Hello F6 Community & Nelson Pass,
Would appreciate your guidance on whether or not this amp would successfully drive / be a good match for my Martin Logan Ethos hybrid electrostatic speakers. The bass driver of these speakers has it's own in-built dedicated amp so the F6 would power the panels only.
I ask because the impedance curve is funky with a low 1 ohm dip albeit at 18kHz - see attached.
92dB / 2.82v /m sensitivity
nominal impedance 4 ohms
Thank you
F6 built and running! Drives the above no sweat. Amazing nuanced and smooth sound yet also toe tapping snappy. Wow.
Build is stock other than IRFP150s and 6v Zeners / 4k7 feeding them.
Many thanks to 6L6, 2pD and Zen Mod for your guidance. And to DIYaudio store for the excellent PCBs.
Last but not least Nelson Pass - thank you so much for so generously sharing your designs and knowledge. Legend.
I remember that Gerd tried some of Lundahls in M2
can't fathom which type exactly , but it wasn't pure repeater ......... anyway , by memory - it was better than Edcor ( must be , considering price difference) but wasn't better than ancient Iskra (telephone installation equipment origin)
in short - it's hard to get anything from specs - just try it
can't fathom which type exactly , but it wasn't pure repeater ......... anyway , by memory - it was better than Edcor ( must be , considering price difference) but wasn't better than ancient Iskra (telephone installation equipment origin)
in short - it's hard to get anything from specs - just try it
Attachments
And now for something completely different... years ago, I built an F6 with variable 2nd harmonic using SJEP120's and Tea-bag's convertible PCBs+cinemag transformers. I am now building a DAC/preamp that has a low output impedance (33ohm) through a transformer, so there won't be any DC out. I should be able to feed that directly into the F6 cinemag transformers, right? This avoids the SJ74/SK170 buffer. If I do that, is there any real need for the R9 or R10 resistors? What kinds of instabilities may arise if I skip R9/R10? Thanks
I'm thinking about dusting off my F6 and experimenting some more and maybe solving an issue I had with it:
Changing the voltage feedback into current feedback as per the schematic in this post worked perfectly. (If I remember correctly I got comparable gain with an 1 ohm resistor, don't forget to add the parallel RC for stability though if you try yourself.
The big problem, however, that got me to put the F6 topology on the shelf is that the current feedback worked too well... It corrected the R1/R2 imbalance and so instead of getting mostly 2nd harmonic and then rising 3rd from 10W and upwards I got a pure 3rd harmonic with no 2nd already at 1W. (I tried to double the resistance of R1, same result.)
So the question, does anyone have a simple idea how how I could re-introduce an imbalance that should be current-feedback-proof?
Changing the voltage feedback into current feedback as per the schematic in this post worked perfectly. (If I remember correctly I got comparable gain with an 1 ohm resistor, don't forget to add the parallel RC for stability though if you try yourself.
The big problem, however, that got me to put the F6 topology on the shelf is that the current feedback worked too well... It corrected the R1/R2 imbalance and so instead of getting mostly 2nd harmonic and then rising 3rd from 10W and upwards I got a pure 3rd harmonic with no 2nd already at 1W. (I tried to double the resistance of R1, same result.)
So the question, does anyone have a simple idea how how I could re-introduce an imbalance that should be current-feedback-proof?
you can use trick with unequal source resistors,, or in case of equal valued source resistors - you can introduce ZAP - Zen Amount Pot
trick solely owned by my Bruder Generg
if you can't remember or you can't find referencing posts, just say
that's practically 50R trimpot across source resistor, where gate reference point is connected to wiper, thus enabling variable amount of local feedback; thingie placed at upper mosfet, one hanging on positive rail
trick solely owned by my Bruder Generg
if you can't remember or you can't find referencing posts, just say
that's practically 50R trimpot across source resistor, where gate reference point is connected to wiper, thus enabling variable amount of local feedback; thingie placed at upper mosfet, one hanging on positive rail
Yes, but fiddling with the source resistors didn't change the h2 h3 balance, when the amp was converted to use current feedback instead of voltage feedback.
The zen amount pot is just variable source resistor if I've understood correctly. I got pure h3 even with 1.2 ohm source resistor on top and 0.47 on the bottom.
