Lightspeed Attenuator a new passive preamp

[Lisandro_P]

This is a great idea. I've been pondering - has anyone trying driving the leds with a PWM source? I'm asking since i'm now in the process of building a digitally controlled (PIC) preamp and have messed with pwm drivers for leds before, and i think you could create a "digital pot" using PWM that would requiere but two output pins...

[georgehifi]

Originally posted by mikelm
Hi George,
The output impedance of my cd player is 20 ohms.

Mike, the output of your cd player, did you measure this also with your DMM from output to ground? If so, WRONG, it is the SERIES resistance from the output back to the opamp amp output and this can only be measured with the cd player on and mute circuit off and NOT with a DMM, or if you have a circuit diagram of the output post it up and I can tell you what the output impedance of your player will be.

Cheers George

[mikelm]

George,

My CD player DAC o/p is buffered with an op-amp. This op-amp has feedback so the the o/p imp is effectively zero.

This op amp is connected directly to the o/p phono sockets with a 20 ohm wirewound non-inductive resistor. This is, more or less, how I always do my CD player o/p's.

The regular transistor operated mute function relies on cheap series resistors of a few hundred ohms which niether of us like.

Please believe me, my CD player o/p impedance is about 20 ohms.

mike

[georgehifi]

Just wanted to make sure, now how can we get rid of your 1vdc of dc offset without a coupling cap, what opamp are you using on the output of the cd? what I/V stage? and what da convertor? Post a diagram if you have one.

Cheers george

[Panelhead]

Mike,
When using a pot for voltage divider, the output impedance always varies.
What stays contant, (maybe not) is the input impedance. Try measuring for open input to ground. Then rotate the pot.
The input impedance is what loads sources and sets the LPF. The Lightspeed is speced at 7K. I would think it varies about 10 -20% as the pot is rotated.

George

[David Garretson]

The following data might interest anyone thinking about making a volume control for an active pre using LDRs. To match up LDRs I used a 5.2V source & a 12-position rotary switch with fixed resistors. Out of 40 factory-sorted LDRs I found four quads matched within 5%-- enough for a balanced standalone passive and a balanced volume control. I'm thinking that to get the LDRs even more closely matched for the balanced appication, I'll put a 10K or 20K PCB trimpot in series with each LED, and use these trimpots to converge the resistances of the LDRs at the spot on the volume control that produces average listening levels. Turns out my tube integrated phono/LS wants a 250K attenuator. I'm not optimistic about the LDR's low power rating and distortion numbers at higher signal voltages but I'll give it a go.

Typical resistance for NSL32SR2S "D" sorted LDRs on 5.2V power:

LED R = Photocell R

5.6K = 195R
26.6K = 623R
81K = 1.9K
148K = 3.9K
219K = 6.4K
470K = 19.2K
690K = 35.7K
1M = 70K
1.18M = 87K
1.5M = 130K
1.76M = 180K

[Blues]

Originally posted by David Garretson
... I'm thinking that to get the LDRs even more closely matched for the balanced appication, I'll put a 10K or 20K PCB trimpot in series with each LED, and use these trimpots to converge the resistances of the LDRs at the spot on the volume control that produces average listening levels.

Please see Lightspeed Attenuator a new passive preamp Post #515.

Thanks for the data.

[georgehifi]

Hey Dave, just out of interest, what sort of ACV comes out of your phono section at say 1k when using a moving coil cartridge, that is if you have a test record with 1k@0db sine cut into it.

Cheers George

[David Garretson]

George, I've been meaning to get in there and check that out and will do so next weekend. I do have a record with 1k test tones.

Regards,

Dave

[mikelm]

Originally posted by Panelhead
Try measuring for open input to ground. Then rotate the pot.
The input impedance is what loads sources and sets the LPF. The Lightspeed is speced at 7K. I would think it varies about 10 -20% as the pot is rotated.

This what I did and I got 46k 38k 48k for right & 66k 39k 44k for left

In each series of three figures the first is with volume at minimum, second is the lowest measured value and the third is full volume.

So your 20% guess is in the right ball park.

mike