Snail Phono Pre

What people are working on at the moment
User avatar
Nick
Site Admin
Posts: 15707
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#136

Post by Nick »

The valve will possibly vary by 15%
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
little eddy
Old Hand
Posts: 693
Joined: Sun Nov 09, 2008 2:06 pm
Location: Manchester

#137

Post by little eddy »

So the valves might be noticeably different as you suggest, as too will be passive component values etc but that doesn't mean I shouldn't 'fix' the current. It's one less variable/inaccuracy.

I'm probably just being lazy. I'll test at least one other cascode pair and if close, can probably assume that the regulator tolerances are quite tight. If not, I'll have different Rset for each CCS.

Rightly or wrongly I try to use CC resistors wherever possible, (from Rapid at +/- 5%). This inaccuracy would directly correlate to the CCS current if used as Rset and therefore I'm thinking of using precision metal film resistors in this instance. With only 3V across Rset, would sonic/noise impact be negligable?
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#138

Post by Mike H »

If the valves are different each will pull up the cathode Voltage until the current matches the CCS, the anode Voltages will quite likely vary a bit in that case. Which shouldn't be a problem, it happens with resistors too.

Bear in mind resistor tolerance and temperature drift as well. Problee be enough effort just to get it working.
Last edited by Mike H on Thu Nov 03, 2011 1:50 pm, edited 1 time in total.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#139

Post by Mike H »

little eddy wrote:With only 3V across Rset, would sonic/noise impact be negligable?
Haven't the faintest idea, only one way to find out :D

I like the Rapid type CC's too but each to his own taste.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#140

Post by pre65 »

Must admit I mainly use Kiwame these days if what I want is not in the box, not that expensive from HFC and next day delivery. :)
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#141

Post by Mike H »

Ah forgot Kiwame, yes like them too, problee over CC if I'm being fussy.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
little eddy
Old Hand
Posts: 693
Joined: Sun Nov 09, 2008 2:06 pm
Location: Manchester

#142

Post by little eddy »

So I have now tested 4 cascoded pairs and a Rset of 550R is predicted to give a spread of currents between 4.72 amd 4.98mA.

I think the 5 - 6% spread warrants me buying separate Rset resistors for each pair.

If I've done my testing right, the cascode arrangment alters the R/I characteristic, in this case by around 10% making Phil's equation I=2.69/Rset.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
Old Hand
Posts: 693
Joined: Sun Nov 09, 2008 2:06 pm
Location: Manchester

#143

Post by little eddy »

So the patient now breathes again albeit rigged up with a few temporary resistors.

I must say, once I worked through how the original builder had put the amp together, I found the changes reasonably easy to implement.

Now need to order the resistors for the CCSs but the hardest thing will be figuring how to fit them below the mounting plate as there isn't much room to the casing.
Attachments
Line Stage phase 1.jpg
Line stage phase 2.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#144

Post by Mike H »

Looking good! What's it based on that's modified?
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
little eddy
Old Hand
Posts: 693
Joined: Sun Nov 09, 2008 2:06 pm
Location: Manchester

#145

Post by little eddy »

The line/output part in its previous incarnation was a 2-stage ECC83 with feedback.

I am also taking the opportunity to replace some key components, hence the large Clarity output caps in the photo.

I brought this amp to Eggborough a couple of years ago when I had just built my Thorens TD-125 deck. The phono part I thought sounded OK (2-stage ECC83 SRPP) but the output level was too low so needed to use the line stage that tonally just killed the sound. Everyone said that it just lacked something so this is my attempt at addressing its shortcomings.

Nick had suggested that while I'm at it, I also rebuild the phono stage hence the overall design covered by this thread.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#146

Post by Mike H »

Ah right gotcha. It's a "work in progress"
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
little eddy
Old Hand
Posts: 693
Joined: Sun Nov 09, 2008 2:06 pm
Location: Manchester

#147

Post by little eddy »

Yes the amp was built for me in return for some work I had done but never having considered a phono/linestage before, I had no input/influence on the original design.

Hindsight is a great thing (I hold an honourary degree in it), but then I wouldn't have learnt anything.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#148

Post by Mike H »

Not arf it is and that's a fact. :D
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
little eddy
Old Hand
Posts: 693
Joined: Sun Nov 09, 2008 2:06 pm
Location: Manchester

#149

Post by little eddy »

Well my eyes are sore and hands have just stopped shaking but feeling happy in that my first cascode BSP129 seems to be working. 674R and 407R for 1.8 and 3mA respectively.

I was bothered that I might damage the fets with the heat from soldering but all seems to have held up (at least on the first one).

Just 3 more to build and test but they will have to wait while I concentrate on the line stage IXYZ CCSs and basic phono stage build. Endeavouring to get it in a half usable state for Owston.
Attachments
BSP129.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20157
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#150

Post by Mike H »

That's the trouble with soldering semis, about 3 - 4 seconds max., if it's taking too long back off to leave it to cool down for a few seconds then try again.

If it's a power device already screwed to a piece of metal you've got more leeway.

It's also possible to have it gripped with long nosed pliers with a laccy band around the handles as a heatsink.

Another thing you gotta watch for with FET's is static charge damaging the gate, hence hold source or drain pin first before touching the gate pin, don't shuffle across a carpet just before touching it, things like that :D


Reason for edit: word missing
Last edited by Mike H on Sat Nov 12, 2011 3:55 pm, edited 1 time in total.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Post Reply