Signal transformer question - 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 August 2010, 03:17 PM   #1
IG81 is online now IG81  Canada
diyAudio Member
 
IG81's Avatar
 
Join Date: Feb 2008
Default Signal transformer question

Hi,

I was not sure where to post this thread, as it mainly pertains to a signal transformer question, but will have a tube amp application and I figure there should be many knowledgeable people in here for my question.

With that said, I have a paire of Cinemag CMMI-5C step up transformers. They are rated to operate with a secondary load of 39kohm, which requires a driving impedance of 150ohm. I know that using lower impedances will quickly degrade the performances of the transformer, but what about higher impedances?

My application is to boost signal prior to my tube amp's input. It's an RCA/Victor console amp I resotred, but the thing has only 6dB of gain and therefore needs strong input signals if I hope to achieve it's full ~10-12W into 8ohm. Since I plan to use it with a CDP and passive pre, 6dB gain is not enough.

The input impedance of my tube amp is approximately 367kohm. The CMMI-5C being 1:5 step-up, reflects that as a ~14.7kohm load on the primary, wich is easily driven by a CDP. I did subjective listening tests and it generally seems fine, no distortion, and seemingly intact frequency response.

Using such a transformer with lower impedances would kill the HF response due to inductance, but higher impedances, as is my case should not cause a decrease in performance the way I see it. Does that sound correct? Are there issues you can see with what I'm doing?

Thanks!

IG
  Reply With Quote
Old 11th August 2010, 03:38 PM   #2
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

It will be interesting to see an experts reply .....

If you are using it at higher voltage levels than its designed for then I
understand things balance out. e.g. if its a 1:5 microphone transformer
then this would well with higher voltages and impedances, the core (sic)
point being the designed normal operating flux or current in each winding.

/Sreten.

Last edited by sreten; 11th August 2010 at 03:44 PM.
  Reply With Quote
Old 11th August 2010, 04:22 PM   #3
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
I think SY is the resident expert about transformers in general and the Cinemags in particular. You may want to ping him.

jd
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 11th August 2010, 07:20 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Actually, Steve Eddy more than me.

In any case, having the load higher than nominal will often cause ultrasonic ringing. If it sounds OK to you, then proceed no further, If you want to tweak and have some fun, you can optimize things by driving the input with a square wave generator and (if needed) a series resistor to pad the source impedance to match the source impedance of your CDP, then attach a series RC network (Zobel) to the transformer secondary. While looking at the secondary voltage with a scope, adjust the R and C values to optimize the square wave response. As starting values, I'd try 47k in series with 100pF.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 11th August 2010, 07:50 PM   #5
IG81 is online now IG81  Canada
diyAudio Member
 
IG81's Avatar
 
Join Date: Feb 2008
Quote:
Originally Posted by SY View Post
Actually, Steve Eddy more than me.

In any case, having the load higher than nominal will often cause ultrasonic ringing. If it sounds OK to you, then proceed no further, If you want to tweak and have some fun, you can optimize things by driving the input with a square wave generator and (if needed) a series resistor to pad the source impedance to match the source impedance of your CDP, then attach a series RC network (Zobel) to the transformer secondary. While looking at the secondary voltage with a scope, adjust the R and C values to optimize the square wave response. As starting values, I'd try 47k in series with 100pF.
If there was ringing, I could not hear it, but I'll definitely test for it, because it cannot be a good thing to have around, wether you hear it or not.

The Cinemag datasheet already shows a Zobel in their test network, optimized for their rated impedances though. I'll look into this...when I get my O'scope working...nearly every one of my projects is getting stalled by this lately.

Thanks for the contribution!

IG
  Reply With Quote
Old 11th August 2010, 08:13 PM   #6
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
I have no experience with this particular transformer, but microphone input transformers when made optimal (and I believe Cinemag transformers are) are not good for higher voltages and resistances. 150 Ohm primary would have too low inductance causing roll-off and saturation on low end of frequencies.

I would suggest to go on Edcor website and buy a couple of transformers under $10 each for levels of signals you need.
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers. -- Plato, 447-367 BCE
  Reply With Quote
Old 11th August 2010, 08:35 PM   #7
IG81 is online now IG81  Canada
diyAudio Member
 
IG81's Avatar
 
Join Date: Feb 2008
Quote:
Originally Posted by Wavebourn View Post
I have no experience with this particular transformer, but microphone input transformers when made optimal (and I believe Cinemag transformers are) are not good for higher voltages and resistances. 150 Ohm primary would have too low inductance causing roll-off and saturation on low end of frequencies.

I would suggest to go on Edcor website and buy a couple of transformers under $10 each for levels of signals you need.
As for signal level, with my CDP's 2V output, I'm slightly under the maximum input signal level of +10dBu for my particular model, so that aspect should be OK. As for higher impedances on their own, I would not know right now.

Thanks for your input. I'll have to do some actual testing when I repair my scope, those tips from all of you guys will give me stuff to look for.

Edit - I might just check out some of these Edcor you suggested, they are pretty cheap! Thanks for the suggestion

Cheers,

IG

Last edited by IG81; 11th August 2010 at 08:41 PM.
  Reply With Quote
Old 11th August 2010, 08:45 PM   #8
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Quote:
Originally Posted by Wavebourn View Post
I have no experience with this particular transformer, but microphone input transformers when made optimal (and I believe Cinemag transformers are) are not good for higher voltages and resistances. 150 Ohm primary would have too low inductance causing roll-off and saturation on low end of frequencies.

I would suggest to go on Edcor website and buy a couple of transformers under $10 each for levels of signals you need.
Hi,

Higher voltages I agree would cause saturation, but I cannot see the case
for when this is combine with higher impedances. If the currents are the same
the voltages do not matter, a transformer is a current operated device IMO.
Capacitance of some sort would be the major issue into higher impedance.

rgds, /Sreten.

FWIW I think you have to chuck nominal loading out of the window and
re-zobel with a far higher resistance + capacitor to keep currents down.

Last edited by sreten; 11th August 2010 at 08:52 PM.
  Reply With Quote
Old 11th August 2010, 08:52 PM   #9
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally Posted by sreten View Post
a transformer is a current operated device IMO.
Sure. And current depends on frequency.

What inductance can you expect from 150 Ohm primary of a microphone transformer?
Can you calculate it's impedance on low end of an audio band?
Can you imagine what happens to signal if it's source is loaded on such an impedance?
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers. -- Plato, 447-367 BCE
  Reply With Quote
Old 11th August 2010, 08:56 PM   #10
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Quote:
Originally Posted by Wavebourn View Post
Sure. And current depends on frequency.

What inductance can you expect from 150 Ohm primary of a microphone transformer?
Can you calculate it's impedance on low end of an audio band?
Can you imagine what happens to signal if it's source is loaded on such an impedance?

Hi,

The primary impedance is reflected from the secondary, its not fixed.
In this case its around 1.5kohms not 150ohms, bass will be extended.
The problem area AIUI will be the treble end.

rgds, /Sreten.

Last edited by sreten; 11th August 2010 at 09:04 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
Mislabeled Signal Transformer? cuibono Parts 7 4th January 2008 11:56 PM
Identifying weird signal transformer Klimon Tubes / Valves 4 28th August 2006 12:50 PM
Help IDing Philips signal transformer planet10 Parts 2 18th August 2004 06:55 PM
DIY signal line transformer ? lumanauw Solid State 58 27th June 2004 04:12 PM


New To Site? Need Help?

All times are GMT. The time now is 04:57 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