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Old 3rd May 2011, 12:06 AM   #121
GregH2 is offline GregH2  Australia
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Quote:
Originally Posted by Cyberschelm View Post
hmm, this looks familiar.

If you change the VAS current mirror for a bootstrap cirucit in swordfish latest schematic, you have a spitting image of rod elliots P101 cirucuit.

I guess like minds travel the same path
Haha, yes you are right. Any thoughts on the listening impressions of Rod's amplifier?

Though any simple 3 stage amp with a fet output is going to look similar.

My design was originally pretty much my own, but after looking to Rod's site for inspiration it naturally morphed into his

My original design was more like lineup's, but the more and more I simmed it, the more I realised a BJT VAS was superior for my purposes.

I wanted a JFET input, but now I have decided that a BJT input is going to be easier (I choose to only use parts in current production). Decent BJTs can be had cheap at any electonics store....2SK170s can't.

While asking about the lateral fets I am going to use someone mentioned using a cap to balance the gate capacitances -which I took on board....and also like P101.

So we were left with a BJT LTP, BJT VAS and balanced FET output... no matter how I tweaked it it was going to look like Rod's design.

Naturally after designing mine, I looked to Rod's site as a reality check. Consequently I borrowed some of his resistor values to help.

So yes, you are right. This is basically a fully current sourced P101!

I wonder how it might sound?
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Old 3rd May 2011, 03:00 AM   #122
tvi is offline tvi  Australia
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Quote:
Looks promising though the Idss is still pretty high. Might be workable though.
2SK117 are $0.76 also RS
Don't know which IDSS class they stock
Y: 1.2~3.0 mA, GR: 2.6~6.5 mA, BL: 6~14 mA

There is a spice model about half way down this page
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Old 3rd May 2011, 06:49 AM   #123
mikelm is offline mikelm  England
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Why did we move away from jfet input devices ? any offset problem is so much easier with jfet or mosfet because they are not affected my impedance so we only have to consider voltage . . . AND jfets have the reputations of sounding better that any other options.

What originally look to me like a nice radical simple design now looks much more like a "vanilla" BJT, cap coupled amp.

DC coupling sounds MUCH better !

I much prefer lineup's final design - to me that is what an amp should be like
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Old 3rd May 2011, 06:53 AM   #124
mikelm is offline mikelm  England
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P.S.

Hi lineup

I am now using multisim V10 and I like it . . . haven't tried V11 yet

but I have no idea what to do when the trials expire

what versions are people using here I wonder - did anyone really pay $2500

thx

mike
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Old 3rd May 2011, 06:56 AM   #125
GregH2 is offline GregH2  Australia
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Mikelm,

I prefer the original one too, especially the one with the single ended input stage.

If you can find me a suitable JFET that is easy to get from a major supplier I'll be happy to build it.

EDIT: My Mistake! I see that Mouser has the 2SK170. Problem solved!

Edit 2: Back to the original design. JFETs ordered!

@tvi:

Thanks for that. I will try to contact RS to find out which model it is....hopefully they will be able to help though I fear they may be ignorant with such matters.

Last edited by GregH2; 3rd May 2011 at 07:11 AM.
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Old 3rd May 2011, 08:32 AM   #126
WuYit is offline WuYit  Sweden
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MiiB,
Quote:
why do you insert an extra driver stage..??
A relevant question as the number of stages should be kept to a minimum. In short, the CFP is much better than the plain source follower.
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Old 3rd May 2011, 08:35 AM   #127
AKSA is offline AKSA  Australia
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Lumba,

The CFP is NOT superior to the plain source follower in a global feedback topology unless it's used in full blown Class A.

Reason: radical instability at switching point, difficulty with reactive loads, particularly capacitive, and serious tetchiness for bias setting
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Old 3rd May 2011, 02:00 PM   #128
GregH2 is offline GregH2  Australia
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Default Is this worthwhile

Lineup, all,

Edit: I just noticed that lineup posted a similar LTP config in an earlier post...oops

2SK170s and Laterals are on their way. If we are lucky I might be able to lash up a channel this weekend.

I have been thinking. There are two potential small problems with the single ended input stage:

1) Changing either the load (DC offset) resistor or source resistor will change the current through the JFET meaning the other resistor will need to be changed again - setting the DC offset and current through the input stage will be an iterative process.

2) The JFET might run a little hot.


I thought a way to solve this might be to cascode the input stage with a BC550 BJT (see attached). This means that:

1) The source resistor can be selected out of circuit to give the desired current.

2) Changing the load (DC offset) resistor will not alter the current significantly.

3) Dissipation in JFET is reduced.

4) Because of point 3, more current can be run in input stage.

5) If desired, because of point 4, a larger VAS fet can be used and run at higher current for better performance at high frequencies or paralled ouput stages.

Does this seem like a worthwhile addition?

Cheers,

Greg.
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Last edited by GregH2; 3rd May 2011 at 02:13 PM.
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Old 3rd May 2011, 02:41 PM   #129
lineup is offline lineup  Sweden
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Exactly, swordfish
You setup outside of amplifier a JFET circuit to select source resistor Rsource.
Select current. I recommend 4 or 5 mA depending on your supply voltage.
2SK170 is rated 400mW.
So for a supply of 35V we get 5mA x 32V = 160mW
if the voltage across Rdrain is 3V
JFET works better at somewhat higher current. This is why 5 mA.

In your JFET circuit you adjust Rsource until you get wanted current.

After this you pick Rfeedback for example 20xRsource, for gain x20.

Then try to calculate Rdrain. The resistor from VAS MOSFET gate.
If ZVP3310A maybe VGS is 3 Volt. For IRF9610 maybe 3.8V
3V / 5mA = 600 Ohm
Set Rdrain initially to 600 Ohm.
You adjust Rdrain until zero offset at output.
Can of course be done with a potentiometer.
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Old 3rd May 2011, 04:13 PM   #130
lineup is offline lineup  Sweden
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swordfishy

I setup your amplifier in my simulator.
With the EXICON Laterals.
For some testing later
Was enough with one only 22p capacitor to get it good stable.
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