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'[EE]: Pages on the web about audio transformers'
2001\09\03@144818 by Edson Brusque

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Hello,

   I'm designing an audio transformer to be used on a direct box. A direct
box is a very simple circuit (basically a box with a transformer inside)
that converts impedance of audio signals. Typically, it's an audio
transformer with 20K-100K input impedance and 150R-600R output impedance.

   Someone knows of web sites that have informations about this? General
informations about transformers, transformers picking EMI, audio
transformers shielding, umbalanced to balanced audio transformers etc would
be very welcome.

   I've done extensivelly searching, but found very few usefull
informations.

   Thanks,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2138
Blumenau  -  SC  -  Brazil
http://www.citronics.com.br
-----------------------------------

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2001\09\03@150312 by Brian Aase

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maybe http://www.jensen-transformers.com/ would
be of some help?

On 3 Sep 2001, at 15:46, Edson Brusque wrote:

{Quote hidden}

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2001\09\03@153013 by Edson Brusque

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Hello Brian,

> maybe http://www.jensen-transformers.com/ would
> be of some help?

   I know Jensen, they're the best audio transformers manufacturers I know,
but there's little on their site about *building* audio transformers. This
is ok, as they don't want to pass their costly informations to the
competition. :)

   Thanks anyway.

   Best regards,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2138
Blumenau  -  SC  -  Brazil
http://www.citronics.com.br
-----------------------------------

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2001\09\03@163244 by Jim

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A couple of questions:

1) What exactly is a 'direct box' (I have never heard
  of one - but I have heard of "impedance matching
  transformers")?

2) What would the source be that you be converting
  from 20,000 Ohms (plus) to the range of 150 to
  600 Ohms? Would this be say, studio/musical gear?

There might be other ways of 'driving' a low impedance load
(like 150 Ohms) other than through the use of a transformer,
such using an op-amp (or two). If the transformer is to
provide some amount of common-mode isolation even this can be
accomplished to degree using op-amps.


Jim

{Original Message removed}

2001\09\03@175321 by Nigel Goodwin

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In message <007d01c134b7$882a0800$0100a8c0@piii500a>, Jim
<EraseMEjvpollspam_OUTspamTakeThisOuTDALLAS.NET> writes
>A couple of questions:
>
>1) What exactly is a 'direct box' (I have never heard
>   of one - but I have heard of "impedance matching
>   transformers")?

It's commonly used in the music field, for example connecting guitars
directly to the PA system.

>2) What would the source be that you be converting
>   from 20,000 Ohms (plus) to the range of 150 to
>   600 Ohms? Would this be say, studio/musical gear?

Yes, almost always, either studio or live performance.

>There might be other ways of 'driving' a low impedance load
>(like 150 Ohms) other than through the use of a transformer,
>such using an op-amp (or two). If the transformer is to
>provide some amount of common-mode isolation even this can be
>accomplished to degree using op-amps.

That's not the main point, it's isolation to prevent earth loops, a
large problem for live performance (I spent a number of years building
and operating PA for some friends in a local rock band).
--

Nigel.

       /--------------------------------------------------------------\
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       | Lower Pilsley   | Web Page : http://www.lpilsley.co.uk       |
       | Chesterfield    | Official site for Shin Ki and New Spirit   |
       | England         |                 Ju Jitsu                   |
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2001\09\04@033746 by Alan B. Pearce

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For information on designing transformers see if you can find a copy of the
"AWV Radiotron Designers Handbook". This book has its origins pre second
world war in Australia and contains all sorts of necessary information. I
believe it has been re-issued recently on CD ROM, and is well worth getting
if you can track it down.

An alternative would be to look for a copy of the ARRL or GSGB handbooks
which may have sufficient information on this for your purpose.

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2001\09\04@043712 by Vasile Surducan

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On Mon, 3 Sep 2001, Edson Brusque wrote:

> Hello,
>
>     I'm designing an audio transformer to be used on a direct box. A direct
> box is a very simple circuit (basically a box with a transformer inside)
> that converts impedance of audio signals. Typically, it's an audio
> transformer with 20K-100K input impedance and 150R-600R output impedance.
>

 Use instead of impedance, potential and currents like you've built a
standard 60Hz transformer but for 20Hz...12KHz range. Apply Faraday's law:

Vp = kNp (BxAc) f x 10 exp -8 where
k = 4.44 for sine waves
Vp = primary's voltage ( computed for high input impedance and mutual low
impedance reflected in primary by secondary coil - can be negligee in your
example )
Np = needed turns in primary
B = density in gauss accepted from your magnetic core without saturation
Ac = cross sectional area of the core
f = minimum output frequency of your signal

the transformer law:  Vp/Vs = Is/Ip = Np/Ns
where p refeer to primary and s to secondary: V = voltage, I = current, N
= numbers of turns

taking care about efficiency n = Ps/Pp ( about 80% to small transformers
less than 20VA and up to 92% at more than 200VA and good magnetic core )
for n secondary coils:
Pp = Ps1 + Ps2 +...+ Psn


using a good empiric formula for determining Ac
Ps = 50...60/Ac,

computing wire sections for coils:
d = 1.1...1.13 sqrt (I/J) where d is diameter in mm, I is ether Ip or Is
and J is accepted current density measured in A/mm2 ( european standard )
( I don't know how you've manage with yours cmils/A )

I hope I don't forget nothing, it's almost all you need to compute your
transformer.

Cheers, Vasile

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2001\09\04@103227 by Jim

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----- Original Message -----
From: "Nigel Goodwin" <KILLspamnigelgKILLspamspamLPILSLEY.CO.UK>
To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
Sent: Monday, September 03, 2001 4:50 PM
Subject: Re: [EE]: Pages on the web about audio transformers


{Quote hidden}

But the original author of this post *may* have had
another application in mind ...

{Quote hidden}

Been there/seen that/done that.

Try eliminating a 'ground loop' caused by the firing of
an airborne-RADAR magnetron sometime while trying to get
a clean 100 millivolt 'pulse' (no PFN (Pulse Forming Network)
after-ringing, 400 Hz power supply ground-loop effects,
etc) out of a Hewlett-Packard (P424A) Ku-nand 'crystal'
microwave (diode) detector ...

Solution to this problem: a piece of paper and #6 screws for
attachment - the paper provided the needed "DC" isolation while
passing RF between the two waveguide 'ports' all the while
insulating anything near DC (and slightly above, like the PFN
'ring' frequency ...

I'm also familiar with 'balanced' audio systems, normally
employing XLR-style connectors and the like, but have seen
a move away from purely 'passive' means to provide common
mode (e.g. ground loop) rejection. For instance, last year
I bought a new dbx 266XL "Compressor/Gate" that does a
marvelous job in an audio chain - and it does not seem to
use "transformers" on it's balanced inputs but rather a
solid state solution (operational amplifiers).


> large problem for live performance (I spent a number of years building
> and operating PA for some friends in a local rock band).
> --
>
> Nigel.

*I knew* the answers to my questions - but wanted explicit
clarification from the author of the question. One *never*
knows what new application one is endeavoring in/on ...

Also -

- I don't mind an answer or amplification from anyone else
either, as that adds to all our knowledge-base and serves
to further all our knowledge in what often times is more
'art' than 'applied science'.


Respecfully, Jim



{Quote hidden}

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2001\09\04@141311 by Peter L. Peres

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You can find some better audio transformer design data in older
(1950-1970) audio project books using valves. Most ideas that apply for
output transformers also apply for impedance transformers. There is also
some treatment of transformer design (aka wideband transformers and
impulse t.) in most any reasonable EE study book or engineering manual.
Valve HiFi entusiasts also talk a lot about transformer construction.

A pair of better stage line transformers will set you back $150 or more
now. imho expect to spend four times that much until you have a working
prototype that satisfies your special needs (I assume that if there would
be an off the shelf one you would buy it). Testing small signal
transformers requires a lot of uncommon test equipment but you can
improvise most of it.

hope this helps,

Peter

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2001\09\04@173249 by Edson Brusque

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Hello Brian, Jim, Nigel, Alan, Vasile, Peter etc :)

Brian wrote:
>    If you're looking for actual transformer design information,
>you might want to search for a copy of "Audio Transformer
>Design Manual" by Robert G. Wolpert.
>At one time, he was selling this book from a website, but I'm
>not sure any more.  Might be worth looking for.

   I've searched on the net, and found that this book is a very usefull
source of reference. Althought, I cannot find it to sell anywhere on the
net. The writer's address is "Robert G. Wolpert, 5200 Irvine Blvd. #107,
Irvine CA 92720" and the book seens to cost about US$36.00. He also wrote
"Transformer Design & Manufacturing Manual".

   The book is available at: http://www.nuenergy.org/orderform.htm for
US$49.00.

   If someone knows another place on the web where I can buy it, or the
writer's email address, please tell.

Jim and Nigel wrote:
>> It's commonly used in the music field, for example connecting guitars
>> directly to the PA system.
>But the original author of this post *may* have had
>another application in mind ...

   No, that's exactly the application I want. I'm a musician and want to
build my own DIs designing the transformers myself. As I'm doing some
research about inductors and transformers, it's all usefull informations
with practical applications for me.

>>>There might be other ways of 'driving' a low impedance load
>>>(like 150 Ohms) other than through the use of a transformer,
>>>such using an op-amp (or two). If the transformer is to
>>>provide some amount of common-mode isolation even this can be
>>>accomplished to degree using op-amps.
>>
>> That's not the main point, it's isolation to prevent earth loops, a
>Been there/seen that/done that.

   An active direct box can be made using op-amps and also is a very
usefull tool. But I like the ground isolation that can be achieved with
transformers. When you're using electronics on stage (like guitar effect
processors, sequencers, computers, keyboards, sound-modules etc), the ground
isolation between your rig and the mixing consoles is an important safety.

Alan wrote:
>For information on designing transformers see if you can find a copy of the
>"AWV Radiotron Designers Handbook". This book has its origins pre second
>world war in Australia and contains all sorts of necessary information. I
>believe it has been re-issued recently on CD ROM, and is well worth getting
>if you can track it down.

   It seens to be a very usefull book. I'll buy one (paper) available at
http://www.bh.com with the title "Radio Designer's Handbook Fourth Edition" for
£35.00 plus postage.

   The CD-ROM is available at http://www.ultranet.com/biz/raddev/#CDROM for
US$29.95 plus postage.

Peter wrote:
>A pair of better stage line transformers will set you back $150 or more
>now.

   Yes, Jensen direct box transformers costs about US$70.00 each.

> imho expect to spend four times that much until you have a working
>prototype that satisfies your special needs (I assume that if there would
>be an off the shelf one you would buy it).

   In Brazil the costs to import electronic components (and everything
else) is very high. Our currency (real) is about US$0.40 and the tributation
is very high (above 60% plus internal taxes) so I preffer doing the
transformers here as there's a very good transformer manufacturer on my
city.

> Testing small signal
>transformers requires a lot of uncommon test equipment but you can
>improvise most of it.

   What uncommon test equipment do you reffer? Anyway, I do have some good
audio tools to make some tests on my computer. Also, I have my ears and the
ears from some friends that works as musician and/or audio tecnician. :)

   Any more infos anyone???

   Thanks,

   Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2138
Blumenau  -  SC  -  Brazil
http://www.citronics.com.br
-----------------------------------

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2001\09\05@091949 by Mark Hull

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www.epanorama.net

Under the audio circuits section is a fair amount about audio transformers etc
Also has links to Jensen audio transformers, who have a lot about DI boxes

Cheers
Mark

---------------------------------------------------------------------
Mark Hull
Engineering / Technical
EMS (Africa) (Pty) Ltd
PO Box 1026, Melville, 2109, South Africa
Ph + 27 11 482 4470, Fax +27 11 726 2552


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2001\09\05@102408 by Jim

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> Hello Brian, Jim, Nigel, Alan, Vasile, Peter etc :)
>

-snip-

>
> Jim and Nigel wrote:
> >> It's commonly used in the music field, for example connecting guitars
> >> directly to the PA system.
> >But the original author of this post *may* have had
> >another application in mind ...
>
>     No, that's exactly the application I want. I'm a musician and want to

You've got to realize that there are *other* activities out
in the world that involve the use of electronics and trans-
formers and that are subject to 'ground loops' as well and
that may use transformers to eliminate this problem.

Unless we sometimes *know* your application - we may fail
in addressing your specific application. Not everyone here
is familiar with the shorthand 'slang' used in specific
fields - say, like in the music/studio business contrasted
with medical applications.

For instance, what if I asked for help on designing and building
a "directional coupler" or a "log IF" - would either of those two
terms make sense in the audio world?


Jim

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2001\09\07@022409 by Peter L. Peres

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> What tools

If you want to characterize the transformers properly you probably need
two complete network analyzers and some auxiliary DC and AC sources to
simulate ground loop, external magnetic fields, DC components in some of
the windings, etc.

If you are satisfied to go 'by ear', get a pair of good ready made
transformers a reel of cable and a switchover device (to switch to your
transformers and to the good ones), and start testing with your friends.
The comparison with the factory made units will remove some of the
siubjectivity in the tests.

Of course you know that overdriving transformers with certain core
materials (soft saturation) is one of the old ways to get a good guitar
fuzz ;-). In fact the core material is extremely important for good sound.

You could also cheat and get two miniature 600 Ohm bal/unbal transformers
for use in audio equipment and build a small preamp to drive them in the
guitar (impedance transformer with a little gain, like 3-5dB). These will
transformers cost $10+ each.

Peter

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2001\09\07@222639 by Jim

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  "two complete network analyzers"

Why two?

I can easily see one - but two?

I could see cases where the concurrent use
of two NAs could/would interfere with each
other, as 'network analyzers' per se, 'excite'
the CUT/DUT (Circuit or Device Under Test)
then observe/display the 'excited' circuit's
response.

Jim

{Original Message removed}

2001\09\08@145047 by Peter L. Peres

picon face
> two network analyzers

Because you probably want to see signals at four points: before the 1st
xformer, after it, after the cable, and after the 2nd xformer. You also
probably want to switch the cable(s) to check several types and several
lengths, switch loads and parasitics on the cable and on the ends, etc.
And you probably want to inject simulated disturbances (electrical and
magnetic), in each of the three (2 xformers and cable). You could probably
analyze only 1 xformer if you knew exactly what the cable does, but in my
(restricted) experience this is not the way it works (i.e. you need to
characterize the cable first to have a model - for each cable). The good
news is that the NAs need to cover only to 100 or 200kHz for audio so you
can do it with a multi-channel fast A/D board and some software.

Why are you picking on me ? Is this some sort of an exam ? Or do I speak
###s$$$ when I quote older manuals verbatim and refer to lab memories ?
;-)

Peter

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2001\09\08@212527 by Jim

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I'm not picking on anyone - I am asking
for clarification, sometimes in the face
of what appears to me to be contradictory
or arbitrary or conflicting recommendations.

As I have done what I thought was a fair amount
of this type of work in the past (from DC
through Ku-band) I naturally have questions
when I see something as I mentioned in my
first paragraph.

I still believe two (or more) 'Network analyzers
would conflict with each other in their use in
attached to apply and measure stimulus to the
same circuit - in fact, I wonder how this would
really be done successfully in actual practice.

Now, if you were referring to spectrum analyzers
or 'wave' analyzers versus NAs - those would be
horses of different colors. Two or more of those
could easily be attached to a circuit excited by
another source (such as NA even).

Jim


{Original Message removed}

2001\09\08@214021 by Jim

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I was going to mention that your A - B test
using factory built xformers versus any
designs that one might wind were a good
suggestion, too. That would also be a very
demanding test - as not all instrumentation
has the dynamic range that the human ear
has.

It *may* also be beneficial to see what
industry turns out as 'professional
product' as well.


Jim



{Original Message removed}

2001\09\09@152212 by Peter L. Peres

picon face
Hi Jim, maybe I need to mention that English is not my native language and
I use the term network analyzer somewhat more loosely than you seem to. A
network analyzer in my parlance is usually a 4-port device (assuming
unbalanced use) that can dynamically measure a number of parameters on a
device to be tested. I did not imply that these were Agilent high end
instruments or something like that.

In my experience a lot of good and very expensive instruments can replace
a lot of lab time and vice versa. Imho there is nothing in particular that
you can't do in 2 weeks with a GDO (or a signal generator), a scope and a
few resistance and capacitor boxes that you can do with 2 NA's in 10
seconds. At least not in the audio to HF range.

Peter

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