MT Audio Design MF-2001

Mains Filter with Isolation Transformer

I continue my effort to make equipment I want myself and can not find at decent price anywhere. I have wanted to try isolation transformers in my system for many years but the commercially available transformers are mostly very large and cost a small fortune. I think these miss the point of using isolation between separate units because you are supposed to connect all units to one large transformer. I am sure you will get better sound with such a transformer too but with separate isolation transformers on all units there is more to be gained.

The intention with this product is to make an affordable mains filter with isolation transformer that can be used on separate units in the Hifi system. You will not get a fancy box since I think it is a waste of money, the mains filter is supposed to be hidden far away from the connected HiFi unit. More important is what is inside it and this is a serious attempt to make the best mains filter you can get for money. It has qualities not found in the mainstream filters sold and a much lower price than other serious filters/isolation transformers. I have seen cables for mains connection at the same price level or even much higher than this high quality filter. How much difference can a mains cable do compared to a well designed filter with isolation transformer?

This product has been planned for a long time now (too long) but we are finally ready to deliver the MF-2001 as a kit. The plans have been slightly changed, the isolation transformer is of higher quality than in the previous description and now includes a Faraday screen to get as good filtering as possible. The transformer is center grounded on the secondary, this is also beneficial for noise levels. Unfortunately this transformer costs almost twice as much as the one we considered previously but quality is most important. Now on to a more detailed description of MF-2001


MF-2001 Description

MF-2003 schematic description

Mains filters purpose:

To reduce the noise and high frequency components in the incoming voltage and to isolate the noise coming in and out of the equipment in your HiFi system. You also get a good protection against shocks from lightening.

Isolation transformer purpose:
To reduce the ground currents between the components connected in your HiFi system. The main transformers in your equipment always have a capacitive coupling to the signal ground and if the primary winding is allowed to float free the ground currents in your cables are reduced significantly. The transformer also acts as a filter, making the noise filtering much better than with only L-C network and the center grounded secondary winding reduce the noise level even more. The transformer also helps to get a good protection against shocks from lightening.

Best method for mains filtering:
I have tried to find the best method for mains filtering together with my friend Martin and we think a mains filter shall be made like this:
1. High performance mains filter on the input, carefully selected
2. Varistor on the input for overvoltage protection
3. Extra RC network on the filter output
4. Isolation transformer with Faraday screen and center grounded secondary
5. Shielded enclosure

MF-2001 output is not intended for use with power amplifiers, the output can be loaded with 100VA and this covers most pre-amplifiers, RIAA-stages, turntables, CD-players, D/A-converters, tape decks, tuners and of course electrostatic loudspeakers and subwoofer crossovers.

Mains filters for power amplifiers need to be made in a different way, the filter must then be able to pass a lot of current for short time periods and this is what mains filters normally prevents. Many have reported that mains filters on power amplifiers tend to make them sound less dynamic even if the sound is also a bit more relaxed.

If this mains filter is a success, I will make an isolation transformer for use with power amplifiers available but at the moment I can not afford the needed investment.


Of course there is an idea behind the construction of this mains filter, and here are some explanations:

Input Mains L-C Filter
On the input we use a high performance filter with high damping. A varistor and RC filter is also included in the input filter for electric shock protection and improved performance.

Separate Isolation Transformer on Each Output
An isolation transformer makes a very significant improvement, the simple reason is that the grounds in your HiFi system may be at different potentials and this causes ground currents. For best result, all connected units shall have its own isolation transformer but this is not cheap. If only one isolation transformer is used, you normally get best result if this is used on the signal source (for example on the CD player). Balanced equipment and cables are less sensitive to potential differences and this may well be the reason for the better sound quality sometimes found with balanced operation, the balanced circuits themselves are not better than single ended if these are made properly. With floating grounds the ground potential will be the same in every connected unit and no (or at least, much less) ground currents will appear, meaning that the cables work as they shall and there is much less strain on the inputs. With one isolation transformer with filter network on each connected unit, the interference between the connected units ought to be minimal.

Separate Input Mains Filters
Separate input filters are used for best noise suppression between the HiFi system parts, noise caused by for instance a CD-player motor has to go through two filters and two isolation transformers before entering for instance the pre-amplifier.

Shielded Enclosure
To prevent radiation from the transformer a grounded steel plate enclosure is used.

Mains Filtering Theory

The method for mains filtering is simple, prevent unwanted frequencies from entering the equipment, but let the 50/60 Hz pass through. This is made with filter components as always, chokes in series and capacitors in parallel. Some parts, not used in ordinary filters, such as varistors and PTC resistors can also be used to reduce the peaks from for instance lightening or switches turning on/off. Some manufacturers of mains filters claim that their filters are made especially for audio and that you get better sound when using these, this can be partly true. When you make filters for use with for instance computers short time current limitation is not harmful, but when used with audio equipment the current flow (50/60 Hz) must NOT be limited, the output impedance of the mains filter must be low enough to permit unlimited current flow (within reason of course).

The formulas used for calculation are the same as in all filters.

Capacitor drop:
, capacitor impedance

Inductor drop:
, inductor impedance

V = voltage
I = current
C = capacitance
L = inductance
f = frequency
Z = impedance
dI = change in current
dV = change in voltage
dt = change in time

From these formulas we can see that the capacitor voltage drop increases with lower value C and slower voltage transients (lower frequency), meaning that the capacitor tries to keep the voltage constant and more so at low frequencies. The Inductor voltage increases with higher value L and faster current transients (higher frequency), meaning that the inductor tries to keep the current constant and more so at high frequencies. Note the similarity of the formulas, the inductor does the same to the current as the capacitor does to the voltage.

In low-pass filters in general, capacitors are connected in parallel and inductors in series, and connected like this the filter action is basically the same with both component types. High frequencies are blocked and low frequencies can pass through.

MF-2001 Features

How does it Sound?

I connected my Audio Note DAC to one MF-2001 instead of the previously used mains filter and this is my impression:
A much more relaxed sound with a more quiet background. At first, I thought it is almost too polite but it did not take long to realize that this is not the case. The transients are sharper than ever, the soundstage appears to be deeper and with better focus on individual sounds. The details appear more natural with no coloration and the whole picture is more lifelike. It is hard to describe in detail what it sounds like, I think the best description is that everything sound more natural. Note that my impression is based on the difference between a mainstream mains filter and the MF-2001 with isolation transformer.

The initial listening tests indicated that this is really a missing link in most HiFi systems, the sound improve a lot. A friend of Martin borrowed the first unit and we did not get it back, he said that every aspect of the sound improved and wanted to buy the MF-2001.

Assembly Pictures

The pictures below show the assembly of my prototype, the production units have a different filter and better plinth but assembly will be made like this in the production units too.

Installed transformer

Transformer and fuse connections

Securing input and output cables

Installation of filter and cable connections

The fully wired MF-2001

Price Information

The price is estimated and may change but I think we can make it.
$140 for kit with input filter and isolation transformer
$70 for separate isolation transformer (without box)
$30 for assembly

S/H price (estimated):

If you are interested in a purchase, contact me via an e-mail.
mailto: MT Audio

For best performance all units connected to the pre-amplifier inputs shall have its own isolated output. Making a mains filter as advanced as this is not cheap but should be well worth the investment in a good HiFi system, the total cost is not higher than many think is decent for a couple of interconnects. I have not seen other ....

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