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Old 4th August 2010, 04:14 AM   #3471
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I would imagine parallel; if cap wired in series it would block DC to the LED!!

I'm not sure where S&B put the cap, but I gather a cap would be there to block DC from entering the TVA. Otherwise the core *could* saturate.

Or I could be wrong!
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Old 4th August 2010, 04:55 AM   #3472
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Default The LDR cap

The cap is there to reduce noise, etc on the LED and has no connection at all to the Variable Resistor.

The way used to describe this device has always been confusing - the Light Emitting Diodes (leds) shine variable amounts of lumens (light) onto the Light Dependent Resistor (ldr) so offering the electrical isolation .....
It's really a "light controlled variable resistor" and should be known as a LCVR, IMO!

The effect you get with adding a cap across the LED pins will vary with different types of circuits that control the LED current (or voltage) - sometimes no change at all, as George found in the original "basic" cct - sometimes not apparent in some systems - so simple to try it for yourself.

... 2cents
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Old 4th August 2010, 11:25 AM   #3473
maximus is offline maximus  Scotland
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Hi Troy,

This is also for any others struggling to understand the application of the VCCS module and the Infra Red Remote modules with regard to control function.

I havenít fitted front panel switches to my unit so I canít provide you with a photo. Instead I will try to explain the circuit operation for you in a way you can understand.

If you look at the circuit diagram for the VCCS module it shows the switch arrangement. The switches are floating above actual ground along with the DS1802, as they have to be referenced to the DS1802 pin 14 to work. If you look at the DS1802 data sheet pin diagram you will notice that pin 14 is designated AGND (analogue ground). The DS1802 has to be floated 3.3 volts above ground to fit within the common mode input range of the LF347 voltage controlled current source. This means that pin 14 on the DS1802 now sits at 3.3 volts above ground. Pin 14 is still the DS1802 chip reference point for all functional pins 16 to 19. Pin 15 is not used in this application.

Pins 16 to 19 are normally pulled high to pin 20 potential internally in the chip. To instruct the chip potentiometers what to do, the relevant instruction pin should be shorted with a momentary action switch to pin 14 even though this pin is not referenced to actual ground in this application. In fact connection of pins 16 to 19 to actual ground could damage the DS1802, as this would place the terminals outside their voltage operating range.

Referring back to the VCCS switch drawing and looking at the VCCS board layout you will notice that connector pins 1-6 are duplicated using a double row of connectors. This duplication allows both the Infra Red Receiver (IRR) board and the front panel momentary push button switches to be connected to the module at the same time. It does not matter whether you use the remote control or the front panel switches to instruct the DS1802, as either option will reference the control pins 16-19 to pin 14. The VCCS circuit diagram switch section shows which of connector pins 1-5 are connected to pins 14, and 16 to 19 on the DS1802 and the function of the pins referred to in the DS1802 data sheet. Pin 6 of the connector is only used to provide power to the IR Receiver board, which is also floating above actual ground along with the DS1802, and it is not involved in any switching actions.

You can use any insulated wire type you wish for the control functions, just make sure you get the pins matched correctly on the VCCS and the IR Receiver board and the front panel momentary switches match the required function pin of the DS1802.

I hope this clears any confusion regarding the connection of the control pins. I understand that some of you may be relatively new to DIY electronics and the operation of the VCCS module may not be easily understood. If my description above is still not clear do say so and I will try to explain in a way you understand. It would help if you can highlight any particular point that is not clear.

Regards
Paul
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Old 5th August 2010, 08:50 AM   #3474
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Quote:
Originally Posted by udailey View Post
Arnold,
Still tired from staying up late listening to music. I missed the part about your amp. Yes a really high gain amp will get loud very early. This is the situation with my high gain amp and high efficiency speakers as the LDRs already start at a minimum of 40Ohms and then mine jump to about 70Ohms nearly instantly. There is quite a difference in volume especially since at the same time the series LDRs are dropping very quickly. They will then smooth out and volume change will be more smooth but at first turn the change is dramatic.
Your solution may not be perfect but it might be great for your system. Can you use your DMM on One Series LDR and tell me its max resistance and min resistance and then do the same thing on One Shunt LDR.
Here my value:

SERIES LDR

RX
MIN 7.30...7.57 kohm
MAX 57.8 ohm

SX
MIN 6.4..6.5 Kohm
MAX 55.1 ohm

SHUNT LDR

SX
MIN 49 ohm
MAX 2590 ohm

DX
MIN 156 ohm
MAX 725 ohm

Measured at 5volt, no audio signal
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Old 5th August 2010, 02:42 PM   #3475
udailey is offline udailey  United States
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First, yes the cap is across, parallel with, the LED. Noise from the power supply comes across the LED and injects into the resistor side of the LDR.

Arnold,
Man DX is messed up. Can I see your set up? Odd that your system should be getting loud fast with these readings, but maybe that messed up DX is going from low to max instantly.
Either way, DX is broken or you have something wired wrong to it. Pics?
Uriah
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Old 5th August 2010, 03:38 PM   #3476
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Uriah -

1 - the link to google docs in post 3458 seems to be broken.
2 - with regard to the snubbers: should they go on all 4 LDRs or just the series??

This thread is always interesting.

Thanks,
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Old 5th August 2010, 04:47 PM   #3477
udailey is offline udailey  United States
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Ray,
I posted another a few pages back. That one works fine.
Snubbers. Interesting. I use them on the 4 but who know? Maybe series only would be fine. Either way its a 40 cent investment. I highly doubt audiophile caps are needed here and also dont use big caps, just 100uf or less.
This one is fine https://docs.google.com/fileview?id=...OTNjMzdm&hl=en
Uriah
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Old 8th August 2010, 02:54 PM   #3478
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Quote:
Originally Posted by maximus View Post
Hi arnold23,

Check out the VCCS module (details on post 1874 page 188). This module provides a voltage controlled current source (VCCS) for each LDR LED. The operating range can be tailored to suit your system by changing the resistance values of resistors R1 to R4. The VCCS module provides volume up/down and balance. You can also add remote control if you wish.

Regards
Paul
Hi Paul, where can one find out more about the VCCS modules and boards and prices?
thanks!
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Old 8th August 2010, 08:02 PM   #3479
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Default The pot

I have tested a number of potentiometers in my Lightspeed attenuator. The result shows that the sound varies greatly from one pot to another.
The tested pots are the cheap carbon film, ALPS blue case, Sfernice ceramic, TKD 2CP601 conductive plastic, and so on. What I found is that the more expensive the pot, the better the sound. The TKD is much better than the ALPS or so in every aspect.

In my humble opinion, the pot still plays the major role even in the LDR scheme.
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Old 8th August 2010, 08:30 PM   #3480
maxw is offline maxw  United Kingdom
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Quote:
Originally Posted by Marlowe View Post
I have tested a number of potentiometers in my Lightspeed attenuator. The result shows that the sound varies greatly from one pot to another.
The tested pots are the cheap carbon film, ALPS blue case, Sfernice ceramic, TKD 2CP601 conductive plastic, and so on. What I found is that the more expensive the pot, the better the sound. The TKD is much better than the ALPS or so in every aspect.

In my humble opinion, the pot still plays the major role even in the LDR scheme.
Care to offer any logical explanation for this theory?
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