Sapphire 300B monoblock

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Our goal with the Sapphire was to design an amplifier around what is probably the most known and popular tube for audio use, namely the 300B. We have also chosen among all that is presently manufactured what we think is the best sounding 300B in the marketplace irrespective of price. That tube is the Full Music TJ mesh plate premium Gold 300B. We also were impressed with the reputation this tube has for reliability. This amplifier also uses another rugged 5687 NOS triode for the voltage & driver gain stages that complements and allows the 300B to perform at its best.

Using a wide bandwidth custom Hammond output transformer has allowed this design to overcome the short comings that a lot of other 300B amplifiers are noted for. You will be surprised to know that the bass is quick, fast and deep, while the highs are smoothly extended to 40 kHz.

Separate high voltage DC power supplies isolate the voltage/driver stages from the output stage, which in a single ended amplifier is a critical attribute.

Our self-biasing stages, while completely eliminating all adjustments, insure long term stability and reliable operation. As in our previous Onyx design, we have implemented what we call a quasi-parallel circuit design where each of the two output triodes have their own independent automatic biasing circuit. Only the plates are tied together to drive the output transformer. Although this doubles the amount of parts in the biasing circuitry that normally would be employed, it results in a purity of sound that one would normally associate when using only a single triode, such as is implemented in our Topaz design.

We also use filtered DC on the filament of the input / driver tube to keep hum and noise at a minimum. The 300B output filaments each have their own regulated DC 5 volt supply that negates the use of hum minimizing adjustments that inferior amplifiers use.

Further refinement comes with a gold plated brass holder with 3M rubber feet inserts to accentuates the styling and to allow proper air flow for the vents under the chassis

We use a delayed DC power sequencing that maximizes tube life by preventing cathode emission deterioration. Low frequency response has been greatly enhanced by using very large signal capacitors and wide bandwidth custom output transformers. These combined attributes reduce overall distortion at higher power levels and give an overall impression of a much more powerful amplifier than what the specifications would suggest.

Unlike solid state amplifiers, that when clipped sound extremely harsh, this amplifier can be pushed beyond it's rating with little noticeable effect due to the tendency to compress when overdriven, which is much more benign than what normally occurs with solid state equipment.

Output Load Impedance Selector Switches

These switches allow selection between 2 and 16 ohm speakers. Using a lower setting than the rating of the speakers increases the damping factor which may increase the performance depending on the speaker being used. In general the best overall setting is usually what the speaker is rated for. The 12 Ohm position can be used to drive speakers rated at 12 Ohms and above. To set the proper impedance you need to insure both switches are in their proper position. Each switch setting shows two possible impedance's. The numbers that match from both switch positions tell you what impedance is selected. Thus four selections are possible, which are 2,4,8 and 12 Ohms.

Input Gain Selection: Low = 20dB, Normal = 25dB, High = 30dB
ABSOLUTE PHASE: None Inverting

Inverted phase can be selected by the Factory or an authorized technician. (This is handy if you own up front equipment that inverts the signal to the amp. By inverting the amp you thereby place the amplifier chain back into absolute phase)

For any noise or other erratic behavior that occurs in only one channel, swap all tubes between the two channels to see if problem moves to the other channel.

Burn-in Procedure for New Output Tubes

If you receive these tubes from Wyetech Labs, they will have already been tested and burned in. Otherwise, plug them in and play immediately. You may notice a subtle improvement over time, but nothing very dramatic, since these tubes do not need an extended burn-in period. If you have received these from an outside source then you should check the bias voltage using a voltmeter. The reading should be within 10% of 78 Volts DC.

Design Goals of the Audio Circuits

We opted to keep the same circuit topology as that found in our more expensive Topaz amplifier, which is plate loaded single ended in all stages. For the signal and driver we use readily available NOS [New Old Stock] triodes that were made in the USA and have met JAN (Joint Army Navy) military specifications. We chose the Full Music premium TJ gold mesh plate 300B's for their superior sound and reliability.

We use a custom Hammond output transformer designed to our specifications which resulted in a wide bandwidth with exceptionally deep low frequency response.

Design Goals of the Power Supply

Single ended amps are more susceptible to power supply aberrations than push-pull amps. For this reason we have split the front end from the output by using completely separate power supplies. This eliminates power supply feedback non-linearity. To keep cost moderate and still uphold sound quality we opted for R-C filtering as opposed to the more expensive L-C used in the Topaz. (R = resistor, C = capacitor, L = inductor) We also use electrolytic capacitors with much larger storage capacity to make up for the loss in filtering when using resistors in lieu of inductors. The radiated heat is distributed inside the chassis, such that, it insures that the chassis runs only warm to the touch. The total power intake and heat dissipation is only 185 watts per channel, for an amplifier operating in pure class A1.

Parts Quality

We use precision quality parts throughout. Our Printed Circuit Boards have a manually designed layout using computers to meet the needs of analog design which are radically different than digital or computer circuit design. We needed large signal and power traces and special footprints for parts like the tube sockets. This has resulted in short signal paths and more precise positioning of components on the board. We use wire wound silicone and ceramic high power resistors and computer grade electrolytic capacitors in our power supplies.

All wiring to the printed circuit boards is done with the use of screw down connectors. This is done for ease of repair should ever the need arise. We use a single OFC co-ax cable for the signal input wiring and Teflon coated silver plated OFC wire for the speaker connection and selector impedance switch wiring.

Warranty

Wyetech Labs warrants to repair or replace any part which proves to be defective through normal use, except external finishes and vacuum tubes ( 1 year ), for a period of 5 years from the date of purchase.

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