Distortion rise on OPT - 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 January 2013, 08:34 PM   #1
rrrs is offline rrrs  United States
diyAudio Member
 
Join Date: Aug 2005
Default Distortion rise on OPT

Hi, I did post this in another thread, but got no response yet.
I am experiencing “strange” behavior on my OPT; it is Lundahl LL1691B, 20K:8ohm used with 211 tube.
Distortion (mainly 2nd order) increases a lot at higher frequency, from 2kHz up.

Here are THD measurements I did in ARTA, all at 1W into 5.7ohm load (this are measurements of actual amplifier, not just OPT):
40Hz – 0.2%
100Hz – 0.11%
1KHz – 0.11%
2KHz – 0.17%
4KHz – 0.35%
6KHz – 0.57%
8KHz – 0.78%
10KHz – 0.94%
12KHz – 1.11%
14KHz – 1.17%
16KHz – 1.14%
18KHz – 0.97%

As this is 2 stage amp using Interstage Transformer I suspected the problem might come from IT, so I did comparison removing IT and using choke loaded first stage, cap coupled to output stage; I got pretty much exactly same distortion figures.

Here is distortion spectrum, showing 10KHz in black and 1KHz in magenta; it is interesting to see that 2nd order distortion is much higher with 10KHz then at 1KHz signal while 3rd order is about the same!?

In frequency response measurement I do not see any strange peaks in that range; black line is IT coupled while magenta is choke loaded capacitor coupled.

211 output stage uses fixed bias, so output impedance of 211 should remain pretty constant across frequency range; around 3.8K at my operating point.

When testing I can hear a clearly audible sound from OPT even at 1W level and this increases with frequency; very similar to my distortion measurements. I understand that this is actually normal behavior of OPT, but is this what causes this increase in distortion and is that increase also something normal?
Attached Images
File Type: jpg Distortion.jpg (68.6 KB, 145 views)
File Type: jpg Frequency.jpg (58.8 KB, 143 views)
  Reply With Quote
Old 11th January 2013, 09:02 PM   #2
cotdt is offline cotdt  United States
diyAudio Member
 
Join Date: Oct 2005
Send a message via AIM to cotdt
Can you hear music through the OPT?
  Reply With Quote
Old 11th January 2013, 09:31 PM   #3
rrrs is offline rrrs  United States
diyAudio Member
 
Join Date: Aug 2005
Never tried to input music with load resistor instead of speaker; will try and report.
But, when doing frequency response measurement (at 1W into 5.7ohm load) I can hear sort of "Pink noise" from OPT; it is not loud, but can hear it from couple of feet distance.
  Reply With Quote
Old 12th January 2013, 02:25 AM   #4
Alexsk is offline Alexsk  Canada
diyAudio Member
 
Join Date: Jan 2012
Quote:
Originally Posted by rrrs View Post
Never tried to input music with load resistor instead of speaker; will try and report.
But, when doing frequency response measurement (at 1W into 5.7ohm load) I can hear sort of "Pink noise" from OPT; it is not loud, but can hear it from couple of feet distance.
Hi,
how's distortions looks like with the higher output signal, let's say 6-8 v RMS ?
  Reply With Quote
Old 12th January 2013, 03:14 AM   #5
diyAudio Member
 
Chris Hornbeck's Avatar
 
Join Date: May 2011
Location: Little Rock
It's normal for output transformers to "sing along". And you know the ancient joke about why amplifiers hum (they don't know the words).

The output tube will make higher distortion into a reactive load than a resistive one, and the output transformer's parasitic capacitances and leakage inductance start to become significant at high frequencies. A transformer for a single type 211 is right on, or just beyond, the raggedy edge of workable over the whole audio range, so this too is normal. The lack of 3rd at 10KHz isn't too surprising given the big notch there.

All good fortune,
Chris
  Reply With Quote
Old 12th January 2013, 03:15 AM   #6
rrrs is offline rrrs  United States
diyAudio Member
 
Join Date: Aug 2005
Hi, it rises proportionally the same.
For example, on 5W into 5.7 ohm (5.3 V-RMS) I get:
40Hz – 0.4% THD
100Hz – 0.34% THD
1KHz – 0.37% THD
10KHz – 3.5% THD

So, at all frequencies distortion increased approximately 3 times in respect to 1W measurement, 10KHz value still being 10 times higher than 1KHz value.
When testing at 5W transformer makes pretty loud sound at higher frequencies 9abone 2-3KHz.

See attached spectrum comparing 1KHz to 10KHz, both at 5W into 5.7ohm.
Attached Images
File Type: jpg 5W distortion.jpg (72.4 KB, 114 views)
  Reply With Quote
Old 12th January 2013, 03:21 AM   #7
rrrs is offline rrrs  United States
diyAudio Member
 
Join Date: Aug 2005
Hi Chris, just as I posted reply seen your post; makes sense.
So, basically should be normal.
What did confuse me was that I spoke to Kevin, Lundahl US distributor who built SE GM70 amp using same transformer and he did not notice any rise in distortion with frequency!?
Perhaps lover plate resistance of GM70 was less sensitive to transformer's reactive load?
  Reply With Quote
Old 12th January 2013, 05:17 AM   #8
regal is offline regal  United States
diyAudio Member
 
Join Date: Jan 2004
Location: MD
I have been having the same issue with my OPT (different manufacturer), I am leaning toward the problem being too much inductance in the secondary.

Mine is a 5k:32 ohm headphone OPT. But it is similiar to yours in that the secondary has a relatively high DCR and inductance.

I have been searching for a real solution. Odd ball transformers like ours I think just need to be custom jobs. Nickle Alloy Parafeed makes a lot of since for these situtations since it allows a much lower dcr/inductance in the secondary. Problem is it is expensive.

Not saying that I am right about this, but disagree that the measurements are "normal".
I also think the transformer "singing" nneeds to be minimized because it causes the tube to distort.

Don't mean to be a sour-puss and am not knocking Lundahl at all because it is an issue for other manufacturers as well. Just as you looking for solutions to age old problems a lot of us tube guys have had been dealing with for years.
  Reply With Quote
Old 12th January 2013, 06:42 AM   #9
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
On my Electra-Print OPTs wound for use with a 300B SET the THD does increase slightly at higher frequencies. Say 0.18 % at 1 kHz and 0.22 % at 10 kHz. Those trannies sing along quite loudly, actually. Louder than the Edcor CXSE25 I used to use. So I wouldn't discard the transformers just because they sing along.

Sand amps sing along too, actually. Though, they don't sing anywhere near as loud as tube amps.

~Tom
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 12th January 2013, 02:50 PM   #10
Alexsk is offline Alexsk  Canada
diyAudio Member
 
Join Date: Jan 2012
Quote:
Originally Posted by tomchr View Post
On my Electra-Print OPTs wound for use with a 300B SET the THD does increase slightly at higher frequencies. Say 0.18 % at 1 kHz and 0.22 % at 10 kHz. Those trannies sing along quite loudly, actually. Louder than the Edcor CXSE25 I used to use. So I wouldn't discard the transformers just because they sing along.

Sand amps sing along too, actually. Though, they don't sing anywhere near as loud as tube amps.

~Tom
That is more or less the same results i got from 300b with Audio Note EI and Hashimoto transformers.
With 2.5v output (correct?) 0.11% at 1kHz looks good, but at 16kHz whould be normal to have something like +2,3 db not +20 db distortions.
In my case at 5v RMS out to 8ohm 1khz and 20 khz are the same -44db, 50hz -40 with Hashimoto, with AN output 20 khz distortion are +2db compared to 1 khz. Again, it is 300b amp.
HTH
  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
Distortion VS OPT impedence Nihilist Tubes / Valves 1 2nd March 2010 11:45 PM
How To rise sensitivity of aleph -x? elviukai Pass Labs 11 2nd June 2006 09:07 PM
impedence rise/amplifiers SkinnyBoy Everything Else 0 14th May 2005 10:48 PM
Reducing impedance rise? JeremyD Multi-Way 6 4th July 2003 10:58 PM


New To Site? Need Help?

All times are GMT. The time now is 04:13 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