DEF Amp

[vdi_nenna]

 

[Dennis Hui]

I have one ready to ship to management of BAF. He will figure out
how to turn it into BAF financing. Previously it was Ebay.

Hi Mr. Pass,

Any hints on the technical specs of this particular amp?

Enquiring minds want to know...

Thanks,
Dennis

 

[Zen Mod]

there is a thread about DEF amp

so , predecessor to SIT-3 , being called exactly DEFiSIT

you have enough info in SissySIT threads , I gave measurements of my exemplar , and I believe Papa's is pretty much the same

at least I made mine ( rail voltage particulary) by his words and there is no anything else to set , besides output node voltage level

in short - if DEF is on sale , grab it - it's damn rare breed and will stay like that

 

[Dennis Hui]

I was curious if Papa put gain in this one. (I saw the basic schematics on
the first page as a follower, but later posts mentioned a transformer for gain.)

Agree with what you said though.

Dennis

 

[Zen Mod]

funny ZM

I forgot that I posted a thread ..... Most Greedy Boy, of them all... or (there is no) DEFiSIT of Papa's Koans

 

[Dennis Hui]

Got it. Thanks Mighty Zen Mod.

 

[Nelson Pass]

Any hints on the technical specs of this particular amp?

It is similar to the DefiSIT, but is direct coupled and uses the SemiSouth
depletion mode R085 Jfet coupled with the Harris IRFP9240, and Jensen
transformer driven by Toshiba Jfet input buffer.

I am preparing a set of curves to accompany this offering, and will post
them shortly.

 

[Antoinel]

It is similar to the DefiSIT, but is direct coupled and uses the SemiSouth
depletion mode R085 Jfet coupled with the Harris IRFP9240, and Jensen
transformer driven by Toshiba Jfet input buffer.

I am preparing a set of curves to accompany this offering, and will post
them shortly.

Look forward to your post.

 

[Zen Mod]

It is similar to the DefiSIT, but is direct coupled and uses the SemiSouth
depletion mode R085 Jfet coupled with the Harris IRFP9240, and Jensen
transformer driven by Toshiba Jfet input buffer.

I am preparing a set of curves to accompany this offering, and will post
them shortly.

FW, White Van Edition

 

[Nelson Pass]

Here are those curves, taken from a BAF talk a couple years ago.

They were taken from this amp or its brothers.

 

[lhquam]

Nelson: How much matching of the FETs was required to get those results?

 

[Nelson Pass]

The input fets was 1 mA Idss match and the outputs were matched to Vds of
about .2V.

 

[Blues]

Vgs, Mr. Pass?

 

[Skorpio]

It is similar to the DefiSIT, but is direct coupled and uses the SemiSouth
depletion mode R085 Jfet coupled with the Harris IRFP9240, and Jensen
transformer driven by Toshiba Jfet input buffer.

Would R100 be usable instead of R085?

 

[Dennis Hui]

SJEP120R100 is an enhancement mode device; for the DEF you want a
depletion mode one. (such as the SJDP120R085)

 

[claudio52]

Here are those curves, taken from a BAF talk a couple years ago.

They were taken from this amp or its brothers.

Very beautiful.
How long for a stable offset?

 

[Zen Mod]

short enough that Pa is declaring as thing worth selling

 

[claudio52]

With a mono amplifier I got a stable offset after 40-50 minutes. I used the best pair chosen between three UJN1208K and twelve IRFP9240.

 

[Nelson Pass]

Vgs, Mr. Pass?

Nice thing about the DEF is that it is only the difference in Vgs that is
critical. That Vgs would be taken at expected operating current and
voltage for perfection. That Vgs difference will alter the bias current
as divided by the sum of the inverse transconductance of the devices.

For example, if the depletion mode device Vgs is 1A at -4V, and has a
transconductance of 2 and the enhancement mode device is +3.9V with
transconductance of 3. The the increase is bias would be seen as

increase I = .1V/(1/2 + 1/3) = .12 amps

If the depletion mode Vgs is greater than the enhancement mode Vgs,
then your bias is high, which is better than otherwise, as you can insert
a little resistance so as to bring it down.

I got some of my pairs down to .1V or so, and the extra bias doesn't
bother me.

 

[Blues]

Nice thing about the DEF is that it is only the ...

Thanks for the detailed explanation!