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Old 28th April 2011, 03:23 PM   #1021
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You guys are getting me fired up to build my second BOZ-J. Using Vishay Naked resistors and Mundorf Supreme. This time using a step attenuator. Still mulling over the 16v PS though. Quiet is key! Need better RCAs too!
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Old 29th April 2011, 06:57 PM   #1022
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Quote:
Originally Posted by Fenris View Post
Your 25K inuput resistor is too high. As it stands, your feedback current will overwhelm your input giving you alot less than the 2x gain you're looking for. The current will flow from the feedback through the volume pot since it is lower resistance the the 1M ground reference resistor. This will also make a variable gain circuit, with gain reducing as you attempt to increase volume. I know because I built one (see 12 posts above). I ended up putting a 4.7K resistor to ground instead of 1M, but that affects the input impedance. My recommendation would be to increase the series input resistor and feedback resistor to about 10x or 20x what they are currently.
I tried with more value for the feedback loop and series resistor, but the high frequency rolloff is huge. Probably the global feedback loop isn't a good idea. In the schematic that I added before, the value of the power supply is too low, I was trying other values (Simulation) and the original 30V to 35V looks the right value, by changing the 1N4747 to a 1N4754 and 30VAC transformer.
The change of R10 from 10 Ohm to 20 Ohm or more increases the distortion, like the change of R7 to 1K.

I'm out of ideas for get less gain with the same amount of distortion (Best less).
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Old 2nd May 2011, 11:43 AM   #1023
anilva is offline anilva  India
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Default Question on volume control

I have finished assembling a jBOZ with a 16V power supply. Pretty much the standard schematic with 25K trimpot and 1M resistor at the input of the preamp.

Amp sounds wonderful. I want to be able to control the gain of the preamp. The amp connects to a F5 and therefore I do not want to put a volume pot at the output of jBOZ. I am planning to connect a volume pot at the input of jBOZ.

Should I remove the 25K trimpot and put a volume control in it's place or can I add a volume control in front of the trimpot. If yes, what value should it be.

Thanks.
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Old 2nd May 2011, 12:14 PM   #1024
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Quote:
Originally Posted by anilva View Post
I have finished assembling a jBOZ with a 16V power supply. Pretty much the standard schematic with 25K trimpot and 1M resistor at the input of the preamp.

Amp sounds wonderful. I want to be able to control the gain of the preamp. The amp connects to a F5 and therefore I do not want to put a volume pot at the output of jBOZ. I am planning to connect a volume pot at the input of jBOZ.

Should I remove the 25K trimpot and put a volume control in it's place or can I add a volume control in front of the trimpot. If yes, what value should it be.

Thanks.
Yes, you must remove the trimpots if you uses a pot. 20K or 25K pot is ok. (The trimpots on original schematic are intended for gain control, but if you use a pot instead the trimpots, these trimpots haven't nonsense.)

I did some simulations and looks that the boz works better with a 30 to 35V supply, I'm waiting capacitors of more voltage for try it.

Happy listening!
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Old 3rd May 2011, 02:22 PM   #1025
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The schematic with 33V and without feedback

Click the image to open in full size.
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Old 3rd May 2011, 03:40 PM   #1026
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18V across a k170 is a lot.
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Old 3rd May 2011, 04:01 PM   #1027
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Quote:
Originally Posted by AndrewT View Post
18V across a k170 is a lot.
Thanks Andrew, this is based on papas design Jfet BOZ, which is the maximum voltage that you would use?.
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Old 3rd May 2011, 04:30 PM   #1028
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ramallo, 120mW on a 400mW device is all right...
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Old 3rd May 2011, 05:07 PM   #1029
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read what JC says about gate leakage current at high Vds.
The datasheet graph also gives some idea of how this varies.
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Old 3rd May 2011, 05:56 PM   #1030
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Quote:
Originally Posted by AndrewT View Post
read what JC says about gate leakage current at high Vds.
The datasheet graph also gives some idea of how this varies.
Thanks Andrew, I'm trying to find "gate leakage current at high Vds" but looking the K170's datasheet, 18V and 6,5mA appears not bad (I can easily be mistaken .

I have paid attention to this post of this thread Jfet BOZ


P.S. With my prototype 16.3V looks too low, I get 1% of THD at 1.4V rms
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