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Pacific RIAA phono pre: failure!
Pacific RIAA phono pre: failure!
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Old 16th May 2018, 01:08 PM   #131
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
Okay, i think i have wired it in the right way. I dont have a rectifier, my power source line looks like this:

24 V Wallwart PSU --> LM317 Circuit with regular 1uF (tantalum) at the output --> capacitiy multiplier --> longish line --> 2200uF Output cap directly mounted on my preamp board. I Use the (-) of the 2200uF cap as my zero reference. Will post a pic of my current setup later.

The reason why i can hear the hum better is that all other noise levels went way down. I should wire the preamp to some other random amps in my house to firgure out if the hum depends on the amp, preamp or turntable.

Where should i connect the turntables "ground" wire? At my zero reference? Should i connect my metal cookie-box to some point on the circuit to achieve a better shielding?
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Old 16th May 2018, 03:26 PM   #132
Salas is offline Salas  Greece
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Pacific RIAA phono pre: failure!
Metal box should be wired to your zero point on the circuit (so they are at same potential, that's also hum critical). Then on the metal box you should have a lug to bind the TT's dedicated ground wire.
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Old 19th May 2018, 04:09 PM   #133
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
okay, i did some days of listening and testing. I am not happy yet and will try to improve the output section again. I want to avoid building a buffer stage but i maybe will have to. But before that, I calculated a bit and thought about the output capacitator size and especially its load resistance in combination with the input resistance of my amp (which is unknown but expected to be between 10k and 47k, so i calculated 25k)

If we simplify things and calculate the output section of the preamp and the input impedance as a loaded voltage divider, i can see how bad the driving capabilitys of my circuit are (see picture 1).
With my highish preamp source impedance of 2k8 ohm and my output cap load resistance of 28k ohm and a 25k'ish input impedance of my amp, only 82% of the voltage arrives at the input of the amp. (picture 1)

Fiddling around with the different values showed me that a first aid would be to put 100k instead of the 28k output coupling cap load resistor i use now. With an input impedance of 25k, 76% of the original signal will arrive at the amps input (picture 2)

But the final conclusion is if i want better drive capability i need to lower my output impedance. To lower my output impedance i would have to replace the drain resistor of Q2 with 1400 Ohm (instead of 2800 Ohm).

I will lower my output coupling cap to .680 uF. I already checked that would be okay if change my circuit to match the scheme to the newest version ( picture 4 )

My question is: will my gain be the same when i use a jfet with an Id of 6.6 mA with a drain resistor of 1400 ohm compared to when i use and Id of 3.3 mA with a drain resistor of 2800 ohm?

second question: did you do your simulations of the circuit considering a reasonable load impedance of the amps input connected to the preamps output? like 10k, 25k or 47k?

Voltage divider - loaded and open-circuit dB calculator damping volts potentiometer circuit impedance damping pad decibel dB voltage attenuator impedance bridging matching - sengpielaudio Sengpiel Berlin
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Old 22nd May 2018, 05:57 PM   #134
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
Little update: after some more testing i located the source oft the hum when my dl-110 sits on the record, but the platter is not turning. It is the transformer oft my 1200 which seems to create a very small vibration at 50hz. The Motor itself works with DC not AC.

So the next big project is to move the transformer out oft the technics case and put it in a external box.

So my next bigger Project will be
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Old 22nd May 2018, 06:03 PM   #135
Salas is offline Salas  Greece
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Pacific RIAA phono pre: failure!
As we speculated it picks it from the TT then. No AC motor in this case but... hello naughty transfo. Alright, nice that you located the source of that little nuisance. Maybe some Mu-Metal shielding application could be less of pain to fix the situation (?)
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Old 22nd May 2018, 08:15 PM   #136
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
I guess the trafo has mechanic vibrations, not electromagnetic hum. Because the hum is only present when i put the needle in the record whilst the platter is NOT moving. The hum seems to be induced in a mechanical way. If i just place the needle next to the platter (using the needle protection plastic as a stand) there is no hum at all. Thats why i dont believe that electromagnetic fields are causing the hum in this case. The problem is very audible with low compliance carts. With Ortofon DJ Cart the hum signal is much loser, maybe because they are more "stiff"?

P.S.: i still love to play records with the preamp, despite wanting to optimise it... I am just so glad that the hum is not caused by the preamp circuit. Thanks for helping me a lot in that way.

Last edited by AddiDub; 22nd May 2018 at 08:20 PM.
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Old 31st May 2018, 05:21 PM   #137
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
Default smps kills my mechanical hum problems...

After our latest reasonings i read a lot of threads about noisy technics 1200 mkII transformers and decided to install a simple SMPS instead of it. I removed the internal transformer and put a Meanwell RS-15-24 inside the deck. That totally solved my hum problem that i had with the needle sitting on the non-rotating record groove. Now it is dead-quiet.

I will improve the implementation of the SMPS further, but for now i am pretty satisfied. Costs: $ 10,- for the ready-made SMPS. Effect: much less noise, hearable better soundstage, more defined and clear bass then before. Biggest bang for the buck i have seen yet.

Next step is to re-adjust the rumble filter of the phono stage to reduce the motorboating-effects...
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Old 31st May 2018, 07:41 PM   #138
Salas is offline Salas  Greece
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Pacific RIAA phono pre: failure!
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Old 5th June 2018, 02:39 PM   #139
csample is offline csample  United States
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Wow so much great information on refining the Le Pacific phonostage.

I would like to build the revised circuit Salas shared in post # 49, but my jfets have an idss of 5.6. Could I still use this circuit? Should I change the bias resistors? Would this change the gain?

Thanks in advance.
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Old 5th June 2018, 05:51 PM   #140
AddiDub is offline AddiDub  Germany
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Pacific RIAA phono pre: failure!
I guess you could do it, but if you use the exact part values given by Salas in post 49 you will end up turning the bias resistor oft the second stage to 5-10 ohms max. I would suggest to use a simple lm317 variable voltage regulator to tune the Vcc until the VDS oft the second jfet stage is 1/2 of the supply voltage that reaches your circuit.
With the idss of 0.0056A, no sources resistor and the drain resistor of 1.5k at the second jfet, you will end up tuning your Vcc to around 17 volt.
Use a 50R trimmer for the source resistance and tune between 0R and 20R to figure out what fits best.

Add the capacitance multiplier that Salas advised me a few pages ago to clean your supply voltage from noise.
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