Go Back   Home > Forums > Source & Line > Analogue Source

Analogue Source Turntables, Tonearms, Cartridges, Phono Stages, Tuners, Tape Recorders, 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 9th July 2007, 01:38 AM   #1
legarem is offline legarem  Canada
diyAudio Member
 
legarem's Avatar
 
Join Date: Jan 2005
Location: Quebec city
Default Changing a deemphasis from 50 uS to 75 uS

I recently bought a Rogers Panthera tuner which has a deemphasis of 50uS. The deemphasis circuit is encapsulated in a module so I canít reach the parts inside this module.

Can I build a RC circuit inserted in the output section so I can have a 75 uS deemphasis.

Do I have to add a 75uS deemphasis with R and C or a 25uS deemphasis because I already have a 50uS deemphasis in the tuner ?

Thanks a lot.
  Reply With Quote
Old 9th July 2007, 02:39 AM   #2
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Your 50uS filter is a 1st order filter ( I assume). Adding another filter in series will increase the ultimate response to a 2nd order slope. The Q of the filter will be another problem.
To change to a 75uS filter you have to get the signal before the 50uS filter and possibly you need to remove the 50uS filter as it will also be loading the circuit.
Do you have a circuit diagram ?
Why would they encapsulate a 50uS filter ?
A simple 50uS filter is just a R and C combination.
L and C if it's 2nd order.

Are you sure what is inside the module ?
Cheers.
__________________
AM
  Reply With Quote
Old 9th July 2007, 10:34 AM   #3
legarem is offline legarem  Canada
diyAudio Member
 
legarem's Avatar
 
Join Date: Jan 2005
Location: Quebec city
Quote:
Originally posted by ashok
Your 50uS filter is a 1st order filter ( I assume). Adding another filter in series will increase the ultimate response to a 2nd order slope. The Q of the filter will be another problem.
To change to a 75uS filter you have to get the signal before the 50uS filter and possibly you need to remove the 50uS filter as it will also be loading the circuit.
Do you have a circuit diagram ?
Why would they encapsulate a 50uS filter ?
A simple 50uS filter is just a R and C combination.
L and C if it's 2nd order.

Are you sure what is inside the module ?
Cheers.

Unfortunatly, I can't get any schematic diagram for this tuner

I looked at the circuit after the MPX chip and there's this module with two output transistors.

Last night I opened the module and there's 4 chokes which can be adjusted (variable) and 6 little capacitors.

For me, this means a lot of trouble to change something. This appears more complicated than a simple R C filter.
  Reply With Quote
Old 9th July 2007, 11:36 AM   #4
oshifis is offline oshifis  Hungary
diyAudio Member
 
Join Date: Mar 2004
Location: Budapest, Hungary
That encapsulated LC filter is the multiplex (MPX) filter, to avoid 38 kHz interference with the bias oscillator at tape recording. The deemphasis is usually a parallel RC complex at the stereo decoder chip. What is the type of the stereo decoder chip? The datasheet should show which pins are the deemphasis RC components connected to.
  Reply With Quote
Old 9th July 2007, 05:25 PM   #5
legarem is offline legarem  Canada
diyAudio Member
 
legarem's Avatar
 
Join Date: Jan 2005
Location: Quebec city
Quote:
Originally posted by oshifis
That encapsulated LC filter is the multiplex (MPX) filter, to avoid 38 kHz interference with the bias oscillator at tape recording. The deemphasis is usually a parallel RC complex at the stereo decoder chip. What is the type of the stereo decoder chip? The datasheet should show which pins are the deemphasis RC components connected to.

As I will never have a tape recorder in my system should it be a great idea to get rid of this LC filter ?

Thanks
  Reply With Quote
Old 9th July 2007, 06:52 PM   #6
diyAudio Member
 
Join Date: Dec 2004
Location: Mt Pleasant Sc
Default De Emphasis

Hey:
To my lousy memory the only difference that you will hear is a lower level of high frequency response.
I think that if you leave the de emph. alone you will never know the difference except that you MAY have to reduce the treble a bit. Now if you are older like me the increased treble sounds good.
Hope this helps
Ed
  Reply With Quote
Old 10th July 2007, 01:12 AM   #7
legarem is offline legarem  Canada
diyAudio Member
 
legarem's Avatar
 
Join Date: Jan 2005
Location: Quebec city
I finally found the R and C which make the deemphasis

There's a 2.7K and a .022 uf cap for each channel as seen in the drawing.

In the National typical application sheet, they show a 3.9K and a .022 uF. I tried a 3.9K instead of the 2.7k and I think that the sound is perharps worse with more or as much treble than it was.

Can someone help me

Thanks
Attached Images
File Type: jpg deem.jpg (9.0 KB, 180 views)
  Reply With Quote
Old 10th July 2007, 01:24 AM   #8
hughmon is offline hughmon  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Los Angeles
Rather than change the resistor, which might effect bias points in the chip, why not just parallel 5.6 nF caps with the existing .022 uF caps? That should be pretty close to a 75 uS time constant.
  Reply With Quote
Old 10th July 2007, 01:53 AM   #9
legarem is offline legarem  Canada
diyAudio Member
 
legarem's Avatar
 
Join Date: Jan 2005
Location: Quebec city
I'll try 5.6nF

With the standard setting of .022 and 2.7k, I calculate a deemphasis of 59.39 uS. This is not near 50 uS I tought this tuner had.

I also tried .033 uF to replace both .022 uF and the multiplex didn't want to decode in stereo with .033 uF caps.
  Reply With Quote
Old 10th July 2007, 10:01 AM   #10
oshifis is offline oshifis  Hungary
diyAudio Member
 
Join Date: Mar 2004
Location: Budapest, Hungary
Something is not correct here... The deemphasis pins not 1 and 2, but 4 and 5. The capacitor value of 33 nF should be OK for 75 us, since there is an internal resistor shunting the external 2.7 k.
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to change deemphasis from 75uS to 50uS for Fisher FM100 DC88 Tubes / Valves 8 24th July 2008 02:02 PM
Deemphasis computer software? Agent327 Everything Else 3 5th September 2007 08:47 PM
Rogers Panthera (T75) deemphasis legarem Analogue Source 0 3rd July 2007 02:37 AM
fm deemphasis? peace brainerd Solid State 3 6th March 2006 04:41 PM
Changing BF's for MJE's Bensen Solid State 2 27th May 2003 05:39 AM


New To Site? Need Help?

All times are GMT. The time now is 05:54 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