Replacement Alternative for Obsolete 3 inch round 25 Ohm 1 Watt Intercom Speakers

Thread Starter

Munson48*w7

Joined Oct 27, 2020
5
Hello,

This is my 1st post, so forgive me if I step out of protocol. I own a vintage Philips wired intercom (made in France) that has served me well for more than 30 years. The outside speakers have given recently up the ghost. The sound quality is nothing short of incredible with the original good speakers. While I did manage to find the original engineering spec sheets for the speakers - (see attachment), and have contacted Philips to no avail, the closest replacement speakers I've found are two inch round, and I think those have already been sold on Ebay. My question is could I expect better results with 16 ohm, or 45 ohm versions shored up with resistors to at least come close to the proper impedance ?. Even better, does anyone know where I might find speakers with the exact attributes as the originals ?. The resonate frequency of the originals is 250 Hz, but at this point I'd gladly settle for matching the specs shown in the title of this post. I'd have no problem retrofitting speakers with square basket frames providing the frame is no larger than 3.5 x 3.5 inches. My main concern using resistors to shore up the impedance is ending up with greatly reduced volume &/or distortion. I am a retired audio tech who should know the effects of matching speaker impedance with resistors - kind of embarrassing. I do know impedance changes with frequency, but this is a narrow (voice) band I'm dealing with.

Many Thanks,
Munson48*w7
 

Attachments

crutschow

Joined Mar 14, 2008
26,073
I expect that the speaker impedance is not that critical.
A 45 ohm speaker will have a lower volume level for a given amp output.
I would try a 16 ohm speaker and add a series resistance only if the sound is not good.
 

Thread Starter

Munson48*w7

Joined Oct 27, 2020
5
I expect that the speaker impedance is not that critical.
A 45 ohm speaker will have a lower volume level for a given amp output.
I would try a 16 ohm speaker and add a series resistance only if the sound is not good.
crutschow,

Thank you so much. We are on the same page. I have qty (4) 16 ohm 3" speakers in my Ebay cart now, along w/some 6.2 ohm non-inductive resistors - gonna give a shot. I will let you know how it goes.

Thanks Again,
Munson48*w7
 

MrChips

Joined Oct 2, 2009
22,584
16Ω speakers should not be hard to find. I have a couple in my junk box.
I would install a 16Ω speaker and leave it at that.
 

LowQCab

Joined Nov 6, 2012
148
If you use anything other than the original Impedance, the output balance of the
"Microphone-Mode", and "Speaker-Mode" will be off to some degree.
This may, or may not, produce acceptable performance.

If the balance is not acceptable with a 16 Ohm Speaker, the proper way to fix the
problem is with a custom made matching transformer, way too expensive.

Another way to fix it involves increasing the gain of the "Microphone-Mode" amplifier,
this may be as simple as changing one Resistor on the Amplifier Board.

A third way to fix it is by physically re-building the deteriorated Speaker Cones using
Glue, Toilet-Tissue, Clear-Spray-Paint, or what ever else you can think of,
to re-establish the integrity, rigidity, and very importantly, the LIGHT WEIGHT, of the Speaker Cones.
(the Toilet Tissue acts like a Fiber-Glass Cloth, and is very effective for repairing Speaker Cones)

Remember that these speakers are also being used as Microphones,
this makes the WEIGHT and RIGIDITY of the Cones critical to adequate Output, and Frequency Response.
Heavy/Soft/Flexible Cones will create a Microphone with a Low, Muffled Output.

Or, just try one of these .........
They're really cheap and may work fine .......
https://www.parts-express.com/cat/m...eca:matches(.,"P_Searchable","1")]&PortalID=1
.
 

Thread Starter

Munson48*w7

Joined Oct 27, 2020
5
Unless it uses the speaker as a microphone too, I would put a 16ohms in .

You can get 25 ohms speakers but from China.
It does, and I am - just ordered minutes ago. Got a great price on four 16 ohm spks, from the US no less - no waiting on that "S

Thanks
If you use anything other than the original Impedance, the output balance of the
"Microphone-Mode", and "Speaker-Mode" will be off to some degree.
This may, or may not, produce acceptable performance.

If the balance is not acceptable with a 16 Ohm Speaker, the proper way to fix the
problem is with a custom made matching transformer, way too expensive.

Another way to fix it involves increasing the gain of the "Microphone-Mode" amplifier,
this may be as simple as changing one Resistor on the Amplifier Board.

A third way to fix it is by physically re-building the deteriorated Speaker Cones using
Glue, Toilet-Tissue, Clear-Spray-Paint, or what ever else you can think of,
to re-establish the integrity, rigidity, and very importantly, the LIGHT WEIGHT, of the Speaker Cones.
(the Toilet Tissue acts like a Fiber-Glass Cloth, and is very effective for repairing Speaker Cones)

Remember that these speakers are also being used as Microphones,
this makes the WEIGHT and RIGIDITY of the Cones critical to adequate Output, and Frequency Response.
Heavy/Soft/Flexible Cones will create a Microphone with a Low, Muffled Output.

Or, just try one of these .........
They're really cheap and may work fine .......
https://www.parts-express.com/cat/mini-speakers/27?N=19815+4294967118+4294964799+4294964608+4294963856+4294964950+4294964094+4294964240&Ne=10166&Nrpp=99999&Nrs=collection()/record[endeca:matches(.,"P_PortalID","1")+and+endeca:matches(.,"P_Searchable","1")]&PortalID=1
.
LoQCab,

Thanks for the comprehensive input, but I've already settled on using these: "4 lot - 16 ohm 1 watt 3" Diameter Transistor Radio Speaker 1" profile Japan NOS" as replacements w/non-inductive 6.04 ohm series resistors. The resistor values are somewhat arbitrary, but brings the total load close enough (22.04 ohms) to satisfy the amp. At 1 watt, & intermittent use, I don't think I'll have any overheating or distortion issues. Enter the description (" ") in the Ebay search bar to see the speakers. They have the same diameter as the originals, & classic paper cone construction. I tried repairing the originals using a 50/50 Damar & Turpentine solution. I found that tip in an old antique speaker repair article, I apparently used too many coats because the results produced clear, but extremely low volume - stiff cones. One of the 3 speakers had an open voice coil, so at that point I decided I simply needed new speakers. I agree with the cost prohibitive aspect of using matching transformers. I have several old 70V transformers left over from my Muzak days, but nothing even close to what I'd need in this case. There was always plenty of headroom with the original good speakers; I could produce feedback @ 3/4 max volume with speaker placement about 30 feet away with solid wood doors, & 2~3 walls in between. I think that kind of headroom will compensate for any reduction in mic or spk mode gain reduction using non-exact speakers. Besides, my days of spending hours or days reverse engineering & modifying are long gone. The older you get, the more precious time becomes. I should have the speakers Monday, and will post the outcome.

Thanks Again,
Munson48*w7
 

Thread Starter

Munson48*w7

Joined Oct 27, 2020
5
Hello All,

It's me - that intercom guy. Well, I installed the 16 ohm 3 inch speakers purchased from an Ebay seller. My results were pretty much as I expected. The speakers look like something you might find in a 1969 Radio Shack transistor radio. The two station system works, but the audio quality is roughly half as good as it was with the original 25 ohm / 250Hz res freq drivers. Audio is intelligible, but that's about it. I'm a bit nostalgic when it comes to old gear that still works. This intercom is well over 50 years old. They don't make em like that anymore. My attempts at matching the impedance gave me nothing more than volume level changes. I plan to try some 45 ohm voice optimized versions in the near future if I can find some with a 3 inch basket. Thanks for all the practical input. I mainly needed some validation, and I got it.

Thanks Again,
Munson48*w7
 

mlsirkis

Joined Aug 11, 2010
29
Hello All,

It's me - that intercom guy. Well, I installed the 16 ohm 3 inch speakers purchased from an Ebay seller. My results were pretty much as I expected. The speakers look like something you might find in a 1969 Radio Shack transistor radio. The two station system works, but the audio quality is roughly half as good as it was with the original 25 ohm / 250Hz res freq drivers. Audio is intelligible, but that's about it. I'm a bit nostalgic when it comes to old gear that still works. This intercom is well over 50 years old. They don't make em like that anymore. My attempts at matching the impedance gave me nothing more than volume level changes. I plan to try some 45 ohm voice optimized versions in the near future if I can find some with a 3 inch basket. Thanks for all the practical input. I mainly needed some validation, and I got it.

Thanks Again,
Munson48*w7
Misco Speakers
https://www.miscospeakers.com/application/files/5315/6391/5179/57RFT-25B-CC.pdf
 

Thread Starter

Munson48*w7

Joined Oct 27, 2020
5
Thanks for the Misco tip, but they are .8 inches too small with a resonant freq well over 4 times that of the originals, but at least the resonant freq is shown.
 

LowQCab

Joined Nov 6, 2012
148
Thanks for the Misco tip, but they are .8 inches too small with a resonant freq well over 4 times that of the originals, but at least the resonant freq is shown.
A resonant frequency around 1k is exactly what you want for best voice intelligibility.
In addition to this, lowering the resonant frequency is much easier than raising it.
A can of Clear Spray Paint is all you need to lower it,
but don't forget that lowering the frequency will also reduce it's output somewhat,
both as a Microphone, and as a Speaker.
But this speaker has extremely high efficiency at 98db/w/m, so that may not be an issue.
Also, don't forget, that the resonant frequency rating is in "Free-Air",
and when you install it in some kind of box, the frequency will change somewhat.
But with a speaker this small, the resonant frequency is not likely to rise very much because of the box volume,
but the resonant frequency of the box itself will just add to the "character" of the sound that you will get.

This Speaker is a lucky find,
all you need to do is make sure the back-side of the Grill, around the outside of the speaker frame,
is closed-off with some sort of plastic material, or maybe Hot-Glue.
This is because you must be sure to isolate the front of the speaker cone from the back side of the cone.
If the front and rear of the Cone are not isolated,
any low frequency response that you may have had will disappear, and the sound will be very "thin".
On the other hand, anything you can do to insure that the box is completely sealed will improve the fidelity.
Also, stuffing the box full of fiber-glass, (or even an old dish-cloth), will improve the sound as well.

Finding a "Drop-In" replacement, especially with matching specs, will probably never happen.
.
.
 
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