Lightspeed Attenuator a new passive preamp

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#5,028
Brought one of my lightspeed attenuators to the local GTA meetup and unfortunately had bad luck as my voltage regulator started smoking.

I thought I had brought the wrong adapter, but I checked it and got about 17vdc out of it (it's rated for 12v).

I've got to replace my voltage regulator (L7805ABV), but should I replace my adapter too? Is it putting out more voltage than the regulator can handle?
 
#5,029
The Lightspeed's internal regulator is 5vdc 1amp and can take from 8-20vdc what happened was the polartity of the wall wart you bought was most probably - negative centre instead of + positive centre, if so this would take out the first electro cap around 100uf 25v and also the L7805 regulator. Should cost no more than ten dollars from a electronics shop, just remember which way the regulator was and also the cap.

Cheers George
 
#5,032
merlin2069er said:
Yes, I noticed that a series diode in front of the regulator would have prevented reserve polarity problems. I've got a few 5v reg circuits I can pop right in as a replacement. I'll be sure to add a few diodes.

What about running it from 2x 9v batteries?
Click to expand...

I run mine from a single 9v battery as a backup when I am messing up the regular supply. I just put a DC connector on the 9V just like the walwort so you can interchange. My standard is to run it from a linear power supply that is toroid to bridge rectifier then CLCRCLC filtered. Last but not least, 150uf Oscons right on the LDR pins also bumps it up. The linear supply is outboard and the last CLC filter before the low noise reg is onboard.

A battery has no ripple but actually a fairly high noise floor.

The regulator is a point to point zener emitter follower from TNT audio site. Also little bump in SQ over the 7805.
 
#5,033
The quietest batteries I found were the rechargeable Lithium-Ion ones in the link, a 6800mA one last for weeks of listening before a 1/2hr recharge is needed. But please make sure if they are for the Lightspeed Attenuator I supply, that you ask the seller if the plug is 2.1mm to connect to the Lightspeed and that centre of the dc plug is + positive.

12vdc Li Rechargable | eBay

Cheers George
 
#5,035
I want to add the level of attenuation of the Lightspeed Attenuator on my LCD-display, etched a little test-rig and measured Vout in relation
to the position of the wiper of the 100K potentiometer.
Since I do not own a precise 100K potentiometer I "simulated" it by 2 x 25 turns trim-potentiometers.
The reference input signal is 300mV. As a load I used the resistor input network (2K / 22K) of my FC-100 amplifier.

After 10 measurements I noticed that the LDRs behave very linearly.
A 1% decrease of the potentiometer's value (going from 100K -> 0K in 1K steps) results in a 1% increase (3mV) of Vout.

Is this really so? Do I have to measure any more?
Is my 300mV input signal a recommended value for any music-source?

Best regards - Rudi_Ratlos
 

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#5,039
Rudi_Ratlos said:
This is my Lightspeed Attenuator PCB.
I have connected an OPAMP MCP6021 to the 100K potentiometer section wiper's output that drives the linear LDR.

Best regards - Rudi_Ratlos
Click to expand...

Rudi, I am looking at your photograph and the MCP602x datasheet. I guess the 8 pin DIP in the upper right corner is your 6021, but I can't visualize the function of a single op-amp in a circuit where there are two pot sections and four legs to be controlled. If the purpose was to offload managing the current from the pot onto the op-amp, I would think you'd use a 6022 or 6024 with two or four amps, so that can't be it. Could you tell me what that single op-amp does, please?
 
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