Woodworking can test my tolerance

I spent 3+ hours obsessing yesterday over mere hundredths of an inch.  My dovetailing jig is cool, but when few hundredth's of an inch make a significant difference in the tightness of a joint, I think Rockler could have done a better job of making their jig more precise.

The general idea is that once you have the jig set up for a given width and thickness of blank, you should be able to clamp a new set of blanks in, route away and Bingo... perfect joint.  One problem is that the fence that sets the depth of the cut is attached to the upper clamp, but the clamp has a substantial amount of play.  So the fence can shift up to about 0.05" between clampings.  It took me a while to figure out that even though two passes had identical adjustments, things were shifting around every time I reclamped.  Ok, so I'll remeasure and readjust the fence depth every time.

The next problem is alignment/squaring of the blanks.  The main magic of a dovetailing jig like this is that you route both blanks at the same time.  There's a little alignment stop which is supposed to keep the two blanks aligned relative to each other and the jig.  My first beef is that it is free to float way out of square to the jig.  Ok, I'll square it each time I adjust it (3 screws).  But wait, it's made of fricken no-so-hard plastic and it flexes!  The upper part of the stop has 2 screws so squaring it pretty much keeps the upper blank square.  The lower portion has only one screw.  Trying to square it is like trying to benchpress with one arm.  Without a second tie point the stop rotates around the single screw and flexes based on the position of the upper.  That would be just fine if it was metal which doesn't flex, but as is, the lower blank is almost never square if you simply rely on the stop to keep it square.  In the end what should take about 1 minute of set up per joint takes about 10 and still may result in a bad (loose, tight or misaligned) joint.

To make up for looseness of tightness, the height of the router bit can be adjusted.  I found that the difference between loose and tight could be as little as 0.015".  You girls take note.  Yeah right, like any girls would read this blog.

Anyway, enough bitchin'.  After about 16 test joints and wasting about 10+ inches of length of my good blanks, I made the final cuts and I'm pretty satisfied with results.  It holds together without glue and only one of the 4 joints needs some filing to get the pins and tails to align.

The next step is to saw a kerf (slot) just below the top of the frame for sliding in the aluminum panel.

Enclosure layout design and mockups

Enclosure layout design, despised by many builders, is actually something I've always enjoyed.  Probably because I started making enclosures for my projects with my Dad when I was about 6 years old.  From cardboard with holes cut with scissors to drilled aluminum.  Those projects usually had just a few switches and lights, buzzers and bells, but the basic concepts are the same.  The main work is drilling the holes, but planning where to put the holes is definitely an art.  It involves ergonomic evaluation to make the device usable, but part placement must consider the functional constraints of the innards.

In an amp, that generally means making sure that components which connect to each other are close to each other and things that could induce noise to each other are far from each other.  Since most components connect to multiple other components, various placement permutations result in the need to consider oodles (technical term) of tradeoffs simultaneously.  Unlike semiconductor block/module placement and routing (which I did professionally for 10 years), the amp layout problem is solvable in polynomial time.  In fact, the problem space is so relatively small that even the meager human brain can figure out a set of component placements which are nearly ideal.

For me, noise is the number one consideration.  Electromagnetic induction is used to our advantage in many components of an amp.  Power transformers, output transformers and chokes all work because of electromagnetic induction.  However, that phenomena is also the leading cause for noise in an amp.  Any conductor (wire, resistor, capacitor, transformer, etc.) is susceptible to induced current from another conductor. To be induced, that current must be oscillating and thus has a frequency which can be heard if it or it's harmonics are within the audible range.  Most hum heard in an amp powered in the US will be either 60Hz like the incoming AC line voltage, or 120Hz (2 x 60Hz).  Either is certainly audible and annoying.

It's near impossible to eliminate the source of electromagnetic induction, but we can shield sensitive components from it, or keep sensitive components far from problematic sources.  Fortunately, induction has an inverse square relationship to distance.  So a small bit of distance between components can have a dramatic effect in the amount of induced current.  Thanks Physics!

Since transformers are the #1 source of EMF in an amp, I feel it's most important to place those first starting with the power tranny.  In this amp, I decided centralize all the power components in the center rear of the amp (see below).  Then the audio portions of the system can exist "far away" on either side. I did allow one twisted pair carrying AC to run along the side, to the front to the power switch and lamp.  This was the single case where I let ergonomics and aesthetics trump performance.

My other layout goals were:

• Arrange all inductors (transformers and choke) for minimal electromagnetic interaction (eg. Orthogonally oriented cores, spaced as far as possible or on opposite sides of a grounded shield)
• Keep AC lines > 2cm from DC rails or signal lines/components (AC lines include power entry/fuse/switch/lamp and tube heater lines)
• Keep AC lines as short as possible (power switch and lamp exempted)
• Arrange signal path components so the path is roughly linear without recrossing (ie. start from front center, out to the power tube, back to the output tranny, then back to the speaker terminals).
• Employ a "Star" grounding scheme with the lug very close to the amp center and close to the power channel
• Optimize heat dissipation.  Keep heat generating components (transformers, power resistors and tubes) away from each other and away from heat sensitive capacitors.

I decided to use Google sketchup for doing my layout.  It's a trip constructing in 3-D and took me a few nights to become productive.  It's an awesome tool though and one of the coolest things is that there's oodles of models for most everything you can think of online.  I found switches, jacks, tubes, etc.  The things I was left to construct myself were the James output transformers, power supply caps, power transformer cover, and circuit boards.  Here's what I came up with for the top view:

The output tubes don't quite look like 6B4G's, but good enough for government work...

... and after lots of jockeying trade-offs, the underside view:

... now "eyeballing" with real (top and bottom) parts:

Time to start planning the final build

With most of my design work complete and measurement and listening tests of my prototype showing nice results, it's time to start planning the final build.  That means final part selection, wood frame prototyping and layout planning.  I've made probably 15 orders in the last 6 months from 6 different US vendors, 1 Chinese, and the rest on Ebay.  Lots of part swapping means my parts bins are brimming with parts to be used on future prototypes and projects.  My current prototype uses all of the components of my most resent design revision except for the power switch, jacks, etc. and it still uses carbon resistors which will be swapped with metal film.

For the wood frame, I've decided to use some Ipe (aka Brazilian Walnut or Iron Wood) boards which my coworker had left over from his very nice deck (Thanks Jeff!).  It's super dense, fairly unforgiving to work and it creates a nasty dust which I shouldn't have been breathing, but it's fricken beautiful wood.  The boards were roughly 6" wide by 3/4" thick.  I had a 2.5' board and a 1.5' board.  I ripped them (roughly) in half and had 4 2.5" strips.  Perfect!  I even justified a new carbide tipped ripping blade which did an amazing clean cut.

I wanted to finally try my hand at some nice half blind dove tail joints.  I've had a jig to do those for years.  I spent a many hours one weekend learning how to set it up and use it.  There's about a dozen adjustments and it's imprecise enough that the only way to really get a tight joint is by iterative trial an error.  Keeping everything aligned and square is the key.  Also logging measurements for each trial.  I had some hemlock blanks which had very similar dimensions to my new Ipe blanks.  I used those to hone my dovetailing skills and made a few small frames.  Here's a "half size" frame about 7" x 5" with a kerf (slot) for the aluminum top panel to slide into:

Driver Design and Fun with Distortion

My original *mono* prototypes had the 6B4G output tube biased using "Grounded Cathode biasing" or "Auto Biasing".  They included a 6SL7 driver tube configured in SRPP (Series Regulated Push-Pull).  The driver preformed pretty well: decent gain and distortion, but because SRPP has the cathode of one triode at half the HT voltage (125v in my case), I had to drive it's heater from a supply separate from the heaters of the output tubes or else I'd exceed the maximum heater-to-cathode voltage of the driver tube.  In the end, my power transformer had 2 heater windings, so I probably could have made it work by using one for the power tubes and one for the driver tubes, but I had already set off into the land of fixed bias for my output tubes with a simple 6SL7 common cathode driver (see my previous post).  This reduced my tube count to three (2 output tubes plus the 6SL7 with one triode to driver each channel). 

My first test with that configuration in October resulted in a much better overall sound.  The details were nice, gain was lower than expected but the distortion profile was generally more pleasing (~4.5% THD@1kHz, -27dB 2nd order, -35dB 3rd - see red spectrum graph below).

Still not outstanding, but better.  However, that was just one channel.  Unfortunately, duplicating the circuit to the second channel resulted in almost double the distortion, but more gain.  WTF!  Both channels were using identical components.  The only difference was the cathode bypass capacitor on the driver triode.  I had used a junk box generic 47uF cap on the original channel (the one with the good distortion) and a decent Nichicon VX series 47uF on the second channel.  Sure enough, swapping the caps pretty much swapped their distortion and gain profiles. So the crappy generic cap had some magic mojo for reducing distortion...

Well, kindof.  Turns out, it's value measured by my multimeter was about 0.02uF.  For a bypass cap, that's essentially not bypassing much other than very high frequencies.  It wasn't a magic cap, it was a blown cap which wasn't far from no cap at all.   Many designers believe that with a proper design, a bypass cap shouldn't really be necessary.  It increases gain, but also increases distortion.  It's used most everywhere because we generally don't want to "throw away" gain.  So to not include a bypass cap means that the lower gain has to be designed into the whole amp.  Fortunately, the 6SL7 driver I'm using has a high transconductance and yields about 25x gain even without a bypass cap.  Thus with an input signal of 1V RMS, I could expect about 25V RMS (~ 35V peak to peak) on the grid of the output tube which is just below the maximum for the operating point that I selected for my output tube.  Almost like the 6SL7 was born to drive the 6B4G!

Look Ma, no driver bypass caps!

Output Section Design

The output section includes the output tubes, the output transformers which drive the speakers, and the biasing circuitry.  The job of the output transformer is to take the high-voltage/low-current/high-impedance output of the output tube and convert it to a low-voltage/high-current/low-impedance output for driving the low impedance (4/8/16 ohms) speaker.  Early on, I settled on a pair of James output transformers, partly because they look awesome in their potted cans, but also because budget conscious Single Ended amp builders seemed to like them, they pair well to the 6B4G output tubes I chose and gave me an option of 3.5kOhm windings and 5k.

In my first mono prototypes, I biased the output tube the good old fashioned way, Grounded Cathode (aka Auto bias).   Because I didn't have enough 6.3v heater windings on my power transformer to heat two output tubes plus two driver tubes in a SRPP configuration, I decided I needed to be able to power both output tube heaters from one 6.3v winding.  That suggested that I should consider using Fixed biasing of the output tubes so that both cathodes could be tied to the same potential (ground).  Fixed biasing adds a negative DC potential to the grid of the output tubes.  That allows a positive AC voltage (the signal) to applied to the grid and still be at a lower potential than the cathode.  The main reason fixed biasing isn't often used is that it requires an extra power supply.  The supply draws almost nothing though (a small handful of microamps in my tests) so the only real design negative is the extra parts count and associated real-estate they will occupy.

I built a quick and dirty (yet remarkably clean) negative supply, reconfigured the output stage, and was amazed by my first listen.  The upper-mid distortion that had bugged me for weeks was almost gone.  When I looked at how other designs (like my Cary SE-1) implemented fixed bias, I was surprised that they usually use the same negative supply to bias both channels.  That seemed odd to me since that basically means they are tied together.  Granted there's 400k ohms or so of resistance between the two grids, but still the signal will take that path with a resulting crosstalk.  I tried shorting the Right channel of my SE-1 to ground and sure enough the Left channel bled to an audible level to the Right speaker.  Granted, it was maybe only 2 to 5% the volume of the Left channel, but still, that's substantial for "Hi-Fi".

I decided I could do better, and I did.  I searched online for hours for a small power transformer with dual primaries.   It's common on large trannies, but not small (5 to 12 volt) trannies.  My plan was to not connect the transformer to the main voltage, but to connect it backwards to a heater winding of my main power transformer keeping it as THE isolation unit and not having to worry about fusing two (or three) power trannies, etc.  So the voltage gets stepped down by the main power tranny from 110v to 5v, then back up to about 90v by the small biasing tranny.  Since it has dual primaries (which I'm using as secondaries), I get two -90v adjustable supplies, one for each channel - zero crosstalk introduced by the biasing.  This was the one idea in my whole design which was completely my own.  I'm sure someone has done it before, but still, it felt great to have that spark of a novel idea and avoid the need for a second transformer.  Below is what the final dual -90v supply looks like:

I needed about -90v so that I could slap a high resistance potentiometer on the end and have the resulting voltage divider produce my target biasing voltage (roughly -40v) somewhere in the middle of the potentiometer's range.  A fixed bias configuration really simplifies the output stage circuitry.  Below is my final output stage schematic with the bias supply applied where you see "-39 v".

Auspicious Day - 10/10/10

My First STEREO prototype...

... Not too pretty, but it sounds pretty darn good!

AC/DC - For Those About to Rock!

My original plan was to employ DC voltage for heating the tubes, but keep prototyping simple by using AC.  DC heater voltage is used mainly because it should reduce the amount of ripple induced into the DC rails and signal path, especially for Directly Heated Tubes like the 6B4G.  That should reduce the audible "hum".  Today I tried DC (added rectification and smoothing) for both the driver and output tubes.  I was surprised that for the driver tube, DC seemed to make absolutely no audible difference to the small hum I hear, and no measurable difference to the measured ripple.  DC on the output tubes, actually increased the hum.  I used one very one high quality 4500uF cap and a low EMF 15,000uF cap, so the DC was about as smooth as it could reasonably be.  With DC, one lead of the heater is grounded while the other is at 6.3v.  By contrast, using AC, both leads are connected to the transformer windings with the center tap at ground, so essentially, each heater lead is at 3.15v.  All I can think is that the symmetry of the AC is preferable even if it's oscillating.  The hum really isn't that bad, so for now, I'm gonna stick with AC heaters.