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Old 8th November 2012, 08:00 PM   #4841
Nanook is offline Nanook  Canada
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Default Wow, what a thread.

This is certainly a very long one.

Thanks to georgehifi for all his support and willingness to share this with us DIY types. Not everyone is so generous.

I have an old QED PCC (passive control centre: Alps Blue based, 6 input, tape monitoring capable) that I quite like. I have this as a "back-up to a McCormack Micro-Line Drive that can be used as a full passive line stage or an active one. Depending on the equipment that I may be listening to, I select the active or passive output (completely separate circuits). I had the opportunity to speak with Steve McCormack via telephone and was pleased to find out that both the MLD and the TDL (Total Line Drive) were designed to be used completely as passive stages with the inclusion of an active output to facilitate the use of powered sub-woofers or remote amplifiers.

A friend is looking at building one (or two) of George's Lightspeed Attenuators. My audio "partner" is building one and I will most likely tag along in the adventure. As this thread is so long, and I haven't obviously read it, can these be implemented within the existing case of an integrated amplifier (a newer T-class amp)? I understand that a power supply may need to be added (or taken from the integrated amp's existing power supply and regulated), but other than that and assuming there is appropriate space to allow the retro-fit, is there a technical reason why this would not work?
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Old 8th November 2012, 08:03 PM   #4842
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Should be a piece of cake. Just regulate a few times and you are good.
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Old 9th November 2012, 12:19 AM   #4843
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Quote:
Originally Posted by Nanook View Post
This is certainly a very long one.

Thanks to georgehifi for all his support and willingness to share this with us DIY types. Not everyone is so generous.

A friend is looking at building one (or two) of George's Lightspeed Attenuators. My audio "partner" is building one and I will most likely tag along in the adventure. As this thread is so long, and I haven't obviously read it, can these be implemented within the existing case of an integrated amplifier (a newer T-class amp)? I understand that a power supply may need to be added (or taken from the integrated amp's existing power supply and regulated), but other than that and assuming there is appropriate space to allow the retro-fit, is there a technical reason why this would not work?

Thanks for your support Nanook.
Inside a cool running Tripath (class D) amp should not be a problem, only in tube amps/preamps or warm running A/B amps there is a problem with heat fluctuations inside is can effect the LDR/LED packs to cause imbalance problems, which can undo the matching/calibration.
The other thing to watch out for is some D class amps are very low input impedance, some I've seen as low as 1k which is not a good match for the Lightspeed Attenuator circuit, you need say over 20kohm input to get the best out it in this situation with short connection to the input stage.

Cheers George
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Old 11th November 2012, 06:23 PM   #4844
Nanook is offline Nanook  Canada
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Default thanks George.

Quote:
Originally Posted by georgehifi View Post
Inside a cool running Tripath (class D) amp should not be a problem.

The other thing to watch out for is some D class amps are very low input impedance, some I've seen as low as 1k which is not a good match for the Lightspeed Attenuator circuit, you need say over 20kohm input to get the best out it in this situation with short connection to the input stage.

Cheers George
Great. I would actually check the input impedance with the manufacturer prior to continuing on. I suppose one could always put a 20 or 30 kΩ resistor in parallel to the input if the input impedance is too low? Or a Zobel of some sort?
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Old 11th November 2012, 07:26 PM   #4845
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Sorry to correct Nanook, if you put a 20k resistor in parallel with the input resistor that goes to ground that's already there lets say it's also 20k for simplicity, what you end up with is an input impedance of 10k, which is even worse.
Now, what you can do with amps that have fet inputs/or tube is to remove the 20kohm that's already there and replace it with 47k or even 100k.
But ask your amp manufacturer before doing this to make sure that will still behave it self. As if it is bi-polar input it could get dc offset on it's output, become unstable, or the bias could change.

Cheers George
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Old 11th November 2012, 07:49 PM   #4846
Nanook is offline Nanook  Canada
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Default George: please no apologies required.

I asked the question to help clarify my understanding, that's all. I asked for help and you have been gracious enough to respond. Thank you.

I will email the manufacturer to check to see what the input and load impedances are of the existing volume control. Else it will be a box similar to your production model.
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Last edited by Nanook; 11th November 2012 at 07:53 PM. Reason: added comment
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Old 12th November 2012, 09:23 PM   #4847
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I am getting all the parts to assemble my preamp and I am having trouble sourcing the 100K dual pot.

My question is, why the dual pot? Is the reason to limit any current via the wiper? I have a hefty good quality single 100K pot. The pot is not acting as voltage divider, but as a reverse variable resistor to limit the current through the led. Am I missing something?
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Old 12th November 2012, 10:22 PM   #4848
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This is what you need avincenty

RV24BF-10-15R1-A100K Alpha (Taiwan) | Mouser

The reason for the 100k dual log is to share the current, and to have a nice progressive feel, slow progressive to start all the way to 2 o'clock then ramping up fast after this.
Mind you this only has this kind of progressive feel if the NSL32SR2S are a close quad matched set, if they are only pair matched it can start too quick or way too slow and have nothing/too much in either half of the rotation.
Cheers George
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Old 13th November 2012, 02:06 AM   #4849
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Thanks for the link.

Can you elaborate why a single pot doesn't accomplish the same thing? If both are linear then the resulting resistance from both sides of the wiper (or common leg on dual pot) should add to 100K. If one side has 75K, the other side will have 25K, etc... Wouldn't the matching of both sides of a single pot be closer than two opposite sides of a dual pot?

I must be missing something right in from of me.

Thanks for your assistance.
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Old 13th November 2012, 03:02 AM   #4850
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The current of two led's on the dual per track against the current of all 4 led's on a single. You must know that pots carbon or plastic don't like DC on them at the best of times, this minimizes the amount of dc.
Try it with a linear 100k dual and it will behaved very differnet, too much too fast, and you have to remember the NSL's are not linear either in their resistance and led intensity overhaul span, this has a big effect on it.

Cheers George
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