B1 Buffer Preamp

[Zen Mod]

Do you have any idea what I might be doing wrong?
Or is this a common problem with simulators?

common problems with simulators .............. less and less brain-time .....
and less gadgets really made

 

[Salas]

Do you have any idea what I might be doing wrong?
Or is this a common problem with simulators?

Jfet model not that efficient maybe.

 

[Melon Head]

I used the jfet model for lsk170 from Linear Systems

 

[Salas]

I have this one. Changing the Beta, Idss changes. Try to see what happens (if it is not the same already).

.model J2sk170 NJF(Beta=59.86m Rs=4.151 Rd=4.151 Betatce=-.5 Lambda=1.923m
+ Vto=-.5024 Vtotc=-2.5m Cgd=20p M=.3805 Pb=.4746 Fc=.5
+ Cgs=25.48p Isr=84.77p Nr=2 Is=8.477p N=1 Xti=3 Alpha=10u Vk=100
+ Kf=111.3E-18 Af=1)

 

[Melon Head]

I just tried your model, but still the same result (ie mostly 3rd Harmonic with no 2nd Harmonic)
If you have LTspice, I could send you the circuit to have a look at.
I am not quite sure if I have earthed everything correctly.

 

[homemodder]

Hi tanh, I havent simulated this circuit but I for one think it would display more h3 than second, playing around with values should make h2 dissapear. I dont think the guys that get h2 to be above have the exact same circuit as in the pass paper, and if I remember correctly NP himself said it would be predominantly h3.

 

[Melon Head]

Well I hope so.
I am learning heaps at the moment using LTspice - changing values or components looking at the effects, determining dc offset, bias currents, input impedance, and also looking at the distortion profile.
It has also been a source of motivation to try tweeking things where as before, I would be totally relying on ohers experience or guessing.

Anyway, I hope as you say it is predominantly H3, or all confidence I have in simulators will be lost.
Unless I can find the source of this discrepancy.

 

[Salas]

I just run a B1 FFT measurement again. Second order is higher than third. Mine has +/- 10V supply and no coupling capacitors. This card has 0.007% residual THD and 100dB noise floor in the last octave.

 

[Nelson Pass]

With a 23-25 V supply and 1 Khz at one volt into 10 Kohm, I routinely
see .0007% with BL parts.

 

[Salas]

I will see near that when I will get my new EMU 24bit I guess. You must be using AP2? The main question is do you still see more second than third harmonic in your FFT like I do? I used 0dBV input, volume fully clockwise, about 3k card input Z.

 

[Nelson Pass]

I use the AP1 - the later products are too fancy for me, but we have an
AP2 on one of our benches here.

 

[Melon Head]

Hi Nelson
It would be nice if you could let us know if the B1 as per the the original design is predominantly 2nd or 3rd order harmonic distortion.

 

[Nelson Pass]

2nd.

 

[Melon Head]

Thanks Nelson
OK then hopefully I have made a mistake somewhere.
Can someone check the way I have drawn the circuit.
I am a little unsure about the signal ground/return and output ground/return, and the DC power ground/return.
Any help would be most appreciated.
Thanks

 

[JimT]

Thanks Nelson
OK then hopefully I have made a mistake somewhere.
Can someone check the way I have drawn the circuit.
I am a little unsure about the signal ground/return and output ground/return, and the DC power ground/return.
Any help would be most appreciated.
Thanks

You do not have things setup completely for an FFT with LT Spice. Try adding the following directives.

.tran 0 30m 0.1u
.step param C list .100u .10100u
.option plotwinsize=0

Test at 1000Hz.

If you want to learn more about LT Spice. Try the forum for it on groups.yahoo.com. I am unable to help you further. I just copied the above from an example I found there. It works for me.

Jim

 

[homemodder]

Just simmed tanh s circuit and indeed it shows that h3 dominates.

Tanh do the sim now at 20Khz and I think youll be even more puzzeled

 

[Nelson Pass]

Whenever spice doesn't match measurement, we look for flaws
in the models first. This would not be the first time that Spice JFET
models have come up a little short.

 

[udailey]

Mr. Pass
I read a post by you that mentioned using the AP1 and some other attenuator after the source and before the amp with a buffer or pre inbetween. I cant for the life of me find it now. You talked about maybe having just a few different resistances to choose from before the buffer/pre and then after it to have, I think, the AP1.
Is this to help 'rectify' any source output/amp input impedance issues or what is the reason?
I think this was with a BOSOZ.
Thanks
Uriah

 

[Zen Mod]

Mr. Pass
I read a post by you that mentioned using the AP1 and some other attenuator after the source and before the amp with a buffer or pre inbetween. I cant for the life of me find it now. You talked about maybe having just a few different resistances to choose from before the buffer/pre and then after it to have, I think, the AP1.
Is this to help 'rectify' any source output/amp input impedance issues or what is the reason?
I think this was with a BOSOZ.
Thanks
Uriah

AP1 is measuring gadget , almost capable of computing 42 .

we don't ( we - spoiled kids ) even need to know exactly what's that beast ..........

anyway - I can just imagine using it as attenuator

 

[udailey]

Ah, I thought it was the Apox-1
Uriah