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Old 20th March 2018, 11:39 PM   #1451
Zen Mod is online now Zen Mod  Serbia
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I'm extremely successful in not guessing right what Papa meant with something .

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Old 21st March 2018, 12:23 AM   #1452
woofertester is offline woofertester  United States
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Quote:
Originally Posted by lhquam View Post
I modified one channel of my spare ZD25 FE boards for 1R0/6 Ohm sense resistors and modified a opto bias circuit. The results were as follows:
  1. OS bias temperature sensitivity: 3mA/C
  2. FE bias temperature sensitivity: 0.15mA/C
  3. OS bias rail voltage sensitivity: -35mA/C
The rail voltage sensitivity appears to be quite high, but if you consider the power dissipation Watts(fet) = Vrail*Ibias, -35mA/C might be close to maintaining constant dissipation/FET with changes in AC line voltage.
That is a beautiful PCB. Or is it fugly in this forum?

Congrats.
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Old 21st March 2018, 12:43 AM   #1453
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most lhquam's builds are Fugly!
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Old 21st March 2018, 12:49 AM   #1454
lhquam is online now lhquam  United States
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That PCB was designed as a "Swiss Army Knife" for experiments, as well as (mostly) conforming the DIYAudio UMS (Universal Mounting Spec). It has lots of connectors for resistor options, such as changing the global feedback network, cascode local feedback, FE load resistor, ... If the circuit design was totally frozen the PCB could be made much simpler and probably mounted directly to the hocky pucks.
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Old 21st March 2018, 01:12 AM   #1455
woofertester is offline woofertester  United States
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Quote:
Originally Posted by lhquam View Post
That PCB was designed as a "Swiss Army Knife" for experiments, as well as (mostly) conforming the DIYAudio UMS (Universal Mounting Spec). It has lots of connectors for resistor options, such as changing the global feedback network, cascode local feedback, FE load resistor, ... If the circuit design was totally frozen the PCB could be made much simpler and probably mounted directly to the hocky pucks.
The swiss army configuration is part of what appeals to me. I am just getting started prototyping an F4 beast and appreciate the utility of being able to swap components and test different ideas.

Other than the swiss army aspect, its an awesome looking board. Its looks like a board you might find in a FW or PL product.
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Old 21st March 2018, 01:44 AM   #1456
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Originally Posted by Zen Mod View Post
most lhquam's builds are Fugly!
Agreed. Seriously fugly.
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Old 21st March 2018, 08:53 AM   #1457
Ian Macmillan is offline Ian Macmillan
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Quote:
Originally Posted by lhquam View Post
Speculation: Perhaps Papa's change of the senst resistors from 0R47 to 1R0/6 was mainly to achieve the negative dI(fet)/dV(rail) character in improve performance for varying AC line voltages.
An alternative speculation is that 0.16 ohm gives 0.64v voltage drop at 2 x Ibias (assuming Ibias = 2A) which matches the voltage drop of the forward biased Zener pretty well. This helps in maintaining a constant bias with peak signals that push the OS into class B, i.e. where one device switches off.

BTW, congrats on completing your build. Mine is progressing rather more slowly.
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Old 21st March 2018, 11:53 AM   #1458
Zen Mod is online now Zen Mod  Serbia
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besides finishing (at last) last iteration of buffer for Iron , I have 4 other amps builds to confirm

then BXA25

life is good

still waiting for damn matching SOT23 socket .... it arrives form China , stuck in Customs procedures
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Old 21st March 2018, 05:54 PM   #1459
lhquam is online now lhquam  United States
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There have been some comments about the Zobel network shown in the 6Moons photos. I do not know what kind of speaker loads (or lack thereof) were causing problems without the Zobel. I found this posting by Bob Cordell which is quite informative Zobel Or No Zobel ?, as well as other postings in the thread.
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Old 24th March 2018, 12:40 AM   #1460
lhquam is online now lhquam  United States
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Default Next Challenge: Good Performance at (sanely) Low Feedback Levels:

My next goal is to modify my (spare) ZD25 PCBs for good performance at global feedback levels of around 12 dB, rather than 40+ dB of my current build. In simulations for the circuit topology below, I have found some very good parameter sets:
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File Type: jpg absrtact-ZD25V2-2.asc.jpg (76.3 KB, 239 views)
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