----------

Why Bob's Devices

Why should I purchase a "Bob's Device"?

 

What do these do?    These transformers are used for connecting a moving coil cartridge (on a turntable) to a phono preamplifier, phono input of a receiver, or your systems preamp.

My experience:  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 you should have me 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.  

Wiring Schemes: SUTs can be wired to the same identical schematic,using identical transformers, and have them sound like different units, by varying the mounting locations, and wiring routing.  Even small moves like locating switches or deciding how to wrap the wires, makes a noticeable difference in the sound quality.  With very low voltages, minor changes make a big difference.   So not all Cinemags sound the same, and not all Sowters or Lundahls sound the same.  It depends on how they are assembled.  I don’t mean for this to sound like voodoo, but there is an art and a science to making each one sound their best. 

The challenge:    After the phono cartridge and before the preamplifier, we are working with an extremely small signal generated by the coil and magnets inside the phono cartridge.  Even the most minute change is literally amplified by your equipment.  Many DIY'ers have taken these same transformers and have used them with varying results.  Just because the schematic is followed and everything is wired correctly, does not mean that you are getting the best quality product.  For example, in product development, I have built up several sets of Cinemag transformers using exactly the same schematic, and had them sound noticably different using different routing schemes for the wires.

Each design is scoped on a dual-trace Oscilloscope, using a low impedance signal generator and a bank of sensitive milli-volt meters, to ensure proper polarity.  I personally listen to each one to make sure it is sonically perfect.  I also custom build units for special situations, such as using XLR connectors for those with balanced inputs and outputs, or with multiple inputs for those with several turntables or tonearms.   

A Little about Matching:   Moving-coil cartridges have more energy than moving-magnet cartridges due the fact that their magnets are stationary, and can be stronger than those in the moving magnet cartridges. They typically have high current and low voltage, where a moving magnet cartridge has high voltage and low current.

To use a moving coil cartridge, you either have to amplify the voltage prior to reaching the phono preamp (by using a Head Amplifier); or to change the current into voltage that is high enough to be used by the phono preamp (by using a transformer). 

What is the difference between a Transformer and a Head-Amp:  The transformer is a passive device and therefore has certain advantages over a head amp or active device, which is most likely built with solid-state electronics (FETs, i.e. Field Effect Transistors).  Moving Coil Cartridges have low voltage but high current.   The transformer uses the extra current that is not needed and converts it to higher voltage to allow the cartridge to match the input of the phono preamp.  Whenever you introduce something into a circuit, there is an insertion loss.  Whether there is a greater insertion loss from a transformer or a head amplifier depends more on the quality of the components than on which type of step up device is used, but a well crafted SUT has fewer parts than a head amp.

Impedance vs. Output voltage:  With transformers used for matching the outputs of MC cartridges, you would want to match the output voltage, rather than the impedance, to get a better match.  (This is different from head amplifiers where you would want to match the impedance only.)  Many folks get caught in the trap of trying to match the cartridge impedance to a step up transformer.  If you do attempt to match the impedance, you will “choke out” the cartridge and it will sound flat. With step up transformers, you match voltage, not impedance, but you do need to make sure that there is sufficient head room with the reflected impedance so it does not “choke out” the cartridge.

A conventional RIAA phono preamp has a nominal reference level of 5mV at 1kHz.  All phono preamps are different, but typically output values below 2.5 mV or above 10 mV will either result in a poor S/N ratio or overload (clipping).  The higher the ratio, the higher the gain (the louder it will sound).  Phono preamp manufacturers usually specify the input voltage as a minimum that is acceptable.  What usually is not specified is the maximum voltage that is acceptable before overloading.  The best sound is usually attained toward the high end of the range.

The MATH Part:  Take the output of your moving coil cartridge which is expressed in milli-Volts (mV) and multiply it by the step-up ratio of the transformer.  If this value falls between 2.5 mV and 10 mV, it will work.  (Remember that the 10mV is not the absolute.  It depends on the phono preamp you are using.)  The standard is 5mV (at 1 Khz), however most folks like to be in the range of around 7mV to match the volume coming from a CD player.  Now matching  is not always this simple, including the reflected impedance to consider, which is why you should take the time to ask what would be a good match for you.   

Please contact me if you are not sure, and I can look up your cartridge and phono preamplifier to ensure a good match.  I have many different transformers in stock and can custom build from a variety of transformers to your specifications.

How I determine the step-up ratios:  I measure final step-up ratios using a 50 ohm audio signal generator at 1kHz with a 47k output resistance, which approximates the real situation of a moving coil cartridge load using this transformer into a standard RIAA Phono Preamplifier.  Then I test each and every one on my own system to make sure it sounds right.

I take pride in each and every set and provide customer support to make sure that they work well in your system. 

What about Grounding?  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 in" mode, the phono system ground is connected to the chassis ground.  In the "ground out" mode, the phono preamp system ground is connected to the chassis ground.  In these modes, 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.