16 or 8 ohm speaker ?

[Dick.1]

Been missing for a while (illness)

I've picked up on a HMV2028 record player that had a couple of problems that I was looking at a couple of months back.

Cartridge had expired so I've replaced that, also the tone arm wiring due to the cart connectors being trashed & not enough length left to repair properly

Got a healthy output now both on cartridge & the "tape & radio" inputs.

BUT....there's a distinct lack of bass.....loads of treble but probably 30/40% less bass than I would have expected

The speaker (16 ohm originally) has been replaced by a previous owner with a cheap looking modernish 8 ohm one.

I'm wondering if it's worth trying to track down a 16 ohm original spec as I'm not sure how tolerant the amp (AC128/176 output stage) is of the load differential & if it would make that much difference sound wise anyway ?

The there's the hum/buzz (sounds around 50hz) that will need looking at later


Dick.

[Mark Hennessy]

The change in impedance might make a slight difference, but probably not as much as you might think in practice.

However, it is a completely different loudspeaker, and that will definitely alter things. There are a lot of parameters in a seemingly simple loudspeaker.

If possible, tracking down the exact unit would be best, but a similar speaker from the same era would probably be a big improvement. What sort of size is it?

Hope you're feeling better now

[Terry]

I'm wondering if it's worth trying to track down a 16 ohm original spec as I'm not sure how tolerant the amp  (AC128/176 output stage) is of the load differential ...

To calculate the peak output power, take the supply voltage and divide by 3 (strictly speaking it should be 2.828 but the supply will almost certainly sag at maximum output) and square the answer. Then divide by the speaker impedance:

P(W) = ((Vs/3)^2)/ZΩ 

To find the current*, divide Vs by 3 again and divide by the speaker impedance:

A = (Vs/3)/ZΩ 

then compare with the answers with the output transistors  data to see if any damage is likely to result.

* This is the RMS value - the peak current will, of course, be Vs/ZΩ 

[Dick.1]

Hi Mark

Just pulled the speaker & it's a 6x4 & doesn't look as shabby as I first thought & is indeed 8 ohm, made in Thailand & rated @ 5w

I've found them being sold as a pair on fleabay @ around £13 delivered described as "shielded magnet" & seemingly for audio use in PC setups but no published specs apart from that.

I need a 16 ohm speaker though

Feeling better now thanks

Dick.

[ppppenguin]

Both Mark and Terry have given good advice. If you've got enough volume then stick an approx 8R resistor in series with the speaker to ensure the amp is safe. May or may not be necessary but if you've still got enough volume then don't worry.

[Mike Watterson]

Lack of bass though is more commonly pickup cartridge, stylus, tracking weight, etc. 8 Ohms vs 16 Ohms won't make that much difference, (coupling capacitor on output). It does mean more risk of abuse to output stage at high volume. It's not like a 3" speaker vs 5" speaker.

You can try 4.7 Ohm, 6.8 Ohm, 8.2 Ohm and all that may change is maximum volume. (I'd use 4 resistors roughly x4 larger, in parallel or 4 in series at 1/4 value if you only have tiny resistors, in any case 1W rating might be fine). I doubt it affects bass much.

[Mark Hennessy]

To calculate the peak output power, take the supply voltage and divide by 3 (strictly speaking it should be 2.828 but the supply will almost certainly sag at maximum output) and square the answer. Then divide by the speaker impedance:

P(W) = ((Vs/3)^2)/ZΩ 

That's not the peak power; rather it's the average sine wave power. What some folk erroneously call "RMS Power"

In practice, the emitter resistors and the preceding electronics results in rather less output. E.g., a typical Hacker output stage running at 18V only gives 1.5W into 15 ohms, rather than the 2.4 or 2.7 watts predicted. These losses will generally increase when driving lower impedances; no amplifier doubles its power into half the impedance.

The AC128/176 pair is good for 1A. So on paper, anything up to 8V peak will be fine, suggesting that a 16V PSU rail would be a sensible max - however, in practice it's probably OK up to 20V or so, given the losses.

But let us not forget that the speaker is not an 8 ohm resistor. It will only be 8 ohms at some frequencies. For more, it'll be substantially higher, and either side of resonance, it'll be a little bit below. So the actual peak current delivered will be dependant on the spectral content of the music.

The lack of bass is most like because the drive unit has a much lower Q than the original. In the correct cabinet (right volume, perhaps with a port), it might well be a decent enough unit. But very different to the original. And I suppose the output coupling capacitor might be a bit on the low side for an 8 ohm loudspeaker, but that'll depend on what it is.

It'll be worth feeding in a signal from an MP3 player or similar to rule out a problem with the cartridge...

[Mark Hennessy]

I've found them being sold as a pair on fleabay @ around £13 delivered described as "shielded magnet" & seemingly for audio use in PC setups but no published specs apart from that.

A link to the unit in question would be most interesting, and very much within the forum rules.

You're allowed to say "eBay" - no need to disguise it as "fleabay". We don't have anti-eBay rules on here, beyond not being rude about a seller

[Dick.1]

Hi Terry

Thanks for the "heads up" .....

When I've looked at the cct diagram properly I see they are of course Germanium transistors & although they are heatsinked using this player at volume will push them where they don't want to be running into 8 ohms as the amp is rated @ 3w with a 15 ohm speaker in the spec sheet & the heat sink is on the small side as well.
But, as they seem to have survived for 45 years or so I'll leave things as they are until I can find a replacement speaker & see where we go from there.

Dick.

[Mark Hennessy]

Do you know what the power supply rail is?