of the vertical dimension, so care is necessary when cutting the holes in the panel that they are not cut too wide.
The tweeter may be set into the panel if required, either by machining with a router or by using a laminated front panel. The latter method is useful when one does not posess a router.
For sealing the joint, use either 6mm wide sticky-backed draught excluder foam tape (from your hardware shop) or a suitable non-setting mastic (applied sparingly).
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an example project using HRL-1

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