Amplifier Stages

Posted on Feb 6, 2014

The XLR or Cannon-type connector is clearly a better audio connector than a 6. 5mm jack plug and socket, but tradition makes the 6. 5mm socket mandatory despite its serious connection problems. Inputs range from a single socket connected directly to the grid of the first stage, no resistors or blocking cap (Marshall PA); and up to six ( Vox AC-30 II), all different. When the input is plugged into

Amplifier Stages
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the upper socket the input has 1M to ground and two 68k in parallel, 34k, in series. In the lower input the input resistance is two 68k in series, 136k, and divides the input in half (-6dB). The cathode bypass capacitor on the lead channel is much lower value, 0. 67 uF, causing more AC negative feedback across the 2k7 at lower frequencies, so producing a `toppy` channel. The outputs are mixed downstream. Ideally the common of the socket should be isolated from the chassis to prevent earth loops. Commonly available isolated sockets are not nearly robust enough for guitar input service. XLR`s are tougher, but they don`t come with switch contacts. Simple amps have only one amplifier channel even when they have a lot of input sockets. Different signals are mixed early in the chain and amplified together. More complex amplifiers provide two or more channels. Normally they were an un-effected channel with its own tone and volume controls; and an effected channel with its own controls including reverb and tremolo/vibrato. These were then mixed into the main power amplifier. There is no reason why channel switching cannot be applied to a valve amp if desired, but it is important to have an `at-a-glance` indicator to show you which channel is selected. The anode and cathode resistors are typically 100k and 1k5 as shown, or 220k and 2k2, always with a 25uF/25V bypass. This was not a great choice since they only have around 2 volts on...

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