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PostPosted: 24 Dec 2020, 18:36 
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Joined: 13 Jan 2018, 21:33
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Location: australia
Very nice equipment. Zen-like lines. I recall Richard Allens name from some of the old magazines. It’s great he has published his circuits for diy use. I am (possibly lazily and naively) hoping something approaching “hi-fi” can be achieved with 2 or 3 vacuum tubes and minimal passive components per channel. I’ll have to work up the courage and improve my diy skills to take on an RTP for instance. Possibly something to consider in retirement a couple of years away if a plague doesn’t intervene in the meantime.

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PostPosted: 25 Dec 2020, 11:05 
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Woodo wrote:
Very nice equipment. Zen-like lines. I recall Richard Allens name from some of the old magazines. It’s great he has published his circuits for diy use. I am (possibly lazily and naively) hoping something approaching “hi-fi” can be achieved with 2 or 3 vacuum tubes and minimal passive components per channel. I’ll have to work up the courage and improve my diy skills to take on an RTP for instance. Possibly something to consider in retirement a couple of years away if a plague doesn’t intervene in the meantime.


Hi For interest:
Again I have no connection to the site.
http://soundmovements.co.uk/diyaudio/RTP3.html

You could build a Croft circuit.
Perhaps regulate it with a Maida reg with a FET on the first stage to improve clarity. (adjustable B+)
Perhaps build a passive pre with one switchable phono RIAA stage and make it so you can replace the module and try different things.

Its been a long while since I used RIAA the last effort was with NAB for tape some years ago. :D

Regards
M. Gregg

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PostPosted: 26 Dec 2020, 19:37 
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Location: australia
This Croft ckt? Does it have the phono stage? I guess the highlighted parts. Looks different to other implementations.

Maida regulator? I’d have to read up on it. I think though I’m a bit of an obstinate tube nut but if it’s worth a try I wouldn’t rule it out. I do have regulators on the 300b heater circuits which efficiently do their job. Im still to get it all clear in my head which way to go, ie how much output do I need, which tubes, operating points effect on distortion and noise etc. I may have to buy a cheaper off the shelf SS phono stage so I can start playing LPs in the short term while I procrastinate a bit more


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PostPosted: 27 Dec 2020, 05:13 
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Woodo wrote:
This Croft ckt? Does it have the phono stage? I guess the highlighted parts. Looks different to other implementations.

Maida regulator? I’d have to read up on it. I think though I’m a bit of an obstinate tube nut but if it’s worth a try I wouldn’t rule it out. I do have regulators on the 300b heater circuits which efficiently do their job. Im still to get it all clear in my head which way to go, ie how much output do I need, which tubes, operating points effect on distortion and noise etc. I may have to buy a cheaper off the shelf SS phono stage so I can start playing LPs in the short term while I procrastinate a bit more


Hi,

The croft super micro 3 has a tube regulator in the link:
http://soundmovements.co.uk/diyaudio/Croft_preamps.html

The Maida regulator with the FET input is shown in the picture.
It was on the internet but the site seems to be down so I'll post the picture

I modified it and mixed two different circuits to create a FET input with a 317 variable regulator so it was adjustable.
But the picture just shows the original circuit of the Maida.

The Maida works very well and can survive a short circuit which after testing quite a few other circuits with the same claim showed they couldn't, :hot:

If you are intending to use moving coil then you will have an interesting ride with the extra gain, I think croft just used MM.
Some tube stages use step up transformers because of the problems. You will often see a FET input stage for MC inputs.
Anyway I thought it might be of interest. I think Audio Note used transformer input but they are just so expensive, the point being in this "new age" of electronics we shouldn't need them but in this case we are trying to use old tech (tubes) with new ideas. :D
In theory it should be easy because tube radios work on much lower signals but...

Regards
M. Gregg


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PostPosted: 11 Jan 2021, 01:39 
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Location: australia
Turner Audio (turneraudio.com.au) has a number of phono preamp designs. One has 10 tubes! And others undertake the RIAA EQ in the feedback. This circuit from 2006 has three including a CF, passive EQ and seems to provide a reasonable output level. A power supply circuit is also available. The circuit had a .47u capacitor in parallel with the 470uf cathode bypass capacitor but I didn't include these.
http://www.turneraudio.com.au/miscellan ... tics1.html


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