Ultrasimple MM/MC RIAA preamp 2

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Congratulations for your build Radioman62. This one you made is flatter than the Le Maison original. With what signal input level did you measure THD? Also what is your gain? Try to listen with that 100R (the one in series with the 33n5 to GND) bypassed or substituted to just 10R, for a comparison.
 
Hi Salas
That resistor gives 0.5dB raise in the upper highs around 20khz. It will stay there because there is no cutting equipment that uses the RIAA beyond 50khz.
This is something that f.e. Allen Wright brought up in one of his articles.
I simulate in LT Spice with a "perfect" RIAA source. Then i get this 0.5dB raise at 20khz. If I simulate with passive components, like the Hagtech inversed RIAA, i get flat responce.
In the real word I have recently built the Hagtech inversed RIAA and used it on a four phonoamps including this one. Flat responce on all of them. One of them being the Luxman LV-105u phonostage. So it seems even the market also use this "practical" 50khz pole to correct the RIAA.

One thing that really can color the sound is how well you load your cartridge. I have measured my cartridge to be 3k 420nH. Together with total capacitance and load, which for the moment is 220pF and 47k, I get a slight raise in the upper frequencys and a drop off before 20khz. It's all easy to simulate.
If I use 56k as load I get a small peak, about 2.5dB at 14khz and drop off starts at 19kHz. This sounds very pleasant. Some of my more dull sounding vinyl's sound better. But is it "the right thing to do"? Maybe i should use graphic eq instead? :)
One thing to strive for with a mm cartridge is to have as low capacitance as possible, so we don't get the peak below 20kHz.

I will come back with figures on the THD. I used my Inversed RIAA and looped with my soundcard. It was late, I was tired .. :)
Gain is around 40dB. I use 50 ohm trimmers on all 4 FETS to fine tune and also lower gain and THD.

All my listenings is with Sennheizer HD650 and my own XPP OTL Headphoneamp.
 

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diyAudio Chief Moderator
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I suggest a 100k trimmer instead of an initial fixed load for finding the best tone for MM sometimes I have been asked. 47K is just not universal and since MM is counting on an electrical resonance to extend to last octave its easier to play with resistive load than adding and subtracting capacitance. So I think you did well to play there. Even less residual Miller gives more leeway with an input load trimmer. Use nominal output of your cart to gauge THD. The Hagtech includes the ''lost constant'' and measures flat with the 100R. The Lipshitz would show a rising trend. With MC carts that extend and even rise in the last octave I have experienced that resistor introducing harsh overtones. With an MM it can be compensating brilliance loss and mixing nicely.

P.S. Is view FFT after running .tran for this particular CCT giving you something like that in the pic BTW? The stock Mad runs higher gain and 2ndH due to no Rs but can play some happy output MC. Better with Rs and MM. Cuts on Miller too. What is your cart model?
 

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Low THD

Hello again

46dB amplification. 5.0mV gives 1volt RMS = 0dBV. As much as my M-audio soundcard can take. Real world figures with the Hagtech style Inverted RIAA in between the soundcard and the amp. 0.015 THD is pretty good :) for a two stage amplifier without global negative feedback.

Regarding the FFT I havent had any succes yet, using the LT Spice. I have to read and learn some more. I can't see any FFT icon in the view tab after a .tran. :confused:
 

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diyAudio Chief Moderator
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Hello again

46dB amplification. 5.0mV gives 1volt RMS = 0dBV. As much as my M-audio soundcard can take. Real world figures with the Hagtech style Inverted RIAA in between the soundcard and the amp. 0.015 THD is pretty good :) for a two stage amplifier without global negative feedback.

Regarding the FFT I havent had any succes yet, using the LT Spice. I have to read and learn some more. I can't see any FFT icon in the view tab after a .tran. :confused:

Looks like the measurement FFT with the LT prediction I posted are in same ballpark for 2ndH. You got PSU and/or ground loop issues with harmonic noise I see also.

Try .tran 0 {Ts+2/f1} {Ts} {100u/f1} uic .param f1=20 f2=20k showst=1700m Ts=7000m freq=1k
 
Hi Salas and thanks for the formula.

About the groundloop or PSU issues, I wouldn't call it an issue of any large significans. The rubbish to the left is around -108dB and it doesn't start with 50 hz or 100 Hz from the PSU. It starts at 75 Hz and then the multiples 150 300. Then there is a 225Hz with multiples.
I have the card un boxed, laying with cables on a table ftm. I can't see how I can get rid of it. I attached a better screenshot where the racket at the right is OK and undisturbed of mousemovements when I snagged the screen.

Batteries or my PSU is the same. Unmeasurable rippel. It means the same as no battery or PSU around 50uV.
 

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If its in the measurement loop and its verified external thing its OK. I got worried because when I see low or higher comb teeth effect all over the FFT window its oscillation in the PSU if not in the audio circuit. Especially if to the right. What kind of PSU? Also if you just loop the card by itself does it still show a ''busy'' noise floor?

To see FFT in the view menu in LT you first hit the running man, and after having analysis results, you should left click on the analysis window first.
 
Interresting Salas

FFT looks the same with battery.
Anyway, FYI, the PSU is well filtered main inlet and then I drop around 10Volts over RC, RC, RC before a LM317 with 10uF Tanthal on adj. and output.
At 20mA I get absolutely unmeasurable rubish on the 24Volt. Tried with both scope and a new Fluke 289 True RMS DMM.

The card is totally clean when I loop without anything.

But... I found the source of the pollution to be my Inverted RIAA Box, probably loading the soundcards output or ... I don't know yet.

I'll be back. Tomorrow. There is a wedding party for me in a couple of hours. My wifes sister is going to get married today. :)
 
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! :eek: ).

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? :confused:

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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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.
 
You got PSU and/or ground loop issues with harmonic noise I see also.
As a matter of fact I had, big time. :eek: I also had some other stuff going on that made my figures way to good to be true. :eek: I'm sorry if my enthusiasm and screendumps promised to much. :(
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.
 
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.
 
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 :D
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" ;) :D

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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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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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 ... :confused:

Something is wrong. I use ideal values on the passive components, exept the output cap.
 

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Found the error

OK
I give this one for free :eek: :)
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 :cool:

Now, how can I proceed? Trying a BC550C cascode to even lower the input cap or...
This is addictive and fun. :p
 

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