Murphy baffle radios [ open baffle ]

[keithpeter]

Hello All

This isn't a hifi project in the usual sense.

Murphy Radio in the UK manufactured a range of console radios with open baffle speaker cabinets, so the radio was quite thin (8 inches or so) but looked substantial (36 inches by about 32 inches). One of these looks OK in a large room as an item of furniture. The radios had a 12 inch full range speaker, a push-pull output stage with negative feedback (made a point of in the brochure).

http://www.murphy-radio.co.uk/murphy...6C_Outside.jpg

http://www.murphy-radio.co.uk/murphy...46C_Inside.jpg

http://www.bonhams.com/cgi-bin/publi...&iSaleNo=14177

The advert above mentions what I might call the forward leaning 'lapels' of the design - do these have any acoustic function?Do they delay the back wavefront same as the side pieces in the classic open baffle design at

http://members.myactv.net/~je2a3/open.htm

I've also noticed how the radio chassis and controls were in a 'pod' in the middle of the upper side of the speaker, just where the 8 inch by 6 inch spoiler is in the classic design.

I'm thinking off adding a t-amp and a DAB radio as a sound source for my home office. In the back of my mind is the idea of acquiring a Murphy in decent condition with non functioning electronics and using it as the case for the radio.....

I've popped a single Visaton BG20 (cheap 8 inch full range with wizzer cone) onto a temporary 'craft wood' baffle that is 850 by 575 mm and I'm kind of happy with the sound for voice and small 'open' instruments, although it is a little harsh to my ears.

[davidallancole]

That is a sweet looking unit. I say go for it.

[Geoff H]

You are spot on with the lapels increasing front / back distance. The overall acoustic dimensions are less like a rectangle.

Re the harshness with the 8" Visaton. The OB tends to have a rise in response in the midrange region, causing voice and percussion to be pronounced. It can be nulled out with a LCR filter.

It's best to avoid concave shapes from the front side. The lapels could be reversed so they fold back. Still easy on the eye, better acoustically.

If you can get a unit with the original amp, it would be worthwhile restoring it. Better at driving a full ranger on OB.

Geoff.

[G.Kennedy]

I love this idea ... what about something like a 8" Hemptone FR8C ... in an OB, with the source being a Tivoli Model one running into a Sonic Impact modified t-amp. All battery powered.

Hmmm ... the gears are turning. I have been looking for a project.

[keithpeter]

Originally posted by Geoff H
Re the harshness with the 8" Visaton. The OB tends to have a rise in response in the midrange region, causing voice and percussion to be pronounced. It can be nulled out with a LCR filter.

Thanks for replies Geoff and all - any pointers as to where I might find a filter circuit with approximate values, or at least the correct term to search for as I'm not finding much? The £20 visaton, a tenner's worth of wood and a couple of shelf brackets is about what I'm spending on this project until I can find a Murphy cabinet in decent nick. I could run to a filter using bog standard parts to tame the shout a little.

Originally posted by Geoff H
If you can get a unit with the original amp, it would be worthwhile restoring it. Better at driving a full ranger on OB.
[/B]

The idea had crossed my mind, but for safety reasons, I'd need to find an isolating transformer for the mains - the Murphy radio has an autotransformer in this model. These were mass production sets so the output transformer may have been manufactured to suit the bandwidth of am broadcasts, as may the 12 inch full range speaker, to save a few bob. I have not heard one yet.

Originally posted by G.Kennedy
I love this idea ... what about something like a 8" Hemptone FR8C ... in an OB, with the source being a Tivoli Model one running into a Sonic Impact modified t-amp. All battery powered.

Hmmm ... the gears are turning. I have been looking for a project.

I'm listening to the cheap and cheerful BG20 on the 'craft wood' baffle now connected to the loudspeaker output of a Pure One dab radio (9 volts battery so about 1 watt into 8 ohms). Sounds OK if not quite loud enough for the Proms, so I imagine that the 5 watts of clean power from a t-amp would be fine for a radio.

My woodwork is 'basic' hence the idea of using a ready made and quite unusual looking cabinet. The Dab radio and t-amp could be fixed to the back of the baffle....

[dnsey]

The idea had crossed my mind, but for safety reasons, I'd need to find an isolating transformer for the mains - the Murphy radio has an autotransformer in this model. These were mass production sets so the output transformer may have been manufactured to suit the bandwidth of am broadcasts, as may the 12 inch full range speaker, to save a few bob. I have not heard one yet.

The Murphy 'big baffles' cover quite a range of models produced over a number of years.

Not all have autotransformers, and the amps range from SE (IIRC) with octal valves to PP 'miniature' valve models. See Dave Grant's site for loads of information on Murphy radios. If you should decide to look for another to make a stereo pair, be aware that the cabinets, although superficially similar, vary quite a lot.

IMO, they all sound remarkably good for their vintage and intended market - there's certainly potential for using the baffles with other speakers. The originals (usually Celestions) have a decent bass response, but not much at the HF end, as you might expect. As you say, the OPT is usually tiny, so a decent one should produce really solid sounding results.

I have an early one of these (perhaps two) tucked away in my workshop - I'll try and dig it out soon. They are very rewarding sets to restore cosmetically, with book veneers and elegant thin brass trim on some models.

[keithpeter]

Originally posted by dnsey

The Murphy 'big baffles' cover quite a range of models produced over a number of years.

Thanks dnsey, I'm looking at the 'art deco' A146C specifically because of its outre looks [ sort of Dr Caligari's Cabinet meets Lyons Cafe ] and I have checked the circuits and it is a push pull output stage and mains autotransformer. Interestingly, Murphy went back to single ended for the VHF set in this series in 1957, with an isolating mains transformer. I linked to David Grant's superb site above for the sales brochure. He has links to the various circuits.

IMO, they all sound remarkably good for their vintage and intended market - there's certainly potential for using the baffles with other speakers. The originals (usually Celestions) have a decent bass response, but not much at the HF end, as you might expect. As you say, the OPT is usually tiny, so a decent one should produce really solid sounding results.

I'd imagine the bass is governed mainly by the OB roll off and the treble would be tailored to suit the AM bandwidth. I'm getting a bit old to be arguing with 300 volt HT and so may wimp out and go for solid state electronics. I have a neighbour who can use a jig saw [unlike me] so getting a piece of wood to mount an 8 inch full range in and bolt to the existing fitting can be managed.

I have an early one of these (perhaps two) tucked away in my workshop - I'll try and dig it out soon. They are very rewarding sets to restore cosmetically, with book veneers and elegant thin brass trim on some models.

Its the A146C which I understand to be the first of the Baffle sets (circa 1946) I'm after and as you say I'd like to restore it as a piece of furniture - see the Bonhams link in the original post, especially the 'arts and crafts' speaker surround! I have no desire for a stereo pair at all, I'm into a mono radio for the 'office' - never seen the point of stereo with the small sweet spot for background/spoken voice - but I take the point on cabinet variations.

If anyone has an example in reasonable cosmetic condition going spare, I'd like to hear off thread.

[Geoff H]

"any pointers as to where I might find a filter circuit with approximate values, or at least the correct term to search for as I'm not finding much?"

Try the Visaton site, under the boxless designs. You will need to adjust to suit your baffle/driver combination.

Geoff.

[dnsey]

I've had a look, and there's definitely a 146C in my shed.

The cabinet needs refinishing, but it's basically sound.

If you can arrange collection from Shropshire, you're welcome to the cab and RF section to play with (I'd like to keep the AF bits).

Let me know if you're interested :-)

Cheers,
Dave.

[keithpeter]

Hello dnsey

Thanks for this kind offer.

http://www.valve.demon.co.uk/Wanted.htm

About 2/3 way down there is an A272C for sale in working order - it hasn't gone yet.

I'm deciding and checking out carriage / courier costs. I won't be able to get down myself very easily.