Making changes to PSU II

[Greg.]

I will shortly be making changes to my Cathode follower converted Pre II. This means I no longer need and HT and heater supply from PSU II for the Pre and it will only then be needed to feed Phono II.

I realise all I need to do is disconnect the umbilical between the PSU and Pre, but wonder if there is any value in removing redundant components from the PSU?

If this is the case, am I right that apart from the supply wiring, I should remove R4, ZD1 and C3? If so, will this have a knock on effect on the Phono supply and will changes need to made to ensure correct voltage? I'm thinking R6 here.

My thinking is that apart from not having any live and redundant wires or connectors, I might gain some improvement to the the quality of supply to Phono II, particularly by getting rid of a probably noisy Zener Diode.

Please let me know what you think and thanks in anticipation.

Oh, and to settle your curiosity, I am converting the Pre II to an LDR (light dependant resistor) passive volume control, because having trialled one in my system, it brings a great improvement to sound quality, albeit as ever IMHO.

Thanks,

Greg

[bob orbell]

Hi Greg, sorry to hear that you are giving up on the CF, but if you are happy with LDR's, then so be it, you can remove your mentioned components and adjust R6 to suit the phono, but I doubt you will hear any difference with them removed, by the way, have you read what Morgan Jones think's of LDR's as a volume control, not good reading. BOB

[david]

LDR passive? Where can I get any details of that? Sounds interesting

[Richard]

I'm sure Greg will have more info but this shows the principle,

http://diyparadise.com/w/all-about-volume-control/

What did MJ have to say Bob, is it in latest edition, I have the second edition but haven't noticed anything in there.

[bob orbell]

Hi Richard and all, MJ's comments are in the third edition, he sums up between light sensitive resistors - switched attenuator as
distortion, benign-none measurable
interchannel tracking error, poor-easily betters 0.05dB
log. law, poor-perfect
of course they may well be much better now.

[Ali Tait]

The one I heard at the last Owston sounded excellent, and was better in the bass than a Music First pre.

[Greg.]

Thank you Bob for confirming what to do to PSU II.

I have read what MJ says in his third edition and remember, this was published eleven years ago. I also personally know others who have experimented with the concept and gave up in frustration. The problem was finding matching LDR's (two per input/output) and also a drift in value that caused channel imbalance. I believe solutions to these problems have now been found. Certainly my trial of a Stereo Coffee volume control from New Zealand was very positive. In comparison with the Pre II CF, the sound presentation was so much more revealing and transparent especially on micro-dynamics in the treble with many instruments and voices sounding more realistic and closer to a live sound. I particularly heard nuances on voices and the classic listening test to high percussion revealing more detail. Bass is better controlled, and in my loaners case, he found it deeper. Furthermore, the sound stage is improved with greater space between instruments and background noise is blacker. I had the unit on loan for over a week and it was also listened to by Nick B and vinylspinner. They both shared my observations. In comparison, the CF Pre II appears to be adding it's own colour to the sound.

Although it is described as a passive volume control, it is a powered unit consisting of a LDR board with a pair of LDR's per input, a separate control board that requires a 12vdc feed. This seems to be reflected in the sound and to my mind meets the need of a pre that is not purely passive as I have found these to be slow, muddy and lacking sparkle which was why I went to CF in the first place. I did this because I found the standard Pre II (3) offered too much gain in my circumstance and being a source of hum through close location to the power amp. The beauty of the Stereo Coffee is that although you have a volume pot and a selector switch, the signal does not pass through either of these components.

I have ordered a Stereo Coffee kit which is available on eBay and it is on it's way to me at the moment. Postage can take over a week. The eBay advert offers a single input model, albeit the LDR board can facilitate up to three inputs. The boards come pre assembled with screw connector terminals for wiring. You need to solder in your signal leads although it is supplied with RCA sockets attached as standard. The proprietor, Chris Daly is a great guy and I have been in regular communication with him. He is supplying me two LDR boards (which can be 'piggy backed') so I have a total of six inputs to compliment the Pre II chassis. I have asked for the RCA sockets not to be fitted to allow my own signal wiring.

Thereafter you need to source your own regulated 12v power supply. On Chris' advise, I have gone for a laptop charger delivering 19vdc to feed a ultra low noise adjustable voltage regulator module to supply the 12vdc. Both readily available on eBay. You can of course, use a battery.

For more reading, DIYaudio has a thread but be warned, it is over 500 pages long
http://www.diyaudio.com/forums/analo...ve-preamp.html

Another thread on Audio-Talk is more concise and the initial post gives links to the Stereo Coffee and a more commercial American product. On page 7, you can read my more extended review.
http://www.audio-talk.co.uk/phpBB2/viewtopic.php?t=5621

There is also a thread on AOS, but that forum is having technical problems at the moment and is currently down. I'll post a link when it is back again.

To be sure, this volume control will not suit everyone. Some will find the sound far too clinical and raw, but after listening, it certainly works for me.

There is currently a Stereo Coffee loan unit doing the rounds and if you want to join the queue, this is the link.
http://www.audio-talk.co.uk/phpBB2/viewtopic.php?t=5682

I hope this has been helpful.

Greg

[Ali Tait]

Nice Greg, I look forward to receiving the loaner unit.

[bob orbell]

Hi Greg and all, yes it sounds good, but as I have speakers that are very revealing, I am positive that this would send my sound over the top just like the shunt pot mod did, and the Pre III, I have said many times that it depends on the rest of one's system, so I hope it works well for you and Ali. I have been playing with a CF with active load lashed up on a matrix board, first impressions were promising but again after an hour and a half, the sound was just too much, so I shall stay with what I have, but it's fun to play BOB

[Greg.]

Here's the link to the AOS thread. Unfortunately it contains quite a lot of forum chaff, but worth working through if you are seriously interested in the LDR concept.

http://theartofsound.net/forum/showt...timate-passive