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rtate 8th July 2008 10:21 PM

BOZ bias adjustment help
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...

Nelson Pass 9th July 2008 01:23 AM

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.

rtate 9th July 2008 04:52 AM

Thank you for the response!!

So should I leave the bias at 40ma and use input attenuation only

Rodeodave 9th July 2008 10:08 AM

A lower value for R104, say 820R (original value 1k 3W), reduces the gain, right?

traw 9th July 2008 01:40 PM

or is it more preferrable to adjust the 100R?..... the 1k || output load / 100 (source resistor) = voltage gain.

Nelson Pass 9th July 2008 07:37 PM

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.

Vix 10th July 2008 08:43 AM

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...?

Manu 10th July 2008 09:34 AM


Originally posted by Vix
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.


Vix 10th July 2008 10:46 AM


Originally posted by Manu

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.


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:


rtate 11th July 2008 03:46 AM

So, is Mr. Pass suggesting to change the value of R106 or is it R108?

What sort a value would be appropriate?

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