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Old 4th May 2007, 12:53 PM   #631
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George,

My pre is configured as active:

Input to LDR's (controlled via 100k DACT pot) to AD815 chip to output

Any benefit in making the change to a lower value pot?

Ryan
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Old 4th May 2007, 01:48 PM   #632
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Default Depends

Ryan,
It depends on your sources. If your sources have strong putput stages that either are dc coupled or have large value coupling caps the lower value might be better in your case. If you have vacuum tube output stages, the 22K might be better.
Measure what it actually is first. This is fairly easy, even with an active stage after the LDR's.
Select an input, then measure from that input to ground. This willl include the series LDR and the shunt LDR together. This is paralleled with the input of the active. I think your active is a 1K series and 100K to ground. So whatever the input of the LDR's is, it is paralleled with 101K. This will lower the measured impedance of the LDR only slightly. I think you will measure about 10K.
Mine was 22K due to accidently ordering the wrong LDR's. The R2 LDR's are lower resistance than the R3 I have.Other were measuring 9K or so, and I was getting 20 -24K.
I seem to remember georgehifi stating that the commercial Lightspeed attenuations are a little lower than 10K.
If you want to lower the impedance of the attenuator, try a 50K pot. This lower lower it to less than half of what is is with a 100K.
Good luck, if you try a different value pot, let us know how it works out.

George
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Old 9th May 2007, 05:27 PM   #633
tunes10 is offline tunes10  Netherlands
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Default high source impedance

Probably been asked before, but what if your source has an output impedance of, say, 2.3 kOhm?

I'm talking about the 47 Labs Shigaraki CDP. I have one. And I want to go passive after hearing the result with an DIY pot/resistor based model from a friend.

Then someone had to spoil it by saying my source output impedance is too high for the Lightspeed...

Any workarounds?

The Shigaraki doesn't seem to mind the mismatch in the current passive set up. Even dynamics are almost good. And tonally it's heaven.

Sorry to just pop in, but this seems to be the thread for Lightspeed questions.

Tunes10
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Old 9th May 2007, 09:29 PM   #634
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Default Re: high source impedance

Quote:
Originally posted by tunes10
Probably been asked before, but what if your source has an output impedance of, say, 2.3 kOhm?

I'm talking about the 47 Labs Shigaraki CDP. I have one. And I want to go passive after hearing the result with an DIY pot/resistor based model from a friend.

Then someone had to spoil it by saying my source output impedance is too high for the Lightspeed...

Any workarounds?

The Shigaraki doesn't seem to mind the mismatch in the current passive set up. Even dynamics are almost good. And tonally it's heaven.

Sorry to just pop in, but this seems to be the thread for Lightspeed questions.

Tunes10
What was the value of your friends pot?
If similar resistance to the Lightspeed then ok you will like the sound of the Lightspeed as well, no difference as the loading will be the same just a better sound.

Cheers George
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Old 10th May 2007, 09:57 AM   #635
tunes10 is offline tunes10  Netherlands
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Thanks, George.

My friend is checking with the DIY builder, I'll get back on it.

This impedance matching is starting to confuse me.

Everyone agrees that mistmatches cause flawed dynamics with traditional resistor based passives.

TVC's are supposed to solve this because source and power amp don't see eachothers impedances. Still, TVC-makers advise low source imputs, <1 kOhm. Than I asked one maker about my high source imp and he answered: no problem, I have clients with 2.2kOhm source impedances and that works fine.

Either the theory is relative or the sales opportunity absolute.

I don't know. But I think the first assumption might be true. I have tried direct injects with players with built in digital and analoque attenuators. These players had lower output impedances than the Shigaraki. But they had all the dynamic problemes that the Shigaraki doesn't. Go figure.
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Old 10th May 2007, 10:19 AM   #636
Nordic is offline Nordic  South Africa
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Getting close to doing one of these... and wondered if there would be any benifit to useing brightness matched LEDs... I still have pack of 5 I bought from RS for some reason, can't remember.
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Old 10th May 2007, 10:56 AM   #637
AndrewT is offline AndrewT  Scotland
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Hi Tunes,
it seems complicated due to a number of different effects all working together to try to reduce the performance of your system.

There is usually a DC blocking capacitor at the output of a source and again at the input of the next stage (amplifier).
This DC blocking cap will act as a high pass filter when feeding into a resistive load (the amplifier).
Two caps in series will reduce the bandwidth more than a single DC blocking capacitor.
Removing the DC block can affect the way the amplifier sets it's output offset. Don't interfere with this unless you know how to measure and correct the output offset.

All cables and inputs have some capacitance.
Any source feeding this capacitance will act as a low pass filter.
Changing the resistances and capacitances will change the effective bandwidth of the system. Some amplifiers rely on accurate bandwidth setting of this low pass filter to ensure stability into any complex load. Again interfering with the low pass filter is only for the experienced DIYer.

To help alleviate these filter problems, a number of rules have developed over the decades.
One of these is that Rs << Rload, at least a 1:5 ratio and preferably 1:10 Rs:Rload is recommended.
If a passive pre (volume control) is in place, then the 1:5 ratio should apply on the source side and again should also apply on the receive side.

Lets look at some real numbers by way of explanation.
Rs=100r, DC block=1uF.
Rload=Zin=50k, Dc block=4.7uF, Cin 1nF, Rin=680r.
Cable capacitance 70pF/m and 3m long (=210pF).
Low pass filters: 1uF & 50K + 1uF & 50k.
combine the two series caps to give 0.83uF.
F-3db=3.8Hz, F-1db~7.6Hz. F=1/2/Pi/R/C (in Farads and ohms).
High pass filters:100r & 1nF + 100r & 210pF + 680r & 1nF,
combine the capacitance to give an effective value of 1.21nF.
F-3db=7.6MHz (100r & 210pF)
F-3db=169kHz (100r+680r & 210p+1nF). F-1db~85kHz.
These two are far enough apart that they do not combine 9in the audio sense) to any significant effect.
The bandwidth for the interconnection is thus 3.8Hz to 169kHz. This will work, but some builders will look for slight alterations both up and down to suit their preference.

Put in some values for high Rs or low Zin or low DCblock and see how narrow the bandwidth can become.

Insert a passive pre into the system and do the numbers before the pre and again after the pre and you will begin to see how complicated it becomes. For even more complication adjust the volume and examine the effect on the low pass filter (yes, frequency moves around with volume).
No wonder there is conflicting advice, particularly from salesmen who care more about their bonus than finding out the audible effect of changes in the interconnects. Fortunately some are very diligent and honest, find them and use them.
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Old 10th May 2007, 11:14 AM   #638
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Quote:
Originally posted by tunes10

This impedance matching is starting to confuse me.

Everyone agrees that mistmatches cause flawed dynamics with traditional resistor based passives.

As you have already discovered it's all rather system specific. The impedance issue is determined by the power amp and so far i haven't noticed you mentioning yours. IME most solid state power amps require an active pre for happiness while tube amps do not.

I've used a high inductance (S&B) TVC for several years and you may drive from a 2.3k source only if you particularly dislike listening to the frequency extremes.

Even if the impedance matching looks great on paper a passive pre may still sound transparent but anaemic.
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Old 10th May 2007, 12:18 PM   #639
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Quote:
Originally posted by analog_sa
Even if the impedance matching looks great on paper a passive pre may still sound transparent but anaemic.

Yes it is the most transparent of all, but your way off with the anaemic statment. This will only happen if you have loaded down the source or the output of the passive too much.

If not loaded down by making sure your source is low <100ohms and your amp input load (impedance) is high >60kohms, there is no active pre that can match the true dynamics of a passive.

Note: I said ( true dynamics ), not the squeezed out shoot you in the face dynamics that active pre's give, that is very impressive for 5 mins then wears you out very quickly, but the big, rich, tight full bodied dynamics that you can listen to cd after cd I hear with a correctly impemented Lightspeed passive.

Cheers George
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Old 10th May 2007, 01:10 PM   #640
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Quote:
Originally posted by georgehifi



Yes it is the most transparent of all, but your way off with the anaemic statment.

Hi George

If you read what i wrote you would notice a "may" qualifier. Excellent dynamics with a passive stage are certainly possible but by far not guaranteed. Good bass is similarly tricky And impedance matching, although essential is not necessarily sufficient. This is a general observation as i have no experience with Lightspeed.

<100ohms source and >60k input impedance will certainly make things much easier, especially if the passive controller is integrated in the power amp. Sadly, a lot of commercial units do not fit this criteria.
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