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Old 22nd June 2013, 03:09 AM   #4671
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Very excellent. The temperature drift probably relates most to the
low voltage across the cascoded Mosfets, and the failure to be able to
match the coefficients of N and P devices.

What Vds are you running on the devices?

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Old 22nd June 2013, 03:53 AM   #4672
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Nelson Pass View Post
Very excellent. The temperature drift probably relates most to the
low voltage across the cascoded Mosfets, and the failure to be able to
match the coefficients of N and P devices.

What Vds are you running on the devices?

Currently Vds is about 3 volts, but easy to adjust with pots.

I believe that the DC offset problem is due to lack of any DC negative feedback. In the F6, the upper bias circuit couples to the output voltage, providing negative feedback to stabilize the offset voltage. In my F6CC, both bias circuits are referenced to the rails. By removing the cascode FETS and jumpering source-to-drain, I can easily verify whether the stability issue is due to cascoding.

Last edited by lhquam; 22nd June 2013 at 03:56 AM.
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Old 22nd June 2013, 07:50 AM   #4673
Zen Mod is offline Zen Mod  Serbia
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it's begging for rails voltage decrease ; even if cascoded , I believe entire shebang will behave sweeter having more proper voltage across cascoded (active) mosfets

that's at least my experience with Jfets - in yore , when I compared Papa Borbely approach ( "voltage starved" ) vs. Papa approach (put them in sweet spot and cascode)
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Old 23rd June 2013, 01:22 PM   #4674
lhquam is offline lhquam  United States
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I found a 2 resistor "solution" to the output offset issue as shown below. R15 and R16 provide negative feedback to the bias voltages sufficient to stabilize the output offset voltage. Because of the symmetric nature of the feedback, the output offset is largely insensitive to equal changes in the rail voltages due to an AC mains voltage change. The downside is that the idle current changes 28ma per volt change on 120V mains. Thus if the amplifier is biased for 1.3A at 120 volts, the bias will drop to 1.244A at 118V on the mains. I have not found a simple circuit that does any better.
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File Type: jpg F6CC-SAK-v1.0-1st-build-stripped-sch.jpg (224.9 KB, 289 views)
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Old 23rd June 2013, 02:49 PM   #4675
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Will the [feedforward] proposed approach in post# 4381 help? Does diyF5 have the difficulty Teaser F6CC have?

Please show on your schematic the voltage drop across R25, R26 [on drains of front end], and Vgs for Q1 and Q2.

Thanks and best regards
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Old 23rd June 2013, 03:28 PM   #4676
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I received my CineMag transformers yesterday, they are much smaller than I was envisioning - rather jewel-like and precious. There is no doubt that they are of the highest quality.

Now that I can determine the height of the PCB assembly, from the face of the output transistors to the top of the signal transformers I can finalize the layout of the chassis. (It's going to be as narrow as I can comfortably make it.)

Here is a photo for scale -
Click the image to open in full size.
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Old 23rd June 2013, 04:07 PM   #4677
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Quote:
Originally Posted by 6L6 View Post
I received my CineMag transformers yesterday, they are much smaller than I was envisioning - rather jewel-like and precious. There is no doubt that they are of the highest quality.

Now that I can determine the height of the PCB assembly, from the face of the output transistors to the top of the signal transformers I can finalize the layout of the chassis. (It's going to be as narrow as I can comfortably make it.)

Here is a photo for scale -
I expected a 6L6 tube for scale.
Click the image to open in full size.
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Old 23rd June 2013, 04:58 PM   #4678
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Quote:
Originally Posted by lhquam View Post
I found a 2 resistor "solution" to the output offset issue.
And there you have it. Probably the next issue you might want to think about
is the increased capacitance of the gain devices operated at 3 volts.

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Old 23rd June 2013, 07:23 PM   #4679
Zen Mod is offline Zen Mod  Serbia
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as said in #4673 ......... but , as usual , nobody is noticing what I'm writing ......... most probably because I already had fair share of writing nonsense

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Old 23rd June 2013, 07:33 PM   #4680
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Antoinel View Post
Will the [feedforward] proposed approach in post# 4381 help? Does diyF5 have the difficulty Teaser F6CC have?

Please show on your schematic the voltage drop across R25, R26 [on drains of front end], and Vgs for Q1 and Q2.

Thanks and best regards
The F5 does not have the output offset problem because it is entirely DC coupled, thus the negative feedback applies to output offset as well as to AC voltage.

Here is the schematic with some voltages added.
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