Spectra 4400, Output, bass response

Read more on the Acoustat Spectra 4400 in our home audio section

Andrew Holtum2004-02-23 22:21

I have a pair of Accoustat Spectra 4400s (love \'em!). The owner\'s manual (I have a photo-copy only) mentions a factory authorized mod to raise bass response by 2 DB or so. I have tried contacting the Hafler factory here in Tempe with no luck. Does anyone know the details of this mod?

I was also wondering about my Hafler XL600 power amp which I drive them with. I am not the original owner of either speakers nor amp. But I was told the Hafler was designed specifically for the speakers and got them both from the same owner. I have seen internal photos of the NYAL Moscode 600 tube/MOSFET hybrid amp and the back end (8 Hitachi MOSFETS, aluminum tunnel heat sink, cooling fan) looks suspiciously like the back end of the Hafler XL600. Also, the power rating (300W @ 8 ohms) is identical and the model names are similar. Do you know if they are in fact the same back end? I know you are an Accoustat expert, but given the ties to Hafler, I was wondering if you knew about this


Andy Szabo2004-02-23 22:21

Please refer to the electrical schematic for the Spectra 4400, posted on this site. Note the multiple taps on the low-frequency secondary winding of the transformers. There are four taps: white is for the Spectra 6600 (least bass boost), green is for the 4400 (more bass boost), and the other two are extra taps to boost bass even further. The blue provides some additional boost, and the yellow tap provides the most bass boost. Don\'t expect this change to extend low-frequency response. It will merely add more of the bass it already has.

Changing these taps is not easy, so make sure you really want to do this before you begin. Since these connections are buried underneath the transformers, significant disassembly of the interface is required. It\'s been awhile since I\'ve seen one of these chassis, so I don\'t remember enough to provide you with a detailed set of instructions. I do remember that you can remove the two transformers and their wood rails, as a single unit. It will probably be necessary to unsolder some wires before you can gain sufficient access.

Since these connections are on the high-voltage side, great care must be taken to thoroughly insulate the ends of the unused taps. All of these wires are sheathed in PVC tubing, and this tubing must be replaced as before, for good arc-over protection.

As factory built, your interfaces should be set on the green taps for the Spectra 4400. I suggest using no more that the blue tap, as the yellow tap will likely provide too much bass. Remember that there are two transformers per interface, so two connections per interface must be changed.

While I disagree that the Hafler XL600 was designed specifically for any particular speaker, (in fact, the XL600 predates the 4400) I believe it is a good, moderately priced amplifier to drive these speakers. Yes, there are amplifiers that might deliver better sound, but unless you are prepared to pay much more, you should be satisfied with this combination.

Hafler engineer Harry Klaus designed the very efficient heatsink assembly in the Hafler XL600. To avoid tooling costs, it was agreed that the aluminum extrusion would then became a catalog item for its manufacturer, allowing anyone to use it. I don\'t remember if NYAL put together their own heatsink/MOSFET assemblies, or whether Hafler sold it to them as a ”kit”. Hafler did sell amplifier modules to other manufacturers, for both audio and industrial applications.

Dairy farmers can have a problem with a slight difference in the potential of the local ”electrical ground” compared to the actual potential of the local ”earth”, that is, the floor that the cows are standing on. This causes a mild current to flow through the cow, causing discomfort, and decreasing milk production. (Cows don\'t complain much, but their happiness is gauged by their milk production.)

It was found that burying a wire grid below the cows, and driving that grid with a signal to nullify that potential difference, would significantly increase milk production. Hafler built amplifier modules for that system, consisting of a standard amplifier card, driving sixteen MOSFETS on a special heatsink. How\'s that for an unusual application for an audio amplifier?

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