More details will come later on this integrated tube hi-fi project that I've been slowly working on since around April of this year, but which is now finally coming together. For now, here are some pictures of the build in progress. The first shows the hand-etched PCB boards mounted to the the chassis top, while the bottom shows a test fitting of the transformers and tubes. The chassis was machined and anodized to my specification by Landfall Systems out of McKinney, TX. While it was on the pricey side, I have to say that they did an amazing job, and that the quality of the workmanship was superb. They were also very nice to work with. The front and back panel will still be sent to TMI Amplification out of Alexandria, VA, as they are my unsurpassed go-to place for laser-engraving. In fact, I would have had TMI do the machining for the top as well, except that Landfall was able to offer post-machining anodization which was a really nice touch for the top.
Tube complement here is 4 x KT-88s power tubes and 2 x 6AN8 triode/pentodes for the power sections (which resemble those found in the Dynaco Mk. III but for the PSU) and 4 x 6DJ8 triodes for the preamps.
Some quick clips recorded awhile ago highlighting some of the clean sounds from the Wombat Mk. I amplifier. My brother Kelly Fawcett playing and singing. No fancy recording or EQ here, just an iPhone camera. Reverb from a Holy Grail pedal in front of the amp.
Some Jazzy/Bluesy noodling on a Strat to give you a good idea of the basic tone:
So it turns out that I was horrible about blogging this project! I managed one preliminary design post, but then nothing. But it's not because I had nothing to say, I just got busy. I actually did a pretty good job of documenting everything as the build progressed and I have plenty to share. So, I hope to redeem myself by instead starting at the end and working backwards! To make amends, I begin by offering a photo gallery of the completed unit featuring some great work by Richmond based commercial photographer Adam Ewing.
Just as a reminder, the EMP-351 is an all-tube microphone preamplifier that is based on a well-known mod of the classic AMPEX 351 reel-to-reel player that was pioneered over at Electrical Audio.
As you can see, I ended up building a dual version, which adds a few bells and whistles to the original version, including additional line level and high impedance inputs, as well as impedance selection and an attenuator bypass switches. It also includes amenities such as 48V phantom power, an output attenuator, phase switch and input selector. This particular unit is was constructed for Andrew Everding, audio engineer at Neil Finn's Roundhead Studios in Auckland, NZ.
Much of the credit for the beautiful appearance of the finished unit rightfully belongs to Mason Wolak of TMI amplification, an Alexandria, VA based company that did the laser engraving of the faceplates. Thanks also to John Morand of Sound of Music Recording Studios for helping out with the final calibration and checkout.
This post will cover the design and construction of the phantom power and DC heater filament board for the EMP-351 microphone preamp project I outlined in an earlier post.
This preamp, which is based on a well-known modification from Electric Audio of the repro and record boards in the old Ampex 351 reel-to-reel player will have relatively high gain. In fact, from the strictly hi-fi point of view, it's a bit of a silly design, since it has two full gain stages prior to an unusual phase inverter which also provides significant gain. In the standard mod, this actually necessitates amplification, then attenuation, followed by further amplification, if it is desired to keep the end-to-end gain of the circuit down to the 65-70 dB or typical required for a mic pre. Without the attenuation offered by the calibration potentiometer after the second gain stage, the end-to-end gain of the circuit would theoretically exceed 90 dB -- too much for practical use. Of course, the penalty for breaking the "no attenuation followed by amplification" rule is an increase in noise.
That issue with signal-to-noise ratio is an inevitable compromise that comes along with this kind of circuit. But it is possible to mitigate the problem, at least a bit. For instance, unlike the original unit, the EMP will employ metal film resistors throughout, which are significantly quieter than the carbon composition resistors found in the original. I also made the decision early on that this project should use high quality regulated 12.6VDC for allof the preamp tube filaments, not just the input tubes. In my experience, DC tube heaters are something that should be done right, or not at all. Unfortunately both the original Ampex and the Electric Audio mod rely on a very crude unregulated DC supply consisting of not much more than the rectifier and a big old 4000uF cap. Needless to say, there will be significant residual ripple with even three preamp tubes, and it's unworkable with the six that I need. Nowadays we can do much better.
The six preamp tube that this preamp will have is admittedly a bit of an issue. They'll be drawing a total of 900mA at 12.6VDC. While that might not seem like that much, because of the generally quite poor power factor of low-voltage supplies, it turns out that you actually need quite a bit higher rating (something like 3A, more on this shortly) in order to safely feed those six tubes nice pristine DC.
So things brings us to power transformers. If you think about it, a project like this represents a bit of an unusual situation. Since we have no current thirsty power tubes to feed, the the current requirements for our high-voltage B+ supply are quite modest -- really just a few tens of mA at very most! But the voltages are still in the high range you would normally associate with tubes. On the other hand, we have a disproportionately large thirst for current from the low voltage supply. Unfortunately, most transformers that supply plenty of current for the low voltage secondaries are also designed to provide waaay more than I need for the high voltage. This makes such transformers a bit too large and unwieldy to be practical for a fairly compact rack mountable unit. I was also not too excited about the other options involving two separate transformers. Aside from the space issue, there's the additional unwelcome complication that this unit has to able to work with both 120V/60Hz power and 230V/50Hz.. a consideration that further limited my transformer options.
The logical thing to do, therefore, was to get a transformer wound just for this job, and indeed that's what I did. That said, I also wanted a transformer design that would be flexible enough to use in the future in a variety of small projects, including potentially things involving a low-wattage power tube. In the end, this meant opting for a bit more B+ current capability than I strictly needed for this project. Also, it meant that I wanted the flexibility of being able to power tube heaters from 6.3VAC. So for my low voltage, I actually opted to have two separate 6.3V secondaries with good regulation that I can run in series for 12.6VAC or rectify for powering a 12.6VDC supply (as I will be doing for this project). The trade off here is that I could have made my life a little bit easier with respect to power factor if I had chosen a higher low-voltage secondary (say 15V), but I would have lost the flexibility of using this transformer in other contexts.
I'm having a small batch of the following transformers constructed for me by Heyboer Transformers, located in Grand Haven, MI. It's a pleasure working with the folks at Heyboer, and they come highly recommended:
Primary 115/230V
Secondary # 1 240 Volts
@ 115 mAmps
Secondary # 2 6.3 Volts
@ 3.0 Amps
Secondary # 3 6.3 Volts
@ 3.0 Amps
Drop me a line if you are interested in one of these transformers, I'll have at least a couple left from this order that I don't have immediate plans for.
Before we go any further, here's a link to the (PDF format) schematic for the filament and phantom board. As you can see from the schematic, this was actually drawn in LTSpice, my favourite (and free!) circuit simulator software. I have, in fact, confirmed the proper operation of this circuit with the simulator. NOTE: a fuse is not shown on this, but is obligatory for any real-world implementation.
A couple of points.. the phantom power is derived from a fairly standard voltage quadrupler circuit. In principle, I suppose I could have used a voltage tripler, and referenced it to the output of the main rectifier. But the inherently half-wave operation of a tripler wasn't appealing, both in terms of the unbalanced current established in the transformer, and in terms of the limited available current. The choice of the TL783C adjustable high voltage regulator, was influenced by Douglas Self's recommendation of this part in his excellent book "Small Signal Audio Design". Every serious audio electronics hobbyist or audio professional should own this book.
As to the heater filaments portion of the design, it's fairly standard, and follows the treatment given by Merlin Blencowe in his excellent (but sadly now out-of-print) book on tube amp power supply design. This will be the first time I've used the Micrel MIC29300-12WT regulator, which is a promising looking high current, low dropout part that is newly available from Mouser. Unlike the TL783, the MIC29400-12WT will be dissipating 3W or so of power, so will be heat-sinked to the aluminum chassis.
Now, of course, a schematic only gets you so far. Long time readers of this blog know that I'm a big fan of the freeware program DIYLC (do-it-yourself layout creator). I do most of my layouts, both for hand-etched PCBs and for entire large amps, using DIYLC. Because of the regulator chips and the small adjustment pot, it made a lot more sense to design this one as a PCB rather than as a turret board. So here's what the PCB layout looked like once it was implemented in DIYLC:
Here's what the mask looks like with no components. Resize this image to 4.75" x 3.75" if you wish to etch your own board based on this. Incidentally, for anyone who wants to build this, I strongly recommend that you chose 105degC rated low-impedance ("low-Z") capacitors for the large electrolytic (1000uF and 4700uF) shown on the schematic.
For one-off or small numbers of boards, I use the Press n' Peel Blue Transfer paper method in conjunction with ferric chloride etching. I wouldn't want to do a dozen boards this way, but it's fine for a couple. Here's a helpful YouTube video made by someone else that demonstrates the method:
Here's what my board looked like at the various stages of this method. First is with the Press-'n-Peel ironed on:
Next, we dump it in the ferric chloride solution and swirl it around for forty minutes or so until all of the exposed copper is etched off:
After it comes out, it'll look this initially. Note that the Press-'n-Peel is still stuck to the copper areas:
This needs to be gently scrubbed off to reveal the copper traces:
You can stop here, but I find that soldering is made much easier if the copper is first coated with a layer of tin. I do this by an application of a product called "Liquid Tin". It's easy to use and works great:
At this point, you are ready to drill the board and mount the components! Once again, here's what the finished board looks like!
Everything here is attached except for the 12V regulator, which as I mentioned will be heat sinked to the aluminum chassis when the board is installed. Needless to say, it's extremely satisfying to start with just a concept, and end up with its physical realization in your hand.
Stay tuned, next time we'll begin looking at the main circuit, and follow along the construction of one of the main turret boards!
I realized today that it's been nearly a year since this blog has
been updated, but that doesn't mean that Wombat Amps hasn't been busy!
On the contrary, we've now launched an improved web site,
I've established a formal Wombat Amps workspace in a building that also
houses dozens of bands -- as a consequence I've been doing a fair
amount of repair work -- more than I had really intended actually.
Nevertheless, I have been working on some interesting projects,
including a cool custom amp that was recently completed and a solid
state equalizer.. But details of these just haven't made it to the blog
for want of time.
However, I'm working on a new microphone preamplifier project that I'm really excited about, and I
hope to document a fairly complete build log here. This mic preamp was commissioned by Andrew Everding, perhaps best known as the keyboard player for the band Thursday. However, with Thursday currently on hiatus, Andrew has moved on to audio engineering, and is currently located in Auckland, New Zealand, where is an audio engineer working directly with Neil Finn (of Crowded House fame) at Neil's well-known Roundhead Studios.
In any case, Andrew is a big fan of tube microphone preamps based on modification of old Ampex reel-to-reel players. As a tube aficionado, this makes perfect sense to me - in a world now flooded with sterile-sounding ultra-low distortion preamp choices, more and more musicians are realizing that a bit of distortion, of the right kind, can be just the ticket for producing a warm, full sound.
One of the preferred platforms for the mod community has been the amplifier from Ampex 351 reel-to-reel player, as shown above. In fact, there's a fair amount of documentation online (especially at the Electric Audio forum, courtesy of Greg Norman), describing how to mod one of these units into a preamp. So why not just mod one of these, and be done? Well, it turns out that there are a number of good reasons. First, the Ampex 351 is now highly sought after, and prices for these units have skyrocketed. You can easily pay $1300 or more for one on fleaBay, and what you get will be in uncertain condition. And this is before the mod process, which turns out to be not at all straightforward. These were constructed on now crufty old first-generation PCBs that are prone to traces lifting from accumulated moisture, and are not particularly amenable to modification. Also, there is a ton of other circuitry in there that gets in the way and is unneeded after the mod, you're for sure going to have to replace all the power supply capacitors anyway, etc., etc.. Even after you go through all the trouble, you're still going to be left with just a single preamp, not a dual. So you'd need to do this all twice to get a dual setup! And this single would not really be optimised in terms of layout, ground scheme, noise, etc. It also wouldn't provide modern amenities such as phantom power and fully regulated DC tube heaters, at least not without having to do a bunch of extra work. But aside from all that, the Ampex is still a pretty damn good reel-to-reel player, and I just hate seeing beautiful old gear being canabilized in that way. So the decision was made early on in the process to scratch-build from new components a dual preamp that stays true to its heritage but incorporates just the components necessary for a preamp. In addition, it should incorporate features such as phantom power and regulated DC tube heater supply, and do it all in an attractive 3U rack-mountable package. Here's the design brief:
Dual microphone preamp inspired by the Ampex 351 circuit;
Each section to combine elements of the Record and Repro
boards of the original in a manner similar to the Electric
Audio preamp mod;
Tube complement of each section will be 2 x 12AX7 and 1 x
12AU7;
Tubes provided will be New Old Stock (NOS) selected for
low noise, gain, and balance;
Custom wound transformers audio transformers from Sowter UK;
Chuck D'Aloia doing his thing at the 2012 LA amp show
As you those that follow this blog now, I spend most of my spare time inhaling solder fumes working on my various amp and effects projects. But once in awhile -- not as often as I should -- I get the urge to actually, you know, play some guitar. The problem is, I'm just not all that good, and not particularly imaginative. Like a lot of guys, I seem to have a hard time breaking out of familiar patterns and riffs. So it's a wonderful tonic when something comes along that can help you break out of a rut and introduce you to some new ideas. This blog post is about one of those discoveries. Chuck D'Aloia is a really great player -- I've long admired his playing, which I first came across when checking out various amplifier demos on the intertubes. But, as it turns out, Chuck is a first rate teacher as well. I highly recommend that you check out his "Blues with Brains" lesson series. Here's a little preview on YouTube, showing how to take on a basic blues progression with a little bit of a more interesting harmonic approach. I bought the rest of the lessons, and it just gets better from there. It was money well spent.. Thanks Chuck!
The Fender Champ is an iconic amplifier with roots going all the way back to 1948. Noted for the simplicity of its circuit and controls (in many versions, just a simple volume knob), it's hard to imagine a simpler or lower-cost tube amplifier. This perhaps explains why, over the years, many manufacturers have produced amplifiers with circuits that vary from that found in the Champ in only incidental ways, and why the circuit is of ongoing interest to DIY hobbyists. Well, that and the great tube tone! While a wide variety of Champ circuit variants have been produced by Fender, the 1958 Tweed-era 5F1 is perhaps the most enduring. However, the layout employed in the 5F1 has a primitive ground scheme, and the amplifier is not particularly quiet. I've been asked a few times how I would lay out such an amplifier with a more modern approach. Note that while it employs a "bussed stars" approach, it has only *one* attachment point to the chassis for the audio circuit right at the input jack (although please note that any real-world circuit MUST also incorporate a completely sound safety earth connection from the third prong / ground of the mains power to the chassis). The input and output jacks themselves should be insulated. The circuit shown here is fairly true to the original. If I were to build one for myself, I would undoubtedly incorporate a few more changes to further modernize things to improve noise performance and reliability -- but purists would undoubtedly complain. Please note that the suggested layout is a draught only, and should be thoroughly error-checked before using it as a launching pad.
