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Lightspeed Attenuator a new passive preamp
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Old 2nd August 2013, 09:34 AM   #5111
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by soongsc View Post
I was talking about signal amplitude into the LDR attenuators. For example, has any tried feeding a 0.5V signal vs a 10V signal and see if the gain is the same. ..................
10Vac is a high signal level.
What is the dissipation in the attenuator, at various volume settings?

Total dissipation and individual dissipations must be low enough not to affect the short term capability and for reliability, the long term dissipation limits may be quite a bit lower.
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Old 2nd August 2013, 10:34 AM   #5112
oenboek is offline oenboek  Belgium
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I've avoided to push the LEDs really hard. When you push them hard, they become really slow and will die soon. The lowest volume I can still change is low enough, even for my wife (and it's an excuse if she wants inaudible level). And when I lower volume further, I have an automatic mute relay kicking in, bringing volume to zero.
I tought of putting in parallel a relais with an extra resistor to reduce by 12 or more dB. In fact it would become a 2-range volume (low volume range with resistor, and high volume without), with a processor behind, everything is possible. But I haven't implemented it as the range is OK now. I couldn't live without the mute though.
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Old 2nd August 2013, 03:42 PM   #5113
udailey is offline udailey  United States
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I dont recall looking for different 'gain' but the distortion is way way different. 200mV least distortion of all in a series/shunt LDR arrangement. 2.5V was as high as I would ever go. 10V distortion was at very high levels but no I dont remember the numbers. I would bet it was in the 8-10% range. Sorry I didnt write down numbers to remember. I just looked at the V vs % and made up some rules of thumb for myself.
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Old 3rd August 2013, 02:05 AM   #5114
wapo54001 is offline wapo54001  United States
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Originally Posted by oenboek View Post
I've avoided to push the LEDs really hard. When you push them hard, they become really slow and will die soon. The lowest volume I can still change is low enough, even for my wife (and it's an excuse if she wants inaudible level). And when I lower volume further, I have an automatic mute relay kicking in, bringing volume to zero.
I tought of putting in parallel a relais with an extra resistor to reduce by 12 or more dB. In fact it would become a 2-range volume (low volume range with resistor, and high volume without), with a processor behind, everything is possible. But I haven't implemented it as the range is OK now. I couldn't live without the mute though.
I have now tested with three LDRs on the shunt side with LDR devices that meet my 50 ohms at 10ma or less criteria (3/4 of the LDRs I get from udaily meet this) I'm getting 17 ohms or less with no loss in control in a circuit that draws less than 10ma total (LDR LEDs in series) so the LDR devices cannot be affected even if you keep the 10K pot set at -56dB continuously. Actually, the control down there is so good that I can control to within 1/100th of an ohm.

I think in future for people wanting substantial attentuation (54~60+ db) paralleled devices will be the way to go. A device costs maybe $3, so a multi-LDR path is reasonable, and it doesn't complicate the circuit unreasonably. The series side requires only one LDR for any configuration.

In my circuit, increasing to eight LDRs for stereo will require value changes for one resistor and two LEDs, otherwise everything remains the same.
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Old 3rd August 2013, 02:54 AM   #5115
georgehifi is offline georgehifi  Australia
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[QUOTE=wapo54001;3583070]I think in future for people wanting substantial attentuation (54~60+ db) paralleled devices will be the way to go. QUOTE]

I have tried 2 and 3 x paralleled ldr's in the past, and to my ear 1 x ldr sounded better, two seemed to soften/smear the sound a touch, not by much but it was detectable. More so than the difference between pure battery power vs linear wall wart, which is hard to pick the difference.

KISS

Cheers George
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Old 3rd August 2013, 03:12 AM   #5116
soongsc is offline soongsc  Taiwan
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Quote:
Originally Posted by udailey View Post
I dont recall looking for different 'gain' but the distortion is way way different. 200mV least distortion of all in a series/shunt LDR arrangement. 2.5V was as high as I would ever go. 10V distortion was at very high levels but no I dont remember the numbers. I would bet it was in the 8-10% range. Sorry I didnt write down numbers to remember. I just looked at the V vs % and made up some rules of thumb for myself.
I think this is good indication that gain testing would prove to be interesting. If the trend indicates higher gain at higher voltages, then the LDRs actually compensate BL curves of speaker drivers. In this case, the LDR should be showing higher distortion at higher input levels as well because it is compensating nonlinear driver with nonlinear characteristics.
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Old 3rd August 2013, 03:44 AM   #5117
wapo54001 is offline wapo54001  United States
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[QUOTE=georgehifi;3583090]
Quote:
Originally Posted by wapo54001 View Post
I think in future for people wanting substantial attentuation (54~60+ db) paralleled devices will be the way to go. QUOTE]

I have tried 2 and 3 x paralleled ldr's in the past, and to my ear 1 x ldr sounded better, two seemed to soften/smear the sound a touch, not by much but it was detectable. More so than the difference between pure battery power vs linear wall wart, which is hard to pick the difference.

KISS

Cheers George
I wondered about that (and agree that KISS is good) and felt it would not be a problem in my system because the paralleled devices are only in the shunt resistor. Overall, my system is quite complex, but the audio signal path is no different from the Lightspeed except for the paralleled LDRs on the shunt side.
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Old 3rd August 2013, 04:45 AM   #5118
georgehifi is offline georgehifi  Australia
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[QUOTE=wapo54001;3583119]
Quote:
Originally Posted by georgehifi View Post

I wondered about that (and agree that KISS is good) and felt it would not be a problem in my system because the paralleled devices are only in the shunt resistor. Overall, my system is quite complex, but the audio signal path is no different from the Lightspeed except for the paralleled LDRs on the shunt side.
This is where I tried it as well as on the series side. Even the quality of the shunt component in speaker xovers make a difference, good caps sound better than cheap caps.

Cheers George
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Old 9th August 2013, 04:17 PM   #5119
Eric is offline Eric  United States
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Lightspeed Attenuator a new passive preamp
Default My LDR based DCB1 Build

I just finished my DCB1 build with LDR attenuation. It still needs a chassis and I haven't had the chance to listen yet - that comes tonight!

DCB1.jpg

The LDRs are mounted directly to the DCB1 board and are powered by Salas' 5V shunt regulator.

LDR Reg.jpg

I built George's LDR control on perf board and have the 100k log pot on a tether for some flexibility.

LDR Board.jpg

The power supply to the LDR control board is a perfect 5.00VDC. With 100ohm resistors and a 100uF caps followed by 0.47uF caps, I get a voltage range of 1.62v to 2.18v for the LDRs. This allows the audio resistance provided by the LDRs to vary from 22R ohms to about 6k7 ohms.

It still needs a nice chassis and bigger sinks so I can push the bias on the DCB1. I can't wait to give it a listen tonight with my AudioGD DAC and Pass a40 amp.

Thanks to George and Uriah for making this available!
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Old 9th August 2013, 09:00 PM   #5120
georgehifi is offline georgehifi  Australia
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Nice work Eric, I'm interested did you manage to build the B1 without using any output coupling caps by matching the fets very tightly. Because from my experiences a coupling cap will mask all the good that comes before it.
Also can/did you measure the output impedance of the B1, as I don't believe it's very low (for a buffer).

Cheers George
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