Step-down from mains to line level - diyAudio
Go Back   Home > Forums > Design & Build > Parts
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 18th December 2011, 06:28 PM   #1
mmerig is offline mmerig  United States
diyAudio Member
Join Date: Jan 2005
Location: south east Idaho, Teton Basin
Default Step-down from mains to line level

I would like to drop power from a stereo set of wires from a power amp (120 watts per channel into 8 ohms) to line level so a line-level headphone can be connected. The headphone impedance would be about 8 to 32 ohms.

A voltage divider would require a high power resister, wouldn't it? If so I would rather avoid this because of potential noise, heat, and cost.

Would a decent audio-quality step-down transformer do a better job (one for each channel)? If so, what would be the specifications?
  Reply With Quote
Old 18th December 2011, 06:40 PM   #2
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
Mooly's Avatar
Join Date: Sep 2007
It not such a problem. An amp like that would need around 330 ohm 2 watt series resistors to the headphone output socket. There's no noise, heat is virtually non existant on normal listening, and cost is ultra cheap.

It's the standard industry accepted way of doing it. The impedance isn't such an issue either.

Try it and see
  Reply With Quote
Old 18th December 2011, 08:01 PM   #3
Lavcat is offline Lavcat  United States
diyAudio Member
Lavcat's Avatar
Join Date: Dec 2009
Location: New Jersey USA
What I do is connect my 'phones to the amplifier outputs directly without using any pads. Of course one has to be careful with the level.
  Reply With Quote
Old 18th December 2011, 09:28 PM   #4
diyAudio Member
Join Date: Sep 2010
ESP easy headphone adapter (project 100):

Headphone Adaptor for Power Amplifiers
  Reply With Quote
Old 19th December 2011, 02:11 PM   #5
mmerig is offline mmerig  United States
diyAudio Member
Join Date: Jan 2005
Location: south east Idaho, Teton Basin
Thanks for all of your helpful suggestions.
  Reply With Quote
Old 19th December 2011, 02:26 PM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
I prefer a ladder attenuator.

Let's suppose the headphones will work well with Rs= 10r.
Now create the ladder attenuator.
The 10r is the lower resistor. It is in parallel to the headphone input impedance.
Let's assume the Headphone must never see a voltage peak exceeding 5Vpk.
Let's further assume that the amplifier is capable of putting out 30Vpk.
The upper resistor of the ladder needs to be 30/5 -1 * 10 = 50r.
Let's check the resistor current to ensure we are not overloading the resistors.
assume the headphones are 10ohm (just to make the sums easier), the parallel lower load is 10//10 = 5r.
The upper resistor is 51r (the nearest to 50r). the total resistance is 56r.
The maximum output voltage is 30Vpk.
Ipk = 30/56 = 833mApk.
The power dissipated in the 51r is 0.833^2 * 51r / 2 (for sinewave signals) ~ 18W.
That's a mighty big resistor to obtain that maximum of 5Vpk at the headphones.
5Vpk at the headphones for more than a few seconds will damage your hearing.

If the average listening level is -20dB ref. maximum, then that resistor has to dissipate an average of ~180mW of power.

See the problem.
Normal listeners listening to normal music at normal levels will never overheat the 600mW resistor.
Design can't easily account for abnormal listeners.
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 19th December 2011, 02:33 PM   #7
diyAudio Member
KatieandDad's Avatar
Join Date: Nov 2011
Location: UK
If you read the threads carefully regarding placing resistors into the headphone circuit, no-one is wrong. The headphones will work but you will lose fidelity.

If you aren't after Hi-Fi then ignore my suggestion.

If you are after Hi-Quality-Hi-Fi then a dedicated Hi-Fi headphone amp is a must and it needn't cost the Earth.

I'll leave it at that unless you answer looking for a Hi-Fi solution.
  Reply With Quote
Old 19th December 2011, 06:23 PM   #8
mmerig is offline mmerig  United States
diyAudio Member
Join Date: Jan 2005
Location: south east Idaho, Teton Basin
I am after hi-fi. Here is the motivation for the original question:

My wife likes to listen to music on headphones while riding a bike on a trainer-stand, which is quite loud. I am getting her noise-cancelling heaphones.

The bike is in a room about 15 feet from the pre-amp and power amp; there are wires from the power amp running to this room, but not-line level wires. The power amp wires are the most convenient source for the headphones.

To me, the problem seems to be impedance and power-matching (either voltage and/or current).

Is a headphone amp appropriate when starting with such a high-power source?

The ESP project 100 looks like a good choice. Why would it be low-fidelity if the output impedance and power are matched to the headphones' input?
  Reply With Quote
Old 19th December 2011, 07:15 PM   #9
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
Mooly's Avatar
Join Date: Sep 2007
Just try the series resistors... for 10p/0.1€$ what have you too lose.

Amps costing 1000 €$ use that method.
  Reply With Quote


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
(minimal) speaker level vs. line level high pass filter noviygera Analog Line Level 1 15th October 2011 09:43 AM
balanced line level baffle step circuit Harolda Analog Line Level 2 5th September 2010 06:13 PM
trouble matching line level to guitar amp input level...any help? mondogenerator Instruments and Amps 5 19th December 2009 10:00 AM
speaker level input vs line level question crippledchicken Subwoofers 4 17th July 2007 03:36 AM
lesser of two evils, high level -> line level, digital volume control mazurek Solid State 3 19th November 2006 12:25 AM

New To Site? Need Help?

All times are GMT. The time now is 10:36 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2