Lightspeed Attenuator a new passive preamp

[georgehifi]

Badge, your problem is the Marchand, at 25k together with your amp 100k you have a total load going back to the Lightspeed of 20k, and with the Lightspeeds approx 7k your cd player is seeing 5.2k this is OK so long as your cd player or dac has a very good output stage, and judging by your remarks about the bass it maybe hasn't. What is your cd player/dac?
Also there are a couple of fixes for this.

One is to raise the input impedence of the Marchand (the easiest) and will help, I don't know why Phil Marchand said it wouldn't.

Two is to get a lower impedence/stronger output on the cd player/dac.

Three is to put an opamp buffer on the input of the Marchand.

Also the interconects between.
Source and Lightspeed ,
Lightspeed and poweramp,
Lightspeed and Marchand,
Marchand and poweramp
must be kept low in capacitance and resistance as you have lots of them in your setup.

In my own bi-amped setup I'm driving two stereo amps of 68k each (34k) total with no problems at all with stunning impactfull tight bass down to 20hz, but I have a dac with a strong output.

Cheers George

[georgehifi]

It just dawned on me Badge, that your source maybe capacitor coupled to the outside world and if so that cap maybe too small in uf with the 5k-6k that it's seeing causing a bass rolloff filter, this could also be the problem.

Cheers George

[Badge]

George, again thanks for all your help. I will raise the input impedance on the crossover. I am also using a back up SACD player which is a modified transformer output stage Sony SACD 775. My regular unit is a Sony SACD 9000 ES with Modwright signature truth modifications which includes a tube output stage and more drive output than the 775. The wire umbilical cord that ties the tube output stage to its own power supply is off to cryogenics at the moment.

[georgehifi]

Badge, I maybe wrong with yours, but a source that has a transformer or a tube coupled output stage, will not have a low enough impedence and strong output to drive into such a low load as you have. I hope I'm wrong for your sake.
Also the tube output stage will have capacitor coupling to the outside world, make sure that cap is at least 10uf.

Cheers George

[Badge]

OK George. Now you have done it. Made me open the closet with all the extra audio equipment. I have two conventional opamp based CD players in there. I will see if there is any difference.

[Panelhead]

Badge,
I get about the same overall loading on my sources, 8K ll 100K. The caps in my tuner are a little too small, the ones in my phono and SA-8260's are fine.
Waiting on a box of 100K limear dual pots to arrive. This should reiple the impedance of my Lightspeed clone from what it is now.
Also while apart I need to jack up my current limiting resistors. Used 121 ohm. But George is using a single control on his. Using two controls each section of the pot is only controlling one LDR. The resistors in mine need to be 200 ohms.
Attracted to this design for use as an attenuator in front of a buffer. But thought it should be tried first as designed. Learned quite a lot about how to rig it. Plus it really sounds nice.
Sometime soon hope one installed in my preamp in front of an AD815 line driver circuit. This will take of any loading issues in my system.

George

[Panelhead]

Tried a couple more things to optimze the unit. I read early in the thread about contact points in the signal path. Since I use multiple sources regularly single input is not an issue.
The case already had a nice vintage ceramic wafer CRL rotary 2 pole. Had a four pole identical switch in the closet so replaced the dual with a quad switch. This switch is used ala Croft, five position, with pos 2 and 4 as mutes. This puts a ground between each input and resduces channel leakthough. Also doubled up the contacts so that each input and the outputs have two high quality contacts.
The switch change is audible. Seemed to even improve bass weight, but cannot see how. Anyway one forward.
Then replaced the dual 50K linear pots with 100K linear pots. This changed my measured input impedance from 8 -9K to 22K - 30K depending on pot position. The impedance is lower in the middle, higher at the extremes of rotation.
This made the Lightspeed clone sound more like the passives tried before. Colored the sound, rounded the transients. It made it smoother, but did not add the fullness to the bottom expected.
It did really help with volume adjustment. With the 50K all listening was in first 20 -30 degrees of rotation. Now normal is 30%, insame levels will hasve to wait for beer!
So after several hours of solderind: the 5 position, 4 pole input switch is good. The 100K dual linear pots are getting removed. Good to look for the 100

[vatofale]

Hi George,

I'm just about to order some of the ldrs, how many of the sorted samples do you suggest I get in order to find 4 matched devices?

Cheers

[georgehifi]

Originally posted by vatofale
Hi George,

I'm just about to order some of the ldrs, how many of the sorted samples do you suggest I get in order to find 4 matched devices?

Cheers

For memory I think the first breakup price was 10, that should see you out of trouble, you save a dollar each maybe, i always buy in lots of 50 or 100, you get a much better price that way.
Which just reminds me we have company RS Components here in Australia, and I wanted to get some quarter watt Vishay resistors from them, get this, the price was $20 each if you buy singles, but if you bought 50 it was $30 for the 50, go figure.

Have fun building it, if you follow my instructions to the letter, you will love the sound also, get this a member of DIY flew to Australia from Singapore to buy one in person then few back that day, talk about on a mission, I hope he posts his findings in this thread soon.

Cheers George

[artaudio]

Thanks George,

I have my new lightspeed installed and having change my Naim NAP-160 power amp to 100K input impedance as suggested by you the results are simply stunning with the Meridian G-08 > lightspeed > NAP-160 > Naim SBL speakers every thing has opened up with great bass, well balance mids and silky smooth highs not something I would have ever believed possible using Naim products. Just goes to show how much pre-amps alter the sound from the CD Players.

Taking into account the number of LDR's needed to match 4 and the time it must take to match and assemble the finished product its a steal at the price your charging.

Cheers
Alan