IC: Croft Micro 25

[Riislingen]

Hi,

As Glenn has just finished updating my beloved Charisma Elite (amazing it is) I´m considering selling my spare Micro 25 pre.

The 25 is formerly modded with an extra switch to support two different gain stages. Therefore the front had been drilled to accommodate said switch.

It has now been reverted to factory trim and I fitted a with new face plate supplied by Glenn.

Not sure where we are price wise so any input will be helpful - speculating around 350 £.

I will up some pictures later tonight as well.

Works absolutely fantastic.

/Mikkel

[DSJR]

Great price and this version IS BETTER than a 25-basic - well it is every time I compare them :lol:

Perhaps you could supply details of the gain-switch mod to the final buyer, since not everyone is a dab hand with a soldering iron and having a lower gain option should help with some surprisingly recent power amps for late-night listening?

Good luck :)

[Riislingen]

Hi Dave,

Thanks - and yes, it´s a killer amp.

The gain mod is removed and everything is back to factory trim along with a new fitted face plate. So no soldering or anything needed.

/Mikkel

[Elephantears]

pm sent with firm offer to buy. Cash ready!

[Elephantears]

By the way, the gain mod would actually be a good thing for me. But I still want the amp in any shape of form!

[The Black Adder]

This should have gone by now... hope so. I too have upgraded from the Basic to the 25 Micro and the upgrade is definitely worth it for someone.

[DSJR]

By the way, the gain mod would actually be a good thing for me. But I still want the amp in any shape of form!

Like I said, any documented details on what to do (preferably by Glenn perhaps) would be very useful I think...

[Riislingen]

Like I said, any documented details on what to do (preferably by Glenn perhaps) would be very useful I think...

Sure, it´s merely about dropping the value of the feedback resistors from 470K to either half or even 100K - which will result in 0 gain.

This could be set up with a 2 way switch, like the one used for mute - in effect switching between these two resistor sets.

So the feedback resistors are the key, this is confirmed by Glenn and goes for the rest of his designs as well it appears. :eyebrows:


/Mikkel

[DSJR]

Thank you :)

[koowat]

Hi Mikkel,

Can you post diagrams how we can do this? I am interested in using the mute toggle for the purpose to high/low sensitivity as well.

How do we switch between the 470k and 100k?

Thanks!

[DSJR]

Better in reality to reduce the sensitivity at the amp end in all fairness, since this keeps plenty of signal down the interconnects and should keep the preamp signal to noise ratio as high as possible (the Crofts aren't unduly noisy under most conditions, despite being valved, but you can never have too high an s/n ratio IMO).

Might be better to try an L-pad on the power amp inpits to start with - a couple of half decent resistors really doesn't affect the sound at all, all other things being equal........

[Riislingen]

Hi Koowat,

I can´t do a diagram, but I´ll post some pics of the inside of the amp and try to explain. Like mentioned earlier the mod is not mounted anymore unfortunately.

My experience is that for the majority, just dropping the FB resistors to about 150K would take care of the rather hefty gain and make the amp more usable.

That operation is a 5 min. task and could be done by almost anyone.

[Riislingen]

Better in reality to reduce the sensitivity at the amp end in all fairness, since this keeps plenty of signal down the interconnects and should keep the preamp signal to noise ratio as high as possible (the Crofts aren't unduly noisy under most conditions, despite being valved, but you can never have too high an s/n ratio IMO).

Might be better to try an L-pad on the power amp inpits to start with - a couple of half decent resistors really doesn't affect the sound at all, all other things being equal........

I´m with Dave here !

An Lpad might solve the issue fast and sufficient. Futher dropping gain to low will impact sound quality I think. My recommendation would be something like:

1. Try an LPAD on the power amp input, if that works - done

2. Use a 5751 tube in the line stage, that will drop the gain about 30 % already and often sound better. Result is also less noise compared to ECC83.

3. Drop the value of the feedback resistors to 150-220K that will load the amp down quite a bit. Doing the 2 way switch is also an option but takes more time and involves drilling the face plate.


I´ll still try to explain the switch thing, no worries:D

[IHP]

By the way, the gain mod would actually be a good thing for me. But I still want the amp in any shape of form!

Nice one Andrew, really pleased you finally got sorted with one of these. Can my guilt finally pass ?!

[Elephantears]

Nice one Andrew, really pleased you finally got sorted with one of these. Can my guilt finally pass ?!

Yes, Ian, its looking like we are both going to be happy now! Can't wait to be reunited with a Croft in the house.

[DSJR]

I'm hoping this is helping people.. I rather prefer to leave everything as-is and maybe try the 5751 line valve option, since that doesn't disturb Glenn's usually excellent and carefully soldered layout. Can be done in minutes and is easily reversed :)

[Riislingen]

I'm hoping this is helping people.. I rather prefer to leave everything as-is and maybe try the 5751 line valve option, since that doesn't disturb Glenn's usually excellent and carefully soldered layout. Can be done in minutes and is easily reversed :)

What Dave says !

Absolutely the best choice in terms of sonics.

The reason for doing dual gain stages was an absurd difference between source output.

Most people would be absolutely fine with the tube change. It will sound much better than the Lpad option for as well.

For the record; Glenn advised on the resistor drop or I would have never done it. :)

[Riislingen]

Amp is sold by the way :)

[koowat]

Hi there,

What Glenn told me is that the 5751 will not work in the single line stage gain - it works on feedback so he says you can use any tube in there 12AX7, 12AU7, 12AT7 and 5751 and the gain would still be substantial.

I've left everything original but all ears to a switch solution - but then it is being put up for sale.

Cheers

[Elephantears]

Very interested to hear any more experiences with changing the gain setting of the Croft pre's, whether this is by resistor or valve changes. I am very new to all this, but I presume it is all very dependent on choice of power amp. I found when I had the Micro 25 previously and tried it with the Series 5 (version 2), the gain was just too much. The result was that it was very difficult to get the correct balance using the dual volume pots. Also it didn't quite agree with my MC phono stage at the time and I had even more gain with my vinyl source. I don't actually know what power amp I'm going to end up with this time, but I'm guessing that lowering the gain is going to be a good idea for me whatever.

Very exciting this though. I said to Ian a while back that whilst I thought I'd had a moment of madness selling the Micro 25 back to him, I also had this feeling fate would bring me another at the right time, when I was ready. And anyway Ian was the original owner of the one I had, so its all working out very nicely.

[Riislingen]

Hi there,

What Glenn told me is that the 5751 will not work in the single line stage gain - it works on feedback so he says you can use any tube in there 12AX7, 12AU7, 12AT7 and 5751 and the gain would still be substantial.

I've left everything original but all ears to a switch solution - but then it is being put up for sale.

Cheers

That´s interesting ! I wasn´t aware. I used the 5157 in my Charisma which is PP with an ECC82. Seemed to make a difference there. Ultimately though, feedback resistors were lowered in both amps to get the right level.

[Riislingen]

So following Glenns advice on lowering the gain of the pre, you need to drop the value of the feedback resistor in the line stage valve. This consist of 2 x 470K Ohms as standard. Dropping them to 100K will set gain at 0.

The amp will sound better with some gain even above average in my opinion - if it´s set too low, some of the grunt and slam of Glenn´s design is lost I think.

Again all of this is subjective. The soldering job is pretty simple and can be easily reverted again.

http://img189.imageshack.us/img189/9152/wp000020gl.jpg

The FB resistors that need replcement are the red ones, they attach to the output leads of your pre (black leads going to out) on the solder tag just behind the valve (marked in green) and on the other end meets the resistors leading to your volume pots (marked in blue).

[southall-1998]

They look like sweets..Yum Yum ;)

[northwest]

So following Glenns advice on lowering the gain of the pre, you need to drop the value of the feedback resistor in the line stage valve. This consist of 2 x 470K Ohms as standard. Dropping them to 100K will set gain at 0.

The amp will sound better with some gain even above average in my opinion - if it´s set too low, some of the grunt and slam of Glenn´s design is lost I think.

Again all of this is subjective. The soldering job is pretty simple and can be easily reverted again.


So wat do you think would be a good alternative value - as a starting point - 250k?

[Riislingen]

Hi Graham,

I´ve tried 115K, 180K and 220K. Preferred the ones around 200K I must say.

[Elephantears]

Just to be sure that I'm following this correctly. Have we established that the 5751 valve can't be used in the Micro 25 line stage?

[northwest]

Just to be sure that I'm following this correctly. Have we established that the 5751 valve can't be used in the Micro 25 line stage?

Bloody hope they better be - I just bought a pair. For a pre-amp I don't own yet too!

[Riislingen]

The 5751 can be used, no worries.

It was merely about whether the 5751 would actually drop gain by the normal 30% in Glenn´s circuit or not.

It will work fine and in many cases sound better than a ECC83/12AX7. Advice is typically to not use them in the phonostage due to the normally lower gain.