Help with input impedance - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 18th December 2015, 08:33 PM   #1
diyAudio Member
 
Join Date: Apr 2010
Default Help with input impedance

Hi folks,

I'm an electronics novice building an ST70-ish home brew amp. I would like to be able to use various input sources, including SS preamp line out, as well as headphone output from a laptop / tablet / smart phone. Would there be a problem in hooking these up directly? Or should I change the input resistor from the traditional 470K in order to present a better input impedance? If I did that, what impact would it have with regard to the grid-to-cathode reference for the input stage? Or maybe I need a buffer of some sort?

Been lurking here a long time, learning lots. Thanks guys.

Dave
  Reply With Quote
Old 18th December 2015, 08:57 PM   #2
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Stop fretting! A problem occurs when then I/P impedance of the downstream unit is too low. Waveform fidelity is preserved, when the O/P impedance of the driving unit is low and the I/P impedance of the driven unit is high.

Close matching of impedances is needed, for max. power transfer. You are dealing with a small signal situation.

BTW, an IHF "standard" requires a SS preamp be capable of driving a power amp whose I/P impedance is 10 Kohms. The 470 Kohm I/P impedance of a ST70 is "a piece of cake".
__________________
Eli D.
  Reply With Quote
Old 18th December 2015, 10:36 PM   #3
diyAudio Member
 
Join Date: Apr 2010
Thanks Eli. Can I assume that the same applies for input from the headphone jacks of the modern appliances I mentioned?

BTW, good luck to your namesake in the Meadowlands on Sunday.
  Reply With Quote
Old 19th December 2015, 01:21 AM   #4
diyAudio Member
 
Join Date: Apr 2010
So as I'm using a basic high gain pentode (EF86) input stage, and assuming I'll be driving it with a headphone level signal, what should I do for the input resistor value? Or should I just go with a lower gain input stage?

Dave
  Reply With Quote
Old 19th December 2015, 02:55 PM   #5
Koonw is offline Koonw  Malaysia
diyAudio Member
 
Join Date: Apr 2013
Quote:
Originally Posted by EL34Dave View Post
So as I'm using a basic high gain pentode (EF86) input stage, and assuming I'll be driving it with a headphone level signal, what should I do for the input resistor value? Or should I just go with a lower gain input stage?

Dave
All the sources input should be properly terminated into their correct impedance. A headphone output from pc/laptop will have 16-600 ohms, you can terminate it with a 600 ohms resistor before it is connected to your amp, But it is likely to have hum and noise from computer switching and audio transformer is only way to remove them. Since you have many sources your should use a selector switch and each correctly terminated. Attached is Dynaco st70 sch, and you can see there is a 10 ohms resistor acting as ground lift, which should improve the ground loop hums and noise. You also need to experiment with high quality interconnect if you experience hums and hiss rather than change the amp gain design which can make it worse than you begin with. The high impedance of ST70 is fine, it can take input from almost any low source.

Sometimes you need to add ground lift to input amp if you don't use audio isolation transformer.
Attached Images
File Type: png Dynaco-ST70-Tube-Amp-Schematic.png (19.7 KB, 117 views)
  Reply With Quote
Old 19th December 2015, 07:02 PM   #6
diyAudio Member
 
Join Date: Apr 2010
Quote:
Originally Posted by Koonw View Post
All the sources input should be properly terminated into their correct impedance. A headphone output from pc/laptop will have 16-600 ohms, you can terminate it with a 600 ohms resistor before it is connected to your amp, But it is likely to have hum and noise from computer switching and audio transformer is only way to remove them. Since you have many sources your should use a selector switch and each correctly terminated. Attached is Dynaco st70 sch, and you can see there is a 10 ohms resistor acting as ground lift, which should improve the ground loop hums and noise. You also need to experiment with high quality interconnect if you experience hums and hiss rather than change the amp gain design which can make it worse than you begin with. The high impedance of ST70 is fine, it can take input from almost any low source.

Sometimes you need to add ground lift to input amp if you don't use audio isolation transformer.
I was actually wondering about the 470K resistors across the inputs. Does this set the input impedance? Is that the same as "terminating it" like you mentioned above. Sorry for these stupid questions. I have no training in electronics other than what I am able to get from sites like this one.

Dave
  Reply With Quote
Old 19th December 2015, 07:09 PM   #7
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 7
Quote:
Originally Posted by EL34Dave View Post
Hi folks,

I'm an electronics novice building an ST70-ish home brew amp. I would like to be able to use various input sources, including SS preamp line out, as well as headphone output from a laptop / tablet / smart phone. Would there be a problem in hooking these up directly? Or should I change the input resistor from the traditional 470K in order to present a better input impedance? If I did that, what impact would it have with regard to the grid-to-cathode reference for the input stage? Or maybe I need a buffer of some sort?

Been lurking here a long time, learning lots. Thanks guys.

Dave
You only need to be concerned with inout impedance matching with RF circuits. At audio frequencies, the input impedance is equal to the value of the DC grid return resistor, which you mention: 470K. The only problem you'd encounter is if the Zo was comparable to that value that would form a voltage divider. None of the devices you mention here should pose any sort of problems.

About the only thing that might pose such a problem would be an unbuffered, hollow state pre. Some such designs do exist.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 19th December 2015, 07:27 PM   #8
Koonw is offline Koonw  Malaysia
diyAudio Member
 
Join Date: Apr 2013
Quote:
Originally Posted by EL34Dave View Post
I was actually wondering about the 470K resistors across the inputs. Does this set the input impedance? Is that the same as "terminating it" like you mentioned above. Sorry for these stupid questions. I have no training in electronics other than what I am able to get from sites like this one.

Dave
Yes it almost will set the input impedance, as the actual input impedance is this resistor in parallel with input impedance of the active device. The resistor is set by design of amp the chosen value has to take grid leak bias into and input source into consideration, you may change it to lower value but not necessarily higher than design as it will disturb the operation, usually 1M the upper limit and 10K the lower limit.

When you connect the headphones to laptop you effectively terminated it correctly if you hear clear and undistorted sound, and it's this terminated condition give you the best sound input to your amp. So you see now 600 ohms termination and 470K they are separate functions even though they are connected together.
  Reply With Quote
Old 20th December 2015, 05:30 PM   #9
diyAudio Member
 
Join Date: Aug 2008
Go with the modern standards and use 10K input impedance. Today's sources are expecting to see that, especially Class-D headphone outputs. Plus, you 'll get less noise and pick up less hum at the input.
  Reply With Quote
Old 20th December 2015, 05:49 PM   #10
diyAudio Member
 
Join Date: Apr 2007
Location: Denver CO US
I would load something like a laptop or iphne with something like 10K if it is a headphone output. They could have startup offsets when loaded with rather high impedance. The coupling cap could take a while to charge.
I ran a ST 70 sirectly connected to a PC for quite a while. The souns was good and I did not exnounter any hum problems.
  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 design input stage with specific input impedance/pot value mahleriana Tubes / Valves 0 16th November 2009 10:37 PM
Input Impedance Bengali Analog Line Level 7 15th August 2007 11:24 PM
How much input impedance is enough? Russ White Chip Amps 5 13th April 2005 08:25 PM
Input pot and input impedance Bricolo Parts 1 19th March 2003 02:59 PM
input impedance castela Solid State 2 17th January 2003 08:02 PM


New To Site? Need Help?

All times are GMT. The time now is 10:21 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki