L-6500 Preamplifier

Like all Allnic Audio products, it uses Permalloy (Iron and nickel alloy) for its transformer cores. Allnic is grateful to Mr. G.W. Elmen of Western Electric for inventing Permalloy for transformer core use, and in so doing, providing an enormous service to recorded music listeners everywhere.

The L-6500 has the following features:

• Single gain stage – A single gain stage circuit uses only one tube for amplification (“gain”). This reproduces a purer, more detailed, more spacious and more dynamic sound than multi gain stages.
• Line output transformer coupling – The L‐6500 is “transformer coupled”. In tube amp circuitry, there are two coupling methods; one is capacitor coupling and the other is transformer coupling. Capacitor coupling is the traditional, low cost method. It is somewhat stable but transfers only voltage, not wattage (i.e., not real energy). With transformer coupling, about 90% of real wattage is transferred (there is still a transformer loss of about10% of wattage – voltage is not affected). Transformer coupling is superior to capacitor coupling. However, all the advantages of transformer coupling depend on the quality of the transformer and on the choice of tubes. Allnic Audio manufactures its own transformers and uses what it views to be the best core material, Permalloy. Allnic Audio’s unprecedented, wide (16Hz ‐ 75kHz, ‐3db), low distortional, and ultra‐flexible (up to 50kHz square wave response) output transformer helps Allnic Audio to realize the ideal transformer coupled preamplifier. Of course, gain tubes are also carefully selected for three critical factors for function with the Permalloy core output transformers: high gm, low internal resistance and high mu.
• Constant and low output impedance – One of the benefits of transformer coupling is that it facilitates constant low output impedance. Low output impedance is critical to the design of a good preamplifier. The L‐6500 has a 150 ohm (150Ω) constant output impedance at all frequencies. In capacitor coupling (C‐R coupling), a “cathode follower circuit” is usually used to lower output impedance. Unfortunately, this method of lowering output impedance is accompanied by high distortion and has an “L” shaped output impedance curve. Please compare the two graphs below, especially for low frequency response.
• Advanced tube technology voltage regulation ‐ For quieter and more dynamic operation, the L‐6500 has an ultra high speed automatic voltage regulation circuit, utilizing vacuum tubes. This also protects the amplifying tubes from change in the AC line supply and copes with any abrupt, internal current demand.