Assembling a pair of Beyer Dynamic microphone transformers into a project case for use as an MC step up.
above: the project case. For these small Beyer Dynamic cans, I can use a case with compact dimensions. Hammond #1590B. This is die-cast aluminum in two pieces. The lid is assembled to the box by four machine screws. Hammond project cases are available in a wide variety of sizes and are affordable. This case cost ~ $5.00 usd. Fwiw, I can't remember what I paid. I found it while browsing through the electronics parts in at a local Fry's. They are available online cheaper, but then you pay shipping to get it home.
After some thought, I chose a "top-loader" layout because all the parts would fit the case, and then I chose this just for subjective aesthetics. If using different sized transformers, I might choose a different arrangement....perhaps with the rca jacks mounted to the sides of the case. But these Beyer Dynamic cans are rather tiny and afford the layout in the renders.
This box will offer two different voltage step-ups. 1:15 and 1:30. This means that if the signal coming off the cartridge is .2mv, it will be 3.0 mv when using the 1:15 tap, and 6.0mv if using the 1:30 tap.
I have noticed that sometimes the method of selecting between a high and low setting on step up transformers is chosen by means of a switch that selects between the turns ratios. However. I chose to use four RCA inputs because by doing so I could avoid sending the very tiny signal voltage coming off the MC phono cartridge through a selector switch before it reaches the transformer. When possible, I want to keep the signal path as simple and short as is possible.
Things needed. The project case. An automatic center-punch. The drill map. And some spray-on adhesive to fix the map to the case. The Elmers's adhesive, seen in the photo, dries quickly and has adhesion adequate for this task.
The drill map printed and cut. For this task, I used presentation matte ink jet paper by Canon. This isn't quite photo grade paper but is stiffer than plain paper and will work well for placing the drilled holes. I got it in letter size 50 sheets for $5.00 usd.
Here's my Starrett 'automatic center-punch'. It has a spring loaded trip hammer that is actuated by simply pushing down on the handle end while the business end, with the point, is centered on the mark. It makes a nice clean punch mark that is easy to find on the drill press. A nice tool to have around if you plan on drilling holes into metal. (old machinist's trick ;-)
Here's a shot of the case after drilling,..... but with the template still in place. It appears that I hit my marks close enough for this project. To remove this template, is squirted some Ronson lighter fluid over the paper. Enough to soak through. Then, after a few minutes, the template lifted off cleanly.
The drilled box after drilling, deburr and clean-up. For the external finish I considered paint, but opted for a "brushed aluminum" finish instead. To get that I used a 6 inch long auto body sanding board with 120 grit sandpaper. I sanded all external surfaces on the box. I chose not to sand the lid.
Prior to assembly, this view shows the individual parts to be assembled into the project box. Six different gold plated RCA jacks. One gold plated ground stud that will accept both spade and banana connectors. Notice the one foot length of interconnect cable forming a circle around some of the components? That is Cardas twin link copper litz. Cardas # 5190. This is 23.5 awg conductor shielded and in teflon dialectric. Pretty good stuff. I'll use that piece to make the two outputs that lead from the SUT into the phono stage.
But wait, there needs to be markers to instruct the user where to plug in the phono cable plugs and ground strap. For a label pattern, I used inkjet transparent media as an overlay. This particular media has an adhesive on its backside. After trim, and before application, just remove the substrate layer, then carefully lay the transparency over the part to line up with the drilled hole pattern. First try, I missed. Second try looks closer and I think will appear good after the box is populated with its rca jacks, ground stud and the tranny cans.
Done. It took a while to solder the individual tranny wires to their respective tags on the RCA jacks and ground post. There needed to be some care. The wires are very fine gage. The insulation is fine. The space inside that small project box is confined. Care must be taken. But all went well. All solder joints hold and deliver signal. Victory.
Action shot! The BD cans deliver signal to the Wright WPP100C phono preamp. Also on this photo my very short output IC's can be seen. These are the above noted Cardas 23.5 awg twin-link copper litz conductors. I've soldered some old(but good) AudioQuest RCA phono plugs to this new wire. This gives me 6 inch long IC's between SUT and phono stage. That should keep the capacitance low. Background, upper left, the Sowter SUT box, supplied by George Wright, that I had been using previously.
Some technical details.
The MC cartridge being stepped up is a Shelter 501 type II that is currently mounted to my TD124. The Shelter is carried by a Zeta tonearm with Incognito/VdH silver phono cables. I'm using the 1:15 tap on the Beyer Dynamics to multiply the .4mv signal up to ~6mv output. This will be an acceptable voltage output for feeding the phono stage.
But what about cartridge loading? The Shelter has an internal resistance of 10 ohms. There is this rule of thumb often quoted by the audio press about MC step ups. Multiply internal resistance by 10 to find optimal loading. In this case, if rule of thumb holds true, the target would be 100 ohms of load.
If I use the 1:15 tapand calculate as follows:
209 ohms. Less than half the load it was seeing with the 1:10 Sowter step ups. But twice the optimal "rule of thumb" load.
One other thing I had anticipated is that the gain on the Wright WPP100C phono preamp would need to be turned down a bit to keep volume levels the same. And this is the case. I now have the gain pots set to 3:00 o'clock. Where before, with the 1:10 Sowters, I had it at 4:00 o'clock.
But how does this sound? The interesting thing is I thought that by halving the load that there would be some audible effect with regard to frequency extension and / or perceived warmth/coolness or otherwise sonic coloring. Not really. Frequency extension seems similar at either end. There is a bit more of inner detail being heard with the Beyer Dynamic cans. Or perhaps it is a different emphasis toward instruments. For instance on Brubeck's "Time In" album, the Morello's Cymbals have "more metal" in the timbre and ring than with the Sowters. And I perceive this as a positive.
I'm still determining of there is any rolling off at the very top. The upper regions at the extreme end of (my) human hearing. The Sowters may offer more detail up there. Probably, that does not matter to me. I should add that this observation is not meant to infer that there is any loss in upper frequency extension. Nothing remarkable other than fractional differences between the Sowter and BD transformers. Either one has good upper frequency extension in combination with the phono stage in use.
There is an increase in punch from the lower frequency regions. Call it a good sense of the visceral on rhythm and drive. More than before. Good atmosphere. Good sparkle. From Beethoven string quartets, to Pink Floyd, a good listen.
Then I tried the BD SUT with an Ortofon Jubilee cartridge.
That cart has 5 ohms internal resistance with .345 Áv output. The Ortofon seemed happiest and most dynamic on the 1:30 tap.
Let's try the rule of thumb on the Ortofon: 5 ohms internal resistance x 10 = 50 ohms for an optimal load. To calc load: 47k / 30▓=52.2 ohms load. Very near optimal. 1:30 is a good turns ratio for this Ortofon. With that in mind, I can't say that the BD cans transformed the Jubilee into something different. Pretty much, it is the same MC cartridge I've known but with slightly more dynamic punch. That much is notable.
The Shelter 501-II and the BD step up on the 1:15 tap. The 501-II has an output of .4 mv and in internal resistance of 10 ohms. While the Shelter did sound good with this step up, and after long term listening, I decided that the mixture seemed a little "hot". A tad too much gain. A bit too edgy. Prone to listener fatigue. Not quite right. But the Shelter seemed more at ease and 'right' while using the 1:10 Sowter SUT. Different step ups for different cartridges.
The DL-103R liked the 1:15 tap on the BD sut. This combination gave the Denon lomc cartridge a bit more force and speed than what the Denon seemed like with the 1:10 Sowter step up. BD and Denon it is!
(Aside: The BD sut seemed equally at home using either the Wright phono preamp or the Hagerman Trumpet phono stage. While the Trumpet delivers a more refined sound than does the Wright regardless which SUT is in use. This goes for either stage using mm phono cartridges. )
Thorens TD124, Zeta tonearm/ vdH silver wire with silk jacket organized in an Incognito harness.(with a few repairs caried out by yours truly)
TT: Thorens TD150 (with mods) Graham 2.2 tonearm, Ortofon MC Jubilee cartridge, Cardas phono cable
Wright WPP100C tubed phono preamp.
Hagerman Trumpet 47 Kohms phono stage.
amp: Classe' CAP 151 integrated (150 wpc into 8 ohms)
speakers: NHT 2.9 (4-way floor standing towers with 10 inch side-firing bass drivers)
Speaker cable: Canare 4S-11 in Bi-Wire configuration and terminated with silver spades.