28 July 2008

DIY 6T9 Tube Amplifier Project

Mark Houston in Australia has put together a compact 6T9 single-ended tube amplifier. The 6T9 is part of the GE compactron vacuum tube family and is a high-Mu triode and power pentode in one bottle. The amplifier and power supply is contained on a PCB which was purchased from Spare Time Gizmos.

6T9 Tube Amplifier PCB
Top keep costs down, the tube amplifier was built into a thin and inexpensive cooking pan. A Hammond power transformer was used along with Edcor audio output transformers.

DIY 6T9 Vacuum Tube / Valve Amp
The power output of this small amplifier is limited (about 4.5W max), so efficient loudspeakers are required. Mark is reporting good sonic results from this simple tube amp when matched a pair of Fostex FE167E bass reflex speakers. For more information, schematics, measurements and photos, see the 6T9 Tube Amplifier Project page.

More DIY Single-Ended Tube Amplifier Projects:
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05 July 2008

DIY Sealed Jordan JX92S Bookshelf Speaker

A while back I purchased a pair of Jordan JX92S fullrange speakers. At the time I purchased them I didn't really know what I was going to do with them, but having read so many positive comments about these drivers, I really wanted to hear them for myself. At $180 each, the drivers are costly, but these drivers have a large Xmax, good low end extension and a fairly smooth frequency response. It is uncommon to find all of these attributes in a medium sized fullrange unit. I was eager to hear these drivers, so I figured I would first try them in a sealed loudspeaker enclosure, with the idea to convert the box to a bass reflex or aperiodic enclosure later on. Figure 1 shows the calculated frequency response for the JX92S driver in a 0.25 ft^3 sealed box. The orange line is the calculated response using the manufacturers T/S parameters and the yellow line is the calculated response using parameters that I measured with a Dayton Audio Woofer Tester 3 (WT3). The calculated impedance is shown in Figure 2. See the Jordan JX92S fullrange driver page for the measured Thiele-Small (T/S) Parameters.

Jordan JX92S Frequency ResponseFigure 1: Calculated Frequency Response - Jordan JX92S in 0.25 ft^3 Sealed Box

Jordan JX92S Impedance Response
Figure 2: Calculated Impedance Response - Jordan JX92S in 0.25 ft^3 Sealed Box

For the loudspeaker enclosure I used prefabricated 0.25 ft^3 curved cabinets from Parts Express
. A rectangular or do-it-yourself (DIY) box of the same volume would also be fine. Damp the walls of the cabinet with upholsterers felt or use poly fill to stuff the box. Adjust the damping materials to suit your taste. The driver is counter sunk, but I didn't have any black low profiles screws on hand and I wasn't about to run out to buy some.

Jordan JX92S Sealed EnclosurePhotograph 1: Jordan JX92S in a 0.25 ft^3 Sealed Loudspeaker Box - Front

Jordan JX92S Sealed Loudspeaker
Photograph 2: Jordan JX92S in a 0.25 ft^3 Sealed Loudspeaker Box - Rear

The measured frequency response is shown in Figure 3. The frequency response measurements were completed using a Radio Shack 33-2050 SPL Meter. On axis, this is a fairly smooth driver. At about 15 degrees and more off axis, the high frequency response drops off
rapidly due to beaming.

Jordan JX92S Measured Frequency ResponseFigure 3: Measured Frequency Response - Jordan JX92S in 0.25 ft^3 Sealed Box

The impedance response was measured using the Dayton Audio Woofer Tester 3 and is shown below in Figure 4.

Jordan JX92S Measured Impedance
Figure 4: Measured Impedance Response - Jordan JX92S in 0.25 ft^3 Sealed Box

The temptation will be run these drivers fullrange and some may actually be happy by only sucking the drivers up against the wall. However, most people will likely prefer some amount of baffle step correction / compensation (BSC). The amount of attenuation required will be a function of the listening room, amplifier and personal taste. I was happy with about 4.5 dB of attenuation (Lbsc=1mH and Rbsc=3R9 for a 7.5" wide baffle). You can use the online baffle step correction calculator to determine a
suitable circuit for your baffle width and desired attenuation.

Jordan JX92S Temporary BSCPhotograph 3: Temporary BSC for Jordan JX92S Sealed Loudspeaker

Most will likely be satisfied with this driver running fullrange plus a simple BSC. However, off-axis the high frequency response starts to quickly disappear. Personally, I like to toe the speakers so they cross just in front of the listening position with Radio Shack 40-1310 Super Tweeters toed out at the same angle. Add a diy active subwoofer (I cross mine in at ~60 Hz) and this makes for a very pleasant and simple 2 channel system. It seems that everyone who hears these drivers prefers them over the Fostex FX120. Myself, I prefer the FX120. I don't find the JX92S as "engaging" as the FX120. Next step is to convert the speaker box into an Aperiodic Loudspeaker Enclosure. Stay tuned.

Related DIY Jordan JX92S Loudspeaker Projects:

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