ABOUT BOB'S DEVICES
WHY BOB'S DEVICES?
I have built hundreds of moving coil step-up transformers and tested more than a dozen of the most popular step up transformers and several step-up pre-amps (head amps) used to match moving coil (MC) Cartridges to phono stage preamps.
WHY SHOULD BOB'S DEVICES BUILD IT?
Since there are many different types of Moving Coil Cartridge, one size does not fit all. Wiring transformers for moving coil cartridges is a tricky process and my designs use tried and true techniques to eliminate hum and ground loops. I also use very sensitive test equipment to properly evaluate and test each device. Even with properly built devices, careful placement and proper interconnect cables and techniques are needed, but I have taken the guesswork out of wiring these devices yourself. There is more to this than just wiring the inputs and outputs of a transformer to the RCA jacks in a box.
MOVING COIL STEP UP TRANSFORMERS
Each step up is individually crafted and tested by Bob Sattin who has built over a thousand step up transformers using dozens of different transformers. He currently features specially designed Cinemag transformers that are not available to anyone else. These are proprietary designs especailly for Moving Coil cartridges. The current reference cartridges include: a an Accuphase AC-2, Miyajima Shilabe, Koetsu Rosewood Signature Platinum, Cardas Myrtlewood Benz Ruby 3 Silver, and Ortofon Cadenza Bronze. Many other cartridges have been used in the past. Bob's Devices as a company has been manufacturing Step up Transformers for 15 years. Bob Sattin has been manufacturing them for many years before that.
Also available are Step up transformers with XLR connectors for those with balanced inputs and outputs, or mono units.
The latest edition is the SKY 10 which has selectable step up ratios of 1:5 and 1:10. This was introduced to meet the needs of Audiophiles who own high gain phono stages including some ARC phono stages with over 55 db of gain.
Use the chart below to select your step up ratio
Grounding and Installing your Step Up
Grounding your Bob's Devices SUT
Each unit includes a grounding post that can be used to connect your turntable ground and your preamplifier ground. It also includes a ground "lift" switch. In all modes, the transformer cases and faraday shield internal to the transformers are connected to the ground screw. In the "GROUND" mode, the minus sides (negative sides or shields) of the output cables are connected to the grounding lug. This configuration works well for those systems where the turntable ground is connected to the negative leads coming from the phono cartridge or where the negative inputs to the preamp are internally connected to ground. In the "LIFT" mode, none of the conductors in the RCA jacks are connected to the case, ground, or shield and there is no electrical connection between channels. This design avoids any transformer induced ground loops regardless of the configuration of your other equipment.
So, you have the Step-Up Device and you want to properly install it into your system:
Step 1 - Connect the leads from your turntable to the input connectors on the Device.
Step 2 - Connect the ground lead from the turntable to the ground lug on the Device.
Step 3 - Use a short pair of interconnect cables to connect the output of the Device to the input of your Phono Preamplifier or Phono Inputs. For best results, this set of cables should be like the ones in this DIAGRAM .
Step 4 - Turn everything on and listen to the background noise. Move the ground / lift switch to whichever position is quietest. Then move the transformer around to find the quietest location.
If you are continuing to have problems, try connecting an ordinary piece of wire between the ground lug on the phono preamplifier and the device (in addition to the one from the turntable). Then try the ground / lift switch in both positions to see what sounds best. Then, if it still is not quiet enough, take a look at the following information:
Start from your cartridge and follow the wires, looking carefully for any places where the negative terminals of the cartridge (lets call them system ground) are connected to equipment grounds (let's call them chassis ground, sometimes referred to as drain wires) We only want one chassis ground connection between pieces of equipment. When there is more than one chassis ground connection, there is the possibility of a ground loop (it doesn't know which way to go, so it keeps on moving and generates noise.)
The other type of noise is interference (you get that from any parts of your system wires running too close to power transformers, power cords, or any kind of AC power.)
Now, since we all use RCA cables from the turntable to the phono preamp, the negative (system ground) wires have to be shielded with a chassis ground. (Read that sentence twice). Some turntables connect the shield of the RCA cables to the chassis ground wire at the turntable, and some do not.
The next 2 paragraphs repeat the initial steps above, but written in narrative format, and in more detail:
If your turntable has a pigtail along with the RCA connectors, then that is the chassis ground from the turntable. Connect that pigtail to the ground lug on the step up transformer. Then connect everything else, without connecting the ground to the phono preamp. Try the ground / lift switch in both positions and see which is quietest. Then, additionally, connect a separate wire between the ground lug on the step up transformer and the ground screw on the phono preamp and see if that is quieter or noisier in either of the ground / lift switch positions. Then you can determine whether you need to ground the phono preamp to the step-up transformer or not. Regardless, you need the proper interconnect as shown in my DIAGRAM to go between the step up transformer and the phono preamp.
You only want them connected that way at one end, hence the ground / lift switch. So, here is the short answer. Remove the ground wire connecting the turntable from the phono preamp. Connect ONLY the ground from the turntable to the Box you got from me. Don't connect the wire from the box to the Phono Preamp just yet. Now, make sure that you use interconnect cable between the transformer and the Phono Preamp that have 2 wires inside (positive and negative) and the shield is connected to the outside (chassis ground of the cable) at only one end. (The cable will have the negative lead and the ground attached to the outside of the RCA connector at one end. The cable will only have the negative lead (Not the ground) attached at the other end.) Now, with everything hooked up, turn it on and listen to the hum. Then move the switch to whichever position offers less hum. Then touch the preamp ground wire to the ground lug and see if it is less or more noisy. Then remove the wire from the phono and see if it is quieter or not. Then touch the phono wire to the preamp wire and see if that is quieter. Sounds like trial and error. It basically is, but the idea is to make sure that everything is grounded, and only grounded once, and the all the audio cables' shields are grounded, and only grounded once.
You will find that the transformers are very susceptible to Electro-Magnetic Radiation, and you may need to move them around a bit to find the quietest location. For me, it is just below my turntable and on top of my preamp.
IF YOU WANT MORE INFORMATION, CHECK OUT
Included on their site are some diagrams on wiring that may be helpful.
HERE'S ANOTHER WAY OF LOOKING AT THE SITUATION
Your cartridge has 4 connectors: + and – for each channel. None of those is a ground wire. After the wires leave the tone arm, there is usually a block or a connection point on the turntable where the tone arm wires are connected to the RCA cables.
Find that point and look to see electrically how that connection is made. Look to see if the connections go directly to RCA + and – on the cable and look to see if there is a ground connection. It may be hard to tell since the RCA connectors may or may not be insulated from the enclosure (you will see a Teflon or other insulated washer on both sides of the connector). If they are not insulated, there is inherently a ground connection there, and the – sides of both cables are also inherently connected together at that point.
The cables from the turntable go to the SUT. The SUT is constructed so that in the LIFT mode, all 4 wires pass independently through the SUT without being connected to ground. The RCA output cables continue to the phono stage.
We don’t know how the wiring is set up inside the phono stage. Things we don’t know are whether the – sides are grounded to the chassis and whether the + and – signals are inverted. This is nearly impossible to determine without a schematic and there is no industry standard. The best way to check for inversion is to switch the + and – wires on the cartridge and see if that fixes the problem or makes it worse.
To top all that off, the signal wires are all subject to stray electromagnetic (EM) interference. That is the main reason why cables are shielded and why we twist the wires. If the wires are not twisted, they act like an antenna and pick up EM. If we twist them, it confuses the EM and makes it a worse antenna. So the best setup is one where the signal wires are twisted and there is a shield around them that is attached to a chassis ground, not to any of the signal cables.
Some tone arm manufacturers take one of the – wires from the cartridge and connect it to chassis ground. I don’t know why, but they sometimes do.
I try to take all this into account with my design which when you move the switch from lift to ground, it connects all the – signal wires to the chassis ground. That takes care of a lot of problems with the inconsistencies in wiring both with tone arms and with phono stages.
You need to understand all of this and clearly look at your system. Take into account that there should only be one chassis ground connection at each unit. If there are multiple connections you get what is known as a ground loop where it also acts like an antenna but the EM signal keeps looping and gets amplified. So you are better off without a ground than with an extra ground.
One of the main challenges is with the interconnect cables. RCA connectors only have a + and – and the – is sometimes connected to chassis ground. So it is important to use cables that have 2 conductors and a shield, with the shield connected to the – conductor on one end only.
WHAT ABOUT IMPEDANCE?
Impedance: There are 2 types of impedance to be concerned about with Moving coil Cartridges: Internal Impedance and Loaded Impedance.
Cartridge Manufacturers are not consistent with how they specify loaded impedance. With Step up transformers, we boost the voltage, since transformers are voltage changing devices. There is also a secondary effect of the internal impedance being the square of the step-up ratio. So, for example, if a transformer has a step up ratio of 1:10, then the internal impedance of the transformer would be 100 (10 x 10). So the reflected impedance back to the cartridge would be 47,000 ohms (using the RIAA standard for a standard MM phono input), divided by the internal impedance of the transformer. So for this example, you take 47,000/100 = 470 ohms impedance at the cartridge. So, the manufacturer sometimes advertises 470 and sometimes advertises less than that, if they choose to use a multiplier.
There is a lot of confusion about how the loading is specified. Some cartridge manufacturers try to take the guess work out of it and put in some kind of multiplier, while others base their loading assuming a head amplifier will be used to boost the signal. For example, Koetsu recommends anything between 5 and 100k. Other manufacturers just specify a minimum, like >100 Ohms. The problem is that we don’t really know whether the manufacturer has provided loading information based on the internal impedance or the loaded impedance of the cartridge, or considered whether we would be using a SUT or head amp. We do know from considerable testing, that you don't want to load a MC cartridge using a SUT to match the internal impedance, or you will “choke-out” the cartridge and it will sound muddy.
Just how close do you want to approach the internal impedance before this happens, and what happens as you approach this ratio? Is resistor loading necessary?
The deal on impedance is that it is not that important with SUTs. Voltage match is what is critical. Having said that, once you make the voltage match, you should look at the reflected impedance. You normally want it to be about 10-times the internal impedance of the cartridge. Now that is not a hard number, it could be 8-times and be fine, and sometimes 4-times can sound great. The important part is that more headroom is better. A lot depends on the characteristics of the phono preamp. Typically if you have enough headroom with impedance, the natural characteristics of the cartridge will be apparent. So, if you have a "bright" sounding cartridge, you may want to have a lower impedance at the cartridge, but if you have the impedance too low, it may sound muddy and flat.
I no longer recommend adding resistors. There is considerable controversy over this and there is a lot of discussion elsewhere. The signal is so low coming from the MC cartridge, that is the one place I would not want to waste any signal. I work really hard to ensure that the output of the transformer has as short of a signal path as possible. That is the most critical part of the setup, where you have already reduced the current coming from the cartridge to gain extra voltage.
I have read several articles that discuss loading of cartridges. I took it a step further and experimented with different loads on several different test cartridges. I both measured the response using a signal generator and dual trace oscilloscope and also conducted many listening tests. What I found was really interesting. Basically if you are just looking at the test equipment, you can tune it pretty accurately, but the real test is in the listening. The biggest challenge is in gathering the correct information on the cartridge you are using. The manufacturer can tell you the impedance and the output voltage, but is frequently wrong with telling you what loading impedance is required. To test that, I used several of my transformers with different cartridges and instead of hard wiring resistors to the input or output of the transformer, I built a set of resistor tees (2 female to 1 male) and inserted them on the inputs and/or outputs of the transformers. I then wired several resistors onto male plugs and inserted them into the tee on the output to try different resistances. Starting with high output values and changing them to lower and lower values, I found that there was a very slight improvement as the resistance came down until there was a major change for the worse when it reached a certain point. This point usually turns out to be about 8-12 times the output of the cartridges I tested but again it was cartridge and internal impedance dependent. Now, I am a purist and don't really want to put anything into a circuit unless it improves it. I have also mounted lugs on the outputs where you can attach resistors as an after-market thing, but they introduced noise, so I stopped doing it. The output of a transformer is the most critical part of the circuit and you have to be very careful with wiring between there and the preamp to eliminate noise.
In most cases where I have tried to load using resistors and tuned them in, I tried an A-B test afterwards both with and without any resistors and did not notice the difference. In my opinion, you usually do not gain anything by adding resistors, and only stand to degrade the sound if not done properly.
So, to make a long story short, I don't recommend messing with loading resistors.
For more reading pleasure, check out these links:
The formulas used on theanalogdept.com site above seem to fall into line with our experiences with listening tests. Email me with your cartridge and phono preamp and I will run the calculations for you and recommend a step up ratio for you to use.