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Preamplifier---Why transformer output?

pdcaceo

Member
2004-06-27 9:58 am
HK
HI guys,

I read BAT VK-53SE preamplifier information. The information is on below

""""""""""""""""""""""""""""""
TRANSFORMER-COUPLED OUTPUTS
The VK-53SE preamplifier now features
transformer-coupled outputs. These transformers
replace BAT’s venerable Six-Pak of output
capacitors with custom-designed amorphous
core output transformers. Each transformer is
encapsulated within a mu-metal shield for the
ultimate in signal purity and noise isolation.
This change to transformer-coupled outputs has
a solid engineering foundation. All devices have
inherent imperfections. Yet, while both capacitors
and transformers can perform the same task of
DC decoupling, in many cases the transformer
can be designed closer to the model of an ideal
device. This benefit, however, does not come
easily.
First - the design of a linear, and close-toideal, transformer is not trivial. Second - some
applications are more suitable for transformers
than others. And third - the highest quality
transformers are also quite high in cost. The
combination of these three elements explains
why good transformers are relatively uncommon
in high-end audio.
In order to achieve this “transformational”
goal, years were spent in prototyping and
testing various output transformer designs. The
result: BAT’s new transformer-coupled output
stage proved itself to be substantially superior
to any capacitor in maintaining a purity of
signal transmission. Electrically, these custom
transformers significantly improve the VK-53SE
preamplifier’s ability to drive low impedance
loads. Sonically, they offer greater dynamics,
transparency, top-to-bottom extension, and
a simultaneously more coherent and organic
portrayal of music.

"""""""""""""""""""""""""""""""""""""""""""""""""""""

My question is what is the advantage of using transformer couple instead of cap couple?


Any one know the reason?
 
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Over the last few years i have been listening to a preamp with a very similar topology to the BAT in preference to anything else. The technical advantage is a significantly lower output impedance if the transformer is a step-down, but more importantly, a really good transformer offers a superior sound than any coupling cap i have tried. This is obviously a personal preference.
 

pdcaceo

Member
2004-06-27 9:58 am
HK
BAT said:

1) This change to transformer-coupled outputs has
a solid engineering foundation.

2) BAT’s new transformer-coupled output
stage proved itself to be substantially superior
to any capacitor in maintaining a purity of
signal transmission. Electrically, these custom
transformers significantly improve the VK-53SE
preamplifier’s ability to drive low impedance
loads.

3) Sonically, they offer greater dynamics,
transparency, top-to-bottom extension, and
a simultaneously more coherent and organic
portrayal of music.


What is the soild engineering reason?
 

pdcaceo

Member
2004-06-27 9:58 am
HK
Over the last few years i have been listening to a preamp with a very similar topology to the BAT in preference to anything else. The technical advantage is a significantly lower output impedance if the transformer is a step-down, but more importantly, a really good transformer offers a superior sound than any coupling cap i have tried. This is obviously a personal preference.


Agreed it can lower impedance if step down transformer used.
 
My question is what is the advantage of using transformer couple instead of cap couple?

Any one know the reason?
Two of them are:

1) you can charge more.

2) you can claim to be unique.

As of lowered impedance, (and probably better power transfer) that´s essential in *power* amps, so you will always find OTs there.

In preamps?

Really needed only if Pre to Power amp distance is LOOOOOONNNNNNGGGG, where balanced is also a desirable feature.

On a regular Hi Fi installation ... not that much.
 
A good quality output transformer can deliver excellent fidelity. It will, however, limit the frequency response at both frequency extremes. This is actually not a bad thing. Subsonic garbage will be attenuated as will supersonic noise.
The problem with a transformer output is that the secondary of the transformer must be terminated properly to avoid high frequency overshoots. Apparently, BAT forgot to include that information.
 
The problem with a transformer output is that the secondary of the transformer must be terminated properly to avoid high frequency overshoots. Apparently, BAT forgot to include that information.


It really depends. I have transformers which perform without any noticeable ringing unloaded. Bandwidth well in excess of 100kHz. Not every transformer is a Lundahl :D
 

pdcaceo

Member
2004-06-27 9:58 am
HK
After reading all those reply and some of my thought:

Transofmer couple advantage are:

1) Ability to drive low impedance loads. As the output impedance is low. For example 200ohm or even lower. So it can provide more driving current.
2) If output is floated, can block the hum by ground loop.
3) Has balance output function
4) Limited bandwidth to reduce ultrasonic noise or subsonic signal
5) The low frequency response should be keeping the same no matter the loading. For example 10K loading or 100K loading. However, cap couple has difference low frequency response between 10K loading and 100K loading.
6) Tube like inductive loading rather than resistive loading. The swing of inductive loading is more. Headroom is more.
 
I don't have a good answer. It may have something to do with the winding geometry and whether one winding effectively shields the other.

Tbh i have never tried the tiny non-gapped Lundahls in this position and knowing how well they are shielded with mu-metal suspect they may work.

Transformers explicitly designed for this particular application like the Tango NP-10, NP-206 hum like crazy with a floating secondary. Their datasheets clearly show the secondary must share the same ground as the primary.

This is probably not an issue for may users, provided the chassis of preamp and power amp share the same grounding through the mains. Of course the output in this case is hardly floating.
 
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