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Old 25th September 2010, 09:41 AM   #271
Salas is offline Salas  Greece
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Nice! Congratulations! You don't need the inverse Riaa to measure 1kHz by the way. Just a shielded voltage divider with parallel resistor as much as your carts Z preferably.
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Old 26th September 2010, 01:19 AM   #272
andyr is offline andyr  Australia
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Guys,

I've come into this thread very late but am extremely interested in what's been discussed (although it's a bit above my head, being a newbie! ).

I have been trying to build (and then modify) a similar circuit - which I believe came out of "Silicon Chip" in the 80s (modify because its RIAA was quite off at both LF & HF) and what has been discussed here is very relevant.

Circuit attached FYI.

I'd be very grateful if you could answer a few questions:

1. The circuit you're discussing has the 1st & 2nd stage JFET Sources connected to ground with a small resistor (20 ohms or so). What is the purpose of these resistors - ie. why not simply connect Source to ground?

2. This is an "SE" circuit, isn't it (as Source is nearly at ground)?

3. The circuit I've been working on, as you can see, has source resistors which are much higher (although the values are far too high, according to a friend of mine - there's not enough current flowing through the JFETs).

Would it be better to change the Source resistors to 20/27 ohms, like the circuit in this thread, and then adjust the value of the Drain resistors to give a Drain-to-ground voltage of 1/2 DC supply (which is 32v, in my case - from 5 x 6v SLAs)?

4. In my original circuit there is also a 47pF poly cap between the Gate & Drain of the 2nd stage JFET. What function does this perform?

Thanks,

Andy
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File Type: pdf Phono Layout 200508.pdf (66.2 KB, 361 views)
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Old 26th September 2010, 03:46 PM   #273
Salas is offline Salas  Greece
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1. Local feedback, gain control. 2. SE stands for single ended i.e. non symmetrical, not source to earth. 3. Weird circuit better don't make hit or miss changes. 4. Reduces bandwidth for some reason. Prone to oscillation with its BJT buffer drive could be.
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Old 26th September 2010, 08:59 PM   #274
andyr is offline andyr  Australia
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Quote:
Originally Posted by Salas View Post
1. Local feedback, gain control. 2. SE stands for single ended i.e. non symmetrical, not source to earth. 3. Weird circuit better don't make hit or miss changes. 4. Reduces bandwidth for some reason. Prone to oscillation with its BJT buffer drive could be.
Thank you, Salas.

Regards,

Andy
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Old 29th September 2010, 10:23 AM   #275
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Quote:
Originally Posted by Salas View Post
You got PSU and/or ground loop issues with harmonic noise I see also.
As a matter of fact I had, big time. I also had some other stuff going on that made my figures way to good to be true. I'm sorry if my enthusiasm and screendumps promised to much.
Quote:
Try .tran 0 {Ts+2/f1} {Ts} {100u/f1} uic .param f1=20 f2=20k showst=1700m Ts=7000m freq=1k
Can't get that one to work. "Unknown parameter on .tran card: "ts+2/f1"

Anyway, Yesterday I did extensive "real live" testings and got consistent and more or less credible figures and readings from my FFT's. I then used the same setup to compare with just the soundcard, the RJM's VSPS phonoamp, another DIY tube phono - the Tetra One from Tubecad, and also the inbuilt phonoamp in a Luxman LV-105u.

This little pacific was the worst when it came to THD figures. Around 1%, almost all second order. The Tetra One 0.06%, the VSPS 0.03 and the Luxman more or less the same as the soundcard loop, around 0.0025 if I recall. I hope to bring those screendumps to a homepage of mine later on about this project.

So after that revealing test I have now lowered the amplification trough higher source resistances (the local fb) and with 3.5mV in I now have 41dB amplification and 0.6% THD. How does it sound? Some of the magic is gone but it sound more like the Tetra One all tubeamp in it's carachter then the clean Luxman amp.

When I first tested this Pacific Phonoamp I suspected much higher THD then the figures first told me, because of the spectacular punchy, all sweet and musical sound of this simple phonoamp. That's why I got surprised and happy when I thought it as such low THD and "still sounded all different from other low THD figured amp.".
But with my latest revealing tests I couldn't believe that 1% second order THD could do such magic to music. A controversial statement I know. But I have been there before with my other project, the CCD tubestage after my DAC with TDA1541A. As I got lower distortion the magic and texture .... once again, was gone.

So, what have I learned?
  • The holy grail to pleasent sound could, at least sometimes, be a pinch of 2nd order Distortion.
  • It's not the local or global feedback that takes away the shimmering sound. It's the lowering of THD
My two cents and I wish you all to experience this yourself. Build the amp and fiddle with it. Listen to it, compare and measure and listen again.

I think most of you will find the same thing as I have done the last year.
This amplifier in it's original form is a perfect project to seek out what I tried to explain in this short comment.
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Old 29th September 2010, 10:33 AM   #276
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That is all very interesting, especially about the 2nd order thd. It also makes some sense. We often strive for low distortion and detail, but with it can come sterility and coldness/hardness. Nobody in their right mind wants that.

What valves are you using in your TDA1541A tubestage? 6N2P by any chance? That's what I'm making right now on DVB Projekt's boards, a la Lampizator.
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Old 29th September 2010, 11:54 AM   #277
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Hi Lucas

This is my project thread for the DAC

In that I/V stage of mine I now use Siemens E88CC goldpins.
But as I discovered and mention in the post#43 of that thread, try the lower value I/V resistor and the higher mu ECC83/12ax7 if you like the 2nd order tubey crisp sound. To experiment with positive feedback you can easily even bring up the 2nd order dist. by 10db more and at the same time have 3more dB amplification.

As you can see I have only got one professed apprentice and believer so far, ide2003
I will jump on that project again and try to make a descent buil-in cabint this winter.
Maybe also a toggle switch "2nd order harmony" or "clean"

By the way, the TetraOne phono amplifier mentioned above uses the same topology as my I/V amp for the DAC. The CCDA style (Constant Current Draw Amplifier)

Back to this Pacific FET amp. Anyone more having live figures measured for THD ?
Or the same listening experience with 2nd order dist?
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Old 29th September 2010, 04:06 PM   #278
Salas is offline Salas  Greece
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I post the .tran window as it shows on LT Spice.

I averagely get 0.02% THD for nominal input in the 0.5mV 56dB MC Simplistic, never built any Pacific so I don't have measured figures. They are different in key areas though. Simplistic CCTs.

High second is veiling detail in my experience, more linear before NFB CCTs retain better subjective expression and don't change character after some NFB than originally worse ones, but beyond 9dB global NFB they tend to shrink image size, not altering tone by my experiments. The lesser ones tend to harden.
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Old 29th September 2010, 07:16 PM   #279
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Question My sim is not OK

Hi Salas

Finally I understand a very little bit how to use LT Spice. I attach my result from the parameters you suggested and the schematic as it is now.
AC responce looks OK and DC operation also seems fine with the right voltage at the drain on both FET's. But the FFT ...

Something is wrong. I use ideal values on the passive components, exept the output cap.
Attached Images
File Type: jpg schematic_salas_FFT.jpg (91.7 KB, 612 views)
File Type: jpg pasific_salas_fft.jpg (87.6 KB, 603 views)
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Old 29th September 2010, 07:33 PM   #280
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Default Found the error

OK
I give this one for free
The problem is a to large signal at V2 on 0.1V when it should be around 0.0035 (=3.5mV) I got confused with the settings for small signal AC setting which I allready have set att 3.5 mV.
The FFT now looks like attached picture below. This is in the ballpark compared to realword and I'm really amased that it is

Now, how can I proceed? Trying a BC550C cascode to even lower the input cap or...
This is addictive and fun.
Attached Images
File Type: jpg schematic_salas_FFT_working.jpg (190.9 KB, 566 views)
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Last edited by Radioman62; 29th September 2010 at 07:37 PM. Reason: Forgot the attachement .. Dooh
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