Ultrasimple SIT PP amp from BAF , or SIT Beast with a Thousand PSUs

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12B4A

12B4A

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Joined 2012
I wonder if the biasing could be done with batteries in some way since there's little gate leakage current (NP gave 1uA as a number in the talk). It seems most step down converters would drain the batteries to maintain chip function. Could I be missing something here?
 
A

Antoinel

Member
Joined 2011
12B4A said:
I wonder if the biasing could be done with batteries in some way since there's little gate leakage current (NP gave 1uA as a number in the talk). It seems most step down converters would drain the batteries to maintain chip function. Could I be missing something here?
Click to expand...
Let us bias the P-Channel SIT per your suggestion as an example. A 12 V Pb-acid battery is connected in series with the 23 V positive rail. The negative end of the battery is joined to the + 23 V rail. Thus the battery floats atop this rail. A potentiometer [10K] is connected across the battery terminals, and its wiper goes to the gate of the SIT. The battery drain is 1.2 mA. An electrolytic decouple capacitor [[for example 1000 uF] is connected between the positive wiper and the negative terminal of the battery which is also the source port of the SIT. One starts with the wiper at the positive terminal and then lowers its voltage relative to the negative terminal to the target Vgs. If you like, a digital voltmeter reads and confirms to you that Vgs is present before the + 23 V is enabled. This bias Vgs can be left on continuously. Later in time, this Pb-acid will need recharging. One Pb-acid battery and two independent potentiometers-decouple caps will service one positive rail [stereo] for a net and continuous 2.4 mA battery drain.
 
dimkasta

dimkasta

Member
Joined 2010
I re-read the entire thing with clear mind, watched the presentation and ordered two pairs of the VFets :)

About the Vgs applied before the Vds I think the amps simplicity demands nothing more than two plain DPDT power switches :)
One in the front to open Vgs and bias, and one hidden in the back under a cover to hit the Vds after we are psychologically ready to believe that Vgs is now ok :)
 
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dsdjoy

Member
Joined 2009
cumulocity.com
dimkasta said:
I re-read the entire thing with clear mind, watched the presentation and ordered two pairs of the VFets :)

About the Vgs applied before the Vds I think the amps simplicity demands nothing more than two plain DPDT power switches :)
One in the front to open Vgs and bias, and one hidden in the back under a cover to hit the Vds after we are psychologically ready to believe that Vgs is now ok :)
Click to expand...

If the switches allow me to do it in the wrong order, I am sure that one day I will do it in the wrong order.

How about one switch, and a time-delay ( capacitor) to switch on Vds via relais after Vgs ? Is that needed anyway or over-engineered ?

If Vgs is established very quickly in the cricuit - let's say only for a very very short time after Vds - then this shouldn't kill the part ?
 
Nelson Pass

Nelson Pass

The one and only
Joined 2001
Paid Member
You could perhaps have a small pair of supplies from the twin secondaries of
a 1 or 2 watt transformer for the bias, and switch a 30 amp main AC relay
for the high current supply using one of them. This way the bias would have
to be in place for the main supply to come up.

:cool:
 
nicoch58

nicoch58

Member
Joined 2004
we must take care of the bias ....

- in case of main AC down , the bias goes fast then caps on B+ ....this will burn the vfet ?
- better do the bias diode bridge as in guitar amp ie with two diode in series for side ie 8device for one bridge ?

@dimkasta are all on the way.....

suggestion for surplus heatsink like papa ?
 
buzzforb

buzzforb

Member
Joined 2010
Zen Mod said:
separate issue , isn't it ;

however , just to spoil part of Buzz's happiness :

:clown:
Click to expand...

Ill try to toot ZM's and Nelson horn at same time on this one. In the case of a bridged version with a single transformer, it is interesting to see just how well the transformer does the job of correcting for inequality between the two sides. It is as if the filar nature of the transformer winding is acting like a crude form of symmetrical error correction. Not saying it is better, just intersting. Cool thing about 5 or 6 filar version is the fact that although the output is bridged, the input can still be driven SE and i dont know that you lose any on the performance side of things. May not be of use, but a friend(Flocchini) pointed these out to me.
http://www.coilcraft.com/pdf_viewer/showpdf.cfm?f=pdf_store:hexa-path.pdf
 
F

flocchini

Member
Joined 2004
dimkasta said:
BTW can anyone comment on the sound of the amp? :)
Click to expand...

I heard the amp with Nelson's OB speakers at Burning Amp. I believe the speakers were bi-amped with a J2 on the low end. I spent about 45 minutes in the room and I can say that I was immediately impressed with the accuracy and the depth that I heard. I think I could listen to this set-up for a long time w/o fatigue. The music choices were really good. I know now that I am drawn to amps that use transformers.

Best

Bob
 
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