Remote controlled line preamp

By 15 Dicembre 2014new, Progetti, Projects

Remote controlled line preamp

AUTOCOSTRUZIONE DIY
Project
10 inputs -remote controlled line preamp.
Project design
Atto E. Rinaldo
Specifications
Line inputs:
10 (8 remote ctrl, 2 manual)
output:
3 Pre-out < 1 Kohm 1 Head set >/= 600 ohm
Rec out
1 (switchable)
Distorsion
Not measurable
Gain
6 db ( 2 x )
Features
inputs and Volume, remotely controlled

This article has been published on Jan 2009 issue of audioXpress magazine www.audioxpress.com
These days with all the possible source of sounds and music, a pre with
5 -6 inputs cannot handle them all. My Yamaha C-80 pre, had 5 line inputs and I badly needed more.
As a first approach I was considering to built a passive pre and select all inputs via rotary switch. A simple, cost effective solution.
Further considerations lead me to believe that the best choice was to place a buffer between sources and the final amplifier. As I got to that point I felt I needed to add also a remote-controlled circuit to select all inputs and to adjust the volume control.
On the May 2005 issue of Audioxpress I found answer to my first point: an all FET Super Buffer and related power supply kits, by Erno Borbely (www.borbelyaudio.com), while from other sources I have found an infrared transmitter/receiver Velleman kit.
The all FET Super Buffer is a simple circuit to build (Photo 1) with excellent specifications and great performance such as:

Specifications
Gain (choice of)
0 or 6 db (2x)
Freq response
> 1Mhz (with unity gain)
Input impedence
1MOhm
Output impedence
< 50 Ohm
Min load impedence
1KOhm
Distortion (6db gain)
unmeasurable ( 0,00XX)
Power req,ts
+/- 24 Volts 25 mA.

The Erno Borbely regulated power supply kit was even easier to build and required a single adjustments to set the correct output voltage. Both circuit worked fine at the first attempt.
The Velleman IR transmitter/receiver (Photo 2 and Photo 5), also came in kit.
Their assembly was very neat and did not require any adjustments. With all that basic functional units on hand, I needed to draw a schematic to put them together. The final circuit is shown in fig 01 and does not exhibit any difficulty to be read.
The concept I have included, is the possibility to select all inputs via relays which could be activated either manually (rotary switch) or from a remote-control.
I could not find any ready made inputs switches so I designed my own including the printed circuit (joy and pain for a hobbyist like me).
The actual version can be seen in fig 02 and photo 4. It has the advantage to be very compact, to minimize the wire lengths from the RCA input connectors to the actual relays switching point and finally to be very close to the volume control and super buffer inputs points. (photo 7)
Each input selections relay performs a double function: to connect selected inputs to the bus, when energized, or short circuit to ground unused inputs via 1 Ohm resistors when in a normal position;
this arrangement prevents stray capacitance to pick up signals from unselected input and mix them with the active one.
Due to the limitation of the Velleman receiver, only 8 inputs ( relays Ry 3 thru 10) can be remotely controlled while other two (relays Ry 1 and 2) can be activated manually via rotary switch.
Each relay, when energized, sends the stereo signal to a bus which feeds the super buffer through two, high quality, 2 Mf Infinity capacitors and a motorized stereo Alps volume control.
The latter can be adjusted via remote control by means of channel 1 or 2 outputs; these outputs energize momentarily either Ry 13 or Ry 14 to provide a CW or CCW rotation. I have built a small handmade wired board to provide this simple function.
Super buffer output feeds three RCA connectors, via Ry 12 n/o contacts, which can be hooked up to an equal number of final amplifier if required; this thanks to its very low output impedence of the Super Buffer.
Ry 12 connect Super Buffer output to the head phone when related switch is “on”; outputs to power amp RCA connectors are disconnected and an high impedence (> 600 ohm) headphone can be used instead.
Similarly, the rec out RCA connectors become live when the “rec out” sw is activated.
Velleman IR receiver, to function, requires +12 volts which are supplied by a “nuova elettronica” modified kit. This power supply provides also a + 5 volt to drive the volume control motor.
When an IR receiver output is activated (say to select tuner, “Tun”input via Ry3 ) the level at pin 8 goes down and Ry3 is energized.
Similarly when manual sw is moved to Tun position (as an example) a ground potential is applied at pin 8 to energize Ry3 to perform required function.
The major task was the physical location of all sub-assemblies in a 4 cm (a bit more than 1” and a half) height chassis. This, in fact required an extensive work to provide cut-away to properly fit all parts in the supporting panel. I recommend to use a higher profile chassis.
Once all subassembly where properly fitted, I begun the wiring with the help of a sketch prepared under fig 3.
Photo 7 and photo 8 highlight the complete work.
As I powered it up everythings was working properly.
I have now 10 inputs to play with and definitely resolved my inputs availability problem. The Super Buffer performs very well.
I have hooked up my Suisse Revox CD directly to the Power Amp and compared its sound when fed through the Super Buffer. Apart for the 6 db gain (2 x) I could not detect any difference; Super buffer does nothing to the sound, it is not there, … and this, to me, is a clear sign that its performance is at the top.

Tests
with other sources, however, did not manifest the same behaviour; the presence of Super Buffer made a significant improvement to the sound which appeared more natural, smooth and dynamic. Probably the optimization of various input/output impedances performed by the Super Buffers, is what makes the difference.

A. E. Rinaldo
Tambre – Italy

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