Gainchooser

A simple preamp with input selector

Part 3: Buffer

Last modified: 20-Aug-2004

Power

First thing to arrange when modifying a passive preamp into an active one: Add power. Fortunately I already had a power supply for Gainpre phonoamp around that I could use. So initially I used that one without modification.

Of-course I added more caps to the board:

• The power rails were bypassed with 4 * 2200uF (Elna)
• Close to each opamp 2 * 100uF (BC) and
• Next to pin 4 and 7 of the OPA627 two caps: 4.7uF and 0.1uF ceramic (stacked)

Buffer

I want to keep the design of the active part of the amp stupid and simple. After all it should bear the name of a gainclone. I plan to use one opamp per channel since I do need minimal gain (in fact I think I'll start with a plain buffer design (gain=1).

Let me first explain a little bit about the design itself. As you can see in the figure above the amp has unity gain which means that the amplification factor is 1. This is because it is an inverted design where the output is directly fed back to the negative input of the opamp. But the gain is not only determined by the opamp alone, the ladder attenuator on the left will make sure that we are able to control the volume in 12 steps.

The 1 M resistor to ground is meant to give the positive input a ground reference at all times, even when the attenuator switch should fail.

I also decided to make the amp DC coupled since I do not expect major offsets in output voltage. After all, less is more, and I can always add aditional components although I cannot see how these should add something to the soundscape. Whould you want to protect your power amp for DC, it is possible to use a 1uF capacitor in series with the resistor of 10 Ohms. Do not forget to give the output a new ground reference by connecting the output with a 470K resistor to ground.

The result

This is how the amp looks after putting the small board in place. Apart from the 4 caps in the center of the board, the layout of the board is pure symmetrical. The 4 caps protect the power rails and are 2200uF/16V each (two, V+ and V- for each channel).

I did also decouple the power pins of the opamps on the pins itself with another 2 small capacitors as descibed above.

It's remarkable so little components are needed to make a good-sounding amplifier.

Well, that is to say, since we're building an amp with unity gain it does not really amplify at all.

The small print board slides in the enclosure, where two small rails in the side panels are designed to hold standard size boards. The plate on which the switches are mounted is an L profile that also slides in these rails, and therefore I do not need to drill holes in the bottom plate of the amp, everything is kept in place by the two rods that connect the switches.

© Maarten@Platenspeler.com, 2004, 2006