Lightspeed Attenuator a new passive preamp

[mikelm]

after I finished listening last night I noticed some residual hum with the isolated arrangement so even though the sound is amazing I guess I have to continue with my experiments

[Panelhead]

Mine two are both diy, thanks to the info provided by georgehifi. They always did have the power supply floating, no ground connection between signal and power.
The passive unit had a low level hum at first. Used resistor from signal ground to chassis to eliminate this issue. 10 - 100 ohms should work fine. Not having a connection from power ground to chassis is not savy, but in this case it is dc at about 17 volts.
In my passive the power comes from a wall wart battery charger. The kind that comes with 12 volt SLA for RC cars. Dual stage LM317's, then chokes to LED's.
The active unit with LRD attenuation powers the LED's off the +/- rails. Uses resistors to buffer the rails from reg noise and has a 1 mH inductor on each side of the LEDs.
I like to think I can hear the sonic effects of the chokes feeding the diodes. But LED's are basically noisey devices, power supply noise most likely is lower than self generated artifacts.
One thing I did to both off mine is to change to switched pots. This way the unit is powered can be up all the time, but the LED's are off. Sholuld help extend the life out to several years of hard usage. Maybe longer. Before I would open the pot up to mid level when not used to keep currents low on both elements.
Just make sure the right side of the pot gets connected to the shunt/series diodes. It needs to be maximum attenuation when switched on.

George

[mikelm]

thanks for the feedback - I will try your earthing arrangment

if the LED's are noisy then a small cap across each one might help ?

[georgehifi]

Ok we just did some blind A/B testing of some power supplies.

1 Standard supply that I give with the Lightspeed Attenuator

2 Gel Cell rechargable 12v (Model car type)

3 Alkiline 9v

4 Super Regulated Linear 12v supply that powers medical instruments, about the size of a Besser Block, and as heavy


All 3 three of us could not reliably tell which was playing.

This prompted me to do some screen shots, the Lightspeed is output is terminated with a 50k resistor, volume at 12 o'clock input attached to a muted cd player, three screen shots will follow.

1st With power supply and led - floating

2nd With power supply and led - earthed to rca earth

3rd No power supply at all

In all cases the noise and frequency did not change when the volume was at min mid or full.

The noise is in all cases .02mV peak to peak at a frequency of 185mhz.
I think that these numbers that I have given are the noise and frequency threshold of my Tektronix 400mhz scope and 400mhz probes, not of the Lightspeed itself. Becuse with the probe shorted I get the same figures.
See for yourself the differences in supply confurations in the 3 screen shots all are left channel top trace right channel bottom trace.

Cheers George


With supply and -ve floating

[georgehifi]

With supply and -ve conected to rca earth.

[georgehifi]

With no supply at all, (non powered Lightspeed).

[georgehifi]

All shots were taken within 2 seconds of each other, camera was tripod mounted and my dear wife was doing the conections of the -ve to rca earth and the disconection of the powersupply, 2nd and thrird shot.
What I can now confidently say is that in my setup there is no influcence on the ldr's (noise or hum) by any of the power supplies, conected or disconected, as for hum, it is hum free, and if any hum is detected is is because of system wiring, (loops and such) as the Lightspeed has no hum of it's own.
I say from this that any difference in "sound" detected is of a personal nature, and not because of a change in residual hum or noise in the Lightspeed itself.

Cheers George

[mikelm]

Hi George,

This noise thing a real mystery to me - I try to understand but in the end I have to go by my ears because my goal is enjoying the sound.

I have some questions for you.

I will email u soon

cheers

mike

[mikelm]

I just put the caps on the i/p side and this seems to have substantially cured the hum problem in isolated mode as well as removing the DC offset issue.

cheers

mike

[georgehifi]

Originally posted by mikelm
Hi George, This noise thing a real mystery to me - I try to understand but in the end I have to go by my ears because my goal is enjoying the sound. cheers mike

Mike, the only thing that may make a difference from what I can see with your "um" decimated Lightspeed that I sent you, is that your modded one has now no advantage of the case shielding, from the power supply transformer and from air born rf, none of my point to point wiring uses shielded cable form rca's in and out to the board, it's just 99'9% oxygen free single strand copper.
Could this be the difference you are hearing?

Cheers George