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Old 25th September 2016, 05:45 PM   #1
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Default Help choosing speakers for vintage radio as enclosure

Hello DIY Audio community.

I have a Firestone 4-A-30 from the 40's. I'd like to make it into a networked/bluetooth audio player. I need help choosing which driver(s) to use.

Currently the amp / AM radio work, but the cloth and speakers were damaged by my toddler. I'd like to add new speaker(s) and drive them from a hifiberry amp+. Inside the console there is a piece of plywood (2ft tall by 17'' & 3/8 wide piece of 1/2) which the ~9" original speaker is attached to. I think I could easily replace the plywood with a new speaker configuration. I would leave the existing amp in place and just not use it. The console/cabinet is open in the back and has no floor inside.

I'm thinking full range speaker(s).

-What speaker(s) would your recommend and in what configuration?

Other information:
- Budget is flexible, but thinking about $100 for the drivers.
- The radio is in a small dinning room with laminate "wood" floors and mostly all hard surfaces.
- I don't have true HIFI audiophile ears/expectations, but would like for music to sound nice (would like Miles Davis Kinda Blue to sound good).
- I don't particularly want to enclose or modify the cabinet extensively so that I can restore it back if ever so inclined.

To see pictures of this model:
4-A-30 Air Chief Radio Firestone Tire & Rubber Co. Air Chief

Amp:
https://www.hifiberry.com/ampplus/
hifiberry amp+ on a Raspberry Pi3.
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Old 25th September 2016, 06:09 PM   #2
GM is offline GM  United States
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It basically looks identical to my '39 Sears Silvertone [RCA] w/12" field coil. As such, it's performance is on a par with some old open back lead guitar amp drivers, so what I'd use for a ~period correct performance; otherwise pick whatever 40-60 Hz high Q [the higher the better] to go low [open/dipole or IB cab recommended] 'full-range' [single or coax] driver that fits your budget/sounds 'right' to you, which in your price range usually means inexpensive car audio or commercial PA [ceiling] drivers.

GM
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Old 25th September 2016, 06:51 PM   #3
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Your guitar speaker comment gave me an idea. I do have a guitar amp with a Celestion G12T-100 in an open back cabinet. Would trying my intended amp to this speaker in the guitar cab be comparable to what would happen in the radio?
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Old 25th September 2016, 07:31 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Guitar speakers simply won't cut the mustard, far
too much upper midrange and no treble generally.

Compare the G12T :

Click the image to open in full size.

To the PA K12H :

Click the image to open in full size.

rgds, sreten.

Mount a FR as high up on your board as it will go.

Last edited by sreten; 25th September 2016 at 07:41 PM.
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Old 25th September 2016, 08:45 PM   #5
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GM & Seren,

Thank you for your help so far. I still have a lot to learn about audio. Let's see if I am understanding and making good assumptions. I apologize in advance if I seem a little elementary.

With the graphs the G12T has too narrow of range near it's rated db's (good for guitar, but not a full band). The PA K12H is much wider frequency response near its db rating

K12H w/ 8 ohm at 200W would work find with a 25W 4 ohm class D amp.

For my application I am better off running the one speaker in Mono than trying to squeeze in multiple small FR drivers to get stereo. Especially considering the cabinet design.

Placing the speaker higher up will help me better hear the full range. Otherwise the treble would be at knee level in a small room.

Would I be better off using plywood or mdf for the front board replacement or would it not make much difference?
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Old 25th September 2016, 09:16 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

You have a large range of choice for a single driver,
I only pointed out the K12H is better than the G12T.

For stereo you would be looking at side mounting
drivers, you can't do anything on that baffle, and
FWIW if you went that route it would be probably
better to go sum and difference, which gives a
mono central channel with all the bass and a
channel to drive small drivers left and right, which
isn't proper stereo but is more spacious than mono.

rgds, sreten
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Old 26th September 2016, 12:16 AM   #7
GM is offline GM  United States
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Quote:
Originally Posted by sreten View Post
Guitar speakers simply won't cut the mustard, far
too much upper midrange and no treble generally.
Worked for me, the old Jensen I used has a near enough same response, which BTW were just cheap clones of the early cinema folded mid-bass horns.

Factor in that mine is a matching impedance SET amp and AM radio was something like 100-5 kHz and they were really only interested in the ~250-2.5 kHz telephone BW having high speech intelligibility and last, but not least, one is always listening pretty far off axis unless laying on the floor firing through a fairly dense cloth and it's a really good match-up, hence the recommendation for a period correct sound.

If driven with a modern amp and sources, then of course yours or mine recommendation is preferred.

All that said, it just dawned on me that if his also has a field coil driver, then re-coning by one of the old radio restorers would be preferred to converting to a permanent magnet driver, so for this reason no modern driver would be a suitable replacement if the original electronics is used.

GM
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Old 26th September 2016, 12:51 AM   #8
GM is offline GM  United States
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Quote:
Originally Posted by morecoffee1 View Post
Thank you for your help so far.
You're welcome, though a bit undeserved so far as noted in my previous response.

Hmm, I'm only familiar with older Class D amps, which typically only had one rating as I learned the hard way, so unless your amp is rated for 8 ohms, then best to check with the manufacturer. You could wire an 8 ohm resistor in parallel with the driver, but best to use a car audio or similar 4 ohm driver for best results.

WRT driver power rating, they are mostly bogus nowadays, so normally as long as its rated as much or more than the amp it's plenty, though some PA drivers may not be rated very high due to being so efficient, so might run you out of the room on 5-10 W or less.

Yes, for modern day electronics, sources FM, etc., the other recommended driver selection is preferred as is moving the driver up as high as it will go. That, or use what we call a horn driver, which has a 'rising on axis' response that's extended much higher in frequency than a guitar amp driver, but I can't think of a cheap one ATM now that RadioShack is long gone. Another way is to use a coax that has a small tweeter that can be adjusted to play louder than its wide range [mid] bass woofer.

Right, mono is your only viable option.

With open back/dipole, baffle board material need only be strong enough to hold the driver without flexing, just if plywood it needs to be void free [$$], so all things considered, 3/4" MDF is fine.

GM
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Old 26th September 2016, 01:41 AM   #9
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GM, At this point I am not really interested in doing a restore job, just keeping it to where I could if I wanted. But since you are interested I did look at the back a little more and describe it.

I have no idea if it has a field coil driver bur,

There are like about 7 wires coming out of the amp and go to two different parts of the speaker.

On the sticker on the center of the back of the driver it says:
Utah
Huntington
Indiana

On a cartridge shaped thing towards the bottom half has a green sticker and says:
A-2936
PRI. 10000 Ω c.t
SEC. 3.5 Ω 10W
Output

On the metal edge of the speaker it is printed:
SE1010 328848
32 Ohm.
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Old 26th September 2016, 05:25 AM   #10
GM is offline GM  United States
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Understood; yes, it's a field coil. Hope the voice coil [VC] didn't get damaged as all the complete re-cone kits are long gone and no repros AFAIK, but there were still some aftermarket universal diaphragm kits, though not sure about 9". All I've ever seen are 10-12".

Note that it won't play 'full-range' though, just period correct with maybe a bit higher HF if driven with a modern signal source.

GM
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Last edited by GM; 26th September 2016 at 05:28 AM.
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