Sadly, I have to say that the Icom SP-34 external loudspeaker intended for the IC-7700 is yet another example of an over-priced loudspeaker with at best only a mediocre performance in its stock form. It does improve on the internal speaker, but only by being larger and pointing forward. It is equipped with 2 low-pass and 2 high-pass filters, but as they are single-section filters their roll-off rate is only 6dB/octave, almost not worth bothering, especially as the narrow filters in the radio largely determine the audio bandwidth anyway. When enabled, their effect is barely noticeable.
With no filters in circuit the audio sounds somewhat muffled and lacking in treble. Not that with only a 3kHz filter for SSB there would be a lot of treble anyway, but even so, speech sounds rather muffled and indistinct. On AM, the radio offers both 6kHz and 9kHz filter options, but even so, AM SW broadcasts sound equally poor.
Opening up the cabinet reveals a driver unit with a decent sized magnet assembly, but again, as with other similar speaker units, no real attempt has been made at making the cabinet an acoustic enclosure. Half way along the cabinet is a 3cm thick piece of semi-rigid foam with a large central hole through which the speaker cable from the rear socket passes. The cable is loosely wrapped in some wadding, but it is unclear exactly what the designers hoped to achieve with this piece of foam, other than perhaps to subdue cabinet resonances.
Having had good success improving two other accessory speakers, the Yaesu SP-101, and an Icom SP-23, I decided to see if I could improve this unit's performance. The first step was to purchase a Phonema Phits I34A speaker enhancement kit that is specifically designed for the SP-34. It consists of a number of precision-cut pieces of acoustic wadding that are placed on the inside of the cabinet. In my case I found that the top and left-hand side pieces were slightly too long, and had to be trimmed in order to fit. Without replacing the driver unit this kit had only a marginal effect upon the performance.
The next step was to change the driver unit. Unfortunately the 12cm SP-34 driver is a somewhat non-standard size, so I could not simply substitute a standard car audio driver as I had done for my Yaesu SP-101 speaker. I managed to find a 12cm driver, type: ESX QE120 intended for a 1984-1997 Mercedes W124 E-Class saloon. These are readily available on eBay. It has a substantial magnet and weighs an impressive 500gms. When this was inserted there was a significant improvement. I also tried the genuine Phonema driver, part number: GP-I34, which weighed 300gms, compared to the Icom unit which weighed 330gms.
I much preferred the ESX QE120, its sound was well balanced, with a nice bass response on AM, and crisp and clear on SSB. There is an old 'rule-of-thumb' that for a well balanced sound the product of the upper and lower cut-off frequencies should be 400,000, hence the 20Hz to 20kHz for a high-fidelity system. Since the ESX QE120 speaker's lower cut-off is 70Hz, this suggests that the upper cut-off should not extend beyond about 5.7kHz, which is nicely met with the 6kHz AM filter bandwidth. In order to fit the ESX QE120 some plastic studs need to be cut off, as well as two small studs intended to secure a speaker, but are not used.
The only electrical difference is that the original Icom driver is 8 ohms, whereas both the Mercedes and the Phonema drivers are 4 ohm units. Fortunately a little 'reverse engineering' of the IC-7700 shows that the audio power amplifier is a Sanyo LA4485 dual power integrated circuit. One half drives the internal or external speakers, whilst the other half drives the headphone output. The amplifier is rated to drive speakers as low as 2 ohms impedance, and so substituting a 4 ohm speaker will not cause any problems. Indeed, from the manufacturer's data sheet the amplifier is designed to withstand a short-circuit to either the power or ground bus, and is protected against over-temperature as well, so is a very rugged device. The series output coupling capacitor is 1,000uF, so the electrical low frequency cut-off is now 40Hz rather than the original 20Hz, but this is of no consequence for communications purposes.
So, after all this expense and effort, was it worth it? The answer is a qualified yes. Using the ESX QE120 driver, the audio is now crisp and clear, there is better bass response despite the higher cut-off frequency, and SSB and AM transmissions are now quite pleasant to listen to. Somewhat surprisingly, both the HP2 high-pass and the LP2 low-pass filters now have a greater effect and can be usefully employed, certainly on AM transmissions.
In summary, this loudspeaker is over-priced, and of indifferent performance in its stock form. Its filters are hardly worth the effort. By replacing the driver unit and packing the case with acoustic wadding a significant improvement can be obtained, but this greatly increases the overall cost. It can only be recommended if you insist on having a matching setup, otherwise there are cheaper, better sounding speakers than this.
Adrian, 5B4AIY |
