studer line driver

I finally managed to box the Studer linedriver board I salvaged from the A727. I also build the step attenuator.

I have a bunch of laptop power supplies and 18V seemed like a suitable voltage to power the board. I did not want to go through the trouble of building a dedicated power supply. Looking at the schematic I decided it should work fine with a resistor divider (and a lower voltage than +12 and -15).

This worked but the listening experience was horrible. The output of such a power supply is so dirty that at some point the studer board even started to produce 2.7Mhz. Bad luck.
I looked for another power supply and found a 12V electronic one with clean output in my junk box. Fortunately this worked flawlessly.

Time for the step attenuator. I opted for the 100K version. The board has 27K to ground at the input, obviously far too low for the attenuator. To solve this I removed the calibration trimmers and used the solderholes to connect the attenuator.

To get rid of the clicking I added R3 and C3. From a noise perspective such a high value resistor is far from ideal but in reality it is not an issue. Anyway adding 3 opamps in the signal path is not ideal either. Everything has its price.

Everything fits nicely in a metal box (no plastic boxes for me).

The attenuator works really well, 12 steps is indeed sufficient for normal use. I measured the accuracy, the step is not so important but I matched the resistors as close as I could.

attenuation in dB

Ch 1	Ch 2
3,36	3,36
6,68	6,68
10	10
13,22	13,21
17,26	17,27
21,78	21,8
26,15	26,16
31,2	31,2
37,91	37,93
46,65	46,66

Values measured with the Fluke 45.

I also measured the frequency response, after all we're dealing with trannies...

frequency response in 10dB attenuation position

freq	unloaded	10K load
10	-0,12	-0,14
20	-0,01	-0,01
35	0,02	0
63	0,03	0,01
125	0,03	0,01
250	0,02	0
500	0,01	0
1000	0,01	0
2000	0	0
4000	0,01	-0,03
8000	-0,04	-0,04
16000	-0,05	-0,05
20000	-0,05	-0,04
32000	-0,03	0,05

Stunning, probably pretty much at the limits of the measurement setup (I used the Hameg 8030 function generator for this)

Now I only need to find a suitable vintage looking knob...

update

I measured a run of the mill stereo potmeter. Values in dB

3,28	3,03
6,45	5,98
10,99	10,02
19,78	20
30,32	30
41,11	40,08