audio transformer, about 30W size, easily available?

[BFoelsch]

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42EE1FD6.6E39722D@hotmail.com...

BFoelsch wrote:

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42ED88D1.242CA476@hotmail.com...

BFoelsch wrote:

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42ED6249.D87912CE@hotmail.com...
mw wrote:

Hi,

Know of any easily available (inexpensive and stocked) 30W audio
transformer to couple to an 8 ohm speaker?

I am experimenting with a custom audio amplifier design that uses
tranformer coupling rather than the usual capacitors. I took a
gamble
and tried to use a power transformer, but that's not working well
because they are designed to work well at 60 Hz and the ones I've
tried
roll off very fast at higher frequencies. This isn't an
audiophile
project, but I want something a little better.

I want something I can get at a place like DigiKey or Newark or
Mouser,
not from a custom house or scrounging at hamfests or one of those
"classic tube audio transformers" that go for $100. I can't use
the
little audio transformers sold in Digikey because their power
handling
is too small.

Would one of those 70V PA distribution transformers work for this?
Is
there a PA transformer vendor with real spec sheets, not just a
catalog
ad?

You don't say what the primary side drive is - i.e. tube or
transistor.

If it's transistor and you want and you want something like a 1:1
transformer
you can pretty much forget it unless you want to pay for a custom
wound
job.
There's simply no market for such things.

70V transformers are designed to be loaded with quite a high
impedance -
no
good for 8 ohms.

Could you clarify this for me? Exactly what impedance should they be
loaded
with?

It's calculated from first principles.

If it's 30W you want and the voltage is 70V then I = W/V = 30/70 = ~
0.43

Zl = V / I = 70 / 0.43 = ~ 163 ohms


Yes, but you are calculating the impedance that the transformer places
across the 70 volt line. You specifically state that

70V transformers are designed to be loaded with quite a high
impedance -
no
good for 8 ohms.

What do your calculations do to support your point? You haven't
mentioned a
single word about the transformer load (secondary winding) other than to
say
that it must be other than 8 ohms.

No I'm talking about the secondary ! The primary is unknown until the OP
gives
some data.

What does the reflected primary impedance
have to do with the optimum secondary load, and why are 70 volt
transformers
no good for 8 ohm loads?

I'm still confused. Why can't a transformer with a 167 ohm primary be
designed to match an 8 ohm speaker?

No you twit ! That's the *SECONDARY* load impedance / voltage ! 160 odd
ohms
isn't 8 ohms. Do you know nothing ?

Are you the original OP btw ?

Graham

Oh, you are talking about the transformer that DRIVES the 70 volt line.
Everybody else is talking about the transformers that connect the 70 volt
line to the speaker.

 

[Rich Grise]

On Sun, 31 Jul 2005 15:08:26 -0400, Tam/WB2TT wrote:

"Chris Carlen" <crobc@BOGUSFIELD.sbcglobal.net> wrote in message
news:dcj0pe02maq@news4.newsguy.com...
jabara wrote:
"mw" <mw@avoidspam.com> wrote in message
news:kv6He.27$3M4.2@newssvr27.news.prodigy.net...
Hi,

Know of any easily available (inexpensive and stocked) 30W audio
transformer to couple to an 8 ohm speaker?

I am experimenting with a custom audio amplifier design that uses
tranformer coupling rather than the usual capacitors. I took a gamble
and tried to use a power transformer, but that's not working well
because they are designed to work well at 60 Hz and the ones I've tried
roll off very fast at higher frequencies. This isn't an audiophile
project, but I want something a little better.

I want something I can get at a place like DigiKey or Newark or Mouser,
not from a custom house or scrounging at hamfests or one of those
"classic tube audio transformers" that go for $100. I can't use the
little audio transformers sold in Digikey because their power handling
is too small.

Would one of those 70V PA distribution transformers work for this? Is
there a PA transformer vendor with real spec sheets, not just a catalog
ad?
Thanks for any ideas,
mw

Good luck finding anything with high frequency response, 4K to 17K That
almost has to be ferrite

?!?!?!?!?!

Then how do all the iron-cored transformer coupled tube amps acheive full
20-20kHz response?



It won't be small. I still have a 25 - 30 W ultralinear output transformer
that I used with a pair of 7034? tubes. It is about 4"square and 5 - 6
inches tall. Must weigh about 5 pounds.

That sounds like exactly what "mw" is looking for. Offer to sell it to

him or her.

Cheers!
Rich

 

[Pooh Bear]

BFoelsch wrote:

Oh, you are talking about the transformer that DRIVES the 70 volt line.
Everybody else is talking about the transformers that connect the 70 volt
line to the speaker.

Errr - no !

Graham

 

[Rich Grise]

On Mon, 01 Aug 2005 23:04:35 +0100, Pooh Bear wrote:

BFoelsch wrote:

Oh, you are talking about the transformer that DRIVES the 70 volt line.
Everybody else is talking about the transformers that connect the 70 volt
line to the speaker.

Errr - no !

Graham

Well, are we talking about 70.7 volt PA transformers? The output
of the PA (Public Address) amp puts out a nominally 70.7 volts.
At each place where there needs to be a speaker, (like, in a
classroom), there's a transformer that kinda comes with the
speaker. Its primary goes right across the 70.7 volt PA line,
and it has taps for the appropriate speaker. A quick web search
turned up a couple that can handle 30 watts, but they're already
attached to loudspeakers, for like three hundred bucks.

Apparently the PA folks aren't used to selling stuff to hobbyists.
)-;

Thanks,
Rich

 

[mw]

Graham Holloway wrote:

Sorry, just checked and found I got them from RS Components (UK), Part No.
257-5017. They are 50VA mains toroids (2x12V secondaries, 2x120V primaries)
and I used two. The secondaries were paralleled with the centre tap formed
going to 0V. A 120V primary of one was placed in series with a 120V primary
of the other and vice versa. This enabled the correct ratio to be achieved
and to cancel the DC component in each valve.

Thanks for the additional info. I ordered the one from Edcor, it will
be here in about 2 weeks.

mw

 

[Pooh Bear]

Rich Grise wrote:

On Mon, 01 Aug 2005 23:04:35 +0100, Pooh Bear wrote:

BFoelsch wrote:

Oh, you are talking about the transformer that DRIVES the 70 volt line.
Everybody else is talking about the transformers that connect the 70 volt
line to the speaker.

Errr - no !

Graham

Well, are we talking about 70.7 volt PA transformers? The output
of the PA (Public Address) amp puts out a nominally 70.7 volts.
At each place where there needs to be a speaker, (like, in a
classroom), there's a transformer that kinda comes with the
speaker. Its primary goes right across the 70.7 volt PA line,
and it has taps for the appropriate speaker. A quick web search
turned up a couple that can handle 30 watts, but they're already
attached to loudspeakers, for like three hundred bucks.

Apparently the PA folks aren't used to selling stuff to hobbyists.
)-;

Uh ?

The BFoelsch guy incorrectly thought I was talking about the 70V transformers
used to connect the speakers to the 70V line.

I mentioned them in passing.

My main point was that a 70V line providing 30 Wattts doesn't look much ( or
even at all ) like 8 ohms ( which is what the OP wants to connect to ) hence '
70V transformers ' are no use to him whatever.

Graham

 

[Rich Grise, but drunk]

On Tue, 02 Aug 2005 01:31:54 +0100, Pooh Bear wrote:

Rich Grise wrote:

On Mon, 01 Aug 2005 23:04:35 +0100, Pooh Bear wrote:

BFoelsch wrote:

Oh, you are talking about the transformer that DRIVES the 70 volt line.
Everybody else is talking about the transformers that connect the 70 volt
line to the speaker.

Errr - no !

Graham

Well, are we talking about 70.7 volt PA transformers? The output
of the PA (Public Address) amp puts out a nominally 70.7 volts.
At each place where there needs to be a speaker, (like, in a
classroom), there's a transformer that kinda comes with the
speaker. Its primary goes right across the 70.7 volt PA line,
and it has taps for the appropriate speaker. A quick web search
turned up a couple that can handle 30 watts, but they're already
attached to loudspeakers, for like three hundred bucks.

Apparently the PA folks aren't used to selling stuff to hobbyists.
)-;

Uh ?

The BFoelsch guy incorrectly thought I was talking about the 70V transformers
used to connect the speakers to the 70V line.

I mentioned them in passing.

My main point was that a 70V line providing 30 Wattts doesn't look much ( or
even at all ) like 8 ohms ( which is what the OP wants to connect to ) hence '
70V transformers ' are no use to him whatever.

Now, wait a minute. I thought I was the one who's been drinking here.

What you just said - a 70.7 V line doesn't look much like 8 ohms.

That's what the transformer is for, duh!

If he's talking about a 70.7 volt line, then he needs a speaker
transformer.

If he's talking about driving that 70.7 volt line with transformers from
his output stage, then all bets are off - that'd be kinda an
exercise in anachronism, given the transistors they have these days.

;-)
Cheers!
Rich

 

[Tony Williams]

In article <TuyHe.336$3M4.272@newssvr27.news.prodigy.net>,
mw <mw@avoidspam.com> wrote:

Graham Holloway wrote:
Sorry, just checked and found I got them from RS Components
(UK), Part No. 257-5017. They are 50VA mains toroids......
[snip]

Mains toroids can have quite a reasonably low
leakage inductance, but the associated downside
is the high capacitance and early LC resonance.

Thanks for the additional info. I ordered the one from Edcor, it
will be here in about 2 weeks.

You really need to state the maximum primary voltage
and source impedance of your transformer driver.

--
Tony Williams.

 

[Adrian Tuddenham]

Rich Grise <richgrise@example.net> wrote:

On Mon, 01 Aug 2005 23:04:35 +0100, Pooh Bear wrote:


BFoelsch wrote:

Oh, you are talking about the transformer that DRIVES the 70 volt line.
Everybody else is talking about the transformers that connect the 70 volt
line to the speaker.

Errr - no !

Graham

Well, are we talking about 70.7 volt PA transformers? The output
of the PA (Public Address) amp puts out a nominally 70.7 volts.
[...]
. A quick web search
turned up a couple that can handle 30 watts, but they're already
attached to loudspeakers, for like three hundred bucks.

RS Components had one in their 2001 catalogue (actually a 100v-line
version for PA systems in the UK) Part Nr. 210-6532 It was priced at
11.98 UKP.

I seem to remeber that this type didn't have particularly well coupled
windings and the output fell off towards 20 Kc/s. Best to ask them for
the spec first if you intend to buy.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

 

[Paul Mathews]

There is so much confusion about audio distribution transformers in
this thread that I just had to put in my two cents.
First, I never did figure out just what turns ratio or (alternatively)
impedance ratio the OP wants, but a great variety of ratios are
available from audio distribution transformer vendors. Unfortunately,
their datasheets don't always make it obvious....you'll have to do some
interpretation.
Second, there is no necessary distinction between transformers used to
boost voltage up from low impedance amplifier outputs to the higher
70.7V and 100V nominal lines levels common for audio distribution and
those used to transformer back down to low impedance drivers at the
loudspeaker end. You simply connect the transformer appropriately at
either end. However, it is true that many audio distribution
loudspeakers come with transformer included, often with a series
capacitor and/or other components wired in. In any case, transformers
suitable for milliwatts to hundreds of watts are readily available in
any qty from pro audio distributors and catalog houses.
To make sense of audio distribution transformer voltage, impedance, and
power specs, simply use Ohms Law. For example, a power amplifier rated
100 watts 8 ohms has a 28.3 volt rms output capability, so a
distribution transformer designed to couple the amplifier output to a
70.7V line has a turns ratio of 28.3:70.7 = 1:2.5. When the 70.7V line
is fully loaded with loudspeakers (each with their own step down
transformer), their impedance is Z = V^2/P = 70.7^2/100 = 5000/100 = 50
ohms. The same transformer can be turned around, so that a driving
source will 'see' a 50 ohm load with 8 ohms on the 'secondary'. Audio
distribution transformers are readily availalble for 25, 70.7, 100, and
140 volt systems, and power ratings range from milliwatts to hundreds
of watts. So, a great many turns ratio and core size combinations are
out there. Note, however, that a good many of them are actually
AUTOFORMER connected, meaning that primary and secondary share turns
and provide no galvanic isolation. Autoformers can be smaller and
less-expensive than true transformers, and they are almost guaranteed
to have lower leakage inductance.

Transformers designed for audio distribution should differ from power
transformers in these respects:
a) less insulation and more intimate coupling between primary and
secondary (by means of multifilar winding and/or interleaving) to
reduce leakage inductance to improve high frequency response
b) thinner laminations and/or better steels to reduce high frequency
core losses
c) power ratings much higher than the corresponding mains transformer,
because audio transformers are not used for continuous power output
(audio average power is much lower than audio peak power for voice and
music)
d) in consideration of c, a well-designed audio transformer will give
much less consideration to temperature rise

For those transformers designed to couple significant power at
frequencies below mains frequencies, the transformer design will also
use greater core cross-sectional areas and/or higher turns counts than
a similar mains transformer. However, this is rarely the case, since
audio distribution systems usually are not called upon to deliver much
power in the lower octaves.

Also, some audio distribution transformers may be designed to tolerate
significant DC offsets present at the outputs of some high power
amplifiers. Deliberate increases in winding resistance and/or the
inclusion of small air gaps in the core accomplish this.

Some toroidal mains transformers will provide good fidelity up to 10kHz
and above, provided that thin insulation was used in their construction
and the windings are close together and well distributed around the
core. However, most will probably saturate badly at frequencies just
below their mains rating and/or voltages slightly higher than rated.

Speaking of low frequency saturation, it's not widely understood that
transformer behavior in saturation, as measured by distortion on the
secondary, depends strongly on power amplifier characteristics. When
the core saturates, coupling between primary and secondary remains
good, but the amplifier sees an incredibly low inductance in PARALLEL
with the reflected secondary load impedance. In effect, the power amp
suddenly has to supply current into the winding resistance. Saturation
is a voltage-times-time phenomenon (that has nothing to do with
secondary loading), with core magnetization flux building continually
during each half cycle of VOLTAGE input. Consequently, the onset of
saturation as input amplitude increases, normally occurs near a voltage
zero-crossing, just prior to the end of a half-cycle. This is the
point of greatest vulnerability for a Class AB power amplifier, where
the voltage across its output transistors is greatest. The sudden
current spike associated with transformer saturation generally causes
some combination of these symptoms: voltage distortion due to finite
output impedance, voltage distortion due to deliberate SOA protection,
output transistor damage. You don't need much equipment to test a
transformer for low frequency saturation: a signal generator, and
oscilloscope, and a low ohms series resistor. You'll also need a power
amplifier if your signal generator doesn't have enough voltage output.
Connect the resistor in series with a low impedance transformer winding
and drive the series combination with the signal generator, with common
connections, including scope ground connected to the open end of the
resistor. Connect one scope channel across the resistor to monitor
current and connect another channel across the signal generator (or
power amp) output. Increase driving voltage and/or lower driving
frequency until the voltage across the resistor exhibits spikes near
voltage zero crossings. You have discovered the saturation limit of
your transformer at the test frequency. Normally, you should expect
the saturation voltage limit to halve for each octave decrease in
frequency, since it is determined by the integral of voltage over time
with units of volt-seconds. Consequently, power handling without
saturation declines by a factor of 4 for each octave. Again, what
happens when an audio signal reaches the volt-seconds limit for a
transformer depends most strongly on amplifier output characteristics,
since the transformer still happily delivers a turns-ratio-modified
replica of driven winding voltage to the other windings. For good
reliability and fidelity, "low-cut" audio signals so that you just
don't go there.

Paul Mathews



Paul Mathews

 

[mw]

Paul Mathews wrote:

There is so much confusion about audio distribution transformers in
this thread that I just had to put in my two cents.

Thanks for your contribution.

First, I never did figure out just what turns ratio or (alternatively)
impedance ratio the OP wants,

That's because in the "big picture" it doesn't matter. While it would
make sense to expert designers that these parameters would be neatly
defined and calculated, I am instead working empirically and changing
things as I go along. That's why I said in my original post "I am
experimenting with a custom audio amplifier design"... that means I have
plenty of flexibility with the design. I can work backwards in a way,
first I'll find a good transformer and then adjust my design to match it.

It's a fun way to do things.

I'll let you know in a couple of weeks how it turns out with the
transformer. The other aspects of the design are working very well, and
even with the 60Hz power transformer I am getting a lot of my hoped-for
results.

mw