Hello all,

I've been given an old Hammarlund SPR-210. It needs a good amount of
restoration but appears complete inside and outside. I've seen on the
manual that it has 600 ohm speaker output and I was wondering where to
find such a spekear with this impedance. Probably there's need for an
external audio transformer to use any low impedance (I have many
in the 16-4 ohm range) speaker?
Any hint is welcome.
Best regards

Frank IZ8DWF

On Oct 6, 8:22*am, frank wrote:
Hello all,

I've been given an old Hammarlund SPR-210. It needs a good amount of
restoration but appears complete inside and outside. I've seen on the
manual that it has 600 ohm speaker output and I was wondering where to
find such a spekear with this impedance. Probably there's need for an
external audio transformer to use any low impedance (I have many
in the 16-4 ohm range) speaker?
Any hint is welcome.
Best regards

Frank IZ8DWF

The usual fix is to use a 10-watt, 70-volt line to voice coil
transformer. The 10-watt tap on a 70-volt line to voice coil
transformer will provide a decent match to a 500/600 ohm output. In a
pinch, an ordinary filament transformer will do. Get a transformer
with a 120-volt primary and 12.6-volt secondary and connect the 120-
volt winding to the receiver and the 12.6-volt winding to the speaker
and you are done! --Ed

frank wrote:

I've been given an old Hammarlund SPR-210. It needs a good amount of
restoration but appears complete inside and outside. I've seen on the
manual that it has 600 ohm speaker output and I was wondering where to
find such a spekear with this impedance. Probably there's need for an
external audio transformer to use any low impedance (I have many
in the 16-4 ohm range) speaker?
Any hint is welcome.

If you wound a 600 ohm voice coil on a speaker, it would be so heavy
you'd have no high end at all.

SO, what we do is to put a 600-8 ohm transformer behind the speaker; lots
of communications speakers have mounting points on the back so you can bolt
the transformer right there.

A 1W transformer for a 70V PA system has about a 600 ohm to 8 ohm ratio,
and will cost less than $5 at an electronics supply house. Or ask a guy
who installs 70V PA systems for piped-in music in supermarkets for an old
one.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Hello Ed!

Ed Engelken wrote:

The usual fix is to use a 10-watt, 70-volt line to voice coil
transformer. The 10-watt tap on a 70-volt line to voice coil
transformer will provide a decent match to a 500/600 ohm output. In a
pinch, an ordinary filament transformer will do. Get a transformer
with a 120-volt primary and 12.6-volt secondary and connect the 120-
volt winding to the receiver and the 12.6-volt winding to the speaker
and you are done! --Ed

thank you for the suggestion, I do have a spare 120V primary transformer
lying around (I live in a 240V country, so no use for it). I could also
wire my own transformer, but that would take some time.
I don't know what a line to voice coil transformer is, can you explain
better this part? Thanks a lot.
Best regards

Frank IZ8DWF

"frank" wrote in message
...
Hello Ed!

Ed Engelken wrote:

The usual fix is to use a 10-watt, 70-volt line to voice
coil
transformer. The 10-watt tap on a 70-volt line to voice
coil
transformer will provide a decent match to a 500/600 ohm
output. In a
pinch, an ordinary filament transformer will do. Get a
transformer
with a 120-volt primary and 12.6-volt secondary and
connect the 120-
volt winding to the receiver and the 12.6-volt winding to
the speaker
and you are done! --Ed

thank you for the suggestion, I do have a spare 120V
primary transformer
lying around (I live in a 240V country, so no use for it).
I could also
wire my own transformer, but that would take some time.
I don't know what a line to voice coil transformer is, can
you explain
better this part? Thanks a lot.
Best regards

Frank IZ8DWF

I've used filament transformers many times and they
work well. While the fidelity may not be as good as a
special purpose audio transformer I doubt you will be able
to tell the difference in this application.
The Hammarlund Super-Pro is a very fine receiver and
will repay your work in restoring it.


--

--
Richard Knoppow
Los Angeles
WB6KBL

On Tue, 6 Oct 2009, Richard Knoppow wrote:

"frank" wrote in message
...
Hello Ed!

Ed Engelken wrote:

The usual fix is to use a 10-watt, 70-volt line to voice
coil
transformer. The 10-watt tap on a 70-volt line to voice
coil
transformer will provide a decent match to a 500/600 ohm
output. In a
pinch, an ordinary filament transformer will do. Get a
transformer
with a 120-volt primary and 12.6-volt secondary and
connect the 120-
volt winding to the receiver and the 12.6-volt winding to
the speaker
and you are done! --Ed

thank you for the suggestion, I do have a spare 120V
primary transformer
lying around (I live in a 240V country, so no use for it).
I could also
wire my own transformer, but that would take some time.
I don't know what a line to voice coil transformer is, can
you explain
better this part? Thanks a lot.
Best regards

Frank IZ8DWF

I've used filament transformers many times and they
work well. While the fidelity may not be as good as a
special purpose audio transformer I doubt you will be able
to tell the difference in this application.

Of course, one solution in the old days was to use a 400Hz transformer,
which were cheap and available as surplus, and which didn't have much
use in the average ham shack at the time. The idea being that since
it's intended for 400Hz, the upper frequencies might get through
the transformer better, and the loss of lower frequenices didn't matter
since for voice you didn't care about much below 300Hz.

Nowadays it may no longer be a solution, given WWII is so far in the past
and the stores that might have them for next to nothing nearby are gone
too.

Michael VE2BVW

Michael Black wrote:

Of course, one solution in the old days was to use a 400Hz transformer,
which were cheap and available as surplus, and which didn't have much
use in the average ham shack at the time. The idea being that since
it's intended for 400Hz, the upper frequencies might get through
the transformer better, and the loss of lower frequenices didn't matter
since for voice you didn't care about much below 300Hz.

Nowadays it may no longer be a solution, given WWII is so far in the past
and the stores that might have them for next to nothing nearby are gone
too.

400 Hz transformers are still very popular for modern aircraft applications.
There are still lots of them on the surplus market.

They catch fire if you connect them to a 60 Hz source because the series
inductance is very low.

They actually aren't very good as audio transformers even.

Really, they aren't good for anything. If you want some, let me know.
I even have a bunch of sealed 120VAC-200VDC supply modules, 400 Hz only.
Not useful.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."