BOZ bias adjustment help

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I just got a boz and want to set up the bias as it has been in storage for a couple of years.

The pass lab diy article says to set up for 40ma by adjusting P102 for 4v across R108.

I have read that in order to prevent clipping with a digital source the bias needs to be lower, around 28-32ma and should be set up with a scope.

I don't have a scope so I was wondering if someone could tell me what the voltage across R108 would be to achieve around 30ma bias...
Thanks
 
The one and only
Joined 2001
Paid Member
It would be 3V (0.03A * 100 = 3).

Actually the more probable issue is input overload, and higher bias
raises the input overload figure.

If overload is an issue, then I suggest some input attenuation. Many
have found that using both input and output pots is useful. A 10K
pot can be adjusted so that the maximum volume that you will actually
use is achieved when the output pot is at max, and thereafter the
output pots becomes your volume control.
 
The one and only
Joined 2001
Paid Member
You can do that, but I will reiterate: The usual problem is input
overload, which is not fixed by adjusting the gain per se.

A higher value that 100 ohms will increase the input overload
voltage, and so could be helpful with the same bias current.
 
A bit OT....

I was looking into ways to improve my BOZ. Actually, it won't be a Boz at all. Instead, I was thinging to add a current source and feedback, transforming it into an Aleph L. However, since I'd like to keep the volume control at the input, I was thinging of adding the B1 buffer in front of BoZ/Aleph L. Not too complicated, and should sound better than a plain BoZ. Or not...?
 
Vix said:
A bit OT....

... Aleph L.


Me too.
ALEPH L is sure one of the best pre I have ever heard.
And remains my fave ... If you look at PassDiy-gallery : among the PL-clone, it seems the L didn't retain so much attention from the DIYer - which I don't understand.

With some high-level sources input overload can be an issue ... in few case I had to provide some attenuation at the input (res in serie ... not tried pot). But generally, when its not expressly needed, I prefer whitout input attenuation as I feel it (somewhat) degrades the gorgiousness of the sound ...

I don't know why.

Manu
 
Manu said:


But generally, when its not expressly needed, I prefer whitout input attenuation as I feel it (somewhat) degrades the gorgiousness of the sound ...

I don't know why.

Manu

Yup. There are some arguments that the opposite should be the case. With the lower input, distortion of the preamp should be lower.
On the other hand, with the attenuation at the output, the noise is lower. Which brings us to a conclusion that we may be more sensitive to circuit noise than to a bit more distortion... :scratch:


Vix
 
It looks to me that you should simply build the Boz as in original schematics, adjust for 40 mA bias, and use some input attenuation, if overload is an issue. You can use 47 K pot first, and then, after you have found the appropriate setting, measure the resistance and replace the pot with fixed resistors. Then, use the pot at the output as a volume control.

Vix
 
to get a ratio you need two resistances.

1r0 and 100r gives -40dB
similarly 5k0 and 500k gives -40dB.
or any value in between can be arranged to work as a -40dB attenuator.

The pot/attenuator is a variable PAIR of resistances that usually give attenuation ratio between 0db an -80dB.
 
rtate said:
How do I determine a value for input attenuation?

I have read post stating anywhere from 47k to100k to 470k
The different values must make a difference....

Your ear will determine what is best for you.
As VIX said, start with 47K ...
and if your layout allows it, try different values ...
My experience is that too much input attenuation makes sound less dynamic...

How should, I say thats not math, thats life...

Manu
;)
 
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