KT88 Push-Pull Mono Block Tube Amps

Bruce from the USA has completed his second follow up to his original OddWatt tube amplifier project (a EL84/6BQ5 Push-Pull Tube Amplifier) with the Odd Blocks. The Odd Blocks are a pair of KT88 Push-Pull Mono Block Tube Amplifiers. Like the Oddwatt 225 (a KT77 SIPP (Push-Pull) Tube Amplifier), the Odd Blocks are also scalable and will work with a number of octal tubes such as KT77, EL34, 6CA7, 6L6GC, KT88, 6550 and KT90. A photograph of the finished mono block tube amplifiers is shown below.


Like the previous OddWatt tube amp projects, the topology of the driver stage is an SRPP this time using 12SL7 tubes. This feeds a self-inverting push-pull (SIPP) output stage operating in Class A Ultra-Linear (UL) mode. The output tubes are Gold Lion KT88. LM317HV regulators are used to form the constant current source (CCS) on the output tubes. An underside view of the amplifier is shown below.


Like the precursors to this project, the KT88 provides very good performance. The SRPP and SIPP output stage combine to produce a very low noise floor. The design can be scaled for use with other tubes by simply adjusting B+ voltage and cathode current.

For complete information about this project, see the Odd Blocks project page - KT88 Push-Pull Mono Block Tube Amplifiers.

Related DIY Tube Amplifier Projects:

• Monoblock 5751 SRPP / KT88 Push-Pull Tube Amp Kits
  
• DIY KT88 Single-Ended (SE) Tube Amplifier Project
  

What's Playing: The Stranglers - Golden Brown

Fostex FE167E Bass Reflex Speakers

by Mark Houston


The very first pair of speakers I put together some thirty-eight years ago was a pair of single-driver loudspeakers using 8” Kaltro twin cone drivers. As I type this, I find myself once again listening to speakers using a single full-range driver. This time it is the 6.5” Fostex FE167E fullrange driver. I had recently completed a small low wattage 6T9 valve amplifier, and asked Gio for some suggestions on speakers to match the amp. He suggested the Fostex FE167E. When I saw they were 94dB efficient I ordered them instantly. Packaged with the drivers were two enclosure plans - a floor-standing unit as well as a bookshelf enclosure. I opted for the smaller bookshelf speaker box. The Fostex Recommended Standard Bass Reflex Type Enclosure is 15L with a 50 by 50 mm port that can exit from the front or rear. The enclosure plan is shown below.


The speaker boxes were put together by a neighbor using 18 mm MDF. The boxes were clamped during the gluing process and the inside joints were sealed with additional glue. The back panel had heavy speaker sound absorbing vinyl glued over its surface and the box was lined with a single layer of thick Dacron (tontine).


The binding post block was fitted and oxygen free copper speaker wire was used for the connections. Sealing sticky back foam was placed under the connecting block and also under the drivers which were screwed in place. The boxes were pre-sprayed with paint. Later grills were added by cutting the center out of a piece of MDF, stretching and stapling speaker cloth over the frames. The grills were attached to the speakers using Velcro.


The baffle step correction / compensation (BSC) calculator can be used to determine a BSC specific to the width of your baffle. The amount of attenuation required will be subject to your listening room and amplifier as well as personal taste. Typically about 4 to 6 dB of attenuation is good. You can experiment with the component values shown below and select a BSC to suit your room, amplifier and personal preference. I am using L=1mH and R=2.7ohms.


Out of the box the first thing you notice about the speaker drivers are the ultra-light the cones and very stiff suspension. The first few days of playing will see the drivers settle though expect to put in twenty to fifty hours before they settle completely. If you are not use to listening to single-driver loudspeakers you may be a little shocked at first. I was, but it doesn’t take long to recognize the all the good qualities of a single-driver speaker system. At a recent Melbourne Audio Club night held at my residence the Fostex FE167E speakers were auditioned with a small 6T9 tube amplifier and they received many accolades from the very discerning listeners. When compared with small 2-way bookshelf speakers the Fostex FE167E produced more bass and equal treble response. Most members could not choose one pair of speakers over the other. Voices however do appear forward on strong vocal tracks. This is not unpleasant just obvious.


When teamed with the 6T9 tube amp, for which they were intended, they really come into their own. They are LOUD with only a few watts driving them, produce good bass and excellent mids and highs. The Fostex recommended enclosure design works well with the drivers and produces some excellent violin and cello gritty earthy notes in the instrument's resonance and character. Piano sounds soft, rounded and musical. Voices tend to come forward a little of the rest of the instruments as do some upper-middle frequencies. This adds some extra dimension to your listening.


Update: 4 May 2009
The plot below shows the measured near field response of the speaker system. The upper line on the plot is the measured response with no filters in place. The lower green line is the response with a baffle step compensation circuit of about 3dB.


See the Fostex FE167E Fullrange Driver page for more information and enclosure ideas for this driver.

More DIY Bookshelf Speaker Projects:

• Fostex FX120 DIY Bass Reflex Bookshelf Speakers
  
• Fostex FE127E DIY Bass Reflex Bookshelf Speakers
• DIY Sealed Jordan JX92S Bookshelf Speakers

DIY Jordan JX92S Tower Speaker

Gary from the United States has put together a fantastic looking pair of tower loudspeakers. His speaker project uses the Jordan JX92S Fullrange Driver which I have used myself and is a fantastic fullrange driver.


The fullrange JX92S speaker is housed in a vented enclosure with a volume of about 1.3 cubic feet and a box tuning of about 35 Hz. As you can see, Gary has done a great job with the finish on the DIY speaker cabinets. The enclosure is constructed using Baltic birch plywood and finished with maple and walnut veneer and a tung oil stain.



Gary reports that the Jordan JX92S drivers can deliver a lot of bass which is uncommon for fullrange drivers of this size class. He also indicates that the speakers sound great, open, tight, fast and as expected, image like crazy. The nice thing about these drivers is that they can also handle all types of music without any signs of strain.

For additional information, pictures and enclosure design, see the Jordan JX92S Tower Loudspeaker Page.

Additional Jordan JX92S DIY Loudspeaker Projects:

• DIY Jordan JX92S Transmission Line Speakers (ML-TL)
  
• DIY Jordan JX92S Bookshelf Speakers (Sealed)

What's Playing: Warsaw - Transmission

Isobaric 6th Order Bandpass Subwoofer

Dan McGrath (from Great Britain) sent an email with a link to his recent DIY Subwoofer project. For the subwoofer project, Dan decided to use an Isobaric Sixth Order Series-Tuned Bandpass enclosure. The basic layout of a 6th order isobaric bandpass enclosure is shown in the figure below.


The enclosure was constructed using Medium-Density Fiberboard (MDF) which is inexpensive and easy to work with. To make the enclosure rigid, Dan doubled up the MDF as shown below (click photo to enlarge).


For the subwoofer project Dan used two Eminence Beta 10 drivers. The Eminence Beta 10 is an 8 ohm, 10 inch bass driver with a power rating of 250W and a sensitivity 97 dB. With the very high efficiency of these drivers, the subwoofer will be able to produce high SPL with only modest power. The Eminence Beta 10 is readily available (often locally) and it is relatively inexpensive. The photo below (click to enlarge) shows the two drivers in the baffle.


The finished subwoofer enclosure is shown in the photo below (click to enlarge). Dan has been using the subwoofer for a few months now and is very pleased with the results. That should not come as a surprise considering the level of effort he put in to this build. Very nice work Dan.


Dan has compiled a very detailed build log of his subwoofer project on his website and there are plenty of photographs. Take a look at his page for more information on this project.

More DIY Subwoofer Projects:

• HiVi Research SP10 - DIY 10 inch Subwoofer
• DIY MAW-10 10" Subwoofer Project

What's Playing: Emillie Mover - Ordinary Day