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mcp6453[_2_] mcp6453[_2_] is offline
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Default Op Amp Labs Transformers

Are the Op Amp Labs audio transformers any good?

http://www.opamplabs.com/products/lo...udio/t-25.html

I'm particularly interested in how badly they ring. What causes an audio
transformer to ring?
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geoff geoff is offline
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Default Op Amp Labs Transformers

On 2/03/2018 9:20 AM, mcp6453 wrote:
Are the Op Amp Labs audio transformers any good?

http://www.opamplabs.com/products/lo...udio/t-25.html


I'm particularly interested in how badly they ring. What causes an audio
transformer to ring?


What it is connected to, either side, impedances, reactances, etc. Or
loose windings or core laminations.

geoff
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Scott Dorsey Scott Dorsey is offline
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Default Op Amp Labs Transformers

mcp6453 wrote:
Are the Op Amp Labs audio transformers any good?

http://www.opamplabs.com/products/lo...udio/t-25.html


They are inexpensive Triad transformers in nice octal cans. I don't remember
which Triad types they are, but they are still in production from Triad I
believe.

I'm particularly interested in how badly they ring. What causes an audio
transformer to ring?


If you terminate them properly and drive them from a low-Z load, they are
okay, but they are not transparent. They are certainly no worse than the
Edcor ones.

Transformers ring because of parasitic resonances caused by capacitance
between windings mixed with the series inductance of the winding. The
higher the impedance you try and get, the harder it is to keep it from
ringing. Fancy winding methods can help some.

Adding a zobel network to the secondary of the transformer can clean things
up a whole lot, but this also limits the range of impedances you can drive
with the transformer. Which is fine for input transformers where you always
have a known load.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default Op Amp Labs Transformers

Geoff wrote:

--------------


I'm particularly interested in how badly they ring. What causes an audio
transformer to ring?



What it is connected to, either side, impedances, reactances, etc.


** In general, having too low an impedance on the input side and/or having a lot of cable capacitance connected to the output side. Mild ringing starts at supersonic frequencies while serious ringing is clearly audible in the high treble range.

Transformers work best when used for stepping DOWN a signal - dropping both signal level and impedance. 1:1 ratio types have a more demanding job and ones that step up for microphone inputs need to be high quality.



..... Phil





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Default Op Amp Labs Transformers

On 3/1/2018 7:45 PM, Scott Dorsey wrote:
zobel network


I'd never heard of a zobel network before, but, of course, I've seen
them many times. After doing a lot of reading after seeing this post,
the best solution seems to be a high-quality transformer, such as a
Jensen, UTC, or Lundahl.

This particular application is an AM radio processor. The output
transformer ringing is rumored to cause a 2% loss in modulation.
Theoretically that's possible, but I wonder how real it is. Transmitters
are such dirty devices.



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Default Op Amp Labs Transformers

On Thursday, March 1, 2018 at 3:21:02 PM UTC-5, mcp6453 wrote:
Are the Op Amp Labs audio transformers any good?

http://www.opamplabs.com/products/lo...udio/t-25.html

I'm particularly interested in how badly they ring. What causes an audio
transformer to ring?


60Hz transformers hum, so at the right audio resonant frequency, I guess the core could resonate and ring, physically!!

Jack

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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default Op Amp Labs Transformers

wrote:

--------------------------



I'm particularly interested in how badly they ring. What causes an audio
transformer to ring?



60Hz transformers hum,


** A lot of force comes from the 60Hz supply acting on anything magnetic with a bunch of wire wound around it. This is single, low frequency, forced vibration, not resonance.


so at the right audio resonant frequency,
I guess the core could resonate and ring, physically!!


** The sort of ringing being referred to here happens at high audio and supersonic frequencies - needs test gear to see.

Folk fuss about it cos it indicates non flat response.


...... Phil
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Scott Dorsey Scott Dorsey is offline
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Default Op Amp Labs Transformers

mcp6453 wrote:
On 3/1/2018 7:45 PM, Scott Dorsey wrote:
zobel network


I'd never heard of a zobel network before, but, of course, I've seen
them many times. After doing a lot of reading after seeing this post,
the best solution seems to be a high-quality transformer, such as a
Jensen, UTC, or Lundahl.


Well, yes. BUT... the key to getting good performance out of cheap
transformers is careful loading. If you can control your load impedance,
you can get away with a $5 Edcor transformer easily.

I am personally pretty fond of the Triad and Murata Mil-T-27E transformers.
The 600:600 ones are $12 on Digi-Key and they are VERY touchy about loading
and output level, but they can be made to work very well.

On the other hand, you can just throw a Jensen JT-11 into an output stage
and it'll be fine. Leave the secondary open, and it'll be mostly okay.
Overload it with a 250 ohm load and it'll still be mostly okay. With a
600 ohm load, the distortion is two orders of magnitude lower than the Edcor.
But you're also paying $50 instead of $5.

This particular application is an AM radio processor. The output
transformer ringing is rumored to cause a 2% loss in modulation.
Theoretically that's possible, but I wonder how real it is. Transmitters
are such dirty devices.


Easy enough to see on the scope. Since the transformer ringing is way out
of band, a shunt network should get rid of it no matter how bad it is.

The ringing of the modulation transformer is a worse issue for AM broadcast
applications... and it can result in overmodulation on peaks from the
overshoot unless the transformer is very well damped with a low primary
impedance.

I'll also point out that in terms of actual perceived loudness at the
listener's radio, a 2% loss in modulation is not very much. I know that
it's a slippery slope and you lose 1% here and 1% there and next thing you
know it builds up, but people worry too much about that stuff when in fact
reducing the limiting would probably make for less transformer ringing too
because you wouldn't be hitting it with so much high frequency crap...
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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[email protected] makolber@yahoo.com is offline
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Default Op Amp Labs Transformers

On Saturday, March 3, 2018 at 8:07:07 PM UTC-5, mcp6453 wrote:
On 3/1/2018 7:45 PM, Scott Dorsey wrote:
zobel network


I'd never heard of a zobel network before, but, of course, I've seen
them many times. After doing a lot of reading after seeing this post,
the best solution seems to be a high-quality transformer, such as a
Jensen, UTC, or Lundahl.

This particular application is an AM radio processor. The output
transformer ringing is rumored to cause a 2% loss in modulation.
Theoretically that's possible, but I wonder how real it is. Transmitters
are such dirty devices.


In that application you are never allowed to go over 100% negative modulation because it causes splatter. You set up the modulation processing limiter so you can't go over 100%, but if you have a transformers or any device that is not exactly flat over the frequency range in the audio path AFTER the limiter, you would need to set the limiter so those frequencies that were peaked were held at 100% and then the rest of the range would be under 100%.


When you need an exactly flat response like this application, the best transformer is no transformer.

m


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Default Op Amp Labs Transformers

On Tuesday, March 6, 2018 at 12:29:49 PM UTC-5, wrote:
On Saturday, March 3, 2018 at 8:07:07 PM UTC-5, mcp6453 wrote:
On 3/1/2018 7:45 PM, Scott Dorsey wrote:
zobel network


I'd never heard of a zobel network before, but, of course, I've seen
them many times. After doing a lot of reading after seeing this post,
the best solution seems to be a high-quality transformer, such as a
Jensen, UTC, or Lundahl.

This particular application is an AM radio processor. The output
transformer ringing is rumored to cause a 2% loss in modulation.
Theoretically that's possible, but I wonder how real it is. Transmitters
are such dirty devices.


In that application you are never allowed to go over 100% negative modulation because it causes splatter.


Spurious emissions?

Jack

You set up the modulation processing limiter so you can't go over 100%, but if you have a transformers or any device that is not exactly flat over the frequency range in the audio path AFTER the limiter, you would need to set the limiter so those frequencies that were peaked were held at 100% and then the rest of the range would be under 100%.


When you need an exactly flat response like this application, the best transformer is no transformer.

m




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Default Op Amp Labs Transformers

On Sunday, March 4, 2018 at 7:49:09 PM UTC-5, Phil Allison wrote:
wrote:

--------------------------



I'm particularly interested in how badly they ring. What causes an audio
transformer to ring?



60Hz transformers hum,


** A lot of force comes from the 60Hz supply acting on anything magnetic with a bunch of wire wound around it. This is single, low frequency, forced vibration, not resonance.


You know, in AC solenoids a shorting ring is beneficial to keep 60 Hz buzzing to a minimum, as the shorting ring provides a CEMF!!!!


Jack



so at the right audio resonant frequency,
I guess the core could resonate and ring, physically!!


** The sort of ringing being referred to here happens at high audio and supersonic frequencies - needs test gear to see.

Folk fuss about it cos it indicates non flat response.


..... Phil


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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default Op Amp Labs Transformers

wrote:


--------------------------



I'm particularly interested in how badly they ring.
What causes an audio
transformer to ring?


60Hz transformers hum,


** A lot of force comes from the 60Hz supply acting on anything
magnetic with a bunch of wire wound around it. This is single,
low frequency, forced vibration, not resonance.



You know, in AC solenoids a shorting ring is beneficial to keep
60 Hz buzzing to a minimum, as the shorting ring provides a CEMF!!!!


** However, solenoids and relays have moving parts - transformers do not, so are normally bolted up tight and run silent.

Had a Chinese made guitar amp here that would suddenly made a loud hum if laid over on its back, standing upright it was OK.

The four bolts holding the power transformer laminations together were barely finger tight. Soon fixed.


..... Phil






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Default Op Amp Labs Transformers

On Saturday, March 10, 2018 at 9:30:43 PM UTC-5, Phil Allison wrote:
wrote:


--------------------------



I'm particularly interested in how badly they ring.
What causes an audio
transformer to ring?


60Hz transformers hum,


** A lot of force comes from the 60Hz supply acting on anything
magnetic with a bunch of wire wound around it. This is single,
low frequency, forced vibration, not resonance.



You know, in AC solenoids a shorting ring is beneficial to keep
60 Hz buzzing to a minimum, as the shorting ring provides a CEMF!!!!


** However, solenoids and relays have moving parts - transformers do not, so are normally bolted up tight and run silent.

Had a Chinese made guitar amp here that would suddenly made a loud hum if laid over on its back, standing upright it was OK.

The four bolts holding the power transformer laminations together were barely finger tight. Soon fixed.


Good deal!

With scraplesss laminations, some xfmr cores aren't interleaved (laminations), but a butt (E & I) stack (Shell type), then the joint is welded.

Working with CVRs (Constant Voltage Regulators) where input can vary +/- 15% but output can only vary +/- 2% (generally used for large tube filaments), were a pain to adjust. Some got out the door out of spec, they were totally varnished. Typically, you remove lamination's from center leg to adjust output voltage, but no way to do it when totally varnished. I suggested to "drill" out the center leg on a drill press while testing!

Jack


.... Phil


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