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Old 27th January 2003, 07:34 PM   #1
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Default Help on DoZ pre-amp

Hi!

I just finished my DoZ-preamp (ESP P37), and I want to adjust the bias, well, the as for the voltage at the collector of Q2/Q3: It is always somewhere between 32 and 32.7 V, no matter how many turns I set the trimpot to (it is a 22 turn trimpot), while it should be 15V. Turning the pot really does not seem to change anything, even if I wait more than 1 hour to let it adjust (is it correct that pin A of the pot is connected to ground?).
One thing I noticed: I could not get BC559 and BC549, but used C559C and BC549C, since I thought that makes no difference, am I wrong?

The stuffing / soldering seems to be OK, I actually build two modules (four channels), and both show the same characteristics...

I do not want to bother R. Elliot, so I first look here for help...

Any comments would be greatly appreciated.

Ciao,

Arndt
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Old 27th January 2003, 08:07 PM   #2
halojoy is offline halojoy  Sweden
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Arrow DoZ pre-amp

Quote:
Originally posted by Cradle22
Hi!I just finished my DoZ-preamp (ESP P37), and I want to adjust the bias, well, the as for the voltage at the collector of Q2/Q3: It is always somewhere between 32 and 32.7 V, no matter how many turns I set the trimpot to (it is a 22 turn trimpot), while it should be 15V. Turning the pot really does not seem to change anything, even if I wait more than 1 hour to let it adjust (is it correct that pin A of the pot is connected to ground?).
One thing I noticed: I could not get BC559 and BC549, but used C559C and BC549C, since I thought that makes no difference, am I wrong?
Ciao, Arndt
Click the image to open in full size.

ESP Project 37 - Minimalistic Discrete Preamplifier

There are several good tweaks you can do on such
a simple Preamp, as DOZ-pre.
I do not like the very low Current in Q1, the Input transistor. Like 80uA ....
But it can not without some other problems
just be raised by lowering R6 10k.

Lowerering R6 means lowering R5 22k and R4 10k, as R5/R4 set the gain.
Lowering R4 means increasing C3 22uF, they set low freq rolloff.
Increase C3, and it is no longer possible to use Polypropylene, or Filmcaps.
-----------------------
So I would do this, to keep low value on C3 (and avoid Electrolytics).
I would lower R6, so set bias in input transistor to 0.5-2 mA.

To compensate, without changing R4-R5-C3,
I would put a Constant Current Source
from Positive Rail into Emitter of Q1.
I am sure it would give this preamp better prestanda.

/halo - has his own version of DOZ-pre - in his mind
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Old 27th January 2003, 08:39 PM   #3
Netlist is offline Netlist  Belgium
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Default DOZ

Hi, Arndt

Did a quick sim on the schematic and give you the results. In the first pic you see the voltage points and on the second one the current flow. I didn't keep the part numbers from the original schema, but that will be no problem i think. The 100k trim pot is at 42%. At 100% i get 29.85V, at 50% 17.45V and at 0% 2.66V
The BC549C and BC559C can't be the problem i think, they have a higher Hfe basically.

/Hugo Hope this helps...a bit
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Old 27th January 2003, 08:41 PM   #4
Netlist is offline Netlist  Belgium
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Default Currents

Next
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Old 28th January 2003, 06:28 AM   #5
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Default Re: DoZ pre-amp

Hi!

Quote:
Originally posted by halojoy

There are several good tweaks you can do on such
a simple Preamp, as DOZ-pre.
I do not like the very low Current in Q1, the Input transistor. Like 80uA ....
But it can not without some other problems
just be raised by lowering R6 10k.

Lowerering R6 means lowering R5 22k and R4 10k, as R5/R4 set the gain.
Lowering R4 means increasing C3 22uF, they set low freq rolloff.
Increase C3, and it is no longer possible to use Polypropylene, or Filmcaps.
-----------------------
So I would do this, to keep low value on C3 (and avoid Electrolytics).
I would lower R6, so set bias in input transistor to 0.5-2 mA.

To compensate, without changing R4-R5-C3,
I would put a Constant Current Source
from Positive Rail into Emitter of Q1.
I am sure it would give this preamp better prestanda.

/halo - has his own version of DOZ-pre - in his mind
Your own version, would it be something like this:

Click the image to open in full size.
?

Right now I do not want to change so much on the original layout, but I plan a preamp based on this OP discussed in a german forum (it is a kind of a ripoff of Thel's MusicAmp 2000) later on.

Quote:
Originally posted by NetList
Hi, Arndt

Did a quick sim on the schematic and give you the results. In the first pic you see the voltage points and on the second one the current flow. I didn't keep the part numbers from the original schema, but that will be no problem i think. The 100k trim pot is at 42%. At 100% i get 29.85V, at 50% 17.45V and at 0% 2.66V
The BC549C and BC559C can't be the problem i think, they have a higher Hfe basically.

/Hugo Hope this helps...a bit
Well, thank you for simulating the schematic with spice (I just recently learned how to use eagle, Spice I have yet to try...), now I can check voltages across the whole circuit, and not just at the collector...

By the way, is a trimpot very easily destroyed by turning the screw some degrees too far? I noticed pressure increasing while turning the screw, but before I stopped, there was a faint "click" in the pot...

Well, thanks for your replies,

Arndt
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Old 28th January 2003, 06:59 AM   #6
Netlist is offline Netlist  Belgium
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The "click" in the pot is normal. Reminds me of the old televisions where the tuning of the stations where done that way".

"and both show the same characteristics..."

You didn't swap transistor pins? BC549C and BC559C is EBC.

/Hugo still listening to a one channel ZenV4 - and happy
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Old 28th January 2003, 07:44 AM   #7
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Quote:
Originally posted by Netlist
The "click" in the pot is normal. Reminds me of the old televisions where the tuning of the stations where done that way".

"and both show the same characteristics..."

You didn't swap transistor pins? BC549C and BC559C is EBC.

/Hugo still listening to a one channel ZenV4 - and happy
EBC, that means, if viewed from button:
......E
B
......C

or

......C
B
......E

?

Quote:
/Hugo still listening to a one channel ZenV4 - and happy
/Cradle still sitting at work, not doing any work, but designing boards with EAGLE *G*

I'd rather be listening to my "modified 70s Marantz" and "End Millenium"....

Thanks,
Arndt
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Old 28th January 2003, 07:55 AM   #8
Netlist is offline Netlist  Belgium
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Viewed from bottom: 1 = E, 2 = B, 3 = C

/ Hugo
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Old 28th January 2003, 12:24 PM   #9
halojoy is offline halojoy  Sweden
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Smile Cradle22 discrete OP

Quote:
To compensate, without changing R4-R5-C3,
I would put a Constant Current Source
from Positive Rail into Emitter of Q1.
I am sure it would give this preamp better prestanda.

/halo - has his own version of DOZ-pre - in his mind
--------------------------------------------------------------------------------
Cradle22: -Your own version, would it be something like this:
No, what I said was that I have my ideas how to make DOZ pre a little better. Raise the Current in Input transistor
form 80uA to something like 500-2000uA.
--------------------------------------------------

Your OPamp project looks very Good.
I put it in my "music & schematics" website.
Hope you do not mind. If you change it, I will put the new version.
DiscreteOP-Cradle22.gif

--------------------------------------------------
About your problem.
It has to be at the input of Q1.
Trimpot-and the resistors to the B of input transistors.
Hope you have found the problem now.
At least Q2 doesn't work - have any current.
So the output goes high.
---------------------------------------------
If I build an OP-amp, it would be a normal 2-transistor input.
Not like yours, symetrical.
And it would probably not be better than "Cradle22-OP".

/halo
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Old 28th January 2003, 01:17 PM   #10
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Default Hi Cradle22

I've built a few DoZ's and when I've had the problem your describing it was due to Q1 being incorrectly connected as mentioned above.
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