Bride of Zen, Bride of Son of Zen

[AndrewT]

R107 is the input impedance setting resistor.
The sch shows 100k.
What resistor are you talking about?
What schematic are you using?

 

[darrr]

Of course i'm comparing originaly schematic from passdiy with received BOZ . In R107 two resistors are installed in parallel and their value is together 1,5k.Qustion is good idea changing back to 100k like on schematic?

 

[AndrewT]

Can any Member explain why one would want to change the input resistor from 100k to 1k5?

 

[darrr]

other diference: in Q1 there isn't tip29- there is c2073. It's good replacment?

 

[darrr]

my measured values

An externally hosted image should be here but it was not working when we last tested it.

 

[Zen Mod]

fiddle with P102 to get 4V at mosfet source , ref. to gnd

that's sole important setting , everything else is secondary

 

[AndrewT]

Those numbers don't compute (stolen from another Thread)
3.6V across 100r gives 36mA
(59-17.6) = 41.4V across 1k0 gives 41.4mA
5.4mA has gone missing?

 

[darrr]

fiddle with P102 to get 4V at mosfet source , ref. to gnd

that's sole important setting , everything else is secondary

Ok. what about r107?There is 1k5

 

[Zen Mod]

input R , R107?

anything between 68K and 330K

 

[AndrewT]

R107 is isolated from the bias/DC conditions on the FET.
All it does is demand excessive current from the Source.

Have you found an explanation for the missing current?

You need to bias up the FET correctly.

 

[darrr]

I'll check it asap after changing all resistors for better one

 

[darrr]

other diference: in Q1 there isn't tip29- there is c2073. It's good replacment?

And what about this?

 

[Zen Mod]

yup
you can use 2SC2073 instead of TIP29

even pinout is the same

 

[darrr]

R107 is isolated from the bias/DC conditions on the FET.
All it does is demand excessive current from the Source.

Have you found an explanation for the missing current?

You need to bias up the FET correctly.

I didn't change anything just set bias to 4v. now I have 57v before r104 and 13,6v after it.

 

[darrr]

I found one more different even r107 1k5 on each input are in series resistors 6k18.

 

[Zen Mod]

with one-line posts , I'm still not getting what problem is

invest some effort to present entire situation and then we can help

 

[darrr]

Sorry for my englishI got the BoZ that was made ca. 10 years ago. I wanted to exchange resistors for the better and I noticed that there are differences in relation to the original scheme. I try to find out why there are such differences and whether they do not affect the whole preamp. The differences are also in measured voltages.

 

[Zen Mod]

that's easy - just follow schematic and change whatever is not by schematic

resistors are cheap , and this way is easier than thinking about every position

 

[darrr]

Ok,this is how the BoZ scheme looks at the moment. Questions:What happens when I change R107 to 100k? I'm afraid to destroy something.
Are the voltages that I have measured acceptable? R104 is composed of three parallel resistors that are very hot, they can not be touched - it's problem?

 

[AndrewT]

R107 is separated from the input device by the DC blocking capacitor.
It does not load the amplifier's input.
Increasing R107 makes for an easier load on the Source.
It should never have been 1k5. Put it back to 100k.
And add some RF attenuation to the input.
0.1nF NPO, or Polypropylene across the 100k.