BOSOZ--variable impedance volume attenuator

[jh6you]

Hi, everybody.

Since I have a chance to get Elma 4-section rotary switch of 11 steps (4 x 11), I am going to make the balanced output volume attenuator for my SOBOZ. By the way, all always say that output impedance of the preamp is preferably to be low. Therefore, I am thinking to make a volume attenuator of variable impedance--i.e. the volume higher, the impedance lower. The sketch is shown in the attatchment. Is this worth trying?

Your advice or opinion would much appreciate.

[wuffwaff]

Hi,

this way your output impedance will always be below 10 Ohms. At higher volumes your Bosoz will see a load of less than 100 ohms though. This will reduce gain quite a lot.

I don´t think this will work.

11 steps is also not really much (some people even find that 24 steps are not enough)

William

[jh6you]

Thanks, William. I like your pessimistic view.

I have actually used the 11-step output volume for long year without inconvenience. Daily, I usually touch the volume knob within 3-5 steps. Meanwhile, if I want to shift up or down the global output level, I could do it by adjusting the input level or adjusting amp gain level. This however seldom happens.

By the way, I want to scratch my head further before I abandon my thinking. Could you give me a bit more specific reason why it will reduce gain quite a lot?

[wuffwaff]

Hi,

just look at the original Balanced Line Stage article by Nelson Pass. Somewhere near the end it is mentioned that loading the amp with 600 Ohms or so won´t worsen the distortion but will reduce the gain.

Loading it with 10 ohms will reduce this even further.

William (realist, not pessimist )

[jh6you]

Yes, I clearly read: "Lower values of output potentiometers will reduce the gain of the circuit, but not otherwise degrade the quality."

And, if I open my eyes big towards Aleph P1.7 service manual, it sepcifies the output impedance as 0-800ohms. What does this explain?

I'm still scratching my head, now with both hands...

[stefanobilliani]

Originally posted by jh6you

And, if I open my eyes big towards Aleph P1.7 service manual, it sepcifies the output impedance as 0-800ohms. What does this explain?

It explains that you can't go up over 800ohm with the output impedance, since it is setted by the volume control circuit.

[wuffwaff]

Jh6you,

if the gain is reduced at higher volume settings you won`t get higher volume at a higher setting! This may not reduce the quality but it won´t be of much use either.

If a volume pot is not buffered the output "sees" the pot and the output impedance changes with the pot setting. Depending on the output impedance of the gain stage your output impedance will go from 0 (pot turned to the left) to a max value (somewhere in the middle) and finally to the out put impedance of the gain stage at the max. setting.

William

[jh6you]

William

I admit that I have not thought about your point. I will study more.

By the way, I have thought about the potentiometer (voltage divider) having almost constantly low R-thevenin at the output node, to make the potentiometer as a kind of "stiff" voltage source. Of course, the V-thevenin at the input node will be variable.

[jh6you]

Hi, William

I am thinking to have one additional register at the input node of the potentiometer, e.g., selectively inbetween 30-220ohms.

My understanding regarding the amp gain reduction:

The original SOBOZ has the current source tails formed by registers. In this case, the value of the output potentiometer will affect the amp gain. In contrast, mine has the CCS tails formed by FETs so that the size of potentiometer could be at far low value. Do you think that now the blind is facing to the right direction?

[wuffwaff]

Hi,

don´t know. I thought you had a normal BOSOZ. Could you show us a schematic?

William