F6 Amplifier

[Antoinel]

DIYers visiting Jensen and Lundahl websites will find schematics of their transformers driven by operational amplifiers [OAs] . The attached schematic is a "new" one to add to their collection. I will not be surprised if this was done exactly somewhere else. A year ago, I needed to simultaneously attenuate, and ground-loop isolate the generous output signal of a compact disc player. I used a stepdown transformer [to attenuate] by Xicon [published response of 300 Hz -3.5Khz]. Two OAs [a dual OPA 2134] drove its primary out of a phase. The loop feedback of the OAs gave an output [at the secondary] frequency response which was flat [30 Hz -10 KHz; as seen on the scope]. So what?

• A certain DIYer may wish to use the general topology of the attached schematic in an F6 application; despite the clear simplicity of the front end of F6.
• Summing junction of OA2 [or its non-inverting port] may be the loop feedback point from the power output stage; speculation on my part.
• A DIYer may wish to lower the power of F6 to make a headphone amp. I have 2 working prototypes [of the attached] which are great headphone [GRADO SR-80i] amps. The lows are rich and the highs are detailed. A testament to the high quality of the topology; despite the limited frequency response of the Xicon transformer.

[lhquam]

I found a very useful book chapter on audio transformers, by Bill Whitlock of Jensen Transformers. http://www.jensen-transformers.com/a...%20Chapter.pdf

[Antoinel]

Originally Posted by lhquam

I found a very useful book chapter on audio transformers, by Bill Whitlock of Jensen Transformers. http://www.jensen-transformers.com/a...%20Chapter.pdf

Thanks for the info. Last time I read it, I paid particular attention to "reflected impedance". Mr. Whitlock is a bright and knowledgeable scientist. He has also written other application notes; for example Jensen AN 003. It is entitled "Interconnection of balanced and unbalanced equipment". In it he showed schematics of Jensen transformers driven by operational amplifiers.

[lhquam]

Originally Posted by Nelson Pass

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.

It is possible to do this with a phase splitter ala JLH/PLH, but if you look
carefully you will see that the bottom output device is run Common-Source
mode and the top device is Common-Drain. It is a rare circuit that will
deliver real symmetry under these conditions.

The Jenson JT-123-FLPCH has inter-winding capacitances of 18nF. When these are taken into account, "pure symmetry" is lost because the secondary for the upper FET is moving with the output signal, vs the secondary of the lower FET is at a nearly constant voltage. The result is that the capacitive coupling to the gates introduces significant 2nd harmonic.

Now I understand why 0.1% THD at 1 watt might be expected, unless there are some tricks to be discovered.

[buzzforb]

Does the lack of symmetry come from the fet phase splitter in the PLH or the load that is presented to it by the two different output fets. Sy's impasse article states that a phase splitter is symmetrical if presented with a symmetrical load. I wonder if it is possible that the use of the transformer has manipulated the output into presenting the transformer with a symmetrical load. Here is a reference for the possiblity of the idea.

6C33C-B OTL Amplifier - Background and OTL Circuits

Nelson says its a classic, but i have a feeling that his "classic" is going to have a clever spin on it. taking something old and making it "new". No student is greater than his teacher.

[Zen Mod]

Originally Posted by buzzforb

....... No student is greater than his teacher.

every good teacher is streaming exactly to opposite , even if just rare ones will better him .

anyway - study of magnetic apparatus's is good thing , but do not expect that numbers will tell most important part of the story ......... which is also hard to measure

feel of continuum is probably most important nominator of said quality .

[Zen Mod]

regarding symmetry - where I'm not loosing my sleep about that issue too much - off course that symmetry will be easier to maintain in F5-like circ but , as with all things in life-one must choose his compromises ;

it's relatively easy to see how many -properly made- sub-blocks are substituted with just one xformer ........

of course that amp will measure better with all that electronic sub-blocks shebang ; but - I have strong feeling that that gizmo will be lacking in one important area of sound - immediacy , comparing with xformer solution

[nicoch58]

Originally Posted by Zen Mod

anyway - study of magnetic apparatus's is good thing , but do not expect that numbers will tell most important part of the story ......... which is also hard to measure

nice word ZM!

PS
we need 3diapason....

PS2
DiyAudio not sure that we need this emoticon :

[buzzforb]

Originally Posted by Zen Mod

every good teacher is streaming exactly to opposite , even if just rare ones will better him .

anyway - study of magnetic apparatus's is good thing , but do not expect that numbers will tell most important part of the story ......... which is also hard to measure

feel of continuum is probably most important nominator of said quality .

[Antoinel]

Originally Posted by lhquam

The Jenson JT-123-FLPCH has inter-winding capacitances of 18nF. When these are taken into account, "pure symmetry" is lost because the secondary for the upper FET is moving with the output signal, vs the secondary of the lower FET is at a nearly constant voltage. The result is that the capacitive coupling to the gates introduces significant 2nd harmonic.

Now I understand why 0.1% THD at 1 watt might be expected, unless there are some tricks to be discovered.

What is the open loop gain of the proposed F6? Some of it needs to be sacrificed in order to lower intrinsic distortion and linearize the operation of the device.