The dreaded impedance thingie.... - diyAudio
Go Back   Home > Forums > General Interest > Everything Else

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

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 12th July 2006, 11:47 PM   #1
diyAudio Member
 
Lostcause's Avatar
 
Join Date: Aug 2005
Location: NW UK
Default The dreaded impedance thingie....

OK don't shoot me, I've read a lot of posts on this subject and I'm in need of clarification. I'm not an EE, not by any means, so please be gentle!

1. Does the input impedance of my amp6 change with the position of my volume pot? I thought it must do but then how can you match it?

2. Can I simply change the output impedance of my modified PS1 by changing the bridging resistor at the outputs?

3. Does anyone know how to easily measure these things?

Hope you can help chaps.

Lee
  Reply With Quote
Old 13th July 2006, 12:45 AM   #2
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
(1). Probably by less than 10%.
(2). No.
(3). Yes, but you need some kit.

Your input impedance is probably the same as that of the volume control, and will be written on the side of the volume control. If it's a transistory thing it will probably be 10k. If it's new valve it will probably be 100k. If it's old valve it might be 500k.

Output impedance is probably harder. If it has op-amps then it's probably less than 100 Ohm. If it has discrete transistors, then it's probably less than 600 Ohm. If it's valve, it could be anything. New valve might be less than 2k Ohm, old valve could be up to 50k Ohm.

Why do you need to know?
__________________
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 13th July 2006, 04:58 AM   #3
diyAudio Member
 
I_Forgot's Avatar
 
Join Date: Jan 2005
Location: Phoenix, Az.
You do not need to worry about impedance matching with line level audio signals. Most devices are designed to have low output impedance and high input impedance, i.e. the outputs "look" like voltage sources to the high resistance/impedance inputs.

Yes the input impedance of the amp varies somewhat with the volume control setting, but unless you are using really long cables, or driving the input from a high impedance source (there are a very few around), you don't have to worry about it.

Phonograph inputs that are driven by relatively high impedance magnetic cartridges do require special attention to get proper frequency response.

I_F
  Reply With Quote
Old 13th July 2006, 09:18 AM   #4
diyAudio Member
 
Lostcause's Avatar
 
Join Date: Aug 2005
Location: NW UK
Cool, so the current set-up is OK? I only listen at very low levels so the pot is only about 30% over most of the time.
Forgive the sketch quality....not exactly a schematic but it should make sense.
Is there anything you would change?
The T-amp has a 25K input resistor......and 25K feedback if that makes any difference.
edit: The mod is as per the "PS1 as a CDP " thread on the digital forum, wired straight from the DAC through a DC blocking cap to the RCA's with a 100k resistor to ground.
Also, does the pot then alter the corner frequency of the RC filter?
Attached Images
File Type: gif 1.gif (5.1 KB, 216 views)
  Reply With Quote
Old 13th July 2006, 10:56 AM   #5
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
You probably get a slightly strange law by loading a 100k pot with 25k - I would use a 10k pot.
__________________
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 13th July 2006, 01:09 PM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
is Tamp wired as inverting?

Has Tamp a DC blocking cap on it's input?

Yes, the following stage (cables and resistors and parasitic capacitance) will affect the turn over frequency of the RC output filter in the DAC.
Any loading less than infinite will have an effect.
Choose components that make the effect inaudible.

From your schematic, it appears you have 100k (DAC resistor) // 25k (input impedance) // 25k (pot-guess) = 11k1.

The turn over frequency (just due to the resistors) is 3Hz rather than the 0.3Hz due to 100k alone.
If there is a second DC block then that effect will aslo have to be calculated.
__________________
regards Andrew T.
  Reply With Quote
Old 13th July 2006, 04:22 PM   #7
diyAudio Member
 
Lostcause's Avatar
 
Join Date: Aug 2005
Location: NW UK
Quote:
Originally posted by AndrewT
Hi,
is Tamp wired as inverting?

Has Tamp a DC blocking cap on it's input?

Yes, the following stage (cables and resistors and parasitic capacitance) will affect the turn over frequency of the RC output filter in the DAC.
Any loading less than infinite will have an effect.
Choose components that make the effect inaudible.

From your schematic, it appears you have 100k (DAC resistor) // 25k (input impedance) // 25k (pot-guess) = 11k1.

The turn over frequency (just due to the resistors) is 3Hz rather than the 0.3Hz due to 100k alone.
If there is a second DC block then that effect will aslo have to be calculated.

OK so I can change the pot to 50K that's not a problem, I have one.
I do currently have another set of DCblockers in the T-amp but I was going to integrate th elot together and just have one set before the pot...or maybe after???
I can change the DAC resistor as well if needed, what do you think?
  Reply With Quote
Old 13th July 2006, 05:29 PM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
a DC block on each item makes them universally compatible with other equipment.
For the above reason I would keep both sets of cap blockers.

You should now calculate what bass roll-off frequency you require and what components can achieve it.

Two equal value series connected caps have an effective capacitance equal to half a single cap.
For different value caps the combination can be calculated using the same technique as parallel resistors.

I would change the DAC to 10uF with 1M0 as the grounding resistor.

I would also put 10uF in the amp with a 2M2 grounding resistor before the cap. These grounding resistors pull any cap leakage to virtually zero volts and can sometimes permit live disconnection and reconnection. Although the preferred method is to always switch off, particularly since some amplifiers can become unstable when there is no source impedance setting the input conditions.

I generally recommend a bass filter roll-off frequency one decade below the lowest usable audio frequency you intend reproducing i.e. about -3db @ 2Hz and -1db @ 4Hz, these reduce phase and reponse errors to virtual inaudibility.
However there is another criterion for the input filter. It should be at least half an octave above the NFB loop bass roll-off frequency and this in turn should be at least another half octave above the PSU roll-off frequency. This rule is generally ignored by many and seems to cause few problems. If you decide to adopt it then it demands a PSU -3db of no higher than 1Hz for an input filter of 2Hz. Some will say that the caps needed to give this will sound less good than smaller caps chosen for good mid range and treble. Experience and trial listening may be your solution.
__________________
regards Andrew T.
  Reply With Quote
Old 13th July 2006, 05:57 PM   #9
diyAudio Member
 
Lostcause's Avatar
 
Join Date: Aug 2005
Location: NW UK
Quote:
Originally posted by AndrewT
Hi,
a DC block on each item makes them universally compatible with other equipment.
For the above reason I would keep both sets of cap blockers.

You should now calculate what bass roll-off frequency you require and what components can achieve it.

Two equal value series connected caps have an effective capacitance equal to half a single cap.
For different value caps the combination can be calculated using the same technique as parallel resistors.

I would change the DAC to 10uF with 1M0 as the grounding resistor.

I would also put 10uF in the amp with a 2M2 grounding resistor before the cap. These grounding resistors pull any cap leakage to virtually zero volts and can sometimes permit live disconnection and reconnection. Although the preferred method is to always switch off, particularly since some amplifiers can become unstable when there is no source impedance setting the input conditions.

I generally recommend a bass filter roll-off frequency one decade below the lowest usable audio frequency you intend reproducing i.e. about -3db @ 2Hz and -1db @ 4Hz, these reduce phase and reponse errors to virtual inaudibility.
However there is another criterion for the input filter. It should be at least half an octave above the NFB loop bass roll-off frequency and this in turn should be at least another half octave above the PSU roll-off frequency. This rule is generally ignored by many and seems to cause few problems. If you decide to adopt it then it demands a PSU -3db of no higher than 1Hz for an input filter of 2Hz. Some will say that the caps needed to give this will sound less good than smaller caps chosen for good mid range and treble. Experience and trial listening may be your solution.

Wow, now things are getting really interesting...and harder to follow....
Nevertheless...I'm thinking of putting the whole thing in one new box so I only need i set of caps (4.7uf should do OK now, yes?. I have some really nice Poly caps and would hate to waste them). I've been told that DC damages the slide contact on a potentiometer so I'll put them before it.
Now for the resistors...Ohms I know, K/Ohms also but what's an M? is it a Mega-Ohm!!!!
As for the PSU, I have a 10,000uf stiffener off board.

Crikey this has taken on a whole new angle for me, I can build, solder and follow directions without fault...I even learned how these Tripath chips function...but this impedance thing has got me in a real spin!!....maybe I should go back to college...no maybe not!
  Reply With Quote
Old 13th July 2006, 06:17 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
keep reading the Forum, it continues to teach me a lot.


M=mega 10^6
k = kilo 10^3
m=milli 10^-3
u~=micro 10^-6 (u should be the greek mu but I don't know how to get it on screen)

But please don't write 10,000 uF when 10mF means the same. Oh dear, that doesn't help, since the Continentals this side of the pond use" ," to mean decimal place not thousands separator.
Except some manufacturers mix m and M, u and m, just to confuse us.
__________________
regards Andrew T.
  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
Impedance rate VS impedance stability TomatoBangBang Tubes / Valves 6 4th September 2008 11:46 PM
Dreaded Pioneer DV-333 Tray Problem DreadPirate Digital Source 4 4th October 2006 02:48 AM
the dreaded c word....(crossovers) xstephanx Full Range 6 19th June 2006 03:55 PM
Need help understanding transformer impedance ratios and impedance matching percy Tubes / Valves 5 28th February 2005 09:35 PM
Generic active filter / crossover... board... thingie... JohnR Parts 5 14th October 2002 11:13 PM


New To Site? Need Help?

All times are GMT. The time now is 03:29 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