Steve asked me to post a little more information on his upcoming Tube amplifier, so here goes: Information on the new Zed Hybrid 4 channel amplifier. This new amplifier combines the attributes of tubes and semiconductors in a unique design. The amplifier is divided into 3 sections, the power supply, the power amplifiers and the pre-amplifiers. Power Supply: The power supply is the driving force behind all amplifiers and is absolutely part of the signal loop and therefore plays as an important part in the sonic signature as any other part of the amplifier. There are a total of 8 separately regulated power supplies within the main power supply block. They read as follows: The main pair of high current rails required for the final output stages. These are choke regulated and use a total of 52,800mfd of capacitance. A second set of higher voltage supplies are used for the main amplifier’s sensitive front end. Low current low voltage supplies used to power the servo control circuits as well as part of the cascaded differential stages. The final pair are the high voltage supplies (+100v and -100v) used for the tubes. These are designed to be ultra quiet as tubes do not have the ability to reject power supply noise very well. These supplies are also choke regulated types for noise immunity and better regulation. Even though the power supply operates at about 85KHz we prefer to regulate all supplies for lower noise and better sound quality. We use very high speed switching diodes (35nsec) in all the above supplies. Capacitors are all low ESR 105 C types. The final supply which is not part of the main switching supply is that used to drive the filaments of the ten tubes. Depending on the tube type installed the filament current varies between 1.8 amps to 3.25 amps. Owing to the fact that the filaments derive their power directly from the battery, we have designed a failsafe circuit which continually monitors the filament supply. Should the control MOSFETs fail even when the amplifier is not powered on, this circuit shall blow open a fuse which will remove the filament circuits from the battery. If this was not done and a failure occurs one will get into their vehicle in the morning or after work with a dead battery! There are various timers to enable the amplifier to carry out a choreographed boot sequence as it is vital that the various supplies come up in a set sequence. Owing to the fact that we have delicate tubes hanging around, they must be handled with the proverbial “kid gloves”. Cathode stripping is a real problem with tubes and their filaments must be powered prior to the main high voltage supplies coming on. As in all Zed amplifiers we use our main switching MOSFETs in a common drain configuration which allows them to operate at maximum bandwidth and also prevents switching supply noise to be injected into the heat sink. The main transformer is capable of delivering over 1.6Kw in a design limited to just under 1Kw. Power Amplifiers: A totally new and unique design has enabled us to achieve performance sonically and on the test bench to which we believe others will aspire to. There are no capacitors in the signal path and so the amplifiers are flat to DC (meaning that they are capable of amplifying a DC signal). Fast acting servo controllers ensure that there is less than 3mV of residual DC on the speaker outputs. Having no capacitors in the signal path allows for instantaneous overload recovery especially with music which is non-symmetrical. The circuit design is fully complementary from input to output which is not unique at all, but it is the various elements which we have designed in which allow us to attain such a level of performance. Low levels of global feedback are used with local feedback at every stage to improve linearity. All amplifying stages are run in pure class A except for the output stage which is run in class B. Note: The amplifiers are run in what is commonly called class A-B. Owing to the fact that pure class A is simply not practical in a mobile amplifier, the amplifiers idle at such a level that class A operation is valid for just a few tens of Milliwatts. The conduction angle is in excess of 180 degree ensuring that the output transistors are never cut off. Our amplifier clips “softly” meaning that we have kept out some of the harmfull artifacts which plague solid state amplifiers. The best way to prevent amplifier clipping is simple – do not drive the amplifier into clipping! To this end we include four separate clip indicators on the front panel. Each of the four amplifier “pods” has its own current delivery system meaning that all current into the pod and out it located entirely within the pod itself. This improves inter channel crosstalk and lowers distortion. We use 20MHz high linearity output devices in a complementary feedback pair configuration combined with common collector circuits. This allows for good thermal stability and very low distortion. The frequency response of the amplifiers is down 3dB at well over 400KHz and THD measures less than 0.0005%. Our Audio Precisions have a residual of 0.0003% and the actual THD of the amplifier is much lower. Whilst Zed has never been obsessed with measurement numbers, we have achieved a level of performance which has allowed us to achieve these figures. Damping Factor (DF) is a modest 100 at 20Hz and we have not attempted to increase this for sonic reasons. It could be easily done by adding more global feedback but this is not the way we choose to go. Pre-amplifiers: An all tube pre-amplifier is what separates this amplifier from all others. There are no semiconductors in the signal path. I have spent almost a year designing and perfecting this design. Running off bi-polar supplies allows for easy biasing of the grids of the tubes. All tubes run in pure class A at considerably higher current levels than the plethora of “12AX7” designs which float around the car audio market. We use high transconductance triodes with a Mu of about 30. These are far more linear and much lower noise than the common “12AX7” and their cousins the 12AU7 and 12AT7. Note: 6BH6 and 6CG7s are also not usable in this design although they are fine tubes. Each pair of channels has a high and low pass pair of 12dB/octave Linkwitz-Riley crossovers variable in frequency from 60Hz to 600Hz and in the X10 mode from 600Hz to 6KHz. The available options are Flat, High Pass, Low Pass or Bandpass per channel pair. The quad ganged level controls (One section at the input and the other at the final gain stage) allow for the setting of optimum signal to noise and overload conditions. These level controls are wired as pure shunt elements and are thus never in the direct signal path. The heaters of the tubes run off pure DC and so no noise can be introduced into the cathode circuits. Timer circuits allow the heaters to get to their operating temperatures before the rail voltages are applied to the plate and cathode circuits. This prevents cathode stripping and allows for a much longer tube life. Muting relays in the shunt mode prevent any turn on/turn off transients from getting into the main amplifiers. Thus the total boot time is 30+ seconds. We use gold plated tube sockets for a corrosion free long life. Tube rolling is not an option on this amplifier as we hand select every tube and install it in the particular position in the circuit. Four tube types will be offered including a Russian military type. Each has slightly different sonic signatures and the final choice of tube will be up to the customer (or leave it to us to select). Cosmetics and features: The main printed circuit card is a four (4) layer design using FR4 (flame retardant) to UL94V-0 standards. Our extensive use of ground planes improves shielding and allows for better RF design. Cosmetically the amplifier will be similar to the others in our line up except that the top shall be made from glass. There will be some other real cool cosmetic “tricks” applied but these shall only be shown just before production commences. The amplifier will be 533mm (21 inches) long with the other dimensions matching those of the other amplifiers. Output power is a modest 100w per channel x 4 at 4 ohms and 200w per channel x 4 at 2 ohm (though we hope that those who purchase this product will shy away from 2 ohm loads). As an added “bonus” we shall also offer an upgrade version for those who prefer to run their drivers at 8 ohms. This will thus be compatible with 8 and 4 ohm drivers and where 2 ohms are not allowed. The name of this amplifier is been kept secret until just before the launch date.
