View Full Version : Radford System
AD Audio
17-07-2018, 07:50
I spotted this nice little system on Stereonet Aus. Radford STA25 monoblocks and studio monitors previously owned by a BBC engineer along with the Radford pre.https://uploads.tapatalk-cdn.com/20180717/71b225fd65c4a2861f22f19c008ec998.jpg
Sent from my ONEPLUS A5000 using Tapatalk
I wonder what source the ex-BBC engineer used? Probably a Garrard 301 or 401 with an SME, or maybe a Decca arm and cartridge. Whatever, it would have been 'state of the art' in the mid to late '60s.
I would love to find a pair of Radford monoblocks, either MA15 or MA25 to replace my STA15/III. I'm a bit of a fan of monoblock amplifiers (I have four pairs).
Bit of thread resurrection here, but just spotted these super rare Radford monoblock amps on Ebay, are these MA-25s?
Speakers look substantial too!
Seller has zero feedback but photos look genuine.
https://www.ebay.co.uk/itm/Radford-Amplifiers-and-speakers/393240513833?hash=item5b8ef60d29:g:oO4AAOSwJKpgbt2 b
Speakers look like they might be some of these
http://ukhhsoc.torrens.org/makers/Radford/Loudspeakers/Loudspeaker_Construction_T90_M90_A90_S90_Leaflet_A 62_Sep73/Page1.jpeg
Amps look similar to these https://www.flickr.com/photos/rvhifi/7169043592/in/photostream/
Definitely different layout from normal Radford amps with valves inside cages. Maybe designed for studio use?
The Radford MA25 monoblock amplifier looks like this
https://www.radfordrevival.co.uk/wordpress/wp-content/uploads/2011/11/Series-3-Leaflet-P1.jpg.
The MA15 looks similar, but uses a GZ34 rectifier valve, located behind the two EL34s on the side valve tray.
The amplifiers in the ebay advert, are versions which I have not seen before, namely having the protective cage covering the valves, as was used in the professional 100W amp Radford made.
It is possible that these are customised MA25 amplifiers made for the professional market (and they are the 25w version, as there is no rectifier valve). It would be helpful to be able to see the connections to confirm; also the mounting of the valve sockets directly onto the PCB is unusual.
It would appear that these are Radford IMA15 (as they use EL34) from the Reference range, have a look at these two threads below. I’m guessing there aren’t too many pairs of these around, super rare!
https://theaudiostandard.net/thread/4776/history-radford-electronics
https://theartofsound.net/forum/showthread.php?51851-Radford-Revival-quot-The-PreAmp-quot/page3
Page 79
http://www.pamphonic.co.uk/HFYB_1964.pdf
Or are they K25 model :scratch:
Yip, sounds like these
https://www.whatsbestforum.com/threads/possibly-selling-radfords.31389/
Martyn Miles
18-04-2021, 20:50
Underrated speakers, Radfords.
I bought and repaired a pair of their ‘Bookshelf’ * models.
A special lightweight Kef B139 and a Celestion HF1300, with a complicated crossover.
Afromosia cabinets ( about an inch thick ! ) with plywood baffles and back panels.
The sound quality was unexpected, to say the least.
* You would need big/strong bookshelves !
Very tempting I have to say. I agree the Radford speakers very underrated. I had a pair of Monitor M.90 speakers, wonderful coherent sound. Currently have a pair of M180 (semi omnidirectional) which for their age and size throws up a surprisingly nice soundstage with instruments well placed.
The speakers the Glasgow seller has are either the Auditorium A.90 or the Studio S.90. The A.90 was 91cm high whilst the S.90 was 115cm high.
Driving them with monos must be pretty decent.
But what is the guy's source? Looks like a piano. :)
Has anyone ever come across either the Radford S.360 or Radford ISO.360 speakers? Love to have a listen to one of those.
But what is the guy's source? Looks like a piano
The preamp’s set to tuner, but maybe that’s for tuning the piano :D
Page 79
http://www.pamphonic.co.uk/HFYB_1964.pdf
Or are they K25 model :scratch:
Yip, sounds like these
https://www.whatsbestforum.com/threads/possibly-selling-radfords.31389/
Agreed - they do look like the Radford IMA15 or IMA25 (models I should have known about, having co-written the article on Radford in AoS's 'The Knowledge'). Without seeing the circuit schematic, I can't tell if the the IMA15 used solid state rectification (whereas the MA15 used valve rectification).
Have a look at this Barry, solid state rectification for IMA series but I’m pretty sure these are K25 amps. You can just see the XLR input in the EBay photographs. It seems the K25 were a modified MA-25 for the BBC specifically for use with the KEF LS5/1A monitors. They amps have a filter for evening out the speaker’s frequency response but this can be switched out. They also have input trim pots but no impedance selector switch.
https://i.imgur.com/hp68Sld.jpg
https://i.imgur.com/qC0EzBM.jpg
Some further reading here
http://freecashmachinemoney.blogspot.com/2011/06/radford-electronics.html
Scroll to bottom of page for this one
https://m.review33.com/forum_msg.php?db=1&tstart=5650&s=&topic=78110527211332&start=120&sort=1&number=295
Oh - this is embarrassing! :o
The table you show is exactly the same as the one shown in our article, there is thus no excuse for me not knowing about it. So definitely a K25, which if the frequency equalisation can be switched out, makes them very desirable items.
I admire your diligence in tracking down the identity of the Radford amp. Well done! :)
I’m sure there’s been some water under the bridge since the article was written Barry :)
oh - this is embarrassing! :o
the table you show is exactly the same as the one shown in our article, there is thus no excuse for me not knowing about it. So definitely a k25, which if the frequency equalisation can be switched out, makes them very desirable items.
I admire your diligence in tracking down the identity of the radford amp. Well done! :)
was a long time ago so i wouldnt worry.. I often forget stuff same day i did it lol
Thanks Guys, however I do pride myself in having an encyclopeadic knowledge about certain audio brands. Anyway, there was no excuse for me not having carefully read the article I co-wrote.
Returning to the eBay sale, £2,500 would IMO seem to be a bit of a bargain for the Radford system.
Barry, it gets worse :), have a look at post #16
https://theartofsound.net/forum/showthread.php?8082-Info-BBC-monitors-1960-s/page2
Looks like the amplifier output impedance is 25 ohm but maybe it’s a multi tap transformer.
The BBC had a habit of taking comercially manufactured items and custom modifying them (or having the manufacturer do it) for their purposes. They did this with Leak, Quad and Radford amplifiers, and with turntables by Garrard, Technics and EMT.
All of these items were given BBC designations; AM_/_ for amplifiers, and RP_/_ for turntables.
Ferrograph were another British company who would supply many specialised/customised versions of their reel-to-reel recorders. These would have been variously for the BBC, the Admiralty, and the secret services and GCHQ.
Got a pair of Radford Monitor speakers a few weeks back - https://www.radiomuseum.org/r/radford_monitor.html
It's a 3 way sealed box design:
- Kef B139 most probably the MKII - 6171 version (rated 40W in closed box),
- Goodmans mid rage driver,
- Celestion HF 1300 tweeter.
Frequency range is quite limited at 45Hz - 15.5kHz
1. Has anyone attempted to modify the crossover and add a Coles 4001G super-tweeter in order to extend the high range beyond 20kHz? Would it be a complicated procedure for a novice?
2. Would it be a good idea to upgrade the KEF - B139 6171 (40w) to one of their more recent 1044SP versions (100W)
Apart from wattage the only other differences other than the size (6171 > 1044) seems to be the Fundamental Resonance (FS) which is 20 for the 6171 and 25 for the 1044SP. Does it matter, would it affect the overall sound signature?!?
I've calculated the enclosure volume and it's about 50L. The 1044SP radiomuseum.com page here says 1044SP is recommended for cabinets 40 - 60L in volume.
3. And lastly, I've checked the capacitors inside. There are 4 caps labeled 8uf that score between 13.5 - 16uf, and 2 caps labeled 4uf that score aruond 12uf on the multimeter. There is another one of 0.47 uf that measures 0.49. I'm guessing the last one is fine but the rest of them are way out of their range. Where do I buy replacements if I'm in the UK?
walpurgis
21-07-2022, 14:39
That's a nice enough combination of drive units.
The HF1300 tweeter cuts off rapidly above 13kHz and a Coles supertweeter can be added on a separate filter without modifying the existing crossover.
If you are over the age of 45, aiming to extend the frequency range up to or beyond 20kHz is likely to be of limited value.
I doubt you'd gain any benefit from changing the B139.
The capacitors are probably in need of replacement, there are so many choices it would be best to look for what meets your needs.
The rear connection terminals are horrible and could be usefully upgraded.
If it was me, I'd just change the caps and terminals in order to retain a fair degree of originality on what is a collectible speaker.
That's a nice enough combination of drive units.
The HF1300 tweeter cuts off rapidly above 13kHz and a Coles supertweeter can be added on a separate filter without modifying the existing crossover.
If you are over the age of 45, aiming to extend the frequency range up to or beyond 20kHz is likely to be of limited value.
I doubt you'd gain any benefit from changing the B139.
The capacitors are probably in need of replacement, there are so many choices it would be best to look for what meets your needs.
The rear connection terminals are horrible and could be usefully upgraded.
If it was me, I'd just change the caps and terminals in order to retain a fair degree of originality on what is a collectible speaker.
I have tested the Coles 4001G on their own on both the B&W DM2 and DM4 and I can well hear them going. I'm 40 so I don't expect to loose my hearing in 5 years time, at least I hope so lol.
I gave up on the idea of having a SP1044 to replace the 6171 version of the KEF B130 which is larger and it would have made more complicated.
Regarding the input terminals I was thinking to replace them however I don't want to get banana jacks as this would imply too much hassle and would have to do a cut-out to mount the plastic box at the back which is just too much work.
I could also just stick banana jack terminals in the existing holes that I don't want to have them stick out so my thought is to buy copper bolts, screws and washers and keep the back flush maintaining the same design and as you say its originality.
I did some math on the economics of it and 4 pairs of copper screw with washers and nuts would cost me £20 however they only come in packs of 10 each from China. I was thinking I could order 10 or 20 and sell the other pairs back on eBay.
Now coming back to mounting the Coles I had a second look at the crossover and noticed in a corner HF2+ / HF-.
Turns out the Radford "Monitor" model shares the same crossover with the bigger brother model "Studio" which employs a Telephones Supertweeter 16ohm.
By the looks of it the only missing pieces I need to add to the existing crossover are a 0,47μF capacitor and an inductor to which I don't know the value. Any idea what μH inductor I would need? The DM4 crossover uses a 0.200mH before the Coles 4001G whereas the DM2 uses 0.3mH
In regards to positioning, my plan is to remove the mids driver, patch the hole and then reposition it further to the side so I can fit the Coles in the middle. They will be quite close packed.
I've made a side by side comparison of the two Radford FN7/5 showing them side by side.3047730478
Lawrence001
24-07-2022, 23:04
Can you not fit the Coles above the HF1300 would be a lot less hassle and they wouldn't sound any different.
Can you not fit the Coles above the HF1300 would be a lot less hassle and they wouldn't sound any different.
It would be much easier wouldn't it?
I would need to build new speaker grills or drill a hole on top. Also they are not mirror image of each other like the B&W DM2's are.
walpurgis
25-07-2022, 06:10
I have tested the Coles 4001G on their own on both the B&W DM2 and DM4 and I can well hear them going.
That tells you little unless you know the frequency the Coles is introduced at and the filter slope.
There are many tweeters that could be used, you don't have to stick with the Coles. Anything with a smoothish response above 13kHz could be considered.
I'd mount the supertweeter in a separate module/baffle above the main speakers. And I'd use a separate filter with it.
Lawrence001
25-07-2022, 10:34
That tells you little unless you know the frequency the Coles is introduced at and the filter slope.
There are many tweeters that could be used, you don't have to stick with the Coles. Anything with a smoothish response above 13kHz could be considered.
I'd mount the supertweeter in a separate module/baffle above the main speakers. And I'd use a separate filter with it.It's more period authentic though. Most of the 60s/70s designs that used the Celestion/Coles combo had switched to a single more modern tweeter by the late 70s as they were much improved in output and frequency range.
If you're looking to fit a more modern tweeter you could drop the HF1300 and just fit a new one there, adjusting the crossover of course.
I was thinking of that however it would still have to be 16 ohm?
I'm not very proficient with crossover design so operating modifications on the existing crossover would need doing some research and I don't have instruments to verify filter slopes etc.
I have a pair of Acoustic Research AR18LS tweeters that are 8ohm and can reach 20kHz on their won but obviously don't know what the implications of replacing the 16 ohm Celestion with the 8ohm AR would entail in terms of crossover modifications.
The only 16ohm tweeter replacement I've found is the Beyma CP22 https://www.ebay.co.uk/itm/133904639804?hash=item1f2d56a73c:g:P70AAOSwZAthaXr W
Don't know if this would be a good match though.
walpurgis
25-07-2022, 12:31
I was thinking of that however it would still have to be 16 ohm?
It doesn't have to be 16 ohm. At higher frequencies, you are dealing with smaller wattages and impedance is relatively less crucial.
There are many interesting tweeters on the second hand market. I favour old non-ferrofluid types myself. The KEF T27 works really well as a supertweeter.
It doesn't have to be 16 ohm. At higher frequencies, you are dealing with smaller wattages and impedance is relatively less crucial.
There are many interesting tweeters on the second hand market. I favour old non-ferrofluid types myself. The KEF T27 works really well as a supertweeter.
My answer was to Lawrence "If you're looking to fit a more modern tweeter you could drop the HF1300 and just fit a new one there, adjusting the
crossover of course." in the idea to drop the HF1300 and replace it with a tweeter that covers the whole top range of the spectrum. The Acoustic
Research AR18LS is a 50w two way acoustic suspension 48Hz to 25kHz crossed at 2000Hz and the tweeter is paper cone however it's 8 ohm so it
wouldn't be a replacement like for like since the HF1300 is 16ohm.
Lawrence001
26-07-2022, 19:13
There may be workarounds like adding a resistor in series. The relative sensitivity is also important. Someone more knowledgeable than me could say more.
Powered by vBulletin® Version 4.2.3 Copyright © 2026 vBulletin Solutions, Inc. All rights reserved.