Thats the one I'm waiting for.For the tube it would be "grid - plate"
It's somewhat closer to jfet than to bjt... high impedance, high linearity, low gain and - you're right - rather high input capacitance.
I will try bjt version a bit later - prototyping the "clean CFA" now
Although it may look strange, my prototypes with Baxandall + Hawksford (with the tube at the input) feel very good. Soldered the second channel, hooked up OPS - just works. Situation with Terry's build is not so easy, but my two channels don't show any problem. What's going on?
I read your thread with great interest , Val. I encountered a situation
where too little gain (or to much) would "excite" my baxandall.
This could be triggered by lowering the 1st stage CCS current too much,
or by degenerating the input pair.
I also experimented with a standard LIN amp / baxandall. no issue here ,
constant Z !
This led me to suspect the very high impedance of the 2 stage was the cause.
still4given's issue might be what tube he is using. My FET simulations also
showed this amp to be "picky" about the input pair.
WITHOUT the baxandall , I can throw any model I want in for the first input
pair (BJT or FET). I just can't let the gain over-run the margin (like a "normal"
amp).
The Baxandall also is very reactive to different OPS's ... E Stuart's "TIS"
IPS design would not sim well with my OPS until I buffered it with an EF stage.
OS
For the tube it would be "grid - plate"
It's somewhat closer to jfet than to bjt... high impedance, high linearity, low gain and - you're right - rather high input capacitance.
I will try bjt version a bit later - prototyping the "clean CFA" now
My baxandall IPS would not work as well before with Jfet's.
Now (updated) , it sings (in simulation). DGS for jfet is CBE for BJT. (below)
Fet's work nicely with a servo'ed amp , as well.
OS
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The only difference between that amp and the "GLA" is the symmetric 2'nd
stage (instead of a single with level shifter).
Even the collector resistances are the same as the sansui. Led's are new ,
dedicated +/-12V supplies.... the sansui "stole" auxiliary voltages from the tuner/
preamp !! A "real" CCS - the sansui just had a resistor from the -12V supply.
That's why this does 10ppm versus the sansui's .02%.
OS
I knew this one would "fly". Can't wait till you and thimios push some
300khz perfect square-waves out of these.
I posted the "ultra safe-mode" design (200v/us). It can be pushed MUCH further ! For 500v/us+ , you might need to use a ferrite RF toroid at the input lead and lift the input filter cap - BUT , it will do it.
PS - you could use 39K-68k fixed resistors for R28 , you are using "new" red led's (1.8V).
My models are "old" red's (1.63V) , I need R28= 80K to get 5.5ma at VAS out.
OS
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R28/R29 "steal" VAS
current from the Cascodes , lowering those values (trimmer) will
lower the cascode current with more (I) for the LED's.
You WON'T be able to see this on Q13/Q16 emitter resistors. To
calculate the cascode current , subtract the LED forward current
from the current you measure on R24/R31.
Another way I told still4given is to place a 10R between IPS/OPS PD+
and measure VAS current directly.
The full range on R28 should be 10K=3.8ma to 100k 5.8ma. Your CCS's
are running "hotter" 1.8ma .... you should have 4.5 - 6.5ma range.
Edit - the RED LED Vf must be 1.8v (newer red's).
R17 - "rbridge" .... you can reduce this to 5 to 10R and get 300khz
square-waves that look like your 50khz ones !!
If you want to "max" this one out - 10-12pf for C7-8 and 5R for R17.
This will give 4.2mhz CLG and 1000V/uS slew with a little overshoot.
PS - this will "flat out" beat most forum CFA's and most definitely the VFA's.
You can even degenerate the diamond (and bridge) Re's - 27-33R and get
>>1Kv/us (1.2Kv/us is the record).
OS
Ok OS,i will test all these and i will report again.
Thanks Mile ,without close matching devices the measurment offset is about +/-6mV.
Regards.
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Ok i will try to trim current today and i will report again.
All these IPS are very good sounding (very high class) and the comparison is very difficult at this time.
If it is interesting yesterday my wife , listening this KRYPTON-V,said.....what a clear crystal sound!
This is a balanced preamp but is use diamond input, diamond differential and folded cascode
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You don't trust me ?? Terry ?
Back when thimios posted his voltage readings , he has 720mV across
one of the main VAS emitter resistors. This deviated from my reading
by .6mA (attributed to the LED's). He also had 33V across R29-47K.
So , 33/47k = .7ma ... he had 6.5ma at pd+. To get .7ma ,the trimmer = >60k.
With the new red leds , your CCS's will most likely be 1.8ma , as well.
You want at least double the VAS LED current - halve what he had on
the trimmer .... 27-33k for R28 would equal 1.5ma. There's your 5.7ma at
pd+ !
Edit , just simmed with 1.81ma CCS. R28=22K gives 5.6ma at PD+. That's at 75V rails.
OS
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Of course I trust you but you gave a range of 39k-68k. Are you saying anything in that range will give the proper output?
Forgive me , I did not take into account what rails thimios is using. I assume
50V (for him).
You use >70V, right ?
Anything ? , that's what the meter is for
. Put in a resistor ,measure V drop R24 (mA) minus V drop R29 (mA) = output.
Use your last IPS as a gauge. "unbias" your OPS ... then even if this
IPS is >6mA , your OPS should be close to home (biased).
I design these modules to be free of the magic smoke , even at either
extreme of the trimmer , you would neither burn your led's or overbias
your OPS to the "smoke" level. I think of these things when I post the
designs.
PS - you could test this IPS without the OPS ... it's a VFA.
Edit - all this would depend on what particular chinese red LED you are using !
OS
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