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Old 30th May 2007, 01:59 PM   #101
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for what it's worth, here's the post that talks about R67/R53 being around 1.0v, as perhaps my last post was too vague to garner a reply.

My new Aleph ONO Phono Section

again, I'm not sure if this value of 1v is needed when the output caps are shorted, or what exactly this is measuring My values are posted above.
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Old 1st June 2007, 06:41 PM   #102
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ok, so I had a chance to measure all the voltages on each channel, about 30 total measurements per channel and then do some simple calculations and what not. I think that I do need to get the values reported above to around 1.0 V. As I understand it, Q16 and Q17 act as a current source and I'm assuming that R67/R53 are used to set their operation point, and ultimately both of these resistors is controlled by the LED at D5.

Here are some calculations and measurements. I have improvised on the notation, and the _schematic notation refers to the attached schematic (not mine) and these numbers are what I'm using for sanity checks... My goal in choosing a new D5, is to get R67/R53 down to 1v, as recommended in the schematic:

V_R67_R = 2.350 V
V_R67_L = 2.361 V
V_R67_schematic = 1.000 V
V_R53_R = 2.374 V
V_R53_L = 2.373 V
V_R53_schematic = 1.000 V

LED_anode_R = -27.05 V
LED_anode_L = -27.04 V
LED_anode_schematic = -27.60 V
LED_cathode_R = -29.99 V
LED_cathode_L = -29.97 V
LED_cathode_schematic = -29.60 V

V_f_LED_R = (29.99-27.05) = 2.94 V
V_f_LED_L = (29.97-27.04) = 2.93 V
V_f_LED_schematic = (29.60-27.60) = 2.00 V

here are my values for one of the two devices acting as current sources:

Q17_collector_R = -3.90 V
Q17_collector_L = -3.93 V
Q17_collector_schematic = -3.90 V
Q17_emitter_R = -27.62 V
Q17_emitter_L = -27.60 V
Q17_emitter_schematic = -28.00 V
Q17_emitter_current_R = (29.99-27.62) V / 150 ohms = 16mA
Q17_emitter_current_L = (29.97-27.60) V / 150 ohms = 16mA
Q17_emitter_current_schematic= (29.60-28.00) V / 150 ohms = 6.7mA

D5 I am current using has the following specs:

V_f_max = 4.0
V_f_typical = 3.5

so, I'm measuring around 2.9v here, so I got one on the low side.. so, am I safe to assume that all I need to do is replace D5 with a device whose V_f is around 1.5 V to get the voltage across R53/R67 to 1v?

This is my first time troubleshooting a circuit like this, so I'd appreciate all the help I can get!

Thanks in advance for the help!!
Attached Files
File Type: pdf ono_preamp_schematic.pdf (48.0 KB, 379 views)
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Old 5th June 2007, 06:43 PM   #103
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in case anyone is following along at home, I got some LEDs that proport to have a V_f_typical of 1.5v today in the mail, and of course they don't... they are measuring around 1.72v... I did pick up some infrared LEDs, but they're measuring way too low, as with these V_R67 / V_R53 is around 0.7v => 4.67mA, and I cannot adjust the trimpot to get V_R28 up to 350mV (well, yet... it takes the circuit about 24 hours to reach equilibrium, so we'll see.. the trimpot is maxed out though)

So my first question to the audience is does this really matter? Also, it it a problem if V_R67 does not equal V_R53? Since I'm going to be using a really low output MC (0.24mV) all of these things kinda scare me, but perhaps they don't matter at all... I need someone more experience to tell me that!

And for my final question, is this problem related to the fact that I cannot get the junction between Q5,R21,R7,C11 down to 0v, or the junction between R62,Q4,R65,Q19,R13 down to 0v, or do I still have another exciting battle ahead of me!
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Old 11th June 2007, 11:58 AM   #104
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The 1.5V led are indeed difficult to source. I found mine by installing a socket for the led on my PCB, then I collected all the led types that I had at home, then I tried all of them. The one I found at exactly 1.5V are small (3mm) red diodes. They are coming from an old PCB that I salvaged years ago. Then I found some more on an other old PCB. It seems that the 1.5V are from old circuits. Maybe the new led design, with higher intensity probably, are using a different junction that is producing a higher voltage. Try to find old PCB with small leds in a surplus store. It may be your solution. Good luck.
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Old 11th June 2007, 03:06 PM   #105
Cobra2 is offline Cobra2  Norway
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Does it need to be a LED ?
Why not 2 x 1N400X in series?

Arne K
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Don't believe everything you think...
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Old 11th June 2007, 04:02 PM   #106
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Leds in this application are used as low noise zener diode. Normal diode forward voltage are more noisy and less stable. I think the noise issue of the led was really a design criteria to get the preamp noise floor as low as possible. At least this is what I guest from the designer intentions.
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Old 11th June 2007, 05:03 PM   #107
PierreG is offline PierreG  Canada
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Quote:
Originally posted by Algar_emi
The previous Bigupload was removed, its delay expired. Here the new one:

Ono_project.Zip by Bigupload.Com
Unavalable again..........
Too bad
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Old 11th June 2007, 09:20 PM   #108
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Send me a email and I'll send you the project file. Bye...
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Old 11th June 2007, 10:30 PM   #109
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Send me a email and I'll send you the project file. Bye...
I e-mailed you to..
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Old 11th June 2007, 11:43 PM   #110
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Quote:
Originally posted by Algar_emi
Leds in this application are used as low noise zener diode. Normal diode forward voltage are more noisy and less stable. I think the noise issue of the led was really a design criteria to get the preamp noise floor as low as possible. At least this is what I guest from the designer intentions.
Here's a good link on the diode versus LED noise:

Some noise measurements for LEDs and zener diodes

I am pretty much giving up on this, I cannot find anything that will work. V_R67/V_R53 is now down to 1.11 V on both channels and this is as low as I could get. There is a .01mV discrepancy between V_R67 and V_R53 in each channel as well.

Do you think this will matter at all?! If anyone has any suggestions at all, I'm all ears!
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