Lightspeed Attenuator a new passive preamp

I think that because your output level dropped that the transformer's primary is too low for the Lightspeed.

Your link is not working for me, but is this the transformer you are using? if so the optimum source it says is 600ohm, your Lightspeed is around 7k that is why the drop in level, it's loaded up the Lightspeed/source, if this is happening then the level will drop and your dynamics will suffer especially in the bottom end.

Cheers George
 

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Output transformer

I agree on this. The transformer is an input transformer. A passive unit, with its variable output impedance may not drive a transformer very well when you attenuate the signal.
Have you considered moving this Lundahl over to the input side of the Lightspeed? You can do your phase swapping there also. The reason I bring this up is that output and input transformers are completely different animals.
I am using Cinemag CLM15K/15K input transformers, they have the same bandwidth as the Lundahl and measure about 1700 ohms dcr. These are around 40.00 each from Cinemag.
The output iron is a big chunk, bandwidth is 10 hz - 3 Mhz, and measures 38 ohms dcr. They are S&B 410's, used to be 35.00 each, long, long ago. Seems like last pair was 100.00 each.


George
 
If this was the transformer, you may improve things a little by series the primaries together instead of paralleling them this should give the ideal source as per their reasoning of about 2.4k this is still not up to the ideal for the Lightspeed of 7k, but it should be better for now, I don't know how it will effect the frequency response though of the transformer. It will be safe to try.
I think Pannelhead/George's way could be the way to go by putting it in front of the Lightspeed, but then in input impedence of your amp should be up around 100k for the Lightspeed to not get loaded up again.


Cheers George
 
Thanks for your comments. Georgehifi that is the correct one. I listened to the setup this afternoon. I really didn't notice any attenuation at either extreme. In fact, the bass seems to have more slam and there is a bit better stage definition. I will move them to the inputs and see how that goes.
 
I have a question :

Is this optocoupler attenuator implementation sensitive to dc offsets from its input stage?

I'm planning to make a 6 channel volume control where I would be using this attenuator directly after a modded Behringer DCX2496 with it AKM DAC output to a 47 ohms resistor and straight to this lightspeed clone. Without a cap after the DCX2496, it'll be in the region of 2.5Vdc offset.
Of course my amp input is with caps so I guess I'm still safe?
 
I was having the same thoughts - so I guess the questions are:

1) what is power rating of the light dependent resistors

2) what is the range of impedances that the lightspeed presents to the source ?

not sure if these have been stated earlier in the thread.

cheers

mike
 
Will said:
I have a question :

Is this optocoupler attenuator implementation sensitive to dc offsets from its input stage?

I'm planning to make a 6 channel volume control where I would be using this attenuator directly after a modded Behringer DCX2496 with it AKM DAC output to a 47 ohms resistor and straight to this lightspeed clone. Without a cap after the DCX2496, it'll be in the region of 2.5Vdc offset.
Of course my amp input is with caps so I guess I'm still safe?

Hi Will, I wouldn't like to have 2.5vdc dc offset sitting on mine, I don't know what it would do to the sound. But it will take it as it has a input max rating of 60ac or dc peak @50mW.



mikelm said:
I was having the same thoughts - so I guess the questions are:

1) what is power rating of the light dependent resistors

2) what is the range of impedances that the lightspeed presents to the source ?

not sure if these have been stated earlier in the thread.

cheers

mike

Hi Mike,
The Lightspeed as I use it gives around 7k load to the source, which in 99% of cd's is fine except maybe for the odd tube output one that has 2k output impedence.
The input rating is 60v ac or dc peak @ 50mW, what you should remember that at cd levels 2v and below the 2nd harmonic distortion is very low but going up from there 10v> or so it gets higher.

Cheers George
 
I ordered a lightspeed a while back from George and it arrived today.

Well, at present the whole thing is not optimized - my cd player output has a quite high resistance and my amp has 20k ohm input.

Even with these apparent drawbacks this thing is really special.

None of the previous glowing reports prepared me for just how real the music now sounds.

The words that come to mind are immediate, intimate and VERY natural.

My previous device was S&B TVC102 mk3.

Many thanks George - this is a fantastic product and well worth the price - :cheerful:

mike
 
Thank you very much Mike for your kind words, even though it's still not fully optimized into your system. You will be in for an even bigger shock, once optimized.

I have a bit more info regarding this impedence optimizing, we had our monthly Audio Club meeting last week and the main theme was to hear the difference with the Lightspeed with different input impedences on the amplifier.

It's important to tell you that the CD player was a stand alone from Belcanto, it's output impedence was 50ohm I was told and the output opamps were of decent current ablity around 50mA.

The first listening test was with the power amp at 300k input, all was good every one loved it, the second test was with the amp at 10k, all said it lost dynamics and the bass had lost power even though it was level matched at the output of the amp to the same as the 300k test. The last test was 50k and everyone again love it, then we blind tested from 50k to 300k and it was obvious that no one could accurately tell which was which but most thought 300k had the edge , so from that I say 50k is ok but the cd has to be low and strong, but 100k to be safe.

Cheers George
 
I’ve built a ‘Lightspeed’ as per George’s drawing and it works fine except for one thing: -

At maximum attenuation with a CD playing, you can still clearly hear sound from the speakers. Could this be an impedance mismatch problem (my amp is a Gainclone based on the Peter Daniels LM3875 kit)?

Would changing the pot on the Lightspeed to a 200K unit help? The original attenuation in the Gainclone was a 20K Alps pot which works fine.

I am using a Marantz CD5400 OSE with PMC FB1+ transmission lines.

I would be really grateful for any advice.

Thanks

Pete
 
Atteneuation

Pete,
The lightspeed doeas not completely mute the signal. The reason is that a normal pot will short up the input, the LDR pair will give you something like 60 ohms to ground and maybe 7K series, so about 1% of the signal passes through.
I just changed my pots to single pots with an on-off switch. The switch kills the LDR's and is wired so the shunt element is next to the off position.
But in my application it is not a real big deal, there is a selector switch with mute positions.

George
 
Goerge & George!, thanks for your replies, the audible sound is not a problem, its just I thought that something may have been wrong but youve put my mind at rest.

Just one further question: -

The impedance of my amp is 20K I think! this a lot lower than the 100K ideal that you stated in your first post. Do you think it would worthwhile (in terms of sound quality) to try and increase the impedance of the Lightspeed by increasing the value of the pot to 200K.

Thanks again.

Pete
 
More data please?

Does anyone have a current versus resistance table/graph for a batch of Siliconix NSL-32SR2S LDRs?

I am putting togther a remote control multichannel preamp that "should" be able to auto calibrate the LDR for each of the channels.

I haven't bought the LDRs yet hence I would appreciate any information you might have.....

Thanks
 
Re: More data please?

barryblue said:
Does anyone have a current versus resistance table/graph for a batch of Siliconix NSL-32SR2S LDRs?

I am putting togther a remote control multichannel preamp that "should" be able to auto calibrate the LDR for each of the channels.

I haven't bought the LDRs yet hence I would appreciate any information you might have.....

Thanks
There was something some body posted somewhere in the middle of this thread, I'm not sure if it was for the Silonex ones though. But also burried somewhere on the Silonex web site there is something as well, but it's in a obscure place from what I remembered and you have to open up a few pages before you find it, please post it up if you find it as I should have saved it myself, from memory it also had a graph on distortion V input voltage up to 80 or so volts.

Cheers George
 
psymo said:
Goerge & George!, thanks for your replies, the audible sound is not a problem, its just I thought that something may have been wrong but youve put my mind at rest.

Just one further question: -

The impedance of my amp is 20K I think! this a lot lower than the 100K ideal that you stated in your first post. Do you think it would worthwhile (in terms of sound quality) to try and increase the impedance of the Lightspeed by increasing the value of the pot to 200K.

Thanks again.

Pete

Pete changing the voltage/current control pot's value will not give you much difference in input v output impedence at the same given volume. You will just have to try to get your amps input impedence higher, to at least 50k each so long as your source is low and strong. Or eek! use opamp buffers.

BTW what are your amps, brand and model please, I'll will see if it's posible to up the impdence.

Cheers George
 
Pete changing the voltage/current control pot's value will not give you much difference in input v output impedence at the same given volume. You will just have to try to get your amps input impedence higher, to at least 50k each so long as your source is low and strong. Or eek! use opamp buffers.

BTW what are your amps, brand and model please, I'll will see if it's posible to up the impdence.

Cheers George

Goerge, the amp Im using with the Lightspeed is a 'Peter Daniels' LM3875 Gainclone.

Information on the amp is here Circuit diagram is on page 3 and components used (premium kit) are on page 5.

Datasheet for the LM3875 is here

Thanks again

Pete
 
psymo said:


Goerge, the amp Im using with the Lightspeed is a 'Peter Daniels' LM3875 Gainclone.

Information on the amp is here Circuit diagram is on page 3 and components used (premium kit) are on page 5.

Datasheet for the LM3875 is here

Thanks again

Pete

Pete I just asked Peter Daniel to comment he knows his amps better than I do, but to me it may not be possible as the LM3875 has a biopolar input and you can only go so high before they start to loose the plot,
But there is a work around because you have the Lightspeed hooked up this maybe ok to run it with say a 100k on the input instead of 20k, but never turn it on without the Lightspeed attached as this gives a 7k ref to ground then for the input bipolars of the LM3875. It maybe ok to run it like this, but a bit deadly if switched on without the Lighspeed attached.

Cheers George