[L0rdGwyn]

So it's been over a week, and it will be a bit longer, going to get back into the swing of things this weekend. After about five days, I had to work on something so it wasn't a complete break.

First, I learned EasyEDA software and created my own CCS PCBs. They are a bit larger than the kits I have been using, but they are sized to take the F series heat sinks from Ohmite. These range from 1 inch tall at a 4.7 C/W thermal resistance to 2.5 inches tall with a 3 C/W thermal resistance. This will allow me to use the CCS at up to 6-7W dissipation with the same board. I also included a dedicated mu output. They are in production at JLCPCB.

I also drafted the source follower / bias supply boards, they will be finalized when the 841 / 801A protoype is completed and tested. Speaking of which, these arrived today.



So that's exciting, unfortunately I am at the start of a series of night shifts, which means I will not get to finishing the build until this weekend, should have a functional prototype by Sunday I would say. I have done a little work to wire up the build sans alligator clips which should make the task a little easier. It has been so oppressively hot lately, let's hope for cool weather this weekend so I don't roast alive in my garage.

On the Snell J/II restoration front, I received my new tweeters and driver units from Atomic Hifi. Well, there was a problem, the tweeters they sent me were not original Snells, as I expected, but they do not even fit in the cabinets if I wanted to use them, the magnet assemblies are 2mm too large and I am not modifying the cut-outs. They are actually Morel MDT-29 stripped of their labels, which if bought from a typical retailer are half the price I was charged. They sent along replacement crossover components as well, but a few caps and a resistor are not worth the premium, somewhat dishonest business practice IMO. Regardless, they allowed me to return them, so I did not press the point. The driver units are originals so I am very happy with them, they will replace the units in the speakers which were in need of a refoam, I decided to start fresh.



The original tweeters were sent off to my Snell restoration guy who is going to replace the diaphragms with a matched set using the original magnet assemblies, the diaphragms he has on hand are original Snells albeit of a later make without the problematic doping material. So that is the plan, should have these done some time next week.

Let's see what else. Over the past week, my home was in a state of chaos while the second floor was remodeled and the hardwood floors restored, original from the 1920s. With the new digs, I was asked to put together a DIY stereo setup for my bedroom so the project queue gets longer and longer. My current crop will need to wrap up before I make any meaningful progress in that direction, but of course I have already started brainstorming. What I am cooking up is a pair of DIY bass reflex full-range bookshelves for my bedroom. Once the speakers are built, or at least the plans are concrete, I will design a small form factor amplifier to pair with them, topology TBD. Listening levels will be low-to-moderate, so the speakers do not need to be overly sensitive.

I started fudging around in WinISD and other speaker modeling software along with some preliminary design education. My very early plan is to use Markaudio's full-range drivers, top choices being the Alpair 10P or the Alpair 11MS. The Alpair 11MS have some nice cabinet schematics available online optimized for high-output impedance amplifers (e.g. SET).

Alpair 10P


Alpair 11MS



Again, this is all very preliminary and like anything in DIY is an iterative process, something I will be digging into in the background, so plans will likely change. Even if I end up using an existing cabinet schematic, I will go through the design process myself as it is something I am interested in learning.

I think that's it, wasn't exactly a break I guess, quite a chaotic week with the house remodel. In non-DIY news, a very sweet stray cat showed up on my back patio on July 4th, we decided to lure him back with some stinky cat food and two days later he returned. He is now part of the family, his name is Lucky.



My other two cats are hissing at him, but they just need time to adjust he doesn't seem to be bothered in the slightest.

That's all until this weekend, can't wait to lay ears on my crazy A2 amplifier

 

[chrisdrop]

So it's been over a week, and it will be a bit longer, going to get back into the swing of things this weekend. After about five days, I had to work on something so it wasn't a complete break.

First, I learned EasyEDA software and created my own CCS PCBs. They are a bit larger than the kits I have been using, but they are sized to take the F series heat sinks from Ohmite. These range from 1 inch tall at a 4.7 C/W thermal resistance to 2.5 inches tall with a 3 C/W thermal resistance. This will allow me to use the CCS at up to 6-7W dissipation with the same board. I also included a dedicated mu output. They are in production at JLCPCB.



I also drafted the source follower / bias supply boards, they will be finalized when the 841 / 801A protoype is completed and tested. Speaking of which, these arrived today.



So that's exciting, unfortunately I am at the start of a series of night shifts, which means I will not get to finishing the build until this weekend, should have a functional prototype by Sunday I would say. I have done a little work to wire up the build sans alligator clips which should make the task a little easier. It has been so oppressively hot lately, let's hope for cool weather this weekend so I don't roast alive in my garage.

On the Snell J/II restoration front, I received my new tweeters and driver units from Atomic Hifi. Well, there was a problem, the tweeters they sent me were not original Snells, as I expected, but they do not even fit in the cabinets if I wanted to use them, the magnet assemblies are 2mm too large and I am not modifying the cut-outs. They are actually Morel MDT-29 stripped of their labels, which if bought from a typical retailer are half the price I was charged. They sent along replacement crossover components as well, but a few caps and a resistor are not worth the premium, somewhat dishonest business practice IMO. Regardless, they allowed me to return them, so I did not press the point. The driver units are originals so I am very happy with them, they will replace the units in the speakers which were in need of a refoam, I decided to start fresh.



The original tweeters were sent off to my Snell restoration guy who is going to replace the diaphragms with a matched set using the original magnet assemblies, the diaphragms he has on hand are original Snells albeit of a later make without the problematic doping material. So that is the plan, should have these done some time next week.

Let's see what else. Over the past week, my home was in a state of chaos while the second floor was remodeled and the hardwood floors restored, original from the 1920s. With the new digs, I was asked to put together a DIY stereo setup for my bedroom so the project queue gets longer and longer. My current crop will need to wrap up before I make any meaningful progress in that direction, but of course I have already started brainstorming. What I am cooking up is a pair of DIY bass reflex full-range bookshelves for my bedroom. Once the speakers are built, or at least the plans are concrete, I will design a small form factor amplifier to pair with them, topology TBD. Listening levels will be low-to-moderate, so the speakers do not need to be overly sensitive.

I started fudging around in WinISD and other speaker modeling software along with some preliminary design education. My very early plan is to use Markaudio's full-range drivers, top choices being the Alpair 10P or the Alpair 11MS. The Alpair 11MS have some nice cabinet schematics available online optimized for high-output impedance amplifers (e.g. SET).

Alpair 10P


Alpair 11MS



Again, this is all very preliminary and like anything in DIY is an iterative process, something I will be digging into in the background, so plans will likely change. Even if I end up using an existing cabinet schematic, I will go through the design process myself as it is something I am interested in learning.

I think that's it, wasn't exactly a break I guess, quite a chaotic week with the house remodel. In non-DIY news, a very sweet stray cat showed up on my back patio on July 4th, we decided to lure him back with some stinky cat food and two days later he returned. He is now part of the family, his name is Lucky.



My other two cats are hissing at him, but they just need time to adjust he doesn't seem to be bothered in the slightest.

That's all until this weekend, can't wait to lay ears on my crazy A2 amplifier

Welcome Lucky. My cats would hiss too, but who could resist.

 

[Xcalibur255]

Double thumbs up for the new kitty. ^_^

I spent quite a bit of time looking into the Markaudio full range drivers myself. Was toying with the idea of trying to build a pair of speakers for fun, but I don't really have the tools or space to do the woodworking and have grown less and less confident over time that my hands are up to the task anyway.

I have this cute little pair of ALO Audio Pandas that use a 3" Dayton audio full range driver. I was thinking about trying to sub a similar size Markaudio driver in as an upgrade, but became fearful of wrecking the nice bamboo cabinets on a gamle that might not pay off. At the least it's unlikely the bass port tuning would be a correct match, I would be rely somewhat on luck for it to work out.

 

[L0rdGwyn]

Welcome Lucky. My cats would hiss too, but who could resist.

It's just all part of the cat experience, they will be fast friends in no time, there is enough salmon pâté to go around

Double thumbs up for the new kitty. ^_^

I spent quite a bit of time looking into the Markaudio full range drivers myself. Was toying with the idea of trying to build a pair of speakers for fun, but I don't really have the tools or space to do the woodworking and have grown less and less confident over time that my hands are up to the task anyway.

I have this cute little pair of ALO Audio Pandas that use a 3" Dayton audio full range driver. I was thinking about trying to sub a similar size Markaudio driver in as an upgrade, but became fearful of wrecking the nice bamboo cabinets on a gamle that might not pay off. At the least it's unlikely the bass port tuning would be a correct match, I would be rely somewhat on luck for it to work out.

The Markaudio drivers are very highly regarded in the DIY community, if you wanted to pursue the driver replacement, you could post your cabinet and port dimensions on diyAudio, there is a dedicated Markaudio driver thread, someone might be able to say how far off the tuning would be. I'm not at that point yet, but I hope to be soon.

 

[L0rdGwyn]

It's alive. IT'S ALIVE!!!

I have tears in my eyes listening for first time, it sounds incredible, moved by the music and relieved after all of the work that went into this project. None of these builds are easy, but this one was extra not easy.



This is a pretty frivolous post, much, much more to come today, there is a lot to say. A gamut of measurements and comments on the final steps of the build. I can't say it is done since there is likely more tweaking to come, but the beast lives.

 

[whirlwind]

Very awesome, a big congrats to you
That is a lot of work.
Enjoy your listening and tweaking.

 

[L0rdGwyn]

On two hours of sleep, I worked on getting the output stage of the amplifier configured all day yesterday, a tedious task, you'll know if you've ever wired a Lundahl OPT.

Finished up and got to biasing the output tubes. This took some time as I was not about to mess around with adjusting trimmers with the amplifier on, I've been very careful while building this prototype given the 450V and 650V B+ supplies, so I had to power down and power up with each adjustment.

Finally, I hit my goal of 420V 40mA on the 801A outputs.



So our tube bias points:

841: Va 500; Ia 9mA; Vg -5
801A: Va 420V; Ia 40mA; Vg -21V

Just as the datasheet describes, love it

Here is another boring video of the startup sequence with the 801A coming up to bias. Again, first click is me turning on power, which immediately sends 120VAC to the filament supplies as well as the source follower / bias supplies. The second click are the relays closing on the time delay boards. These boards sit between the wall AC and the 841 / 801A B+ transformers, allowing the filaments to come completely up to voltage / temperature before B+ is applied. The time delay boards are each powered by the 6.3VAC windings of the source follower transformer. The voltage drop across a 1ohm current sense resistor on the filament of the 801A tells us how much DC current is passing through the tube by Ohm's law (1mV = 1ohm * 1mA), so we are looking for a 40mV drop across that resistor.



After doing my final voltage checks, there was only one more thing to do...listen? No! Measurements of course.

The amplifier starts to clip between 18.8Vpk-pk and 19.8Vpk-pk (full output) into 8ohms, making this roughly a 5.5-6W amplifier.

Here is a 1kHz sine wave at 18.8Vpk-pk.



Before the next graph, a quick recap on class A2. Most single-ended class A designs are class A1, meaning the grid always operates at a negative voltage. In a class A1 design, the grid will begin to draw current at the most positive swings (closest to 0V). Without a source for that current, the amplifier will clip, this typically defines one of the two boundaries for the max output swing a tube can generate.

But what if we could supply that extra current and drive the grid positive that is the basis of a class A2 design, which are much less common given their...peculiarities. In this amplifier, the positive grid current is supplied by a transistor known as a source follower. This is the transistor equivalent of a cathode follower, more commonly known to tubeheads. Since the source follower is directly in the signal path, that makes this a hybrid amplifier with each of the respective devices doing what they do best: tubes for voltage gain, transistors for current gain. The source follower provides an easy capacitive load for the input tube and can generate more than enough current to push the output grid - to which it is direct-coupled - far into class A2 territory. Also worth noting that some tubes, like the 801A, were designed to operate in A2 regions.

So how far does that grid go? Here is the full peak-to-peak swing of the grid at max output before clipping. 144Vpk-pk with its most positive peak at +46V, where it will draw roughly 10mA.



Next is the frequency response. Good bass extension down to 20Hz, but we do have some HF rolloff. We hit -3dB on the high end at about 16.5kHz, 20kHz is down at -4.4dB. I did experiment with taking the plate output vs the mu output of the 841 CCS given its high plate resistance without any appreciable change, I believe this rolloff is due to output transformer parasitics. Appropriate transformers for the 801A that will provide a sufficient output impedance are hard to come by, so this is likely a consequence of the 37:1 turns ratio. My Lundahl LL1620 in my other amp didn't quite hit their spec either. I will look a bit further into it and check some Bode plots along the circuit path to be sure there is not another low-pass bottleneck. TBH though, I am not concerned, it is not at all noticeable on subjective listening and I can't hear beyond 16.5kHz anyway.



I did do some distortion measurements as well, here is what we got:

1mW into 300ohms (with paralleled 8.2ohm resistors on the output): 0.25% THD
1W into 8 ohms: 0.77% THD
Full output 5.86W into 8 ohms: 4.54% THD

I have only listened with headphones so far. The most immediately obvious characteristic is this amp is fast and the bass is very tight. It has a very clean sound, this is not your warm and gooey type of tube amplifier, but very dynamic, airy, and well-extended in both directions (subjectively speaking ).

Once my Snells are back together, which very well could be tomorrow, I will lug this beast into my living room, quarantine my cats so they don't get fried, and give it a proper two-channel listen.

So far, I am very pleased with my monstrosity, but it is only halfway done. Phase 2 will be getting it into a proper chassis with a separate enclosure for the power supply. The layout is going to be an extreme challenge, but the show must go on!

 

[Tjj226 Angel]

Great job!

Im even willing to bet your high frequency roll off is caused by something other than your transformer which means it should be fixable giving you even better performance.

I can't wait to see this thing finalized.

I will be curious how this sounds on the snells vs the omegas. You seemed to imply that the omegas had a different presentation than the snells, and I would bet money that this amp will accentuate those differences. It could make the omegas come to life or it could give the snells the little extra oomph they need.

 

[L0rdGwyn]

Great job!

Im even willing to bet your high frequency roll off is caused by something other than your transformer which means it should be fixable giving you even better performance.

I can't wait to see this thing finalized.

I will be curious how this sounds on the snells vs the omegas. You seemed to imply that the omegas had a different presentation than the snells, and I would bet money that this amp will accentuate those differences. It could make the omegas come to life or it could give the snells the little extra oomph they need.

Thanks! That would be nice, I will save that hunt for next weekend. Looking forward to finalizing as well, can't wait to hide the rats nest of wires and leave four pretty thoriated tungsten DHTs on top. I'll have to go to a meet and seek out the tube purists, I'll tell them there is a transistor in the signal path after the fact the Omegas and Snells are pretty different, I'll do a proper head-to-head once I get the tweeters back in the J/IIs. This amp has some truly killer bass (at least on headphones) should be a good fit for the revamped Js

 

[evonimos]

Lovely!
Cats, tube projects and Snell 2way speakers.

Keep us updated.

 

[L0rdGwyn]

My prediction came true, the restored Snell J/II tweeters arrived today!



The original magnet assemblies were maintained, however the diaphragms were replaced. A very nice touch that I did not expect, new faceplates with sealing foam around the perimeter (the screw holes on the original face plates were a bit tired).

The gentleman who did these for me is baco99 on Audiokarma, going to plug him since he did such a great job and has been so helpful to me, he has a company called Holt Hill Audio. He designs and sells his own speakers, but will also restore any vintage speaker you throw at him, so if you find something you want to restore, he's your guy. He has a large stock of these OEM Snell diaphgragms, a perfect replacement as mine were mismatched.

Holt Hill Audio: http://holthill.com/#

All there was to do was put everything back together. Here is my revamped fancy crossover back inside the cabinet.



And the speaker just before closing.



Oh man, these look good



Because I am a psycho, the only things original in the speakers are the cabinets, two out of the three inductors, and the damping material LOL they got the works. I know its over-the-top, but I know what I like, still below MSRP in 2020 dollars

Back where they belong.



BIG improvement across the board, the speakers are now well-matched, the treble is so much improved, I expected that but I would have been happy if it was simply balanced, better imaging, clear, articulate and detailed, the midrange too, it was quite recessed with the speakers as I received them. Going to give them some time today and tomorrow for burn in.

 

[L0rdGwyn]

Just one more thing.

My package from JLCPCB arrived with the CCS boards I designed.



I added some nice-to-haves, there is a dedicated mu output pad and these take the Ohmite F-series TO-220 heat sinks, same as what is pictured but they can get taller, up to 3 C/W thermal resistance at the largest size, giving these CCS to flexibility to run at highish currents. I placed the standoff holes right at the center of the heat sink, such that a single pair of standoffs can support the largest model which are 2.5 inches tall.

These will load the 841 in the finalized A2 amplifier. Need to finish my design of the source follower / bias supply boards next.

 

[L0rdGwyn]

Sorry, I know people are probably tired of me already but I found some interesting tubes.

One of my current tube infatuations (there are several) is the Telefunken EL156, highly regarded tubes for SET or PP use, but quite pricey, go for around $400 a pair, here is a picture.



EL156 datasheet: http://frank.yueksel.org/sheets/118/e/EL156.pdf

But as I was poking around, I came across a pentode that I think is interesting and relatively inexpensive! The Telefunken EL152 (6.3V heaters) and FL152 (12.6V heaters), I think they look rather nice!



I had never seen them before, they are descendants of the WWII era Telefunken LS50. Interestingly the USSR reverse-engineered the LS50 during the war and created the GU50, which is popular with some folks over on diyAudio given they are still plentiful in Russian surplus and are dirt cheap. You can get a pair of FL152 for $80, a pair of GU50 for around $20!

EL152 / FL152 datasheet: https://frank.pocnet.net/sheets/118/e/EL152.pdf

Here are the EL152 / FL152 triode curves. The datasheet doesn't give tube constants for triode strapping, but from the curves, mu looks to be about 5, Ri around 1.5K.



Could make for a cool amplifier, looks pretty linear to me. I have seen bias points in SET config suggested around 350V 100mA with a 40W max plate dissipation. Doing some eyeballing, a 5K OPT and a driver that can swing ~100Vpk-pk would get you something like a 4.5-5W SET amp, could be done on the cheap using the GU50 instead of the Tele's. Anyway, thought I'd share, I'm always on the look out for weird audio tubes to fantasize about.

 

[L0rdGwyn]

I brought the beast into my house for some two-channel goodness. I am never doing that again HA!! Omg it is heavy.

It's...beautiful.....lol.



It sounds great with the Snells, again very clean, tight bass, very focused and dynamic. The 6A5G amp by comparison is more soft, a little more..ethereal? diffuse? Hard to nail down the differences after one listen.

As promised, I threw the Omegas into the mix to compare to the new-and-improved Snell J/II.



So going low to high...the bass. Maybe it was the amp pairing, maybe it was my room being back in its proper arrangement, but the Omegas initially surprised me in terms of the bass quantity, it is more than I remember, but still not as high-output as the Snells. In terms of quality, it goes to the Snells for sure. With the Omegas, there is bass, but what instrument am I hearing again? The Snells do detailed bass, and legitimately have usable bass down to 50Hz, I mean these things really rumble, frequency sweeps show it is even audible down to 40Hz. What the Omegas lack in bass is detail and dynamics, there is a low end thump, but the nuance and attack just isn't there.

In the mids, it is mixed. Replacing the Snell drivers definitely closed the gap in terms of midrange detail, which previously I would have given to the Omegas. The Snells' midrange is very cohesive with the rest of the frequency spectrum and sounds very realistic. The Omegas are mid-forward, which lends itself very well to the human voice, they do female vocals extremely well, but due to the forward nature, they can sound disjointed at times. For acoustic music or music with vocal emphasis (stereotypical "audiophile" music, you know what I'm talking about), the Omegas kill it, but the Snells are more well-rounded and realistic sounding.

In the treble, again, the new tweeters really closed the gap with the Snells. Hard to say who pulls ahead here, they are both very detailed, the Snells do better with bad treble in bad recordings. Since the Omegas are a bit more aggressive, when it sounds bad, it sounds BAD, which might be viewed as a strength, they are more revealing in that regard. I'm going to call it a draw, the treble is very satisfying and airy in both cases, nothing to complain about here.

In terms of overall character / tone, the Snells to me have more of a "realism" to them, the Omegas have a very apparent character, it is a bit jarring going from the Snells to the Omegas, but then my ears adjust and it seems to fall away. I am going to call it clarity, which goes to the Snells.

For imaging / soundstage, Snells again. Those wide baffles really throw a huge stage and they have pretty darn good off-axis performance. The Omegas are very sensitive to on-axis listening, they have a much smaller "sweet spot", which is a disadvantage considering these are in my living room and there are often others enjoying the music with me. The Omegas have a pretty impressive stage themselves, but to a greater degree have more of a "two point source" image. The center image is well-defined for the Omegas, but it is all relative, the Snells fill in that center image more fully.

So the verdict? It might be obvious, I prefer the Snells. No doubt there is some bias considering I put a lot of time and effort into these speakers. Like I said before, it is all relative and the Omegas are very nice speakers, if I did not have the Snells I would be very happy with them. Also worth noting this is not Omegas top-of-the-line offering as these are the ferrite magnet drivers, the alnico drivers are Omegas fanciest. I suspect the perceived deficiencies in clarity would be gone with the alnicos.

The Snells just have a killer bass, tone, soundstage, imaging and are a great lifestyle fit for me given my space and the type of amplifiers I am building, so there it is.

Not sure if the differences could be picked up, but something I wish I had is a microphone setup such that I could get high-quality recordings of this system to post for comparison, would also allow me to take FR measurements...something to think about

 

[Tjj226 Angel]

Not bad for a pair of old speakers from the 90s.

I know this is just going to make you bang your head into a wall, but I am playing around with different tweeter upgrades on my snells. Im thinking a good planar tweeter might be a good choice.

IDK.

I showed you those cardboard horns, and even with as ugly and messed up as they are, they DESTROY the snells and even the tannoys in mid range clarity. Its not even a competition. So needless to say I am really tempted to try and perfect the horn and use the snells as a sort of bass cabinet.