Rogers BBC Studio Monitor LS3/6 info. - UK Vintage Radio Repair and Restoration Discussion Forum

Valvepower · 26th Feb 2021, 3:14 pm

Hello All,

I am looking for any schematics and info on the crossover etc., in the Rogers BBC Studio Monitor LS3/6.

The reason for asking the speaker I have here is labelled up as 8 Ohms, however, the bass driver is labelled as 15 Ohms. I know there is a hulking big (auto) transformer used in the crossover to adjust levels etc., however, I wondered if this transformer has tapings so the speaker can be set as 8 and 15 Ohms? As I have read, they were available in 8 and 15 Ohms.

These are low serial number units being completely untouched and in exceptional condition with fully functioning drivers. I think when I took the back of this morning, the last time it would have been off, it would have been when it was first made!

This came via a friend this week who runs a house clearance company and I had to pay a fair price as he knows the true value of speakers like this.

Regards
Terry

Edward Huggins · 26th Feb 2021, 4:53 pm

To get a learned second opinion as to the crossovers, do speak to Falcon Acoustics in Norfolk. They have in-depth knowledge of these.

mhennessy · 26th Feb 2021, 6:29 pm

Falcon haven't been in Norfolk for a while - Malcolm Jones retired and sold the company to Jerry Bloomfield, who is based in Oxfordshire. Though Jerry ensures that Malcolm has a pretty active retirement! Funnily enough, it wasn't until I moved away from Norfolk that I discovered that they were based in the same village I lived in for 6 years - not sure how I missed that!

https://www.falconacoustics.co.uk/fa...olm-jones.html

According to a brochure from Rogers, it was normally supplied in 15 ohm form, but 8 and 25 ohms were options. According to a BBC sheet I have here, it was normally 25 ohms.

I'm guessing the autotransformer must have allowed for this. All the data I have says that the tappings are 1dB apart, and are used to match the sensitivity of the drive units. But one diagram states that an additional tapping was added - though no details were given. That same sheet doesn't include anything about the super-tweeter, which was added later at Rogers' suggestion, so it's an early version of the documentation. I know that my collection of data is far from complete, and am always looking for anything that helps to fill in the gaps. But data about the transformers is likely to be impossible to find - measuring an example would be the best way to figure out what it can do.

It's actually quite rare to see the original die-cast woofers in an LS3/6, and rarer still to find them in working condition. Treat them with kid gloves - they are extremely fragile. The voice coil former is paper, and they are very prone to overheating.

There was a custom version of a Dalesford woofer that was used in the Export Monitor, and that (or a version of it) was also used to repair LS3/6s. It's easy to spot because it has a pressed steel frame, and those are what turn up in LS3/6s most often.

Valvepower · 27th Feb 2021, 2:05 pm

Hello,

Many thanks for the replies, greatly appreciated.

I had a look at the LS3/6 information on Mark Hennessy cracking and informative site and found a picture of the crossover assembly for a 15 Ohm speaker. I was able to just make out the connections on the PCB assembly although the resolution of the pictures on the site were against me though! and noted the input to the speakers on Marks site goes to a different connection/pin to that of the pair I have here.

The +ve input goes to the + INPUT pin on the pair on Marks site, whereas the +ve input goes to pin 3 (Orange wire to transformer) on my pair.

As best I could, using a DVM, I was able to track the route of the track on the PCB connected to the + INPUT pin and ascertain this connects to pin 1 (Brown lead to transformer), which also links to pin Y, via a yellow lead, and the Y2 (input) connection on the super tweeter HF2 PCB.

To make sense of all this I’ll remove the crossover assembly and measure the auto transformer and crossover, however, going by what I’ve found so far it supports the idea the auto transformer alters the impedance.

I am not sure if there would have been 8 and 15 Ohm HF units – anyway the DC resistance of the HF units are: HF driver = 11.7 Ohm and super tweeter 3.7 Ohm. I rather doubt this would be the case and as I said above the whole speaker impedance is altered by the auto transformer.

I did ask Andy Whittle, current incarnation of Rogers, and he could not help and pointed me in the direction of Mark Hennessy site.

As time goes on, I’m realizing these are a bit of a find, more so in original working condition. I was intending to use these with my valve amplifiers like the Radford STA15/25, Quad II and Leak TL25+ et. al. so the power will be limited but seeing the construction of the drivers I feel I’ going to have to be careful even with these amplifiers.

Finally, these were part of a Probate house clearance and I know nothing about the history of the speakers. As my friend said when dealing with Probate sale/clearance you don’t ask too many questions given the prevailing circumstances.

I feel these had been looked after and used sparingly (for want of a better word) as the Leak ST70 amplifier (see pictures), also with the speakers, does not show any of the normal signs of wear on the front panel screen printing.

Regards
Terry

AC/HL · 27th Feb 2021, 5:59 pm

Found it. The first drawing was sent to me, I can't remember where from possibly Rogers. The second is my scribble. The speakers themselves were passed on via the forum some time ago.

Valvepower · 27th Feb 2021, 7:10 pm

Hi,

AC/HL, thanks for that, appreciated.

I spent a bit of time this afternoon working out the circuit and it would be interesting to see how close I'd got with the circuit!

Mine is a very, very rough and ready sketch with no component values as it was just the taps on the transformer I was interested in.

I took some DC resistance measurements of the auto transformer and they agree with the circuit etc.

Regards
Terry

majoconz · 27th Feb 2021, 10:48 pm

I assume (careful!) that you have been here....

https://pinkfishmedia.net/forum/thre...-ls3-6.238588/

Valvepower · 1st Mar 2021, 4:16 pm

Hi

Martin, yes, I had, but many thanks for the heads-up though.

There are lots of useful information regards BBC speaker design on the BBC Eng. site. https://bbceng.info/ Sadly there was only one entry for the LS3/6, which was more just an information sheet.

Thanks to everyone who replied on this one.

Regards
Terry

26th Feb 2021, 3:14 pm	#1
Valvepower Octode Join Date: Aug 2008 Location: Rayleigh near Southend-On-Sea, Essex, UK. Posts: 1,880	Rogers BBC Studio Monitor LS3/6 info. Hello All, I am looking for any schematics and info on the crossover etc., in the Rogers BBC Studio Monitor LS3/6. The reason for asking the speaker I have here is labelled up as 8 Ohms, however, the bass driver is labelled as 15 Ohms. I know there is a hulking big (auto) transformer used in the crossover to adjust levels etc., however, I wondered if this transformer has tapings so the speaker can be set as 8 and 15 Ohms? As I have read, they were available in 8 and 15 Ohms. These are low serial number units being completely untouched and in exceptional condition with fully functioning drivers. I think when I took the back of this morning, the last time it would have been off, it would have been when it was first made! This came via a friend this week who runs a house clearance company and I had to pay a fair price as he knows the true value of speakers like this. Regards Terry Attached Thumbnails

26th Feb 2021, 4:53 pm	#2
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,328	Re: Rogers BBC Studio Monitor LS3/6 info. To get a learned second opinion as to the crossovers, do speak to Falcon Acoustics in Norfolk. They have in-depth knowledge of these. __________________ Edward.

26th Feb 2021, 6:29 pm	#3
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Rogers BBC Studio Monitor LS3/6 info. Falcon haven't been in Norfolk for a while - Malcolm Jones retired and sold the company to Jerry Bloomfield, who is based in Oxfordshire. Though Jerry ensures that Malcolm has a pretty active retirement! Funnily enough, it wasn't until I moved away from Norfolk that I discovered that they were based in the same village I lived in for 6 years - not sure how I missed that! https://www.falconacoustics.co.uk/fa...olm-jones.html According to a brochure from Rogers, it was normally supplied in 15 ohm form, but 8 and 25 ohms were options. According to a BBC sheet I have here, it was normally 25 ohms. I'm guessing the autotransformer must have allowed for this. All the data I have says that the tappings are 1dB apart, and are used to match the sensitivity of the drive units. But one diagram states that an additional tapping was added - though no details were given. That same sheet doesn't include anything about the super-tweeter, which was added later at Rogers' suggestion, so it's an early version of the documentation. I know that my collection of data is far from complete, and am always looking for anything that helps to fill in the gaps. But data about the transformers is likely to be impossible to find - measuring an example would be the best way to figure out what it can do. It's actually quite rare to see the original die-cast woofers in an LS3/6, and rarer still to find them in working condition. Treat them with kid gloves - they are extremely fragile. The voice coil former is paper, and they are very prone to overheating. There was a custom version of a Dalesford woofer that was used in the Export Monitor, and that (or a version of it) was also used to repair LS3/6s. It's easy to spot because it has a pressed steel frame, and those are what turn up in LS3/6s most often.

27th Feb 2021, 2:05 pm	#4
Valvepower Octode Join Date: Aug 2008 Location: Rayleigh near Southend-On-Sea, Essex, UK. Posts: 1,880	Re: Rogers BBC Studio Monitor LS3/6 info. Hello, Many thanks for the replies, greatly appreciated. I had a look at the LS3/6 information on Mark Hennessy cracking and informative site and found a picture of the crossover assembly for a 15 Ohm speaker. I was able to just make out the connections on the PCB assembly although the resolution of the pictures on the site were against me though! and noted the input to the speakers on Marks site goes to a different connection/pin to that of the pair I have here. The +ve input goes to the + INPUT pin on the pair on Marks site, whereas the +ve input goes to pin 3 (Orange wire to transformer) on my pair. As best I could, using a DVM, I was able to track the route of the track on the PCB connected to the + INPUT pin and ascertain this connects to pin 1 (Brown lead to transformer), which also links to pin Y, via a yellow lead, and the Y2 (input) connection on the super tweeter HF2 PCB. To make sense of all this I’ll remove the crossover assembly and measure the auto transformer and crossover, however, going by what I’ve found so far it supports the idea the auto transformer alters the impedance. I am not sure if there would have been 8 and 15 Ohm HF units – anyway the DC resistance of the HF units are: HF driver = 11.7 Ohm and super tweeter 3.7 Ohm. I rather doubt this would be the case and as I said above the whole speaker impedance is altered by the auto transformer. I did ask Andy Whittle, current incarnation of Rogers, and he could not help and pointed me in the direction of Mark Hennessy site. As time goes on, I’m realizing these are a bit of a find, more so in original working condition. I was intending to use these with my valve amplifiers like the Radford STA15/25, Quad II and Leak TL25+ et. al. so the power will be limited but seeing the construction of the drivers I feel I’ going to have to be careful even with these amplifiers. Finally, these were part of a Probate house clearance and I know nothing about the history of the speakers. As my friend said when dealing with Probate sale/clearance you don’t ask too many questions given the prevailing circumstances. I feel these had been looked after and used sparingly (for want of a better word) as the Leak ST70 amplifier (see pictures), also with the speakers, does not show any of the normal signs of wear on the front panel screen printing. Regards Terry Attached Thumbnails

27th Feb 2021, 10:48 pm	#7
majoconz Heptode Join Date: Jan 2008 Location: Ashhurst, Manawatu, New Zealand Posts: 571	Re: Rogers BBC Studio Monitor LS3/6 info. I assume (careful!) that you have been here.... https://pinkfishmedia.net/forum/thre...-ls3-6.238588/ __________________ Cheers - Martin ZL2MC

1st Mar 2021, 4:16 pm	#8
Valvepower Octode Join Date: Aug 2008 Location: Rayleigh near Southend-On-Sea, Essex, UK. Posts: 1,880	Re: Rogers BBC Studio Monitor LS3/6 info. Hi Martin, yes, I had, but many thanks for the heads-up though. There are lots of useful information regards BBC speaker design on the BBC Eng. site. https://bbceng.info/ Sadly there was only one entry for the LS3/6, which was more just an information sheet. Thanks to everyone who replied on this one. Regards Terry