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Old 7th April 2007, 06:23 PM   #591
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Default Re: Where?

Quote:
Originally posted by Panelhead
The LDR MAY have a very fast response, just the tempco effect takes a long time on top of actual LED current to resistive material function.
George, my speculation precisely!
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Old 7th April 2007, 06:25 PM   #592
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Default Re: Re: Where?

Quote:
Originally posted by mikelm


Yes right on the LEDs - ( with polarity observed )

The difference between 1uF in this position & 100uF is major !

I mean M A J OR ! ! ! like yesterday I thought I needed a new CD player . . .

1uF sounded like it might need snubber resistors. This is another reason I thought about a larger value cap. Snubbers are not needed for higher values.

I suggest you just put ur favourite 100uF cap in and see what you think.

Mike, you're confirming my speculation in an early post in this thread. My intuition leans to thinking PSU noise *has to* affect the LDR.
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Old 7th April 2007, 07:29 PM   #593
mikelm is offline mikelm  England
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Default Another Clue ...

.

I was turning my pot today and I heard a crackle ! ! !

...so I would agree with you tom but I also suspect that some of the noise contribution is from the LED itself, and that it is some variety of broadband noise. So a bigger cap across the LED may address noise issues that caps & chokes in other positions cannot address.

The difference I noticed with the 100uF cap was across the bandwidth with a fuller bodied, more etched out, 3D kind of a sound. Not 3D imaging as such but rather each instrument have depth & occupying a 3D space.

I would be interested to hear what others notice with this mod if you try it out.

mike

ps tom did u get the email I sent ?
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Old 7th April 2007, 10:40 PM   #594
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Default Crackle

Mike,
That crackle may be your pots. Most pots do not like dc across them for long periods.
I have tried several values and tapers so far. All the 2 watt pots have been quiet. The only 1/2 watt pots tried also made noise when adjusting volume.
This is another reason I like the switched pots now installed. Keeps power off the pot unless actually listening.
Gotta install those electros across the leds soon. If it gets much better I will not be able to stand it.
And, thanks again to Georgehifi for sharing his circuit and this wonderfull method of attenuating input signals.

George
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Old 7th April 2007, 10:48 PM   #595
mikelm is offline mikelm  England
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Default Re: Crackle

Quote:
Originally posted by Panelhead
That crackle may be your pots.
Yes I agree, but the point I am making is that the noise - even with my 100uF caps in place, was audible - thus strengthening the case that these LDR's are noise sensitive.

I await your impression of the cap mod with interest

mike
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Old 8th April 2007, 12:14 AM   #596
peufeu is offline peufeu  France
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Makes sense that the LED light output stability is important. After all, it is multiplying the audio signal to control the volume, so if it varies according to the audio signal or other noise, you'll hear that...

Has anyone tried to feed the LEDs with a constant current source ? (keeping the // cap) ?
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Old 8th April 2007, 12:28 AM   #597
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Quote:
Originally posted by peufeu
Has anyone tried to feed the LEDs with a constant current source ? (keeping the // cap) ?
Mmmm ?

A constant current source feeding 4 potentially noisy LED's - isn't that a recipe for interaction ?

...and it would also be cross channel

Perhaps that's why you suggest keeping the caps in the circuit ?

I think it would need 1 CCS per channel - it adds complications, what advantages would you predict ?

mike
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Old 17th April 2007, 09:11 AM   #598
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Default quick question

Hi guys,

Have not looked at this thread for a long time (other distractions). The good news is that I got the LDR pre to work (I had connected the thing incorrectly the first time...). I am impressed by the mid-range clarity as well as the clean treble. But, the pre-amp is just to loud, even at the lowest setting.

My pre consists of balanced inputs which run into THAT1200 balanced to single-ended converters. This then runs into the LDR volume pot and this then runs into an AD815 chip which runs to single ended outputs

Balanced input-THAT1200-LDR volume pot-AD815-single ended output

Should I increase the size of the 100 ohm resistors used in series with the LDR's or rather adjsut the gain on the AD815?

Thanks
Ryan
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Old 17th April 2007, 10:27 AM   #599
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Default Current limiting resistors

Ryan,
I think that lowering the value of the 100 ohm resistors will increase the maximum attenuation. This will lower the shunt resistance at maximum attenuation.
I have one built with an AD815 also. Great combo!
My best recommendation is to make the gain of the AD815 2. Sounds better to me than it did as a buffer, unity gain. Depends on system gain, I do not need any gain, all sources are plenty loud with unity gain or passive.
Try checking how the impedance varies. Measure from inut to ground and rotate pot to see how constant the impedance is. Then mesure from output to grpound to see how low the shunt resistance will drop.


George
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Old 17th April 2007, 11:42 AM   #600
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Thanks George.

Does your passive use the AD815 as a buffer and do you prefer it that way or with the AD815 set to provide gain?

Have you tried the pot without the AD815?
I think I'll try without the pot this evening.

My power amp has an input impedance of 27k. My source is a Theta Gen Va which I believe has amazing driving ability.

Ryan
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