Lightspeed Attenuator a new passive preamp

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#3,144
The only problem in matching series pairs and shunt pairs is you will end up with a very odd feel to the volume control, it will either come on too quick at the begining and nothing at the end or very slow at the begining and then a big rush at the end.
If you match all 4 you will get a feel like a logarithmic pot slow then gradually building.
Cheeers George
 
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#3,145
Hello George,

In my implementation I am having a switch between the regulated supply board and the light speed board. When the switch is turned ON, light speed is working fine. But, the odd thing is that when the switch is turned OFF (Supply is still available), I can hear some odd distortion/noise. Is that because of the missing ground point or due to the switch ? Please suggest. Mean while, I will try removing the switch from the circuit and will check again.

Best regards,
Bins.
 
#3,146
when you switch off power to the LDRs, they all go to dark resistance. i.e. very high resistance.
The effect is like having a 2M0 pot set to -6dB on the input pin of the next stage.

It will amplify all the interference and signal that it can manage from that Meg Ohm source impedance.
 
#3,147
Hello Andrew,

So, is that not a ground/switch issue ? Is there any way to fix that ?

Best regards,
Bins.

AndrewT said:
when you switch off power to the LDRs, they all go to dark resistance. i.e. very high resistance.
The effect is like having a 2M0 pot set to -6dB on the input pin of the next stage.

It will amplify all the interference and signal that it can manage from that Meg Ohm source impedance.
Click to expand...
 
#3,149
Hi George,

There is a very small amount of noise emitted from the light speed even when the supply is ON. I can hear it only when I am so close to the speakers. My light speed body is composed entirely of wood and so there is no earthing/ground. Will that be an issue ? Can it be solved by providing an explicit earth point for the supply/audio section ? Please suggest...

Today, we tested the light speed against some of the best passive and tube pre amps. The light speed is the one we felt having the best frequency response, imaging, channel seperation and staging. The sources we tried include Merlin CD Transport, Audio Aero and Northstar. The power amplifiers were tube mono blocks.

Best regards,
Bins.

georgehifi said:
It's like having an open circuit input on your amp, you will get noise, esspecially if you use non sheilded interconnects as I do. (Eichman Xpress 6 )

Cheers George
Click to expand...
 
#3,150
binspaul said:
Hi George,

There is a very small amount of noise emitted from the light speed even when the supply is ON. I can hear it only when I am so close to the speakers. My light speed body is composed entirely of wood and so there is no earthing/ground. Will that be an issue ? Can it be solved by providing an explicit earth point for the supply/audio section ? Please suggest...

Best regards,
Bins.
Click to expand...

Could be, my production Lightspeed Attenuators are in RF shielded extruded aluminium chassis and are absolutely dead quiet.
Try wrapping some aluminium foil around it and earthing the foil to the body of the output rca leads.
Or I hope your not using a SMP wall wart?
Cheers George
 
#3,151
This sound is emitted from the speakers or the lightspeed itself? When using a switching power supply I could hear the LEDs whine from switching on and off. Other than that I dont attach my signal GND to earth GND in the Lightspeed at all and I dont connect 0V to earth in the Lightspeed. I do get a tiny amount of sound, ear right to the cone of the 96db speaker, with Lightspeed on and volume down all the way and source not playing but on. I imagine that is probably a grounding problem in my amp or transformer hum but its so quiet. No reason for me to think its the Lightspeed. If the power going to the LEDs in the LDRs is dirty the voltage spikes, etc can actually be heard. I know this sounds like bull*r)@ but its true and you can test it by giving it a junk supply vs a decent supply. Its the same reason there is so much talk on this thread about different power supplies.
Uriah
 
#3,152
Matching Results

After receiving some nice pairs from Uriah, I matched some more LDRs on my own.
2x30 LDrs in series with an 80V CC source. So, every LED had exactly the same current.
8 steps of current, from 10mA down to 0,005mA. The results were fed in a spreadsheet and the criteria were set to compare the R at the different current levels. The worst deviation counts.
In the spreadsheet , I set the max allowable deviation to 0,3dB which brought me only two pairs! Maybe this was due the higher deviations with 0,005mA current?

 42 42 127 126 199 196 733 718 1566 1536 4140 4130 17700 18200 88700 89800

and the other pair

 38 39 122 121 192 191 711 692 1514 1473 3960 3860 17000 16600 85700 84000



when allowing 0,5dB I get a lot more pairs.

What do you think?
Cheers , Frank









 
#3,154
Hi,

1. The sound is emitted from the speakers. It is not from light speed.
2. My power supply is based on uA7810 & uA7805.
3. Exactly, the sound is tiny and is observed only when the light speed, source and power amps are on.
4. The main power supply is regulated using servo controlled stabilizers. Will they contribute to the noise ?

Best regards,
Bins.

udailey said:
This sound is emitted from the speakers or the lightspeed itself? When using a switching power supply I could hear the LEDs whine from switching on and off. Other than that I dont attach my signal GND to earth GND in the Lightspeed at all and I dont connect 0V to earth in the Lightspeed. I do get a tiny amount of sound, ear right to the cone of the 96db speaker, with Lightspeed on and volume down all the way and source not playing but on. I imagine that is probably a grounding problem in my amp or transformer hum but its so quiet. No reason for me to think its the Lightspeed. If the power going to the LEDs in the LDRs is dirty the voltage spikes, etc can actually be heard. I know this sounds like bull*r)@ but its true and you can test it by giving it a junk supply vs a decent supply. Its the same reason there is so much talk on this thread about different power supplies.
Uriah
Click to expand...
 
#3,156
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#3,157
Hi George,

Is the noise due to supply circuit ? Even Uriah is getting some noise. I still think that it is due to some ground or RF interference. Please comment.

Best regards,
Bins.

georgehifi said:
Then you should be able to match up a quad set, and you won't regret it, much better control usability.
Bins maybe grab one of these, cleanest pure DC power you'll find. Good for 12hrs before recharge
12V Lithium-ion Li-ion Rechargeable Battery for CCTV - eBay (item 380219619869 end time May-03-10 21:00:17 PDT)

Cheers George
Click to expand...
 
#3,159
fbee said:
I set the max allowable deviation to 0,3dB which brought me only two pairs!
Click to expand...

Hi fbee,

I feel your pain. Although I matched mine using a different method, for my batch of 50 I took 2500 readings over 6 currents, I even monitored the temperature for each reading, then wrote a program to crunch through the results and presto... 2 pairs. 😀

I'm feeding the output of the LDRs into a 10 bit ADC which I thought would sufficently smoothen the numbers to match exactly. I had to losen the tolerances a bit just to get a couple of quads. I still don't know if my quads are good enough.

Have you tried retesting all your LDRs to prove your testing procedure is accurate? I will as soon as I get a day of 23.1 to 23.8 degrees celcius. 😛

regards
 
#3,160
Hi George,

I am using the light speed MK II version. The supply circuit is based on uA7810 & uA7805. It is feed from a simple 12V DC wall wart adapter.

Best regards,
Bins.

georgehifi said:
Post up your supply circuit including ldr's and all earth points.

Cheers George
Click to expand...
 

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