Optical Volume Control Professional PCB

[AndrewT]

Hi,
the averaging will make for better stereo image balance. But I have no idea how many devices would be needed to achieve adequate balance over the whole range of attenuation.

But before you go any further, consider the following.

Assume the max LDR resistance is 30k.
Assume the min LDR resistance is 40r.

The max attenuation with
one pair is 20*log[40r / 30k04] ~-57.5dB
two pair is 20*log[20r/60k02] ~-69.5dB
10pair is 20*log[4r/300k004] ~-97.5dB
The minimum attenuation with control at max volume is
one pair is 20*log[30k/30k04] ~-0.01dB
two pair is 20*log[15k/15k08] ~-0.05dB
10pair is 20*log[3k/3k4] ~-1.1dB.
That voltage loss of -1.1dB is not a real listening problem.
Go and look at the effect all this paralleling/serialing has on input and output impedance. It becomes a monster that needs tamed.

 

[AndrewT]

I must be missing what you are saying.

Wire the shunt LEDs in series.
Wire the series LEDs in series.

 

[Salsero]

Are LDR PCB and parts kits still available? What is the price?
Cheers
Salsero

 

[dvb projekt]

Hello Salsero,

all is still available. The price is as followed:

Professional 2-Layer PCB: $15
Parts-Kit (excl. LDR´s): $30
Worldwide shipping: $4 (only the PCB); $9 with parts kit
paypal fee: 3,9%

Best regards,
Oliver

 

[korben69]

Hi Oliver
Can you publish a scheme pointing how to bypass LM7805.
I'm wondering about feeding the LDR's using MiniGold Reg.
As my knowledge is weak I don't know how to proceed...
Thanks for your help

 

[dvb projekt]

Hi Oliver
Can you publish a scheme pointing how to bypass LM7805.
I'm wondering about feeding the LDR's using MiniGold Reg.
As my knowledge is weak I don't know how to proceed...
Thanks for your help

That´s simple!

1. remove the 7805 and the three caps from the board
2. insert a bridge between the outer solder points from the 7805

After that, you could feed +5VDC from any other external reg. to the pcb.


Best,
Oliver

 

[Salsero]

Oliver,
Where do I order the 4 matched LDRs?
Thanks
Salsero

 

[dvb projekt]

You could order the LDR´s from Uriah here.

 

[Ted205]

Hi guys

has anyone improved the attenuation.

in its loudest setting i have the same ouput as the input (3vpk-pk). At it's lowest setting i have about 20mV. Is it ok to lower the 100ohm resistors (increase the current through the LED and lower the LDR resistance nearer to zero) to improve this ?

 

[AndrewT]

Hi,
measure the current through the 100r resistors.
The maximum LED current is 25mA (Vr=2.50Vdc), but I suspect if left at this maximum value for a longer time will increase the risk of failure.
This may get you down to 30 to 35ohms for the LDR and achieve an extra 1dB of attenuation.
Is it worth taking the risk?
What is the minimum current through the LEDs? It may be that the series LDR is not getting to a high enough resistance due to using a pot that does not have sufficient resistance.
What is the excess gain through your system?

 

[Ted205]

i've tried increasing the LED current and it does improve things but it goes beyond the LDR max current rating.

My amplifer has a gain of 40 (BPA300) the input is fixed from a DAC (3vPK-PK).

 

[AndrewT]

My amplifer has a gain of 40 (BPA300) the input is fixed from a DAC (3vPK-PK).

it looks like your system has an excess of gain.
Before spending any more money, find out how much extra gain you can remove to get back to sensible signal levels.

 

[udailey]

Also, please get your 100R resistors back in place. Lowering the LDRs min resistance by allowing to much current through will obviously damage the LEDs but before this damage occurs the matching will be gone and can not be fixed.
Uriah

 

[Ted205]

thanks guys.

I'm going to see how i get on with a gain of 30

 

[AndrewT]

That reduces your overall system gain by just -2.5dB

 

[Ted205]

the lowest i can go is -6db at a gain of 20.

 

[AndrewT]

and that prompts the question:
Why adopt a BPA300?

What is speaker impedance and sensitivity?

What Power Input (or voltage) to the speaker is your normal level listening?

 

[Ted205]

and that prompts the question:
Why adopt a BPA300?

What is speaker impedance and sensitivity?

What Power Input (or voltage) to the speaker is your normal level listening?

In my opinion it prompts the question; why use the the Optical Volume Control..... It's not ideal for high gain systems as i have discovered.

 

[AndrewT]

..... It's not ideal for high gain systems as i have discovered.

I will go further, it is not recommended for high gain systems.

Have you read the Lightspeed thread?
There is recent comment on obtaining higher attenuation from the LED/LDR combination. I can recall at least three methods for increasing attenuation, some switchable to suit listening SPL.

 

[Blues]

In my opinion it prompts the question; why use the the Optical Volume Control..... It's not ideal for high gain systems as i have discovered.

The OVC/LS/or other LS-based designs used with high gain systems, you will find that at its lowest possible volume position it might still be loud and will eventually burn out the LEDs if kept there...otherwise it sounds great. Using the OVC, at what volume position do you usually listen? If you do find it too loud just above minimum then it is a problem (on a romantic evening) of too much system gain.

In this regard, I find the LS design flawed...