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Discrete phono stage, single supply.
Discrete phono stage, single supply.
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Old 7th March 2018, 04:35 PM   #31
Salas is offline Salas  Greece
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Discrete phono stage, single supply.
Quote:
Originally Posted by LLPe View Post
@Salas
You have accidentaly swapped collector and emitter for the BC327.

Lars
Good catch. Thanks. I restored the same mistake and replaced the sims both for original and new tweaked values in post#19
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Old 7th March 2018, 04:49 PM   #32
Salas is offline Salas  Greece
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Discrete phono stage, single supply.
Quote:
Originally Posted by catd View Post
good morning...

I have already put in BC550 + BC560 because I had these and there were no transistors on the circuit board when I found it again. Probably used for other things over the years.
When I compared the CDIL BC414 / 415 and BC550 / 560 datasheets, I found them pretty similar.

When I have time and the right values C & R, I will try your suggestions in one channel to switch between the previous and the new version and listen if one can hear a difference.

If it turns out that the thing has any value, then a new PCB layout is probably appropriate because the original does not seem very useful.
They should be very similar. Please look back in post #19 for the changed values suggestions after Q2's orientation mistake was restored. Pay attention to the lower C1 value too. Can be a small Wima MKP now.

That Q2 orientation mistake made #28 wrong and invalid as well. There's no gain or noise difference if changing to the 337/327 types anymore.
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Old 8th March 2018, 03:39 PM   #33
catd is offline catd  Europe
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Hello...
@Salas: thank you for your efforts.

If anyone is interested in an even simpler circuit: these I just have found in my notes.
If I remember correctly:
I bought it some decades ago as ready to use device, it came in an enclosure along with a built in small power supply.
I have only leftover this circuit drawing, and don't know the exact supply voltage, must be around 24Volts.
It sounded a bit bass accentuated, but perhaps only in the context of the then used Ortofon F15.
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File Type: jpg phonopre.jpg (19.2 KB, 165 views)
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Old 9th March 2018, 07:34 AM   #34
Salas is offline Salas  Greece
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Discrete phono stage, single supply.
Hello

The way T2's collector DC voltage centers between +Rail and GND when simulating with BC550C substitutes in Spice, better use a 18V PSU.

The R3 rheostat's setting affects gain and frequency response. 0dB is 1V in the attached chart for three different good R3 settings. 500Ω 750Ω 1kΩ top to bottom. Input is 5mV. Most normal for MM gain and better hiss is the 1kΩ red curve setting. At 100Ω (not shown) the circuit produces much total output with a very shallow under 1kHz curve and much tilted up treble. Obviously loses control with a too low R3 value. 350Ω should be R3's border balanced setting.

So when looking at proper setting area frequency response curves, they simulate as upper bass & low mid range little accentuated indeed. Although there is not much low bass in this phono, to the human ear the upper bass area is where a fuller lows feeling comes from. If in combination with some high octave recess also, that fullness hump becomes more present. A little like when listening with headphones as Sennheizer HD 650 I would dare say. Can't sustain real lows level due to a small diaphragm in an open frame but tricks us with an under 1kHz progressive hump to satisfy for lower tones feel in music. While its a little veiled in the treble also. But if it has to be a compromise, rounder is safer than brighter to most people. You seem to remember that phono circuit's sound well.
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File Type: png Catd_Unknown_Var_R3.png (11.3 KB, 114 views)
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Old 13th March 2018, 10:26 AM   #35
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Default Installed and in use.

Hello again, the experimental "Yang-Riaa" preamp is now installed in my diy amp and lid closed. As mentioned before, it's perfect, quite noisy and has some strange "pumping" sound in the right channel. I tried to localize the source but failed, so we will have to live with that for a while. But hum is not a problem, since there is a regulator, large decoupling capacitors, proper star grounding and "balanced" wiring all the way to the pick-up.

This is my first riaa preamp and and it makes it possible to listen to vinyl again, oh yeah! And it is definitely not the weakest link in the signal chain, I believe that may be the recording or vinyl itself. In other words, a good recording sounds wonderful and a bad one horrible.....

So thank you all for your input good advice during this design and build! Attached is modified schematics, installation photo and RIAA response and noise plots. Comments are welcome!

@catd: That's a cute old device, please tell about your resuls. Maybe an Olson Mini will be my next project...?

@LLPe: I am not sure if it's motorboating, since that usually comes as a very distinct and "pumping" sound at a more constant frequency. This is very weak, similar to the other noise level, and with more random frequency. That's why I suspect it may be a faulty device, they sometimes behave like that. Anyway, your version looks interesting too, please post results...
Attached Images
File Type: jpg Yang-RIAA.jpg (51.4 KB, 116 views)
File Type: jpg Yang-RIAA-installed.JPG (171.5 KB, 110 views)
File Type: jpg Yang_Riaa_Plot.jpg (128.0 KB, 102 views)
File Type: jpg Yang_Riaa_Noise.jpg (125.3 KB, 21 views)
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Old 13th March 2018, 05:52 PM   #36
catd is offline catd  Europe
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Quote:
Originally Posted by Salas View Post
... Although there is not much low bass in this phono...
C4/R7 at the output form a 44 Hz high pass. If one increase or omit R7, the curve would decrease less in the lower range, I think.
Probably this should be a very simple subsonic filter.
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Old 13th March 2018, 06:05 PM   #37
Salas is offline Salas  Greece
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Discrete phono stage, single supply.
As I have seen in simulation analysis without that specific R7 value the upper bass booms. That's why the designer probably put it there so to fix that filter for any higher following line preamp's impedance. See here with R7 100kΩ
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File Type: png Catd_Unknown_R7100k_R3_500_750_1000.png (11.0 KB, 31 views)
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Old 13th March 2018, 06:32 PM   #38
Salas is offline Salas  Greece
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Discrete phono stage, single supply.
@nnx

Change your R36 to 560Ω and your R37 to 40Ω and run Spice again so to see if you got more HF in reality than in the original simulation thus more hiss. Pick signal from top of R36 for MM like attenuation, top of R37 for MC like attenuation. Also, yes same type transistor samples can have more noise than other samples. Or weird noises that the others lack. If you can manage a higher value C1 with still good overall frequency response by manipulating your phono's RIAA values you will probably deflate that noise bump at VLF to a useful degree
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Old 13th March 2018, 06:51 PM   #39
stocktrader200 is offline stocktrader200  Canada
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I used a 2N5210 as a first stage flat gain into an op amp RIAA in 1986. so far the best I've heard I am upgrading the sx780 preamp to this but only have 2SC1815Y which appear to have slightly more noise.
you could change R6 to 4.7k and R3 to 1.5m for more gain

Last edited by stocktrader200; 13th March 2018 at 07:02 PM.
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Old 17th March 2018, 11:42 AM   #40
nnx is offline nnx
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@ Salas:
Thank you for your advice!

I have made the modification to the Riaa generator as you described, and it'a quite surprising to see the frequency response in the HF area. The green curve shows +20db around 1 Mhz and that is quite a lot. I wonder if adding a zobel network as suggested will make any difference (blue curve)?

I also made a noise simulation with a different input capacitor, C1=500nF. It shows less noise, but only at the very low frequencies around 20Hz, and has almost no effect above 50Hz. The Riaa curve doesn't change much, only minor adjustments may be needed.

The strange noise described above seems to be a transistor related phenomenon called "popcorn noise", and the cure is finding and replacing the transistor.

@ Stocktrader200:
Changing the first transistor may be an option, I have 2N5209, 2SC945 and BC550 in my collection. Reducing R6 to 4k7 will double the gain and reduce the noise a little. It will also alter the riaa equalization, so some adjustment will be needed there too.

On the other hand, if we reduce R4~R6 and R24 with a factor 100, and increase C2 and C2 100 times, then the noise will be reduced around 4 times. Maybe that is the way to go?

Anyway, the preamp is now inside the main amp case, so any modifications will be made next time it is opened for service and modifications. In the meantime, maybe it would be fun to start on another Riaa preamp project.....?
Attached Images
File Type: jpg Yang_Riaa_5.jpg (137.7 KB, 41 views)
File Type: jpg Yang_Riaa_5_Plot.jpg (163.2 KB, 34 views)
File Type: jpg Yang_Riaa_5_Noise.jpg (107.0 KB, 16 views)
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