Fostex Backloaded Horns - BK20

[jh4db536]

I went to a few show and meetups and i loved the way these high efficiency BLH's sounded so i decided to try the 8" version. Ordered up flatpacks from Madisound and some Sigma F208 full range drivers. Will amp these with a custom 300B directly heated triode SET amp that i am currently building right now. At 96db/w efficiency it should loud enough...i seen these run with a portable headphone amp at a meet before.

These are some examples of 6" BK16 versions and modified custom stuff.




The flatpacks come in a nice relatively small but dense package. It's all Elmers glue and clamps available from your local HD supply store. This is my first time working with wood so bare with me.



Basically you glue a piece at a time and clamp





Bought some parts to make PEQ and Crossover


To finish this, i think i will trim route the edges and finish it matte black. Waiting for the tax bill to determine whether i will buy t500 super tweeters this year.

[novisnick]

Thanks so much for posting your thoughts and pictures are much appreciated!

[mountain]

Another with motivation and skill!  Impressive!  Thanks and enjoy.

[jh4db536]

Here's the amp for the BK20's that im building - just flea power of 8 watts
DHT SET, capacitor coupled interstage, fixed bias...im still a noob at tubes. This is my 2nd time building a tube amp, first time power amp.
My next one could be full blown point to point or on turret board now that i'm beginning to understand what makes these sound good (obviously $, custom made iron, the right tubes...not necessarily NOS, and good simple design).

unregulated section of power supply - 400v/250v for the tube B+ and 20v for the filament heaters


High Voltage Regulator B+


Main Tube Driver. Configured for 300B and PCC88 Driver Tubes


30lbs of Iron...well and some silver too. Custom made Electraprint silver output transformers
Default Power transformer
Also using Jensen Line In transformers (not pictured) this amp will be differential balance input only.


DIYA Pessante 3U 400mm will be used. i had this laying around for like 10 years. Time to put it to use.

[jh4db536]

Adding a little something here (still in progress). This is a lot of work...second skin is very easy to work with luckily. You need about 20-30sqft to do a pair of bk20. Get some B stock DamplifierPro2. The wood feels and sounds 2x thicker/denser, it's just solid. I also ensured everything is sealed at the edges with silicon caulk.





Got some isoacoustic stands from a friend who upgraded to the aluminum ones. I think these are going to help with hardwood floor when i bring them to my house.

[vneal]

nicely done

[jh4db536]

Been real busy, but any free time has been spent finishing the amp for this. It's fresh off the CNC machine.
From drawings

To actual




Will probably get to plug it into the wall and verify functionality of the PSU and regulator modules this weekend.

[mgbpuff]

Congratulations on a very well built project. I didn't build my low power / high efficiency system, but I enjoy it a lot. My system is Cain & Cain Abbey speakers with a single Fostex FE166 full range , George Wright 308b triode SET monoblocks with DC directly heated cathodes , and a W4S passive preamp. I aid the speakers with separate Townshend supertweeters and with a pair of powered subs. There are no crossovers except of course the subs have a low pass that I match up to blend in with the natural roll off of the Abbeys. Sources are TT, CD player, XM radio, and a new OPPO Sonic streamer. It is all in a small room with a single lounge chair and it is very relaxing to just chill out and listen to the music.

[jh4db536]

Thanks.

This is my first time building speakers, crossovers, and power amps; I have experience with DAC, preamp and headamp builds.

Most of my experience is with solid state stuff, but I'm really happy to learn the alternative philosophies.

[vcautokid]

Wow great job and congrats. I love to see DIY projects like this. Nice to see something not made at the factory. Looks great.

[jh4db536]

I'm already thinking of the next build after i determine if these modules achieve the level of sound i expect:

I bought the 2a3MKIV and built this 300b which seems kind of redundant, but they are radically different in design.

What i intend to learn from this "modern" tube amp is to compare the sound solid state rectification vs tube and choke filtered (CLC) rectification. Obviously solid state rectification is a lot easier and cheaper to implement cause i can easily just buy bridge rectifiers or discrete schottkeys from mouser for a few cents each. Large value crossover grade film capacitors in the PSU are not cheap and custom wound chokes aren't either.

Another thing i wanted to learn about the different filament heater technologies. The 300b heaters i built feature high frequency DC and the 2a3MKIV feature high frequency AC. I am sure both are quiet compared to using unfiltered and unregulated AC straight off the transformer to heat directly heated tubes. This is pretty high tech modern and vintage stuff...im sure SMD and modern switchers in tube amps is rarity. I think HF AC heaters are unique in tubes as well and i've only seen them as experimental PCB's and schematics.

I am thinking to make third parallel 45 tube amp (probably only good for headphones...est max 3watts) and there's some considerations.

What tubes to use? I've been using fairly cheap new production in most of my builds so far. I've had an opportunity to audition the vintage mono plates, grid plates, and other high end stuff. There's no doubt in my mind that certain vintage stuff sounds superior. This stuff costs $500-$1000 each and who knows how long it lasts (and yes im going to daily drive them). I would have to invest and build a tester/curve tracer if i ever pursued these.

What transformer do i use? Based on my experience with using upper mid-grade output transformers (off the shelf and custom made EP) i am fairly comfortable now looking on ebay for vintage stuff that works. Should i go exotic high end magnetics i.e. monolith, cinemag, EPrint, or Lundahl? Which philosophy do i subscribe to? nickel core, silver, amorphous, nano crystalline, copper foil, computer algorithm designed? Monoliths are probably going to be 2-4 grand each. EPrint fully loaded (silver sec, honeycomb winding) probably near 1.5k each. Could also just go on ebay and get a pair of vintage Tango's overnight from japan for 2k.

diy is not about saving money  hobbies are $

[jh4db536]

It's about that time where you're pretty confident that everything functions as intended. I had a few miscalculations with the CNC, like the power transformer wires don't all fit through a 3/8" hole so i'm gonna have to ream and deburr it. We're very close...as soon as i get through this work week i'll finish this thing cause i'm dying for the next project.

The power transformer really hums. I hope the sorbothane isolation washers i implemented in this are good enough to prevent microphonics. I have a feeling that genalex might be similar to EH 300b.

Verified function of B+ HV Regulator 400v, filament regulators for the output tubes and driver tubes.




I got a really cool vintage project coming up next.

[jackpine]

Apr 30, 2017 19:34:34 GMT -5 jh4db536 said:

It's about that time where you're pretty confident that everything functions as intended. I had a few miscalculations with the CNC, like the power transformer wires don't all fit through a 3/8" hole so i'm gonna have to ream and deburr it. We're very close...as soon as i get through this work week i'll finish this thing cause i'm dying for the next project.

The power transformer really hums. I hope the sorbothane isolation washers i implemented in this are good enough to prevent microphonics. I have a feeling that genalex might be similar to EH 300b.

Verified function of B+ HV Regulator 400v, filament regulators for the output tubes and driver tubes.

I got a really cool vintage project coming up next.

If your transformer is humming a CMX-2 might be the cure. emotiva.com/products/accessories/cmx-2

[jh4db536]

Still have to finish the amp, but this project is close enough to the finish line.
Raw wood, but it's functional. Going to assess performance before making them look pretty.

[jh4db536]

It's almost done. Waiting for some cheap blank chassis pieces from Italy


I was going to diy it, but the bjc Canare was cheaper than what I could make myself. Can't beat the ultrasonic welding.

[jh4db536]

It's pretty much ALIVE


This thing is Massive. it could be a lot bigger...the transformers are still a bit close to each other unless i staggered them.

[jh4db536]

The listening room is far from ideal, but it works. The room is so small i am only about 5 ft from the horns.

Sounded a little bit thin at first, toed it in made a big difference. I need to put the stuffing in it and bring out the REW + microphone to kill some of the midbass.
It's missing the sparkle of the super tweeters, but I could probably live without subs.

This chain is completely DIY Gamma3 DAC > dg300b > BK20 + F208EZ

Tubes are Siemens PCC88 Drivers and Genelax 300b (which aren't microphonic)

It's going to be interesting when i bring the Yggy and the VRDS here.


My enclosure is not done yet so this is just mockup. I decided that im going to put some gauges (Ammeters) on it for Tube Bias.



Added some stuffing to cut down the extra midbass. Only starting out with .25oz and 1oz in different parts of the horn.

[jh4db536]

Still learning to use my tools. Learning as i go along but playing with REW and microphones (USB UMIK) is a blast. Since i have the whole house to myself for now room dimensions are 12.5 x 18 with a large opening to a larger living room on the right side. I marked the floor so i could push it back to get the bass right. I could stuff it a little more to get rid of the slight rasp and characteristic honk (rising response through hf rolloff).

noaudiophile.com/speakercalc/

I used the rule of thirds to position the cabinets making basically a equilateral triangle of 5'9" in the middle of the room. Toe in is minimal because i thought the center lost focus. Also reduced the upward tilt because it made the image too high. In this position, the focus is right above the fireplace or beyond. There's depth and width is a little limited to just the wall about 3 feet from the cabinets.

Norah Jones, Karen Carpenter, and John Mayer sound like theyre here performing in this room for me. Imaging is great.

BK20 on axis mic 24" from cab, orientation horizontal/parallel to the floor
No subs no tweeter, bass falls off the cliff at 55 although i don't crave a sub yet. Down the road i could see myself with a pair of 18" dayton ultimax kits and hypeX amp.
DG300B amp, Gamma3 DAC
Super tweeters and lpad parts are in the mail. I will fix that high freq rolloff.









With and without super tweeter at listening position. Tweeter unplugged and measurement overlayed


Again at listening position - this is where i ended up in optimizing the stuffing. Except for the bass performance I could probably quit now right here and be content. I removed the tilt (completely level now) and the bass hump came down.


According to the Schroeder Freq it's around 211hz for a room of this dimension and would be similar to the other room i would put my setup in. I ran a sinewave at 200hz and started walking around with the microphone.

If i were ever to treat a small room, the most effective spot to position a low freq tuned bass absorbing panel would be the wall directly behind the cabinets (+10dB). Second place would be in the front corners at tweeter height (+7dB). Third would be on the side walls in front of the cabinet about 4 feet (+5dB).