Old Goodmans 10" speaker

[Bushbaby]

Hi All,
Can anyone recognise what this Goodman 10" speaker is please? I managed to buy this quite cheaply and it was indicated that it was 12 ohms which seems strange to me (did the original owner measure the impedance across the terminals and got 12 ohms?) think its more like to be 15 ohm from the 1960's (indicating Goodmans Made in England)

Many thanks

Rob

               

[ppppenguin]

The DC resistance is not the same as the impedance. The quoted impedance figure is only nominal and it varies widely with frequency. Probably 15R nominal.

[Diabolical Artificer]

If you connect your speaker up to an amp and sig gen then measure the voltage across the speaker terminals at various frequencies starting from 20hz say you'll find the voltage varies with frequency. This means as Jeffrey says impedance is changing with frequency though the DC resistance might always be 12 ohm.

At some point you'll find a peak, where the coil/loadspeaker is resonant, or to put it another way at some frequency the stars align, the moons in the right quarter and the speaker is having a good hair day. Because Z (impedance) changes with frequency, the nominal frequency will be under the peak frequency, it's a fudge, from 20hz to 1khz say the Z will be about 15 ohms, ish. This gives you an idea what the driver was designed for, IE guitar or Hifi.

Z changes too when you put the speaker into a box, the resonant frequency will change too depending on the box design.

Mark knows this stuff better than I do, he's the speaker guru,hopefully he'll have the time to chip in.

Andy.

[ppppenguin]

Andy, almost but not quite. Most amplifiers (almost all solid state and some valve) behave as a constant voltage source. So the current through the speaker will change with frequency while voltage stays almost constant.

[radioman]

It'll work if you include a series resistor !

Andy

[Bushbaby]

Thanks all and much appreciated.
Does anyone have any idea with regards to the specification for this speaker? I've tried hunting for information but cannot find anything about it?

[Mark Hennessy]

With no model number, you'll struggle to find anything (the numbers on the gasket and diaphragm are probably not model numbers).

First, what are your expectations? It's clearly not hi-fi. And I wouldn't expect it to last long as a guitar amp, given that it'll have a paper voice coil former and ancient glues. What is the diameter of the voice coil - 3/4", perhaps? I wouldn't put more than 5 watts into that, honestly. Perhaps OK as a practice amp fed from a 1 or 2 watt transistor radio amp, but definitely not for stage use. I'm guessing it was originally intended for use in a large radio or radiogram.

If you wanted to use if for a guitar practice amp, just bung it in a typical open-back cabinet. But if you want to use it more seriously, you'll have to measure the T/S parameters: https://en.wikipedia.org/wiki/Thiele/Small_parameters

Don't let that article put you off - you only really need Fs, Qts and Vas to be able to design a box for it. There's lots more detail here, including a spreadsheet that helps you with the maths: https://sound-au.com/tsp.htm

With those parameters, you can design a sealed or ported box. That is simple today - use something like WinISD to do the maths. Back in the '80s I did it by hand - the maths isn't hard (it couldn't have been, because I was able to cope with it!).

Next, you'd need to measure the acoustic output of the drive unit to determine how it'll sound, and what sort of crossover you'll need. That's more tricky, but I seem to recall that you do have the kit for that. I'm not expecting an especially smooth curve from something like that. It'll become very directional about about 1.5kHz, give or take, so crossing over low will help.

Oh, and if you're tempted to tackle the rust, be careful that nothing gets in the magnetic gap. I'd probably just paint on a bit of Kurust or similar - rubbing it down would be too risky IMHO...

[Bushbaby]

I was thinking about this for a small guitar amp project of a couple of watts. Another project for the future now I'm retired.

Best Regards

[Diabolical Artificer]

If it was a Celestion driver you might find out what it was from the cone number, not sure there's a site the same for Goodmans. Two ways to find out if it's any good for guitar, bung it inna box, connect to an amp and plug in the axe or use that method I was waffling onnabout taking into account the following posts. Can't remember using a series R but must have done I suppose.

Having said all that it looks like a cheap low powered 12/15" guitar driver,but then the surround is a bit small as is the coil diameter. Similar 12" & 15" Celestion driver I have are beefier.

Andy.

[Mark Hennessy]

Ah - if "low-powered guitar amp" was the original thought, mentioning that in the question would have saved me including the stuff about T/S parameters (normally when someone asks for specs of a drive unit, that is what they mean). The T/S parameters are about sorting out the bass end, but obviously that's not relevant to a guitar amp. Still, hopefully it helps someone else one day...

Andy, if you didn't use a resistor, I suspect you used a signal generator to drive the speaker directly. The built in source impedance (50ohm, 600ohm, or whatever) would be fine for seeing the resonant peak. It's what I do all the time.

As I say, I suspect this is more a radio speaker than a guitar speaker. Too feeble for serious use, and too much like other Goodmans radio speakers I've seen. As such, it won't be as efficient as a "proper" guitar speaker, but I'm sure it'll be fine for practice. After all, people convert radios with smaller speakers:

https://www.youtube.com/watch?v=HVdG8UF2UMU

As a Hacker collector it's a bit tricky to condone it, but radio side was dead and there's so little on AM these days anyway. The only mod appears to be a hole in the side for the socket (perhaps he used a switched socket that would allow the radio side to work if ever he fixed it?), but a future radio collector could replace that with a DC input socket. Of course, if he'd found a Sovereign II instead, it would already have the input socket.

Based on that video, I'd say that a Hacker A205 would be the ideal amplifier for this project. Just add an input socket and a volume control, plus a couple of PP3s (and perhaps a DC input socket?). The amp has bass and treble controls too - take those from the donor radio. That takes care of the electronics - meaning it's now just a woodwork project. Simple

Of course, if you don't want to "rob" (sorry!) an amp from a working Hacker, just copy the circuit. It's simple enough to knock up on Veroboard or even tagstrip, and the schematic is widely available:

https://www.markhennessy.co.uk/articles/hacker_a205.jpg

Transistors aren't critical - it would be quite possible to use silicon instead of germanium for T3-6, depending on your beliefs.