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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

LDR Pre MkII - LDR volume control and I/O switching
LDR Pre MkII - LDR volume control and I/O switching
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Old 20th July 2019, 04:57 PM   #231
sabre66 is offline sabre66  Canada
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So after a little bench testing I swear its the oled, if i turn on the unit (hold down play button)with garbage on screen the relays chatter (sometimes)if i remove the oled I can hear the unit turn on and i can cycle through inputs ( relays switching tested with DMM) without any issues. Voltages seem fine 5vdc across all inputs and it works (its actually usable) except for oled not coming on.Tried a capacitor across D, nothing. Oh well I guess its just a waiting game for now.
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Old 10th August 2019, 07:24 AM   #232
niss_man is offline niss_man  Australia
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Hi Zdr

I have been using my ldr mk2 for the past 2 months now and it is going well but. When I compare to my previous ldr, this one has a higher noise floor. Are there any modifications I can do to try and lower the noise floor? I am guessing it may be dac/arduino noise coming through the system. I just tried moving the power supply away from the main board but that made no difference. I did also add 1000uf capacitors to each of the 3 5v power supply output terminals but that didn't do much either.
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Old 11th August 2019, 10:37 PM   #233
niss_man is offline niss_man  Australia
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@zdr.
Can a 50Hz mains filter be put into the software code to help lower the idle noise of the LDR passive pre?
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Old 12th August 2019, 07:34 AM   #234
zdr is offline zdr  Belgium
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Quote:
Originally Posted by niss_man View Post
@zdr.
Can a 50Hz mains filter be put into the software code to help lower the idle noise of the LDR passive pre?


Iím not sure what you mean. I donít hear any noise in my setup.
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Old 14th August 2019, 08:00 AM   #235
niss_man is offline niss_man  Australia
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Hi Zdr

When I havent got any music and turn the volume up on the LDR then the noise floor increases. I have made this a little bit better by connecting my metal chassis to the 0v dc point at the power supply board. It is worst between 25-49 level of volume but when it reaches 50, the noise disappears. My mains supply is 50Hz so I was thinking maybe the arduino can be programmed to reject this 50Hz noise somehow?
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Old 14th August 2019, 08:16 AM   #236
zdr is offline zdr  Belgium
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I donít see how could 50Hz noise possibly get into audio signal, since audio ground and digital ground are not connected. There is also no connection between any of the supply lines and audio signal. It could be entering audio path through other parts of the system though.
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Old 15th August 2019, 03:41 AM   #237
niss_man is offline niss_man  Australia
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In the original thread forum member "nikosokey" had same problems as I have. That is he was getting noise. This is seen here
Arduino based LDR volume and source selection controller
You did actuaally reply to him zdr.
Nikosokey says he did fix his noise problem by connecting chassis gnd(earth) to 0v power supply gnd. Which is what I have done as well.

Mains frequency hum will almost certainly be there on most amplifiers and preamplifiers in varying amount depending on the quality of the power supply/filters/circuit etc.

I ask you to do this. turn your amplifier and ldr on then with your ear to your speaker start to turn the volume on the ldr passive pre up. You will hear the noise floor through your speakers when it is turned up right. This is the noise I am talking about. I find it is loudest between ldr volume levels 25-48 (gradually increasing) then when it reaches 50(max) the noise floor I did hear suddenly "dissapears". Can someone explain why this happens?
Now lets show some FFT plots I have done.

This is without the chassis to ov/gnd connected. We can easily see a high 50Hz Harmonic as well as a fairly high 3rd harmonic as well. Not to mention all the other harmonics in there.
lightspeed2 14-8 original.png


So now the chassis to ov is connected and it is looking much better. The 50Hz has dropped and alot of other noise is also decreased. 3Khz (3rd) harmonic is still higher than 2nd harmonic 2KHz.
lightspeed2 14-8 frame to ps.png


Here is my signal generator only. 3rd order harmonics are lower than the 2nd order here.
Sig gen only 1Khz.png
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Old 15th August 2019, 04:07 AM   #238
niss_man is offline niss_man  Australia
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Some more distortion graphs.

This 1st pic is using my 1st ldr passive pre by "Rudi" as shown in this thread
Versatile and comfortable passive pre-amp
THD - 0.073% This volume level is equivalent to around around 44 on the new ldr attenuator(zdr type).
lightspeed 1 15-08.png


Now the next few pics are with the second smarter ldr as shown in this current thread.
Volume level 9. THD-0.29% although it is not really heard at this low volume.
lightspeed 2 15-08 vol 09.png

Volume level=35 This is loud and about the level I play my music at
lightspeed 2 15-08 vol 35.png

Volume = 44. This is the level I was testing when you can see the 1KHz harmonic at -30db
lightspeed 2 15-08 vol 44.png

Vol=48 We can see THD dropping as we near the maximum volume.
lightspeed 2 15-08 vol 48.png

Vol=49 Distortion lowering more.
lightspeed 2 15-08 vol 49.png

Vol=50 Maximum volume. Now is when I hear no noise on my speakers. Also distortion is only THD- 0.011% and also now 3rd harmonic is lower than the 2nd harmonic.
lightspeed 2 15-08 vol 50 max.png

A thing to note before people ask. These measurement were done with only my signal generator connected to a ldr passive pre (old or new type) and then to my behringer mic mixer/sound card as shown on this thread by xrk971.
Howto - Distortion Measurements with REW
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Old 15th August 2019, 05:46 AM   #239
zdr is offline zdr  Belgium
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My reply was about the ďvolume bumpĒ which was present in original design. I will check the buzzing noise when I get back from vacation on 23rd. In theory, I really donít see how can any signal pass over to audio part, as itís galvanically isolated. Only during calibration the two grounds are connected with the relay.
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Old 15th August 2019, 06:35 AM   #240
niss_man is offline niss_man  Australia
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Through inductance. The measurements dont lie. This is what I hear and measure.
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