John
Rack’em Up!
That would be my guess as well, I would be concerned about a faulty amp taking out my speakers. Can resistors be checked for accuracy without removing them from the circuit?
That would be my guess as well, I would be concerned about a faulty amp taking out my speakers. Can resistors be checked for accuracy without removing them from the circuit?
Jonathan Ribee
Unavailable at present
Tony L
Administrator
I don't think it's anything like that faulty to be honest, plus as I understand it faulty tube amps don't take out speakers, e.g. they can't 'go DC' the way many poorly protected transistor amps can as there is a very hefty output transformer between the live bits and the speaker. At least that's how I understand it anyway. I understand the 303 is vaguely similar in that it uses coupling caps between the output stage and the speakers. I still plan to get it looked at fairly soon. I may even consider replacing the resistors myself - I can certainly solder neatly enough and the tag board is very simple and easy to access, I'd just need someone to check my work before I turned it on! The only thing stopping me sending it to Classique Sounds, Arkless or whoever is that I don't drive and I'm simply not prepared to ship one in such good physical condition through the mail. I need to get it done locally, or ideally get someone round to do it here.
Richard C.
pfm Member
Yes Tony, quite right.
In a decent system, the output transformer will be quite the most expensive component, particularly in the case of Partridge transformers originally designed in conjunction with D T N Williamson for his famous amp - not to mention his equally famous KT66 pentode. It does serve as a barrier between the "guts" of the amplifier and the speakers and in the view of Harold Leak, getting rid of the output transformer was the biggest single advantage obtained by the adoption of solid state. Sad that the HiFi world became saddled with the transistor largely to satisfy production economics.
Richard.
TheDecameron
Unicorns fart glitter.
And an anti valve evangelist would say the sound is ruined by going through miles of thin wire inside those OP transformers blah blah bah.
Barrymagrec
pfm Member
I could never understand why the one transformer at the end of the amplifier was so despised, ignoring the thirty or forty or more transformers in the recording chain.
Robert
Tapehead
Probably because it is dealing with high level signals and significantly larger currents than transformers further down the line (excluding mains). Issues such as phase changes impeding the operation of NFB and of course core saturation.
For designers back in the 50s and 60s, driven to design ever better performing amplifiers in terms of response, stability, distortion, phase and output capability, the output transformer represented a bottleneck.
Of course there are good and bad transformers, and if you only need to produce a clean 10w you can make something very nice indeed without it costing huge sums.
The Leak and Radford designs of the day illustrate this very well.
PigletsDad
My intelligence test came back negative.
It is worth noting that transformer design becomes lot easier if it doesn't have to cover the full audio band (10 octaves or so) at full power.
If a valve design was optimised for use in a system with a subwoofer, or in some other multi-way active design, the transformer has a much easier life. If the bottom couple of octaves don't need to be handled at full amplitude, you can use a lower inductance, a slightly smaller core, and get much better behaviour round the top end of the audio band, in terms of phase shift and stability for feedback designs.
If a valve design was optimised for use in a system with a subwoofer, or in some other multi-way active design, the transformer has a much easier life. If the bottom couple of octaves don't need to be handled at full amplitude, you can use a lower inductance, a slightly smaller core, and get much better behaviour round the top end of the audio band, in terms of phase shift and stability for feedback designs.
Arkless Electronics
Trade: Amp design and repairs.
Between zero and 2 or 3 on good quality audiophile type recordings often...
Barrymagrec
pfm Member
Not back when the O/P transformer argument was current, each time the signal was recorded and played back there were probably 4 transformers per channel in the tape machine alone, desk, limiters, cutting amplifier etc - all adds up pretty quick.
Tony L
Administrator
Don't think you'll be finding many of those on prime period Blue Notes, Impulse, Decca SXLs, Living Stereos etc!
Arkless Electronics
Trade: Amp design and repairs.
I was on about today's technology...
Barrymagrec
pfm Member
Fair enough - I refer to the time - thirty or forty years ago - when a much higher percentage than now of recordings being made were of excellent all round quality, at least on the classical and opera side. I don`t blame the equipment, by the way - it is the way it is used...
Tony L
Administrator
The Croft Micro 25R turned up yesterday, and very nice it is too:
Initially I got a loud hum (well, everything is loud through large 95db efficient speakers in a small room!). This was, eventually, tracked down to an earth-loop. Lifting the earth wire at the mains plug on the Leak which has cured it, and I still have electrical continuity from the Leak's chassis to mains earth via the interconnects and Croft, so all should still be safe (I've checked this with a multimeter). All seems to be working. There is a little increase in background noise as to be expected, and the volume knobs are pretty twitchy but still usable – they start at 12 o'clock, and as pictured above are giving around 75-80db average at the listening seat from CD. Even though I'd like more travel the pots are nice and smooth and it's certainly easy enough to set level and balance accurately. It's very nice to have a balance facility, I've not had that for about a decade now!
This high gain is obviously 100% the issue of the Leak, not the Croft. 125mv for full output is simply insane. I can't for the life of me work out why it is like this – that's the kind of gain one would expect on a bloody guitar amp! It was originally partnered with a tube pre, the Leak Varioslope, so why did the Stereo 20 need such high gain? I think I'll look into getting the Leak's gain dropped down a bit as I assume it will drop the noise-floor a bit too, which would certainly be a bonus. Would changing the ECC83 in the Leak's input position to a 5751 help here? As long as it's reversible I'd far prefer to gently tweak the Leak as I need the Croft to work in other contexts, e.g. if I land a pair of active monitors at some point (which is still on the cards). I briefly tried the Croft with my 303 too (which behaved itself despite not being fixed yet), and it sounded lovely, it's actually a combo I could happily live with. Volume knobs were far more manageable too, and it's really quiet too.
Will report more on the Croft later, but I've heard enough to know it is a very good preamp indeed, and will be just the thing for any future plans as well as in it's current context. The next stage is to buy an MC step-up transformer and get to hear that phono stage...
Initially I got a loud hum (well, everything is loud through large 95db efficient speakers in a small room!). This was, eventually, tracked down to an earth-loop. Lifting the earth wire at the mains plug on the Leak which has cured it, and I still have electrical continuity from the Leak's chassis to mains earth via the interconnects and Croft, so all should still be safe (I've checked this with a multimeter). All seems to be working. There is a little increase in background noise as to be expected, and the volume knobs are pretty twitchy but still usable – they start at 12 o'clock, and as pictured above are giving around 75-80db average at the listening seat from CD. Even though I'd like more travel the pots are nice and smooth and it's certainly easy enough to set level and balance accurately. It's very nice to have a balance facility, I've not had that for about a decade now!
This high gain is obviously 100% the issue of the Leak, not the Croft. 125mv for full output is simply insane. I can't for the life of me work out why it is like this – that's the kind of gain one would expect on a bloody guitar amp! It was originally partnered with a tube pre, the Leak Varioslope, so why did the Stereo 20 need such high gain? I think I'll look into getting the Leak's gain dropped down a bit as I assume it will drop the noise-floor a bit too, which would certainly be a bonus. Would changing the ECC83 in the Leak's input position to a 5751 help here? As long as it's reversible I'd far prefer to gently tweak the Leak as I need the Croft to work in other contexts, e.g. if I land a pair of active monitors at some point (which is still on the cards). I briefly tried the Croft with my 303 too (which behaved itself despite not being fixed yet), and it sounded lovely, it's actually a combo I could happily live with. Volume knobs were far more manageable too, and it's really quiet too.
Will report more on the Croft later, but I've heard enough to know it is a very good preamp indeed, and will be just the thing for any future plans as well as in it's current context. The next stage is to buy an MC step-up transformer and get to hear that phono stage...
sideshowbob
Champagne fascia aficionado
Nice. New Crofts certainly look better than the old ones. And he's finally put volume knobs on with clear positional indicators. It's was always a bugger to set balance properly with the old unmarked round knobs.
jimb0
Jelly Roll Soul
They are still pretty agricultural looking close up IMO. I had the basic pre (with upgraded shiny knobs!) as a phono stage for a year or so. Too much volume with the Quad 306 I tried it with as a pre so I'm interested to hear that it works well with the 303.
In my limited experience (no audiophile) the phono stage in the basic version was really excellent with a palpable presence I have not heard in the other integrated phono stages I have owned. As I have now settled back in with Tannoys I am sorely tempted to see if I can get a dem with better Croft gear... Lack of tone controls holds me back though!
In my limited experience (no audiophile) the phono stage in the basic version was really excellent with a palpable presence I have not heard in the other integrated phono stages I have owned. As I have now settled back in with Tannoys I am sorely tempted to see if I can get a dem with better Croft gear... Lack of tone controls holds me back though!
Tony L
Administrator
Remember my Croft has the reduced output mod, I imagine a stock one would be largely unusable with the Leak and high-efficiency speakers! As I understand it the mod is simply one resistor change (and could easily be retrofitted), apparently Glen doesn't like doing anything else to drop it down further as it screws up the sound of the pre. The Quad 303's input is 500mv for full out, so it's quite a lot less twitchy than the Leak, which, as I say, is simply insane in any modern context - it's bad enough with a passive, the little DIY stepped attenuator hadn't enough adjustment, the two usable clicks falling between 'just too quiet' and 'just too loud'. My Audio Synthesis was fine though, but that's pretty much unique in that it has a dual ganged attenuator giving IIRC 138 possible positions. I'd recommend you still went for a Croft with the output reduction mod if you use it with a Quad, it feels fine with somewhere around ten past giving about 75-80db out of the Tannoys from CD. Perfectly usable IMO. From what I can tell the phono stage is pretty high gain too - I tried plugging my MC into it just to check it actually worked and, despite sounding thin and lifeless, as one would expect a low output MC to do into a MM stage, it went plenty loud!