F6 Amplifier

[Zen Mod]

shoot

[buzzforb]

All ears!

[bobodioulasso]

Originally Posted by lhquam

The sweet spot?

On your schematic L3 has one end open circuit.

[lhquam]

Here it goes...

In post #88 Papa says: "One of the charms about the transformer vs JLH/PLH phase splitter is that it delivers pure symmetry in the same way a circlotron does - each like polarity part is subjected to the same conditions, although in opposite phase..."

In order to achieve "pure symmetry" the effects of the parasitic currents due to the transformer windings to have must have a symmetric effect on the two output FETs.

There are numerous formulations for the optimal modulation voltage for the "spare" winding: an approximation that assumes the open-loop gain (OLG) is very high and more exact formulas that takes OLG into account. I present here the simpler approximation.

Assumptions:

• The interwinding capacitance is C and is the same between any 2 windings.
• Winding 1: The driven primary winding has an AC voltage of Vin.
• Winding 2: The "spare" primary winding has AC voltage X.
• Winding 3: The upper secondary winding has an AC voltage of Vout=Gain*Vin.
• Winding 4: The lower secondary winding has an AC voltage of 0.

The AC current I[j,k] between winding j and winding k is 2*PI*f*C*(Vj-Vk), where Vj and Vk are the AC voltages of windings j and k.

Thus the AC current into the upper winding 3 is the sum of the currents from other other 3 windings which is Itotal[3] = I[3,1]+I[3,2]+I[3,4]

The AC current into the lower winding 4 is Itotal[4] = I[4,1]+I[4,2]+I[4,3]

The AC currents have a symmetric effect on the output FETs when Itotal[4] = -Itotal[3]

0 = Itotal[3] + Itotal[4] = I[3,1]+I[3,2]+I[3,4] + I[4,1]+I[4,2]+I[4,3]
= I[3,1]+I[3,2] + I[4,1]+I[4,2]
= 2*PI*f*C*(V[3]-V[1] + V[3]-X + V[4]-V[1] + V[4]-X)
= 2*PI*f*C*(2*Gain*Vin-2*Vin - 2*X + 2*0)
= 4*PI*f*C*(Gain*Vin-Vin-X)

X = (Gain-1)*Vin

[Zen Mod]

have no objection

there is still one dilemma which you probably need to solve - is it that , evidently measurable progress , beneficiary to sound ?

what I grasp till now from Pa's writings , it's desirable to have K2 above K3 (and greater), but their ratio is still Golden one ......

considering that I believe in his ear/brain combo more than to mine ( if nothing else , mileage is greater ) , I also believe that we all are waiting to see his choice regarding that ....

great work ,lhquam

[lhquam]

I am not sure that the ratio of H2 to H3 can be held constant across frequency with H2>H3. It is certainly possible to hold THD nearly constant ans shown in post #2237.

Originally Posted by Zen Mod

have no objection
...
what I grasp till now from Pa's writings , it's desirable to have K2 above K3 (and greater), but their ratio is still Golden one ......
...
great work ,lhquam

[bobodioulasso]

Ihquam,
i can follow your calc but do not understand your statements 3 and 4

Winding 3: The upper secondary winding has an AC voltage of Vout=Gain*Vin.
Winding 4: The lower secondary winding has an AC voltage of 0.

Having four identical windings, i thought each were showing same AC voltage.
What "Gain" exactly define?

[lhquam]

I am referring to AC voltage between the winding and ground (or other windings) not the AC voltage across the winding.

By Gain, I am referring to the closed-loop gain of the amplifier. Actually, the ratio of Vout to the voltage into the top of the driven primary winding.

Originally Posted by bobodioulasso

Ihquam,
i can follow your calc but do not understand your statements 3 and 4

Winding 3: The upper secondary winding has an AC voltage of Vout=Gain*Vin.
Winding 4: The lower secondary winding has an AC voltage of 0.

Having four identical windings, i thought each were showing same AC voltage.
What "Gain" exactly define?

[lhquam]

I have been listening to the flat 0.1% THD version with Gain=5, Rs=0R12.
I am not happy with it.

Not having 4 instrumented pots I have not done serious listening at THD levels other that 0.1%. I find that I cannot maintain a flat THD vs. Frequency spectrum without adjusting both P3 and P4, the upper and lower Zen pots.

Since Papa (and others) have been suggesting THD (particularly H2) up to 0.1%, I will shift away from high open-loop gain and feedback to more degeneration and possibly higher gain to reduce feedback. My next experiment will be with 0R47 source resistors.

Originally Posted by buzzforb

Which do you like best?

[lhquam]

After switching back to my F5 balanced with minimal THD, I am less happy with it now. High frequencies are too too sharp.

By first F5 build had badly mismatched FETs. But there was a satisfying sweetness to its sound that I have not been able to replicate. At that time I did not have equipment to measure the harmonics, so I do not know what the harmonic mix might have been. So far, my tweaking of the Teaser-6 does not come anywhere near to that sound.