MM phono stage capacitance matching - diyAudio
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 8th June 2007, 03:51 PM   #1
diyAudio Member
 
Join Date: Mar 2005
Location: UK
Default MM phono stage capacitance matching

My phono stage has an input capacitance of 470 pF. The cart I want to use has a recommended input capacitance of 60 pF. So I need to put a 68 pF capacitor in series with the hot line - what material/spec would work best?

Thanks
  Reply With Quote
Old 8th June 2007, 04:49 PM   #2
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

I do not think it works like that, the response would be "flat"
on the connected side of the capacitor, not at the input.
And your forgetting the parallel capacitive loading of the cable.

What cartridge do you have ?
100pF loading is low for the tonearm cable.

/sreten.
  Reply With Quote
Old 8th June 2007, 05:53 PM   #3
diyAudio Member
 
Join Date: Mar 2005
Location: UK
I hadn't forgotten the cable capacitance, just assuming it's a constant. I'm figuring that for the price of a few caps it's worth trying. The bigger question really is what type of cap and tolerance would it be best to experiment with?

It's an Empire LTD 750 - NOS. The phono stage is a Dynavector P-75.
  Reply With Quote
Old 8th June 2007, 06:02 PM   #4
mlloyd1 is offline mlloyd1  United States
diyAudio Member
 
mlloyd1's Avatar
 
Join Date: Feb 2001
Location: illinois
paralleled capacitance adds.
maybe i don't understand what you want to do ...


you say your cartridge wants a load of 60pF
you say your preamp presents a load of 470pF.
sounds to me like your preamp needs to have some input capacitance removed.

yes?

mlloyd1
  Reply With Quote
Old 8th June 2007, 07:17 PM   #5
diyAudio Member
 
Join Date: Mar 2005
Location: UK
You're right, I do want to reduce capacitance. Isn't the formula for series caps the same as for resistors in parallel? i.e. 1/C=1/C1+1/C2. Plugging 470 pF and 68 pF into that equation gives an effective capacitance of 59.405 pF.

As I said, I'm comfortable with the theory, prepared to experiment and would appreciate some guidance on what type of caps/tolerances, etc. Obviously at 4.5 mV output, voltage rating isn't relevant.

I don't really want to remove capacitance from the phono pre as I also use it with other carts.
  Reply With Quote
Old 9th June 2007, 03:13 PM   #6
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Your cartridge load would be correct but you would not be reading
the output of the cartridge - you would have a capacitor in series.

It will not work .......

/sreten.
  Reply With Quote
Old 10th June 2007, 01:38 AM   #7
AuroraB is offline AuroraB  Norway
diyAudio Member
 
AuroraB's Avatar
 
Join Date: May 2001
Location: Norway, -north of the moral circle..
I totally agree with Sreten..there is something seriously wrong in this idea.
But---since you insist on trying,-any cheap capacitor, 20% or better, just for the sake of experimenting. If your idea holds water at all, the type or brand of cap will only affect the subtleties of the matter.
I'm not at all saying that capacitor types don't matter - just that their influence is grossly overrated, in general terms......
  Reply With Quote
Old 10th June 2007, 03:11 AM   #8
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
If your preamp has a 470pF input capacitance , then it must be due to a capacitor soldered across it's input. No standard circuit will have an input capacitance of 470pF in a phono stage without the addition of an external capacitor.
Your solution would be to find the input capacitor and remove it.
Note that this must be a capacitor in parallel with the input and not any series capacitor which might be there to block dc .

The tone arm cables generally have a capacitance in the region of + 100pF per meter. Finding cables lower than 70pF per meter might not be easy. So even with zero capacitance in the preamp you might not be able to get a 60pF load.
Unless of course you install the phono preamp board right under the turntable ( inside).

Then the contributing capacitance will be only the wires in the tone arm and the preamp input. You might have to change the input capacitor in the preamp to achieve the 60pF loading.

In any case you probably have a very rolled off HF performance right now with a 470 pF plus 120pF cable = 590pF

You could
1. Remove the input capacitance on the preamp and
2. keep the phono preamp right next to the turntable with a cable less than 1 foot in length.
You might just be able to manage close to 60 pF with a low capacitance cable. Check Belden or Canare for a low capacitance coaxial cable.

The best method would of course be to install the phono board inside the turntable . Some turntables do come with inbuilt phono stages. I have no idea how good those are. But it certainly helps with capacitance AND eliminating the performance of phono cables (?).
Cheers.
__________________
AM
  Reply With Quote
Old 11th June 2007, 07:30 AM   #9
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
Given all the problems ashok has outlined in reducing capacitance, you have to wonder about a cartridge needing 60pF. Is it a typo, or was the manufacturer completely unaware of the real world?
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 11th June 2007, 08:24 AM   #10
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
I think the 60pF requirement must be a typo .
Many older generation MM cartridges needed 470 pF but nowdays they are closer to 250pF. Aren't there some requiring 125pF capacitance ?
In any case cable capacitance + tone arm wiring capacitance+preamp input capacitance need to be accounted for.
It's easy to add capacitance but impossible to remove what is already there.

It's clear the only two places where you can do something to reduce capacitance is
1. Reduce length or type of coaxial interconnect cable
2. Change the capacitor at the input of the preamp.
Cheers.
__________________
AM
  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
Mosfet Output Stage Capacitance Nelson Pass Pass Labs 17 25th May 2012 11:47 AM
DIY phono to replace roksan phono stage seroxatmad Analog Line Level 6 14th December 2008 06:46 PM
Should I replace Scott 222C phono stage with Claret phono stage? Bing Yang Analogue Source 0 22nd August 2005 07:41 AM
Pearl Phono stage output matching? Gregorio Analogue Source 1 30th June 2004 03:06 PM
Pearl Phono stage cartridge matching Gregorio Pass Labs 3 30th June 2004 11:10 AM


New To Site? Need Help?

All times are GMT. The time now is 04:12 PM.


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