Lightspeed Attenuator a new passive preamp - Page 488 - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th December 2012, 11:20 PM   #4871
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
This is, for instance, how digital isolators work. They use either transformers, radiofrequency, light or capacitance to isolate either DC or unwanted voltages from the signal. Common mode noise is especially a problem with the capacitive isolators and its effect is heightened with more current. In our situation we use a lot of current compared to what digital isolators are using. Capacitive coupling is a great way to transfer digital AND analog signals. This is one of the reasons that capacitors across the LED side are an advantage. Use a big cap for any decent sized fluctuations, which would be noise, and put a tiny, really fast cap, in parallel with it. The two together will deaden a lot of the problem. The reason to use current sources is that they reject noise so well.
Just read the first page of this article
http://waltjung.org/PDFs/Sources_101_P1.pdf
If we can reject noise at all audio frequencies, great. If we can reject above audio frequencies, also great. Even if its out of our hearing it interacts with the frequencies we can hear, causing distortions.
If you guys don't believe me, then just get the crappiest, oldest, lowest frequency switching supply you've got and replace the LDRs power supply with this thing and then tell me you don't hear the noise from the supply. The fact that it will be painfully obvious just means that noise does couple from the LED to the resistive side of the LDR. A 7805 has decent noise rejection but its rather poor compared to what we can get and for a cost only a few times more than the 7805 itself. Look at Jungs supplies. Complicated? No. You can all do it. There isn't even any tuning involved. Besides a couple of them that might oscillate this is childs play now that Walt has done the work for us.
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
  Reply With Quote
Old 5th December 2012, 11:50 PM   #4872
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
Lets look at a 7805.
Cheapest one is 69 cents on Mouser. Its a Fairchild.
Line and load regulation between 4 and 100mV. That's not so hot.
Output noise is 42microVolts per output volt. So 210uV. 210/1000000 of a Volt. .00021V. This is down a little more than 70dB.
Walt Jungs current sources, at 18V, are putting out between -110dB and -140dB, sometimes hitting -150dB and should cost about the same as the LM7805 as long as you don't buy the big mosfets he uses in some of them. Just use the little TO92 ones. So add a minimum of 2 zeroes after the decimal as an improvement over the 7805.
Output resistance is about equal on both.
With the 7805, by the time we get to 10k its -55db (ripple rejection) and keeps right on going up.
In my estimation the 7805 is a good starting point. Make your Lightspeed with a regular TO92 or TO220 regulator. It will sound amazing. Then start putting small circuits to clean up the regulated power after it. They should just plug n play. This will reap dividends in improved sound.
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
  Reply With Quote
Old 6th December 2012, 02:11 AM   #4873
wlowes is offline wlowes  Canada
diyAudio Member
 
wlowes's Avatar
 
Join Date: Mar 2007
Location: Toronto
Blog Entries: 9
Default I took the approach by udailey

I started with a 7805. Sound was very good. It did not seem to make much difference if there was a walwort before the 7805 or a 9 volt battery.

Just for interest sake, I played around with different caps after the 7805 that happened to be in my current cap supply. Be it psychological or voodoo, there were definite preferences to be seen just based on the cap.

I have not built a Salas shunt. Seems a bit much to build a $50 regulator to power $10 worth of LDRs. Reading the TNT article cited above, it was clear from their measurements that the Zener + Emitter Follower measures very well vs 7805, 317 and various optimizations of the 417. The parts cost $5 and you can knock one out in a few minutes. I replaced the 7805 with it and the sound is better.

I could not wrap my head around why power supply to LDR would have any impact on LDR resistor. If Udailey's theory of capacitive connection is accurate, then it could explain what my ears have heard.

Seems very reasonable that Salas shunts and current source supplies could yield a flavour to the sound that some would find pleasing. For me, I am not experimenting further as the sound is so darn good now there are other areas of the system that offer far greater gains.

Another thought.. why to we build +5v regulator and then add 100r resistors? Why not tune the regulator down to +4v and drop the resistors. Would a better quality pot have any advantage in reducing noise in the power supply? I currently use the cheapest off the rack part that can be found. I like the simplicity, but it begs the question if there are optimizations to be had in this dpt?
  Reply With Quote
Old 6th December 2012, 09:07 AM   #4874
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
omitting the current limiting resistor is likely to lead to damaged LDRs.
__________________
regards Andrew T.
  Reply With Quote
Old 6th December 2012, 09:12 AM   #4875
diyAudio Member
 
Join Date: Nov 2009
Location: UK, Manchester
damn, I must build separate Salas then
bad thing is that all five-six positive regs I've built slightly change the output voltage by 5-10% after 10-15mins of operating time.
  Reply With Quote
Old 6th December 2012, 10:18 AM   #4876
diyAudio Member
 
Join Date: Feb 2011
Blog Entries: 3
What about a decent wallwart feeding an amb sigma 25?
__________________
"Simplicity is the key to brilliance"
  Reply With Quote
Old 6th December 2012, 11:11 AM   #4877
Marra is offline Marra  United Kingdom
diyAudio Member
 
Join Date: Nov 2009
Location: Near Lincoln
Quote:
Originally Posted by dtses View Post
damn, I must build separate Salas then
bad thing is that all five-six positive regs I've built slightly change the output voltage by 5-10% after 10-15mins of operating time.
You will be pleased with the result.
  Reply With Quote
Old 6th December 2012, 12:40 PM   #4878
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
Quote:
Originally Posted by dtses View Post
damn, I must build separate Salas then
bad thing is that all five-six positive regs I've built slightly change the output voltage by 5-10% after 10-15mins of operating time.
This might fix your problem, although I have only watched this effect after considerable 'warm up' time. Put a cap on each leg of the reg in addition to your input and output electrolytics. These new caps should be film or better yet tantalum. Great results with .47uf films but even better with a few uf of tantalum on each leg. Do not go overboard as to much capacitance on especially the adjust leg will screw it up. Each of these will go to ground. The one on adjust is parallel to the adjust resistor, so you are making sure that the positive lead of the cap is basically on the regulator leg. This will stabilize the voltage a LOT. However you cant put a tantalum on the adjust pin on a fixed regulator.
Also, put some resistance to ground after the regulator. For a regulator to operate correctly it needs a load on it. Some really good ones only need a mA or two but some of them really need to be shown the right path and need 5-10mA of a load which the LDRs will not give them in most volume positions. So put a correctly sized (watts) resistor from output to ground ( a LED in series with this resistor will give you ON indication) to load this guy. By the way you guys can get away with the TO92 regulators in this circuit as they can handle 100mA as long as you make sure you don't drop to much voltage, which in the case of running only LDRs you wont have a problem.
Oh, another way to help improve regulation is to heatsink the regulator in addition to the caps and loading.
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com

Last edited by udailey; 6th December 2012 at 12:46 PM.
  Reply With Quote
Old 6th December 2012, 12:48 PM   #4879
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
Quote:
Originally Posted by MagnumOpus View Post
What about a decent wallwart feeding an amb sigma 25?
http://www.amb.org/audio/sigma25/
This regulator is simply the 7805 we are talking about but with input and output electrolytics, a load resistor and a heatsink. Its a very good start but you should still put caps on the LED side of the LDR at a minimum and you can follow the regulator with a current source which can simply be a LM317 in CCS mode which is show in Walt Jungs paper and also can be found in the LM317 datasheet.
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
  Reply With Quote
Old 6th December 2012, 01:04 PM   #4880
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
Quote:
Originally Posted by wlowes View Post

I have not built a Salas shunt. Seems a bit much to build a $50 regulator to power $10 worth of LDRs. Reading the TNT article cited above, it was clear from their measurements that the Zener + Emitter Follower measures very well vs 7805, 317 and various optimizations of the 417. The parts cost $5 and you can knock one out in a few minutes. I replaced the 7805 with it and the sound is better.
If you are now looking for other parts of your signal chain to improve, I have a suggestion:
Another LDR on each channel. There is likely a series resistor after the Lightspeed, probably directly after the RCA input on your amp. It is in series with the signal and directly between the RCA and input of your amp circuit. Replace this guy with an LDR. A tiny supply that feeds off your amps supply can run this LDR. A LM334 current source with a trimmer as Rset will be a good way to regulate the value of this LDR. It will give another sonic improvement. You can do this on the output of your source as well since there is likely a 47R resistor or higher on the source's output. It will be the last thing the source sees before it gets to the RCA.
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 02:32 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2