Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Lightspeed Attenuator a new passive preamp
Lightspeed Attenuator a new passive preamp
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 17th September 2011, 02:24 AM   #4241
soongsc is offline soongsc  Taiwan
diyAudio Member
 
soongsc's Avatar
 
Join Date: Mar 2005
Location: Taiwan
The LDRs themselves seem to have a log characteristic.
__________________
Hear the real thing!
  Reply With Quote
Old 17th September 2011, 09:06 AM   #4242
portreathbeach is offline portreathbeach  United Kingdom
diyAudio Member
 
Join Date: Jan 2010
Location: Cornwall
Thanks George, your schematic in post #4240 is the one in which I have created in Eagle.

Obviously this design only really needs 2 matched pairs, but as you said, quad matched will obviously be better. As for the 5 volt supply, can this be on the same PCB as the LDRs? or is it best to keep it separate for noise?

You also show in your drawing that the feed to the 7805 is 9 - 18v, these have a max input of 35v according to the datasheet, so would my 25 v be OK, if the regulator was bolted to my heat sink along with the amp chip?

Your MKII circuit shows only 1 100K log pot, this should work with the MKI schematic also shouldn't it? 5v to the wiper, one end to one pair of devices and the other to the other pair.
  Reply With Quote
Old 17th September 2011, 12:20 PM   #4243
georgehifi is offline georgehifi  Australia
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Quote:
Originally Posted by portreathbeach View Post
Thanks George, your schematic in post #4240 is the one in which I have created in Eagle.

Obviously this design only really needs 2 matched pairs, but as you said, quad matched will obviously be better. As for the 5 volt supply, can this be on the same PCB as the LDRs? or is it best to keep it separate for noise?Supply for the led's is totaly isolated from the ldr's so yes it can be on the same board

You also show in your drawing that the feed to the 7805 is 9 - 18v, these have a max input of 35v according to the datasheet, so would my 25 v be OK, if the regulator was bolted to my heat sink along with the amp chip?Yes the more you sink the hotter it will get

Your MKII circuit shows only 1 100K log pot, this should work with the MKI schematic also shouldn't it? 5v to the wiper, one end to one pair of devices and the other to the other pair.
Same as for both, one trimmer only on the louder channel
Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com

Last edited by georgehifi; 17th September 2011 at 12:23 PM.
  Reply With Quote
Old 17th September 2011, 04:33 PM   #4244
portreathbeach is offline portreathbeach  United Kingdom
diyAudio Member
 
Join Date: Jan 2010
Location: Cornwall
Thanks George. In your schematics, you have signal GND and the 5v power GND together, would it not be better to keep them separate, like this:

Click the image to open in full size.

Also, just looking at the LM7805 regulators, because the current this circuit uses is so small (not much more than 50mA) and the regulators are rated at 1A or some at 1.5A output, it shouldn't get very hot, especially if it connected to the heat sink.

Last edited by portreathbeach; 17th September 2011 at 04:53 PM.
  Reply With Quote
Old 17th September 2011, 05:57 PM   #4245
portreathbeach is offline portreathbeach  United Kingdom
diyAudio Member
 
Join Date: Jan 2010
Location: Cornwall
Another quick question. What is the need for a series and a shunt LDR?

Could you not substitute the series LDRs for (1K resistors for example) and then alter the resistance of the shunt LDRs, therefore only needing 2 devices?

I understand in a normal input pot setup, it is 'seen' by the previous piece of audio equipment as the total of resistance of the pot and this is always the same. But surely using 2 x LDRs, you are not guaranteeing a constant input impedance, are you?

Again, sorry if these questions have been answered previously, I have only just got to post #951
  Reply With Quote
Old 17th September 2011, 06:47 PM   #4246
Marra is offline Marra  United Kingdom
diyAudio Member
 
Join Date: Nov 2009
Location: Near Lincoln
Lightspeed Attenuator a new passive preamp
Quote:
Originally Posted by portreathbeach View Post
Another quick question. What is the need for a series and a shunt LDR?

Could you not substitute the series LDRs for (1K resistors for example) and then alter the resistance of the shunt LDRs, therefore only needing 2 devices?

I understand in a normal input pot setup, it is 'seen' by the previous piece of audio equipment as the total of resistance of the pot and this is always the same. But surely using 2 x LDRs, you are not guaranteeing a constant input impedance, are you?

Again, sorry if these questions have been answered previously, I have only just got to post #951
I believe that was Georges mk1 version. The mk2 and better sounding ? version uses the four ldr's. This thing does sound good; just gets out of the way of the music.
  Reply With Quote
Old 17th September 2011, 07:27 PM   #4247
portreathbeach is offline portreathbeach  United Kingdom
diyAudio Member
 
Join Date: Jan 2010
Location: Cornwall
Hi Marra. The drawing I have done is a copy of the MKI version (post #33), the MKII version uses a single pot and one trimmer I believe.

Are there any reasons why having a varying impedance (using a fixed resistor and an LDR) other than a more steady input impedance (series/shunt setup) would benefit. Does audio equipment NEED to 'see' a fixed impedance? If it doesn't, then wouldn't a single LDR and resistor on each channel be easier to implement and give a smoother curve, as you are only adjusting one resistance, not 2.

Last edited by portreathbeach; 17th September 2011 at 07:39 PM.
  Reply With Quote
Old 17th September 2011, 08:59 PM   #4248
georgehifi is offline georgehifi  Australia
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Quote:
Originally Posted by portreathbeach View Post
Thanks George. In your schematics, you have signal GND and the 5v power GND together, would it not be better to keep them separate, like this:

Click the image to open in full size.

Also, just looking at the LM7805 regulators, because the current this circuit uses is so small (not much more than 50mA) and the regulators are rated at 1A or some at 1.5A output, it shouldn't get very hot, especially if it connected to the heat sink.
Sorry the powersupply is not grounded to the chassis or signal. It makes no discernable difference if it is, I just let it float because it can, as wall warts have no earth and the led and it's powersupply have no electrical connection to chassis or rca earth.
Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  Reply With Quote
Old 17th September 2011, 09:05 PM   #4249
portreathbeach is offline portreathbeach  United Kingdom
diyAudio Member
 
Join Date: Jan 2010
Location: Cornwall
Hi George, thanks for your input. What would happen with a circuit like this:

Click the image to open in full size.

Not sure if 200K would be high enough.

Any thoughts on how this would work

Last edited by portreathbeach; 17th September 2011 at 09:09 PM.
  Reply With Quote
Old 17th September 2011, 09:05 PM   #4250
georgehifi is offline georgehifi  Australia
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Quote:
Originally Posted by portreathbeach View Post
Another quick question. What is the need for a series and a shunt LDR?

Could you not substitute the series LDRs for (1K resistors for example) and then alter the resistance of the shunt LDRs, therefore only needing 2 devices?

I understand in a normal input pot setup, it is 'seen' by the previous piece of audio equipment as the total of resistance of the pot and this is always the same. But surely using 2 x LDRs, you are not guaranteeing a constant input impedance, are you?

Again, sorry if these questions have been answered previously, I have only just got to post #951
There are many advantages mentioned for the MKII over the MKI, but the MKII is much harder and costlier to make, otherwise I would have stuck with the MKI.
Cheers George

Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  Reply With Quote

Reply


Lightspeed Attenuator a new passive preampHide 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

Forum Jump


New To Site? Need Help?

All times are GMT. The time now is 04:22 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki