What is the purpose of resistive load at line output trans? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 11th September 2009, 02:16 AM   #1
diyAudio Member
 
Join Date: Oct 2002
Location: SINGAPORE
Default What is the purpose of resistive load at line output trans?

I am looking at some linestage with output transformer coupling. it seems that some designs incorporate a resistive load at the secondary of the linestage output transformers. eg. 600ohms , 1k etc. one example would be the AudioNote M10.

Click the image to open in full size.

there are also plenty of designs without the resistive load at the secondary.

can anyone advise the purpose of this resistor?

Last edited by commstech; 11th September 2009 at 02:22 AM.
  Reply With Quote
Old 11th September 2009, 02:35 AM   #2
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
In the schematic you have provided, and most others like it, it is to maintain the correct load impedance reflected back to the primary and plate of the output tube. This is done in case the external load is insufficient to perform the task. If you should connect an external load of the correct matching value, then a resistor is unnecessary. The resistor shown in the circuit is called a terminating resistor that can be switched in or out as needed. In the case of a tape recorder playback amplifier that has a VU meter connected across the secondary, proper termination is necessary for correct meter calibration.
__________________
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." ~Thomas Sowell

Last edited by HollowState; 11th September 2009 at 02:44 AM.
  Reply With Quote
Old 11th September 2009, 02:49 AM   #3
diyAudio Member
 
Join Date: Oct 2002
Location: SINGAPORE
Quote:
Originally Posted by HollowState View Post
In the schematic you have provided, and most others like it, it is to maintain the correct load impedance reflected back to the primary and plate of the output tube. This is done in case the external load is insufficient to perform the task. If you should connect an external load of the correct matching value, then a resistor is unnecessary. The resistor shown in the circuit is called a terminating resistor that can be switched in or out as needed. In the case of a tape recorder playback amplifier that has a VU meter connected across the secondary, proper termination is necessary for correct meter calibration.
any method to determine if the external load is sufficient or of correct matching value & anyway to know what this value must be?
  Reply With Quote
Old 11th September 2009, 03:14 AM   #4
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Quote:
Originally Posted by commstech View Post
any method to determine if the external load is sufficient or of correct matching value & anyway to know what this value must be?
Consulting the manufacture's specifications is the best way. In the case of a transformer coupled output like the M-10, the secondary impedance is the value to use. And the terminating resistor will (or should) match it. In this case it is 600 ohms which is standard for most professional audio line amps.
__________________
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." ~Thomas Sowell
  Reply With Quote
Old 11th September 2009, 04:34 PM   #5
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
The resistor is there to provide a termination should it be required, note as well that it will reduce the gain significantly (depending on primary impedance relative to the driver tube rp) which may be its real purpose. I serviced a lot of AN stuff and indeed most of my system is transformer coupled. In just about all cases with UTC, AN and Magnequest transformers I have found it generally sounds slightly better when the transformer is driving a relatively high load impedance. (10X -100X the impedance the transformer is designed to drive) Remember we are mainly interested in voltage gain here and significant power transfer is not involved. (I have never seen an M10 where the switch was closed and the resistor was in circuit.)

Poorer quality transformers may behave and sound better with load impedances relatively close to their designed load impedance.

There should generally be no problem with running these into loads that are close to open circuit relative to the impedance the transformer is designed to drive. Note that this is not true in the case where the driver can develop appreciable power (plate voltages and current capability) sufficient to damage the transformer. I have yet to see this in my somewhat limited experience. (I run with a crowd of people who use transformer coupling extensively, although at this point I am certainly the most extreme.. )
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan

Last edited by kevinkr; 11th September 2009 at 04:40 PM.
  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
Tubes load resistance and output trans impedance Jeffy74 Tubes / Valves 4 22nd December 2012 08:53 PM
Output trans plate load question 5687 vs 5965 phrarod Tubes / Valves 0 7th June 2009 02:24 AM
Parafeed with resistive load mashaffer Tubes / Valves 9 16th September 2005 07:19 PM


New To Site? Need Help?

All times are GMT. The time now is 10:00 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