Most Greedy Boy, of them all... or (there is no) DEFiSIT of Papa's Koans

[Zen Mod]

que?

 

[indra1]

In post #432 you mentioned an issue with the hall chip in combination with 3N type opto would need some sort of gain stage to maintain Iq stability vs supply and temp.

Recently the Wizard disclosed that he calls a different opto for bias duty in the XA25. The best part is that by his infinite wisdom, Srajan managed to publish a very interesting photo of the secret part, just enough to show that it is an 8 pin dip complete with a perfect blur on the part number nobody can read.

I suspect the 8 pin dip to be a 6N type with very high CTR that would very likely overcome the issue of #432. Another piece of crumb towards Halley Babelfish Pucks.

 

[indra1]

Since I could not find the spice model of the part, I use external npn for verification of the idea. Please check to see if in your opinion this is good enough for further effort.

 

[Zen Mod]

if you say it works , no reason to doubt it

however - main interest with any improved Hall based thingie - finding either better or supplemented Opto - is with IXYS pucks

IRFPs are well tamed with sissy optos , beasts are the problem

 

[indra1]

with IXYS pucks ...

With the pucks, @ ~2A Iq, sim shows 0.1A Iq drift with 11-35V supply and 150mv offset drift with 3V supply variation.

 

[Zen Mod]

brilliant , as usual

 

[Zen Mod]

waidaminit!

rummaging through zillion iterations of Babelfish XA25 ....... dummy ZM tried that approach some moons ago (in sim only) and I found already developed solution for pucks (last schm (again - sim only!) I posted in berserker F4 thread) with CMirrors -superior in multiple ways - Iq , offset stability in temp and rails change domain , same as bias stability after long burst

so , time will tell ....... we are young , and we'll try them all in vivo

 

[indra1]

Yepp. The Pucks are really interesting, perhaps even better with SEF.

 

[Zen Mod]

too bad that driving them through buffer-xformer combo is not looking promissing

 

[indra1]

Just as the XA25 is Pass Labs product, we want to think BIG when driving the pucks. A couple of KP903A as buffers to a 2W 32:600 xformer could be the minimum.

 

[indra1]

Bias stability vs supply is a bit reduced to gain better stability vs signal and lower distortion by adding emitter resistor and C3 to bootstrap the base.

 

[lhquam]

too bad that driving them through buffer-xformer combo is not looking promissing

I also considered that approach, but there are fundamental problems. If the pucks are operated common-drain (source followers), then the transformer secondaries must produce the same large voltage swing as the puck output, which is too much for any of the line transformers. That leaves us with operating the pucks in common-source mode and fighting the puck gate capacitances, particularly Ciss. We really want to operate the pucks as followers.

 

[Zen Mod]

if one can drive M2 and SIT-3 with xformer , no problem with swing

 

[lhquam]

Yes. I forgot that the auto-transformer has a swing per winding that is total-swing/N-windings. That means for 20 Watts and a transformer with 4 windings each winding sees 5V peak.

The Jensen JT-123-FLPCH datasheet http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-123-flpch.pdf, shows distortion vs. level for a test circuit with 2 windings in series as the primary and 2 windings in series for the secondary. The maximum output level is +20dBu = 11V peak, or 5.5V peak per winding. The 20Hz distortion is 1%, but decreases to below .01% above 70Hz.

 

[Zen Mod]

there are some measurements of DEFiSIT and SissySIT and Babelfish M25 around , so you can compare all of them with known figures of M2

B. M25 made with Edcor , rest of them with Cinemags

 

[woody]

What is the value of C2 and I see in the M2 amp a small 680pf cap and a 10k resistor is used across the edcor transformer secondary why is this not needed in the DEFISIT ?

 

[Zen Mod]

it is specific RC cell for Edcor 600:15K , to tame transfer characteristic ( linearisation)

DEFiSIT is made with Cinemag, not needing RC

 

[Zen Mod]

some house-cleaning

superduper nice small xformers of Yore, which I put in my little Jewel, they are working, but their contribution to overall heat of amp simply becomes too high

they were not buzzing, but they're simply too hot for my liking

who knew - supposedly 120VA jobbies, but evidently purely intended to feed B Class amp (being exactly pulled from pair of these), and juicing 60W for DEFiSIT channel is simply too much

just a remainder of little Cuties .......

 

[Zen Mod]

So, I've sent e-mail to my guys at Trafomatic and ordered pair of new ones

these - 150VA, single 24Vac secondary, decreased flux, static shield, magnetic shield, black finishing tape

addition of small NTC board, so each having own mains NTC

they're practically cold per se