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 20th February 2012, 04:35 PM   #1
diyAudio Member
 
Join Date: Dec 2008
Smile Very Simple Attenuator For Audio Card

Hi, Folks!

Most of DIY amplifier designers have been using audio cards to make audio measurements. This is really cheapest method to get the most important parameters: Frequency Response, THD, THD+N. It provides to get spectrogram and sonogram picture.

The only problem with audio card is that it has a very high linear input sensitivity. Depending on the manufacturer this ranges from 0.5V to 1.5v.

There is a great Test & Measurement interface from Pete Millet, but sometimes we need something very simple.

So I was decided to build a very Simple Attenuator with minimum parts.
It consists of 9 resistors and 1 rotary switch.

The rotary switch allow to you to switch input voltages between 2.83v, 4v, 5.66v values.
Which corresponds the amplifier output at 1w, 2w, 4w (on 8 ohm dummy load)
and at 1w, 2w (on 16 ohm dummy load).
At the output is configured to produce 0.775v RMS (0 dBu) (unloaded!).

As rotary switch I have used hi-quality made Grayhill 71BD36-01-1-AJN (PDF datasheet).
It looks very solid: 25,000 cycles at rated load, gold-plated rotor contacts, Military Qualified.

As resistors I have chose non-inductive wire-wound Mills. Very well made and not so expensive as Vishay.

The GND must be the same for IN and OUT.
Please see the schematic.
Any comments are welcomed!
Attached Images
File Type: png ba79583c426c8160f3328c0e9e7903e6.png (6.6 KB, 193 views)
File Type: jpg e75a6dead3fd2c150968d61fb0d079c9.jpg (141.8 KB, 185 views)
File Type: jpg 5c2019c3543e5df6216e18ef072a8c6f.jpg (133.0 KB, 182 views)
File Type: jpg attenuator-levelfive-2311.jpg (163.5 KB, 182 views)
File Type: jpg 1f4d744f5eecc0d70a1aa69b91d4773c.jpg (166.5 KB, 172 views)
File Type: jpg a726e74565b6e5740c9d3de7cb998b1a.jpg (190.9 KB, 45 views)
File Type: jpg ba700bb0806ba2c343a36fcf2d5e3ccf.jpg (132.9 KB, 39 views)
File Type: jpg attenuator-levelfive-5902.jpg (209.2 KB, 28 views)
__________________
www.Levelfive-Audio.com

Last edited by levelfive; 20th February 2012 at 04:52 PM. Reason: Images were added.
  Reply With Quote
Old 20th February 2012, 04:51 PM   #2
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
You might want to place a set of diodes antiparallel across the output as clamp protection. Something like cheap red LEDs would not conduct much (uA) at low voltage, but would clamp at 1.6V to protect the sound card input.
  Reply With Quote
Old 20th February 2012, 04:57 PM   #3
diyAudio Member
 
Join Date: Dec 2008
Quote:
Originally Posted by TheGimp View Post
You might want to place a set of diodes antiparallel across the output as clamp protection.
Hello, The Gimp!
I thought about zener clamp, but fear that it will be itself a source of distortion.
__________________
www.Levelfive-Audio.com
  Reply With Quote
Old 20th February 2012, 05:08 PM   #4
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
I measured the voltage across a yellow LED with a 470K/470K resistor divider powered from a 1.5V alkaline battery. I only measured .001V difference with the LED across one resistor vs no LED. When I touched the leads my body conductance made for .004V difference.

I think it would not contribute significantly to distortion, but one would have to measure it in the actual circuit to be sure.
  Reply With Quote
Old 21st February 2012, 09:19 AM   #5
diyAudio Member
 
Join Date: Dec 2008
Quote:
Originally Posted by TheGimp View Post
...but one would have to measure it in the actual circuit to be sure.
You are right! But Zener diode is semiconductor, which in itself is not good for the "pure" audio circuit.

I have read this: Zener diode protects FPGA inputs
The key phrase is:
Quote:
"Some parasitic effects in the zener diode will create other distortion to the waveform."
And also found this:
Quote:
The noise of VR tubes is only 1mV of very smooth broadband noise, while the more common Zener diodes have 3 to 5mV of spectrally nonflat noise, occasional LF bumps and pops from "popcorn" noise, problems with temperature coefficients, and a huge amount of grossly nonlinear capacitance.
(c) Lynn Olson
__________________
www.Levelfive-Audio.com
  Reply With Quote
Old 21st February 2012, 09:56 AM   #6
diyAudio Member
 
Join Date: Aug 2008
Quote:
Originally Posted by levelfive View Post
Hello, The Gimp!
I thought about zener clamp, but fear that it will be itself a source of distortion.
Yes, it WILL. Verified.
  Reply With Quote
Old 21st February 2012, 12:29 PM   #7
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Do LEDs suffer from the same noise issues?

I think not based on RLD and other use in cathode bias circuits for minimum distortion.
  Reply With Quote
Old 21st February 2012, 03:20 PM   #8
diyAudio Member
 
hidnplayr's Avatar
 
Join Date: Sep 2008
Another question is wheter the zeners would produce this noise when they are not conducting (not clamping).
I think not, or a lot less at least
__________________
Real tubes have top-caps
  Reply With Quote
Old 21st February 2012, 04:17 PM   #9
diyAudio Member
 
Join Date: Nov 2011
Location: Europe
Simple THD measurement will tell you almost nothing about the sound and other problems, you need to get into details about harmonic distortion and many more things. Like meassuring input, driver and output stages separately, looking for oscillations, etc.
Let's say you got xx THD, how do you know what adds to your reading? And does it really tell you what to do to get better sound?
For simple meassurements you need at least P.Millet's interface, DWM and oscilloscope. Mr. Millet's description of his interface will give you many hints about measuring with soundcard. I stopped thinking about THD and such things after reading this article:
tvst1
  Reply With Quote
Old 21st February 2012, 05:54 PM   #10
diyAudio Member
 
Join Date: Dec 2008
Quote:
Originally Posted by GlowSignal View Post
Simple THD measurement will tell you almost nothing about the sound and other problems
I did't say about only "simple THD measurement" without taking other factors into account. You do not quite understand topics subject.

And thanks for link
__________________
www.Levelfive-Audio.com
  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
Simple and inexpensive Passive Balanced Attenuator possible? cersepn Analog Line Level 14 3rd April 2014 08:30 PM
Measure THD, etc of power amp with sound card - attenuator? CBRworm Solid State 36 18th September 2009 05:55 AM
lighspeed attenuator w/Simple SE AMP Salsero Tubes / Valves 6 17th July 2009 03:20 PM
Attenuator for sound card measurement AR2 Parts 8 9th January 2006 10:13 PM
Would like to build simple volume control or attenuator Barry R Multi-Way 4 20th August 2005 12:28 PM


New To Site? Need Help?

All times are GMT. The time now is 10:22 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