BBC LS1/2/121 monitors - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Draker4c · 18th Nov 2022, 6:33 pm

I have a battered pair of LS3/1 which were the OB configuration of the LS3/4. Mark how were they regarded?

mhennessy · 18th Nov 2022, 11:52 pm

I suspect you mean the OB version of the LS5/1.

Both used a 15" woofer and a pair of tweeters. In the case of the LS3/1, the tweeters are mounted in front of the woofer to give better results when you're sat close to it. For the studio version, where greater listening distances are expected, the two tweeters are placed above the woofer in the usual arrangement.

Two tweeters were used to get greater power handling. For the 5/1, but not the 3/1 as far as I can establish, only one tweeter is operational at high frequencies (when the wavelengths approach the spacing between them, and acoustic lobing would result).

The tweeters are the GEC "presence unit", which is a forerunner to the Celestion HF1300. They don't go very high by today's standards (about 13kHz IIRC).

The original woofer was from Plessey, but for the A models a Goodmans woofer was used, with changes to the crossover. Slot loading was used to widen the dispersion at higher frequencies.

The LS3/1 is a sealed enclosure; the LS5/1 is a larger, ported box.

The LS3/1 uses the AM8/1 amplifier; the LS5/1 uses the AM8/4. Both are push-pull types giving around 15 watts. The latter incorporates HF equalisation for the LS5/1 (compensating for the move to single tweeter operation at high frequencies). This amplifier is also used for the LS2/1 - returning back to the original topic briefly - but with different EQ - it is designated AM8/4A.

But I must be honest: I've never seen or heard either, so I'm unable to comment on how they perform. No-one I've met over the years has mentioned them to me. Perhaps unfairly, I wouldn't expect great things from them - I know the BBC had a lot of problems with the consistency of paper-coned speakers supplied by third-parties, which is what led them to research plastic alternatives, leading first to Bextrene, then polypropylene. As such, they probably don't do stereo very well - not that they were expected to, in fairness.

Draker4c · 19th Nov 2022, 7:38 pm

Thanks for your detailed response Mark the units I have are indeed closed box type with tweeters mounted in front of the woofer, response tails off at around 13 KHz and the sound is BBC brown. However mid band is wonderfully crisp and detailed. Not bad for unloved and unmaintained speakers of around 60 years old!

Draker4c · 19th Nov 2022, 7:51 pm

My favourite speakers for general listening are KEF P60 which although not an official BBC type were used in self op BBC radio studios in conjunction with a Quad K300 amplifier employing active crossovers. Specified SPL Of 113 db at 1 metre. Apparently used in place of
BC1A s for “the happy sound “.

18th Nov 2022, 6:33 pm	#21
Draker4c Pentode Join Date: Jun 2017 Location: Basildon, Essex, UK. Posts: 189	Re: BBC LS1/2/121 monitors I have a battered pair of LS3/1 which were the OB configuration of the LS3/4. Mark how were they regarded?

18th Nov 2022, 11:52 pm	#22
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: BBC LS1/2/121 monitors I suspect you mean the OB version of the LS5/1. Both used a 15" woofer and a pair of tweeters. In the case of the LS3/1, the tweeters are mounted in front of the woofer to give better results when you're sat close to it. For the studio version, where greater listening distances are expected, the two tweeters are placed above the woofer in the usual arrangement. Two tweeters were used to get greater power handling. For the 5/1, but not the 3/1 as far as I can establish, only one tweeter is operational at high frequencies (when the wavelengths approach the spacing between them, and acoustic lobing would result). The tweeters are the GEC "presence unit", which is a forerunner to the Celestion HF1300. They don't go very high by today's standards (about 13kHz IIRC). The original woofer was from Plessey, but for the A models a Goodmans woofer was used, with changes to the crossover. Slot loading was used to widen the dispersion at higher frequencies. The LS3/1 is a sealed enclosure; the LS5/1 is a larger, ported box. The LS3/1 uses the AM8/1 amplifier; the LS5/1 uses the AM8/4. Both are push-pull types giving around 15 watts. The latter incorporates HF equalisation for the LS5/1 (compensating for the move to single tweeter operation at high frequencies). This amplifier is also used for the LS2/1 - returning back to the original topic briefly - but with different EQ - it is designated AM8/4A. But I must be honest: I've never seen or heard either, so I'm unable to comment on how they perform. No-one I've met over the years has mentioned them to me. Perhaps unfairly, I wouldn't expect great things from them - I know the BBC had a lot of problems with the consistency of paper-coned speakers supplied by third-parties, which is what led them to research plastic alternatives, leading first to Bextrene, then polypropylene. As such, they probably don't do stereo very well - not that they were expected to, in fairness.

19th Nov 2022, 7:38 pm	#23
Draker4c Pentode Join Date: Jun 2017 Location: Basildon, Essex, UK. Posts: 189	Re: BBC LS1/2/121 monitors Thanks for your detailed response Mark the units I have are indeed closed box type with tweeters mounted in front of the woofer, response tails off at around 13 KHz and the sound is BBC brown. However mid band is wonderfully crisp and detailed. Not bad for unloved and unmaintained speakers of around 60 years old!

19th Nov 2022, 7:51 pm	#24
Draker4c Pentode Join Date: Jun 2017 Location: Basildon, Essex, UK. Posts: 189	Re: BBC LS1/2/121 monitors My favourite speakers for general listening are KEF P60 which although not an official BBC type were used in self op BBC radio studios in conjunction with a Quad K300 amplifier employing active crossovers. Specified SPL Of 113 db at 1 metre. Apparently used in place of BC1A s for “the happy sound “.