Zero Feedback Impedance Amplifiers

[mikelm]

Originally posted by jkeny


I had intended to run off 12V SLA battery anyway (PSRR of design is low or undetermined I think - so quality PS is necessary for quality performance).

I don't know what the power from this setup will be but SI T-amp is loud enough for me even with Rogers LS3/5A ineffecient speakers.

John

actually the PSRR of this cct is excellent - so power supply can be very simple - but battery should also be fine. Actually SLA batteries are not very quiet but with this design it does not really matter ...

don't forget a fuse ! !

mike

[Sheldon]

Originally posted by jkeny


I had intended to run off 12V SLA battery anyway (PSRR of design is low or undetermined I think - so quality PS is necessary for quality performance).

I don't know what the power from this setup will be but SI T-amp is loud enough for me even with Rogers LS3/5A ineffecient speakers.

John

Assuming a minimum output of 20V PP and a 1:1 OPT, that would give you about 7V rms, so about 6 watts into 8 ohms. In the original Zeus, best distortion performance was with the 4:1 OPT, a little less with the 2:1. Don't know if she showed 1:1, but you might ask.

Sheldon

edit: Oops, forgot the negative rail myself. At 1:1 power would be about 25 watts, 6 watts for 2:1.

[mikelm]

Originally posted by Sheldon


In the original Zeus, best distortion performance was with the 4:1 OPT, a little less with the 2:1. Don't know if she showed 1:1, but you might ask.
Sheldon

Yes this is true - but in spice experiments I found if you lower the power supply voltage and increase standing the current using the o/p transformer for reducing distortion seems to be unecessary - the transformer may have other advantages but I think distortion reduction is not one of them in these circumstances.

an o/p choke of course is still absolutely essential

mike

[Sheldon]

Thanks Mikelm and jkeny. This is interesting. I do plan to do a version of this amp when I finish a couple of other projects. Here is an interesting post which bears on the Zout requirements of the driving stage. Your thoughts? http://www.diyaudio.com/forums/showt...010#post999010

Sheldon

[Sheldon]

Originally posted by suiraMB


The LU1014D have an Rds(on) of 0.007 Ohm, so they should fit that criteria very well.

They have a very low gate charge (~10nC), and ~1nF capacitance, which should allow for very good high frequency response and slew rate. In fact, parasitic capacitances in the secondary winding of the input transformer may very well swamp the JFET parasitics.

Finally, with regards to component requirements, they are depletion mode devices. You do not need a positive gate voltage to drive them. Also, being JFETs, they are less sensitive to static discharge and such, so you could simply connect a center tapped input transformer to the gates and the negative rail, for a grand total of 2 transformers and 2 semiconductors.

The simplest way to bias them, is to make sure the transformer primaries have a sufficiently high primary DC impedance; this is just like cathode biasing a valve. You need to develop this negative gate voltage, because they will give you 25 amps at 0Vgs with a 1V supply.

With a transconductance on the order of ~20S at low voltages and a triode-like curve, the potential for low distortion should be significant. You could even use a lower voltage power supply and a step up transformer. At 1:3, you would still have a more than sufficient damping factor.

Hmm, the more I think about this, the more interesting it is. In my previous post, I referenced a discussion about variation of Crss with output and its possible effect on phase related distortion. It looks like these FET's go from about 300pf at 20Vds to 600pf at 0Vds. That's a much lower ratio than the Mosfets I've looked at, especially as Vds approachs zero (and sometimes the steep change starts at 20 or 30Vds). The tube driver I have for this has a swing of 200V and a Zout of about 8K. So I could step than down 4:1 for a 25V swing and a Zout of 500, which would give a corner frequency of 500kHz. Even with a 2:1 step down on the OPT, I'd have 6 watts or so. Should be plenty for my application. If you are right about the distortion suiraMB, it should be no problem even going 1:1. I have little concern about the output impedence as I will be using these for driving only frequencies higher than about 300Hz. The only trick would seem to be designing the OPT to give the desired bias current. That should be possible from wire resistance tables. But, the bias power would have to be dissipated by the transformer. Wouldn't it be easier just to add some series resistance and get rid of the heat there?

Thanks again,
Sheldon

[Sheldon]

Originally posted by Roscoe Primrose
actually, 0.5W into a 105dB/W/m driver ought to play louder than any sane person wants in a reasonable size living room...Peace

According to our forum tag line, the issue of sanity might not be a primary consideration in this sport.




Sheldon

[jkeny]

Hey Sheldon,
I don't think a European group buy is going to happen anytime soon. Any chance you might order an extra 3 matched pairs from GRollins in US when you are ordering your own and posting to me. WIll pay in advance of course.

John

[Sheldon]

If I decide to go this way, and I'm leaning hard just because it seems very cool, I'd have no problem getting some extra's and sending them along. You can help me if I do, by pointing me to the post with the latest ordering and shipping info. I looked at the GB thread and it's a long one to dig through.

Sheldon

[jkeny]

Sheldon,
Thanks, I would be delighted if you could do that.
As far as I know the info on the first post of GB thread is still current prices + shipping info http://www.diyaudio.com/forums/showt...387#post814387

John

[Sheldon]

Originally posted by jkeny
Sheldon,
Thanks, I would be delighted if you could do that.
As far as I know the info on the first post of GB thread is still current prices + shipping info http://www.diyaudio.com/forums/showt...387#post814387

John

I'll let you know if I go this route, and I'll try to decide soon. No big gotcha's so far? And the simple biasing scheme is very attractive. I'm very new to ss device characteristics, but it looks like these FET's have a negative temp. coefficient, so, unlike the mosfet's, no special measures are required to avoid thermal runaway. Am I reading that right? If so, that's another nice feature.

Sheldon