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Old 16th February 2001, 08:05 AM   #1
diyAudio Member
 
Join Date: Feb 2001
Location: Lublin/Poland
Hi there,
This is my first post.
I have finished putting my Bride of Zen (1st design by NP)
together and I connected it to my (one channel only - not enough heatsinking yet :-( ) Zen. It works fine but now I have a few questions.
Before my BOZ was operational, my CD player was driving Zen
directly. Not all CDs sounded great.
Q1. Now (with BOZ) everything has changed BUT loud passages on some CDs clip somewhere (I don't know whether it is in BOZ or in Zen,
probably the latter). This surprises me since I haven't noticed this without the BOZ, especially that this happens when the volume pot is set for overall level lower than it would have been without the BOZ. I suspect that without the BOZ at loud passages the output of my CD player simply dropped. This is not the case with BOZ. Can anyone confirm this?
Q2. This is a minor one. There is an ambiguity in NP's on-line docs on BOZ. In some sources C102 is 47uF and in the others 100uF. This has to filter some noise. I use 47uF.
What changes if I use 100uF? (I want to know even if I probably would not hear that.)
Q3. I undersatnd that BOZ has more gain than the usual preamp. Can I possible destroy non-ZEN amp driving it
with the BOZ? Well, I tried this already with no ill effects.

Greetings,
Greg
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Old 16th February 2001, 01:20 PM   #2
Freddie is offline Freddie  Sweden
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Join Date: Oct 2000
Location: Sweden
Hmmm...

Sounds like the BoZ clips.
Try to lower the bias current to about 30mA, adjust the pot so that you will get about 30V across the 1000 Ohm resistor at the drain of the MosFet.
Hope this helps! (I've heard other people saying that the BoZ sometimes clips.)

I have a little modified BoZ myself (increased gain, increased bias) running at a supply voltage of 100V.

/Freddie
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Old 16th February 2001, 10:06 PM   #3
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Join Date: Feb 2001
Location: Vancouver, B.C., Canada
The bias value as stated in the BOZ article at the Pass Labs site is incorrect and causes un-symetrical clipping of the Mosfet.

If you have access to an occiliscope you can bias the BOZ for symetrical clipping which on my BOZ is 28mv and which I have been told can be anywhere from 26mv to 30mv.

If you don't have an O-scope you can do the biasing by ear by playing a loud passage and adjusting the bias for the smoothest sound.

The credit for the information in this post goes to a fellow at the VT52 home page (who's name I unfortunatly forget) as he was the one who told me about the biasing problem with the BOZ.

Take Care.


Tony D.
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Old 17th February 2001, 09:29 AM   #4
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Join Date: Feb 2001
Location: Lublin/Poland
Thanks guys,

I have played a little bit with different voltages
at 1kOhm resistor as Freddie suggested.
It helped, but not much.
Then I replaced the 1kOhm resistor with a 500Ohm one
(this reduces gain, as I understand).
This helped a lot.
But playing my CD player with the volume pot at min
position is not possible (I hardly play at 1/2 volume).
With the sources which have lower output level
(like my tuner) it works much better.
Also replacing the volume pot with a lower value helps.
However, NP writes that "higher values [of the pot] are OK",
so I am not sure about this modification.

Greg
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Old 20th February 2001, 12:23 AM   #5
FYC is online now FYC  Hong Kong
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Join Date: Jan 2001
Location: Hong Kong
Resetting the bias current will solve. I have exactly the same problem and solve it by reducing bias to about 28mA. When measured across the 1K ohm, you should see 28V in my case.
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Old 20th February 2001, 07:28 AM   #6
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Join Date: Feb 2001
Location: Lublin/Poland
I rechecked my last tweaking. Must have been something wrong there. Now with the reduced bias it works finally.
The sound is gorgeous.
(and it works with 10kOhm volume pot too)

Thanks,
Greg
PS. I had a member of a small orchestra at my place
2 days ago. She could not believe that you can produce
such sound yourself at home. She liked the sound of all
instruments and said it is as it should be.
We tried piano, chamber orchestras, symphonic, vocal
(male and female), violins, cello, brass,
classical, jazz, pop... you name it.
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Old 27th February 2001, 07:09 PM   #7
awj is offline awj
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Join Date: Feb 2001
Location: US
Hi all,
this is my first post.
I just built zen and boz, and found that boz did clip.
I managed to solve this problem by limiting the input signal. I put 22k resistors at inputs so my boz has voltage divider before mosfet gate (basically a fixed vol control).
Try to experiment with different values as resistor network at input acts as a RC network (with mosfet capacitance and cd player capacitance) and will affect the frequency response.

regards
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Old 1st March 2001, 03:11 AM   #8
FYC is online now FYC  Hong Kong
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Join Date: Jan 2001
Location: Hong Kong
Another option is to move the volume control to the front instead of the end, this will solve the overload problem.

I do experience clipping in BOZ quite offen. Setting the bias to 30mA will solve asymmetric clipping but will not solve overload. Moving the volume will cure this.
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Old 2nd March 2001, 06:52 AM   #9
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Join Date: Feb 2001
Location: Lublin/Poland
You are right,
decreased bias solves problem with majority of CDs,
but there are still some that do clip. I will
attenuate the input signal or reduce the gain.
I did not use any "special" capacitors at the input
and output (of course they are not electrolits),
so I bypassed them with the smallest ones.
I don't know what to think of the result. The sound
is more crisp, but now the preamp lisps. So I think
I will remove them.

Greg
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Old 2nd March 2001, 09:11 AM   #10
FYC is online now FYC  Hong Kong
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Join Date: Jan 2001
Location: Hong Kong
I do not think it is advisable to play around with the gain in BOZ, actually there is not much gain to further reduce. Moving the volume control to the front should address all the clipping issues.
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