 |
|
What makes The Calla 60w. amplifier special?
There are many reasons: The circuit, the choice of tubes and the components.
The circuit: (in a nutshell)
The Naked Truth Amplifier
combines two old and time tested ideas, the Williamson front end
and a circlotron. The Williamson is one of the
simplest ways to combine a voltage amplifier with a phase
inverter. Being simple it uses very few components in the signal
path and is therefor quieter and has less deleterious effect of
the signal.
In balanced input mode no phase inverter is needed but a second voltage amp is need so the phase inverter is turned into the second voltage amplifier.
The circlotron is a circuit originally developed in the ‘50’s by Mr. Alpha Wiggins and the Electro-Voice Co. It is a novel design that has all the advantages of a single ended amplifier and is push pull and in this case class A. Below is a drawing of the basic concept of a circlotron.
 |
If you look at one half figs 2 and 3 you have a single ended amplifier. The problem with this is you have current constantly flowing in one direction. If you hook this up to a speaker the current (dc) will pull the speaker to one side of its throw, this is not good.
 |
 |
By combining two single ended amplifiers together you can balance them so that the current no longer flows through the speaker or the matching transformer. This is very adventagous. It gives you the ability to run class A triode push pull transformerless.
 |
Why a triode? Triodes are inherently linear devices. They do not require feedback to correct their faults as beam power tubes do. Feedback in an amplifier makes it sound thin and lacking in dynamics. Not needing feedback is a wonderful thing. Using power triodes like the 6AS7 in parallel sets and driving off the cathode brings the output impedance down. There are two triodes per envelope in the 6AS7 or 16 triodes in the amplifier. This brings the output impedance down low enough that it is possible to drive a speakerdirectly.
The Tubes
The output tube is a 6AS7. It is a tube that was used for years commercially. It is rated at 13w. per triode, two triodes per envelope. Each triode is individually fused at the cathode so that an internal overload will blow the fuse. Most triodes and this one in particular are linear devices, that is; whatever you put in at the grid will come out the same but bigger at the cathode. These tubes were made in very large quantities and are still available as New Old Stock, as 6AS7'S,6AS7G'S,6AS7GA'S and 6080's, or new from Svetlana. They are a reasonably priced tube.
The 6SN7 is the original tube of the famed Williamson circuit. It is a low noise dual triode. It is well known for its favorable sonic character and reliability. These tubes were also made to be used commercially by the truckloads and are still available N. O. S. They are also a reasonably priced tube. There are so many variations of this tube it would be tiresome to list them.
The components.
At Naked Truth Audio we have used very high quality components. The transformers are all toroids. The coupling capacitors are all polypropylene with many of them metal foil mulitcaps®. All of the electrolytic decoupling capacitors are side capped with small polypropylene capacitors. All the transformers are oversized. All the signal path resisters are metal film. The balance and bias potentiometers are high quality multi-turn for reliability and ease of operation. The bias/balance meter is a panel mount LCD meter again for ease of operation. All connectors are gold plated for long life. The wire is silver coated low oxygen copper with Teflon insulated. The chassis is of a heavy gauge steel, welded and painted, for long life and beauty. Steel is also the best shield from emf interference.
Naked Truth Audio
P.O. Bx. 1353
Ojai CA 93024
805.646.8383
FAX 805.646.9683