View Full Version : Croft Series 7 power amp
I am pondering the purchase of one of these. I know the two Daves (DSJR and hi-fidave) rate Croft's current range highly; any thoughts on the particular strengths and weaknesses of the Series 7 (if any)?
Haselsh1
03-10-2010, 11:33
Joe, I have one of these along with a Micro 25 Basic preamp and still get absolutely staggered by the detail this power amp uncovers in the music. It is capable of driving some quite difficult loads with my own Dali speakers dipping to 3.8 Ohms in places and has a serious authority and control over its output.
In pure music terms, the amplifier stuns with the amount of detail it uncovers and the way in which low level backing vocals become so obvious. I love the band Ezio and their sound is truly fabulous from this amplifier especially the attack on their acoustic guitars. The amplifier does not quite have the 'air' and 'space' of a good 300B valve setup but it is a lot more powerful and authoritative in its appreciation of sounds.
I do not want another amplifier setup. I have now found the best I have ever heard in around thirty years of using Hi-Fi.
hifi_dave
03-10-2010, 11:45
It does have a weakness - the case is a plain black box..:violin:
technobear
03-10-2010, 12:31
I have a Syntegra - the last integrated that Glenn made, I think - it's a very similar design, valve driver stage and MOSFET power stage with an input selector and volume control up front. This is a very musical amp with great transient attack, great timing, superb transparency, tight deep bass and a very quiet background. It responded well to tube rolling too but that's a matter of taste. Yes the latest casework is a little bit plain but the price/performance ratio is sky high as a result.
Haselsh1
04-10-2010, 15:00
One of the guys from Musical Fidelity once told me around 1982 that if one was to spend 80% of the development budget on how it looks it as sure as hell isn't going to sound much cop. I tend to agree.
I guess limited current drive ability into difficult loads would be my only negative.what speakers do you have?
Thanks all for the replies.
Darren, speakers are Audiovector Mi3 Signature floorstanders, fairly efficient (91db).
Haselsh1
04-10-2010, 18:23
Thanks all for the replies.
Darren, speakers are Audiovector Mi3 Signature floorstanders, fairly efficient (91db).
My speakers are also 91db/W/m but do dip down quite low in the bass. The Croft has no problem with them. It can easily manage such things.
technobear
04-10-2010, 18:25
I guess limited current drive ability into difficult loads would be my only negative.what speakers do you have?
Zu Druids - 12 ohms impedance and sensitivity in the high 90's.
The Croft drives them to insane levels http://www.technobear.btinternet.co.uk/emoticons/slayer.gif
Before the Zu's, it used to drive Triangle Celius Esw's to semi-insane levels and they were 92dB.
High-ish sensitivity speakers and valve amps is, of course, a classic route to audio nirvana.
I love audio vectors: I bet this is a match made in heaven.
Thanks again folks. Looks like this might be the start of a beautiful relationship!
I'm looking forward to listening to the 7 series myself.
The only thing I will struggle to overcome is the 'dual mono' volume controls.
I seem to recall seeing these years ago on another amp, and the solution to keeping them in 'balance' and enable just turning one dial, was to stretch an elastic band over the two knobs.
hifi_dave
05-10-2010, 10:00
You can try the elastic band dodge but I have found (after selling hundreds of Croft pre's over the years), that users rarely find it a problem in every day use. You don't need to align both to the exact same point to enjoy great imagery.
You can try the elastic band dodge but I have found (after selling hundreds of Croft pre's over the years), that users rarely find it a problem in every day use. You don't need to align both to the exact same point to enjoy great imagery.
Fair enough Dave.
Truth is, I'm still trying to imagine life without a remote.
Sad really, isn't it.
The twin uncalibrated volume controls are a PAIN IN THE A*SE, but a very necessary evil to endure because Glenn's preamps sound so good. No point in modifying mine, as it's now a vintage item.
I still reckon Glenn should maybe put twin or a stereo stepped attenuator in his "R" series pre, as it would aid volume tracking at least.
Lol, if you know Glenn, you'll know there's about as much chance of that as him fitting tone controls! :lol:
Dual-mono volume pots are an integral part of Croft preamps, love 'em or loathe 'em.
Like you say, though, the sonic performance of his amps is such that there is simply no contest between 'enduring' having two separate volume controls, and buying a worse sounding, and no doubt more expensive, preamp with a stereo pot.
Joe,
Congratulations - you've made a superb choice. Make sure you let us know how you get on when the new amp is in your system :)
Marco.
Ali Tait
05-10-2010, 15:26
I'd have thought it would be pretty straightforward to change one of the mono pots for a stereo. Just leave the other in situ for looks,the other would control both channels.
Indeed, mate, but not everyone would want to mess around with their brand new kit.
Take those DIY specs off for a second and embrace the reality of the majority who buy (mostly new) commercial gear from a dealer and to whom such fettling of spanking brand new kit would be an anathema! ;)
That's not your world, I know, and to an extent it's not mine either, but it is for most people :)
Marco.
Ali Tait
05-10-2010, 16:34
Perhaps, but if something bugs you that much,it would make sense to do something about it. Could always be reversed if you wanted to sell.
hifi_dave
05-10-2010, 16:38
We went through all this a few months ago and the answer from Glenn is simply that 'it sounds best this way'. Obviously, Glenn is well aware that stereo pots exist but he is adamant that the very best results at the price are obtained with dual pots.
I have been selling Croft products for almost 30 years now and I can honestly say that I have never had anyone refuse to buy because of the dual pots. A few minutes use and you forget it.
All you need do is take a listen to realise how good this gear is and then take a look at the price.
Yep, but that doesn't detract from the fact that it's a bit of a bodge....
Personally, like you, it wouldn't bother me if it solved the 'problem', although I'm not even sure there is one, as there are valid sonic reasons why Glenn insists on fitting dual-mono volume pots to his designs.
You have to remember that when you're buying Croft gear, you're buying kit aimed soley at enthusiasts who value ultimate sonic performance more than anything else.
As commercial gear goes, these are truly 'race-tuned' products with no pandering whatsoever to (sonically) unnecessary fripperies, or even the convenience of a stereo volume pot! ;)
Remote control? Hahahahaha... You're joking, aren't you?
Marco.
I have been selling Croft products for almost 30 years now and I can honestly say that I have never had anyone refuse to buy because of the dual pots. A few minutes use and you forget it.
All you need do is take a listen to realise how good this gear is and then take a look at the price.
Wot 'e said!
Marco.
To be fair to Glenn, once my phono stage is modified to drive lower impedances (something he knows about) he then suggested I could use a good quality stereo pot if I wish (currently 1M pots are fitted).
I've found 1M attenuators, but the audio log ones I've discovered so far are stereo, rather than mono ones.
Reid Malenfant
05-10-2010, 17:46
Even very good quality stereo potentiometers are often mismatched around the middle of there travel. They are often very good at the low end & high volume but where the control will be used most they can be more than enough out to cause a channel inbalance ;)
I didn't know they were probably 1Mohm as well, this precludes most good stereo pots :scratch:
The pots on MY Croft preamp are [edit - 500K]. The current preamps, as far as I know, have been re-designed on the phono stage to avoid this necessity, hence the offer to update mine (and the recent story of woes when I used a second power amp from the record outputs for headphones, lost bass and set about using a line buffer which turned out to be a modified MF X10-D into a refurbed Quad 33. No trouble now and I have bass extension aplenty as the record output is seeing half a Meg input impedance now.
I'm enjoying my Croft so much right now I really don't want to send it to him. I also don't want to take it to hifi dave's to compare with the Micro "R" as I'll probably be disappointed with no funds for magor upgrades....
Techno Commander
05-10-2010, 18:21
I suppose that if you were unduly worried about the dual pots, then the 2 knobs could be "coupled" with a nice thick rubber band and would allow both to turn simultaneously. :)
Doesn't work on mine as the pots are spaced too far apart.
Hi Mark,
I didn't know they were probably 1Mohm as well, this precludes most good stereo pots :scratch:
Excuse my technical ignorance, but what is "1Mohm", and why would that preclude most good stereo pots? This is a non-techie learning here :)
My modded Croft Charisma X uses a 100K Glasshouse Seiden metal film stepped attenuator, shown here:
http://www.hificollective.co.uk/images/seidenatt3l.jpg
It sounds superb, and was a real improvement on the Alps Black carbon pot fitted before, that Glenn prefers to use, which in comprison to the Glasshouse, made my preamp sound rather lifeless and shut-in.
I've never understood what relevance the 'K' value (10, 25, 50, 100 or whatever) has with volume pots and in turn its relationship with a preamp, specifically the gain thing, too.
Apparently the gain changes depending on the 'K' value of the pot?
Educate me, dude! :cool:
Marco.
P.S Ali (or anyone else able to explain), feel free to answer this, too.
I can't get technical 'cos I'm right out of my comfort zone, but in my preamp, the phono stage output impedance is too high for a 100K pot to be used. Glenn has changed this on all subsequent phono stages so it won't apply to you Marco, or any of the current ones. Apparently, too low a value of pot on mine causes bass to disappear. I tried this on my first 4PP, geting some rather nice wire-wound pots via hifi dave (I forget their value as it was a very long time ago). The sound screamed and went all "ringy" with them, so I put the originals back.
This doesn't apply to current models, but perhaps stereo pots at the price level aren't as good as the separate versions?
The chap who sold be his croft preamp has a web page http://www.soton.ac.uk/~apm3/diyaudio/Croft_preamps.html devoted to the early models from Glenn. He drew the basic circuit (but used 1M on the pot value)
http://i132.photobucket.com/albums/q8/DSJR_photos/Croft_SMA4_signal_circuit-1.gif
The 100K resistors to ground on the input selector have now been removed at Glenn's request
OH CRAP, I've got it wrong!!! Mine has 500K not 1Meg (1,000K)......
My preamp now (I've since removed the 2A rated mains filter Glenn built in, again on his recommendation -
http://i132.photobucket.com/albums/q8/DSJR_photos/Croft4PPrecap1.jpg
Current Croft 25 -
http://i132.photobucket.com/albums/q8/DSJR_photos/croft_25.jpg
Spectral Morn
05-10-2010, 18:33
Dual pots also allow you to adjust levels as a balance control.
Sometimes odd room layouts or one wall being solid and another being plaster board can cause balance issues. In cases like that the ability to adjust levels is great to have.
Regards D S D L
Reid Malenfant
05-10-2010, 18:51
Excuse my technical ignorance, but what is "1Mohm", and why would that preclude most good stereo pots? This is a non-techie learning here :)
I've never understood what relevance the 'K' value (10, 25, 50 or whatever) has with volume pots and in turn its relationship with a preamp, specifically the gain thing, too.
Apparently the gain changes depending on the 'K' value of the pot?
Educate me, dude! :cool:
Marco.
Ok, i'll try ;)
1Mohm would be 1 meg ohm or if you like 1 million ohms :)
As you may well have observed with the Caiman passive output mod thread different capacitors & resistors in a circuit can effect the low (& depending on how they are connected) the high frequency rolloff of a circuit. In other words these components create an upper & lower frequency barrier depending on what they are feeding.
In the case of the croft i should imagine that the 1Mohm pot is a design choice as it'll probably be feeding a very high impedance stage & the pot will to an extent take control of this. The capacitors on any inputs which are used for DC blocking could be an order of magnitude below the capacitance that you'd need to feed a 100Kohm (100,000 ohm) potentiometer to get the same low frequency rolloff.
Because of this you could fit much smaller value but also much higher quality capacitors as DC blocking on the inputs (design concerns again). You might even be able to use some polystyrene capacitors which are very high quality indeed :)
There is no "gain value" as you call it which will alter depending on the potentiometers value, whether this is 500 ohms or 1M ohm. The same thing (the audio output from whatever equipment is playing) is fed to one end of the pot & the other end is grounded, the output is taken from a wiper that can move from one end (ground at zero volume) all the way through to the full input voltage where the wiper is at the same potential as the input (where you'll probably be clipping your power amp to all hell :eyebrows:). We all know how far we can turn the volume up, there is usually headroom to go higher though with older analogue gear as this has a lower output than CD etc, so you might turn the volume control further than you normally would compared to listening to a CD.
The actual potentiometer value is selected by the manufacturer so as to allow most if not all equipment to drive the thing properly, in the case of the croft it's probable that the input impedance was made high (1Mohm) to allow other valve gear to drive it easily. As you know valve equipment tends in general to have a higher output impedance, so it makes sense to cater for this & fit a high impedance potentiometer so that there is no lacking in bass response.
Transistorised stuff often has a low ouput impedance (10 ohms isn't unheard of) so to keep system noise to a minimum (johnson noise, which all resistors produce) it makes sense to keep it fairly low, but then you have problems finding good DC blocking caps :eyebrows:
It's all a compromise Marco, just depends which way you lean ;)
Hope that helps, but probably raises more questions :doh:
Ali Tait
05-10-2010, 18:53
Yep, but that doesn't detract from the fact that it's a bit of a bodge....
Personally, like you, it wouldn't bother me if it solved the 'problem', although I'm not even sure there is one, as there are valid sonic reasons why Glenn insists on fitting dual-mono volume pots to his designs.
You have to remember that when you're buying Croft gear, you're buying kit aimed soley at enthusiasts who value ultimate sonic performance more than anything else.
As commercial gear goes, these are truly 'race-tuned' products with no pandering whatsoever to (sonically) unnecessary fripperies, or even the convenience of a stereo volume pot! ;)
Remote control? Hahahahaha... You're joking, aren't you?
Marco.
Yep,I'm well aware of the type of person Glenn's gear is aimed at. I was just suggesting a fix for folks that are really annoyed by dual mono pots. Doesn't sound like anyone is bothered to that extent though.
Indeed, mate. Incidentally, the "you" I was referring to in that post, after the second paragraph, was the collective "one", not you personally, just in case there was any misunderstanding :)
Mark,
Thanks for that. It's a *little* clearer now!
Marco.
If you want remote control and a single volume pot you could always buy one of my Audionote pre's, problem solved
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