audio-talk

The Barbarik schematic shows the cathode resistors for the 12SJ7's as 620 ohms. That would imply the feedback resistor for the output tubes needs to be in the 6~12K range, which will load down the output tubes a bit, although not as much as the speaker load. The feedback resistor will also need a co...

A minor digression on loudspeaker measurement: The biggest difference between RTA measurements and more sophisticated techniques is that the pink-noise RTA measurement is an open-loop measurement. In other words, the measuring device that generates the RTA display has no idea what the stimulus is; t...

The straight pentode connection, with the screens regulated is, for me, the best way of running pentode valves. IME so far, ultra-linear just isn't as good. In isolation, ultra-linear sounds perfectly OK; it's only when you apply the pentode connection, that you begin to realise that straight pento...

Thanks for posting your amplifier, Steve! There are times when the sheer cost and complexity of the Karna amplifier gets to me ... matched quads of 300B's, for one thing, not to mention the hassles with the filament power supplies. It's especially gratifying that the new KT77's (with local feedback)...

To sum up: don't use sinewaves to test speakers. Put two feet of acoustical absorbent (blankets, coats, pillows, etc.) on the floor midway between the mike and the speaker. If you hand-hold the mike, hold it as far away from your body as possible, and hold it steady. Mike-stands with an extended boo...

I'm sorry if I disappoint anyone with this reply, but here's my two cents: * Sinewave testing is only useful for amplifiers, and then only in the context of distortion analyzers that can separate out harmonics from the 2nd to the 10th. * For speakers in rooms, steady-state sinewave amplitudes swing ...

You could adopt a hybrid filter for the Fostex. By hybrid I mean a filter that's partly in the amplifier, and partly at speaker level. For example, there could be a 1st-order lowpass in the amplifier, and a synchronous-tuned inductor going to the Fostex driver, with the inductor followed by a passiv...

Thanks for your posting, IslandPink! Just for review, what's the order of the highpass filter for the AH425? I'm contemplating an OB-mounted Altec/GPA 416 (Alnico), operating mostly through the baffle-peak region, with an additional 416 in a closed-box beneath it, so I'm watching your experiments wi...

I think Slowmotion is on to something. A 288 with a good 1" compression driver on top is a very traditional solution. I've even heard suggestions of using a 604 Duplex this way: large-format horn from 700 Hz to 7 kHz, than the internal horn of the 604 above that. However, with a new Alnico Dupl...

I've gotten a lot of heat in the "other" forum about designing the AH425 around a large-format driver instead of a small-format (1" exit) compression driver. After all, the small-format drivers can easily reach 15 kHz or better, even with plain old Mylar diaphragms. The real reason is...

Good thing I have a pair of GPA 288 Alnico's on hand, as an alternate to the Radian 745P's. I'll follow your lead and try both. The 288, with its crimped-aluminum tangential surround, has more diaphragm compliance than the Mylar-suspended Radian. This lowers the Fs of the 288 by 100~200 Hz, and resu...

Kind of hard to see a good way to do a notch filter in a power amp. Active notch filters usually require a positive feedback loop, which doesn't do any favors to the sound (distortion rises sharply in the positive-feedback region, which is very audible). That's the big sonic problem with textbook Sa...

Looks really nice, Mark. Congratulations!!! The last time I messed around with a fullrange Fostex, the response had several peaks in the 1~5 kHz region, nothing at all like the published curves. If you're going to hit it with a notch filters, I'd find the first (lowest frequency) peak, attenuate by ...

I don't think the esthetics of curve-matching will be too difficult. It's a lot easier to bend perspex/acrylic than plywood, and it machines more accurately too. It was your comments (and comments made by other posters) than made me realize that perspex had acceptable sonic qualities, at least compa...

Well, I'm still open to the option of a perspex curved-wing dipole for the upper driver, and using a GPA/Altec 416 Alnico in a 3.5 cubic foot closed box for the lower driver. EQ applied to the lower driver compensates for the dipole LF rolloff of the upper driver, and if substantial EQ is required, ...