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Old 21st November 2012, 12:50 AM   #3001
The Dastardly Dad of Three
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Quote:
Originally Posted by Antoinel View Post
I proposed in a past post of this thread the attached schematic in the event additional voltage gain was needed in diyF6. I already bought the PCBs from the DIYAudio store. I hope the schematic is useful for DIYers. One pending question hung: Where's one inject overall loop feedback in it?
Brgds.
why not just like the f6 unbuffered - ie R2 resistor between tx primary and gnd and R1 from o/p (F6 Unbuffered schematic) ? The Ba-3 front end doesn't look like it needs much in the way of feedback.... actually come to think of it you have followers in the op stage so you might be able to escape overall fb altogether imo.
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Old 21st November 2012, 03:42 AM   #3002
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Quote:
Originally Posted by Tea-Bag View Post
Here is my P2P F6. Not exactly totally safe construction.

I biased at what I think if 1.5A.

Measuring V across resistors in standard Pass supply.
1.5 x .(4x.47R) in power supply = .176

No big sound evaluation yet, just one mono CHR-70 running no baffle.

I have ordered The Cree SIC Mosfets, and will modify this to use them adding more LED's.
Because I used a choke filter to take advantage of my 30-0-30 transformers I just lifted the lead at Q1's drain and inserted the DMM in line. Worked well with the PTP.

Looking forward to experimenting with the harmonics.

This is a fun amp! and it sounds great too

Best

Bob
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Old 21st November 2012, 06:06 AM   #3003
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by Nelson Pass View Post
Especially complimentary followers.

.... not me , Bubba .

Mighty ZM hate any sort of authority

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Old 21st November 2012, 01:27 PM   #3004
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Quote:
Originally Posted by kasey197 View Post
why not just like the f6 unbuffered - ie R2 resistor between tx primary and gnd and R1 from o/p (F6 Unbuffered schematic) ? The Ba-3 front end doesn't look like it needs much in the way of feedback.... actually come to think of it you have followers in the op stage so you might be able to escape overall fb altogether imo.
Feedback in certain Pass FET circuits [ACA, F5 and F6] is like Schade Feedback in tube circuits with possible and welcome good side effects. Any modification of F6 must [imo] have the following prerequisites:
  • An output stage [e.g. with MOSFETs and degen] like that with R100A. Either case is a current source amp prior to loop feedback with intrinsic pentode-like output characteristics. Complementary symmetry is doable but will not cut it because it is intrinsically a voltage source amp.
  • Overall loop feedback to a summing [voltage substracting] junction like the novel one in F6. This is the special Pass Feedback. It carries the high probability of reorganizing the intrinsic pentode-like characteristics [open loop] to triode-like characteristics in the resultant F6 voltage source amp [Pass Feedback].
Clearly, Pass F6 and its diy clones are great sounding amplifiers whether my humble postulates above flush or not.
Brgds
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Old 21st November 2012, 01:54 PM   #3005
Tea-Bag is offline Tea-Bag  United States
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Default More Cree stuff, I can't give it up!!

Again with the Cree stuff. CMF10120D
Quote:
Originally Posted by Tea-Bag View Post
To do this, I would guess that the LED's would need to be a string of 5 or so, to allow for its 20V+ gate charge. We are assuming it also has less distortion than IRF240 and similiar sound to R100. Won't know until you try.
So I know it will turn on fully at lower VGS, however, in looking to tell if the tempco allows one to run without source degeneration, I can across this at the end of the speadsheet. Any comments? I am beginning to think an IRF240 might be better.


There are two key characteristics that need to be kept in mind when applying the
SiC DMOSFETs; modest transconductance and no turn-off tail. The modest transconductance requires that VGS
needs to be 20V to optimize performance. This can
be seen the Output and Transfer Characteristics shown in Figures 1-3. The modest
transconductance also affects the transition where the device behaves as a voltage
controlled resistance to where it behaves as a voltage controlled current source
as a function of VDS
. The result is that the transition occurs over higher values of
VDS
than is usually experienced with Si MOSFETs and IGBTs. This might affect the
operation anti-desaturation circuits, especially if the circuit takes advantage of the
device entering the constant current region at low values of forward voltage.
The modest transconductance needs to be carefully considered in the design of the
gate drive circuit. The first obvious requirement is that the gate driver be capable
of a 22V (or higher) swing. The recommended on state VGS
is +20V and the recommended off state VGS
is between -2V to -5V. Please carefully note that although
the gate voltage swing is higher than typical silicon MOSFETs and IGBTs, the total gate charge of the SiC DMOSFET is considerably lower. In fact, the product of
gate voltage swing and gate charge for the SiC DMOSFET is lower than comparable
silicon devices. The gate voltage must have a fast dV/dt to achieve fast switching
times which indicates that a very low impedance driver is necessary. Lastly, the
fidelity of the gate drive pulse must be carefully controlled. The nominal threshold
voltage is 2.3V and the device is not fully on (dVDS
/dt ≈ 0) until the VGS
is above
16V. This is a noticeably wider range than what is typically experienced with silicon MOSFETs and IGBTs. The net result of this is that the SiC DMOSFET has a
somewhat lower ‘noise margin’.
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Old 21st November 2012, 02:33 PM   #3006
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Tea-Bag View Post
Again with the Cree stuff. CMF10120D
...
Looking at the CMF10120D datasheet, figure 3 suggests an undesirable temperature coefficient for use without degeneration and/or thermistors.

I haven't attempted to estimate the Beta (or Kappa) parameter from fig 1 and fig 2, but the transconductance looks to be approximately 1 at 1.5 amps Id. That would mean an F6 open-loop gain of only 16 with an 8 ohm load.

To me the CMF10120D doesn't look appropriate for the F6.
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Old 21st November 2012, 04:17 PM   #3007
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Quote:
Originally Posted by lhquam View Post
Looking at the CMF10120D datasheet, figure 3 suggests an undesirable temperature coefficient for use without degeneration and/or thermistors.

I haven't attempted to estimate the Beta (or Kappa) parameter from fig 1 and fig 2, but the transconductance looks to be approximately 1 at 1.5 amps Id. That would mean an F6 open-loop gain of only 16 with an 8 ohm load.

To me thedoesn't look appropriate for the F6.
lhquam: kasey97 suggested in an earlier thread to use a step-up transformer to generate added open loop gain. Will it and/or a BA-3 gain front end help the solution with an IRFP240 output stage? Where and how will Mr. Pass find an open loop gain of 38 dB for the mod F6 like in original F6?
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Old 21st November 2012, 06:00 PM   #3008
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Has anybody heard the Cree parts?
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Old 21st November 2012, 07:06 PM   #3009
labjr is offline labjr  United States
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Quote:
Originally Posted by buzzforb View Post
Has anybody heard the Cree parts?
I heard they sound Cree-py!

Seriously, I can't imagine Nelson hasn't auditioned everything currently available.
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Old 22nd November 2012, 12:15 AM   #3010
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The attached schematic diyACAF6.pdf maybe a temporary solution for diyF6 using IRFP240s in the output stage:
  • Use ACA for the output stage. This amp is highly acclaimed.
  • Connect the secondary windings of the Jensen in series. The net winding is the input to 2SK170. This may double the open loop gain. Or, one may triple the open loop gain by connecting one of the two primary windings in series with the series-connected secondary windings; clearly if this electrically proper as I am not skilled in transformers. Note that I used the novel Pass Feedback summing junction twice; in the primary and secondary coils.
Happy thanksgiving to y'all. Kind regards.
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