DIY speaker Bake Off


All fixed now


Anyone fancy bringing a deck? I’d like to try out the phono stage on the Linn DSM


I’m happy to take along my Denon DP55F, it has an RB300 and an AT-F7 on it.


Postpone the xmas fair,it’s still November


sounds like a plan, I’ll see if I can find some records, I know I put them somewhere


I’ve already confessed to Stefan that the Linkwitz dipole subs won’t be ready for Saturday :sob:. He very generously asked if I’d like to bring some consolation electronics along instead. So I’ll put my Leak TL/12 Point One amps in the back of the car and also my Dynakit Stereo70. The former were designed just after WWII and went on sale in 1948. They were the first production amps, on this side of the pond at least, which delivered the hi-fi standard of 0.1% distortion at full output. The Stereo70 is far and away the most popular valve amplifier, by numbers sold, in the world ever. Not many of them were made for 240V though, so they’re not as common over here as the global sales volume might suggest.



[@Stu placeholder]

Think I’ve got a 12" signed EP from these somewhere.


You don’t really need that st70 do you? Quite happy to give it a new home.


Embarrassingly it’s been in my loft for 4-5 years. Having got it down and plugged it in I am remembering how much I like it though (except for its buzzy mains transformer - a very well-known issue, particularly on the UK’s 120V stepped-down mains).

EDIT: They’re still in production of course Go on, it’ll be Christmas soon …



I’ll try and find a suitable source to partner them with :grinning:


I can bring a Squeezebox Touch with a USB stick full of music. This can act as the all-important volume control.



DC offset? Gappy windings? Loose laminations? [8 other options I’ve forgotten?] Your public needs to know!


Got most stuff working last night, Phil (Linn) is coming today to get his stuff set up, see you all tomorrow.


Give that a go by all means, I’ve got a Linn DSM here with line out which obviously does the volume as well. I’ve also got my trusty old Buffalo 3 DAC with a PC which can do the same. I meant a suitably vintage bit of kit to plug into it. I guess all of the above can pretty easily clip the input of those amps?


I don’t think it’s loose lams - there’s no sign of that and I’ve tightened everything on the tranny itself to within an inch of its life. I get a bit of DC here which varies through the day - worse on weekday evenings. And DC will of course make any other problem worse. But the tranny still buzzes when the DC’s at its lowest. It could be gappy windings I suppose. But I fear it’s simply the old problem of ‘not quite adequate’. The Stereo70 is a 35W/ch valve amp and in this case that is what it delivers. But the mains tranny isn’t very big. It runs hot. If the problem was a defect somewhere then we might expect that most Stereo70’s would be quiet but some would be noisy whereas it seems they all do it. The problem is so widespread that there’s a commercial isolator product available for Dynakit products to help reduce it And of course (final nails in coffin) the UK’s higher mains voltage and 50Hz frequency will both make this problem worse. The solution is almost certainly one of these - a replacement transformer specified for 240V primary. It’s probably not a coincidence that the main mechanical difference between this and the original is that it has a taller lamination stack. Nothing’s as good as More Iron.



Your sources sound fine Stefan. TBH most vintage active sources (pre-amps and to a lesser extent tuners) can be a bit disappointing. They certainly don’t measure up to the performance of the power amps. As far as sensitivity goes the Leaks are only specified for 12W/ch output but they’re very sensitive - just 160mV RMS in is needed to reach max power. The Stereo70 is at the other end of the traditional range - 1.3V RMS required for full output. But now that’s 35W/ch so in practice again there’s rarely a sensitivity issue.



Cool, I’ll look forward to it then


Ta for that. Barely-adequate mains transformers seem to be all too common with valve amps. Odd that more manufacturers don’t use toroidals for the mains…


Yes. A few do. Perhaps it’s just old-fashionedness - the folks who can remember how to wind really high-voltage windings not having invested in the specialised machinery needed to thread a toroid (there are exceptions of course - Canterbury Windings notably). The thing with toroids of course is that they tend to be better than E-Is up to the limit, at which point they suddenly become a good deal worse.

Valve people who want to improve on E-I seem to go in the direction of C-cores. I suppose they’re a sort-of toroid, but now the iron is threaded through the windings rather than the other way round.



My WAD Kel80’s have toroids. I am sure I read that early ones had toroid opt’s too.


Yes. The Vellemann K4000, K4040 etc have toroids both as power and OP transformers too. There are a few about. I’ve got a very nice-looking set designed for a stereo PP EL34 30W/ch integrated. One day I’ll build an amp with them. (They were a gift from a guy who’d had them for a decade or so, thinking “One day I’ll build an amp with them”.)