What am I missing? - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, 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 19th June 2005, 07:52 AM   #1
diyAudio Member
 
jeff mai's Avatar
 
Join Date: Aug 2002
Location: Melbourne
Default What am I missing?

Here is the scenario:

Passive I/V stage (2K7 to ground) --> 4.7uF coupling cap --> 12H log tapped autotransformer (100 ohms DCR) as volume control --> 22K input impedance at amplifier.

I figured on some easily measurable (and possibly audible) LF roll-off due to:

1) lower AC impedance to ground at LF for the I/V stage
2) voltage divider effect of the cap + autotransformer at LF

Tried it - couldn't hear any roll-off. Measured it (with CRO) - I get 1 or 2db down at 10Hz. No humps, bumps or dips from 20Hz to 20kHz.

It could be that I don't fully understand how the passive I to V works from the DAC, but I'd guess that the DAC's current output causes a voltage to develop across the resistor to ground.

I guess the other possibility is that the autotransformer is much more than 12H, but I don't think so. That figure came from the maker.

Any thoughts on what I'm missing?
  Reply With Quote
Old 19th June 2005, 08:54 AM   #2
diyAudio Member
 
jeff mai's Avatar
 
Join Date: Aug 2002
Location: Melbourne
Default Re: What am I missing?

More info I forgot to mention:

The I/V stage is attached to a TDA1543 DAC chip.

The position of the volume control during the test is such that the reflected load will be very, very high, making coil inductance + DCR the dominant portion of the autotransformer impedance at low frequency. Even at a 1:1 ratio (full volume), the reflected load of 22K will be 10 times the autotransformer impedance at 20Hz.
  Reply With Quote
Old 19th June 2005, 12:48 PM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Dumb question- I've never used the 1543, but I did a lot of playing around with passive I/V using a 1541A. For that DAC, the resistor used as the I/V had to be MUCH smaller than 2k7, otherwise the linearity and dynamic range went to hell. Something like 100 ohms was appropriate. The small resistor load meant more voltage amplification was needed, so that was a big trade-off. Does the 1543 have a spec for the maximum acceptable resistance load?
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 19th June 2005, 09:41 PM   #4
diyAudio Member
 
jeff mai's Avatar
 
Join Date: Aug 2002
Location: Melbourne
Quote:
Originally posted by SY
Does the 1543 have a spec for the maximum acceptable resistance load?
Don't know, but even were 2K7 too high, I don't think this would affect the FR. It certainly wouldn't flatten out by coincidence!

What I hear and measure make no sense. I must be overlooking something.
  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
X/SuperSymmetry - Am I Missing Something? Simon G Best Pass Labs 127 17th July 2006 11:38 PM
Missing schematics Måsen Solid State 2 24th January 2005 05:14 AM
am i missing something theChris Analogue Source 1 22nd June 2003 12:25 PM
Missing center tap Herrmann Everything Else 3 30th April 2002 06:54 PM


New To Site? Need Help?

All times are GMT. The time now is 04:21 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