Snail Phono Pre

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little eddy
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Post by little eddy » Sat Nov 12, 2011 3:48 pm

I've now got the IXYZ CCSs fitted to the line stage.

A test CD I have at the amp input measures 2.34V (I'm presuming that this is RMS). The output of the gain stage measures 41.9Vac so I have a gain of 17.8.

My question is do I have enough voltage headspace over the gain stage CCS in order not to have clipping? Vout is 143Vdc so if I add 1.41*41.9 I get 202V peak. The supply to the CCS also measures 202V!!!!!!!!

Each time I reduce the power supply RC filter resistor value to increase the supply voltage, the gain just seems to increase thus leaving me in a similar position.

But isn't CD max output 2Vrms in which case Vout peak would be 193V, in theory giving me 9V headroom so:

Am I right about the max CD input level?
Do I need to increase the voltage to the gain stage?
Or should I just add a 1M in series with the CD input therefore haling Vin to the gain stage?
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Line stage with CCS.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Nick
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Post by Nick » Sat Nov 12, 2011 3:53 pm

CD is meant to be 2v out, but some are more ( I suspect the louder sound is a selling point at the dealers). But I would ask what the max output you will need is, if 2v in gives you 17v out, and your power amp clips with 5v input, you will just turn the volume down.

Sounds to me like you need more headroom for the CCS though, he gain may be increasing as you give it more headroom because its clipping otherwise. I would use the current though the CCS and a target dissapation given they are not heatsinked to give you a max voltage across the CCS and work backward from there.
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Mike H
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Post by Mike H » Sat Nov 12, 2011 4:02 pm

Ah I see the problem now (but took a while!), the line stage has Voltage gain, so you will need the volume control to be IN FRONT of it, not after.

If it were just a cathode follower say, it wouldn't matter.
 
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little eddy
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Post by little eddy » Sat Nov 12, 2011 4:22 pm

Yes I don't think I would have had a problem if I had the POT in front of the gain stage but if you remember the discussion, it was a dockyard job to alter from the original amp configuration so I decided to leave as-is.

Obviously the 2.34V is a function of the old Technics player I have in the garage. Not sure what my AN DAC is so in with a 910k resistor for the time being, it should be more than enough attenuation to get me going.

I was expecting that when I have the phono part finished, I could equalise the input levels anyway by attenuating the CD so halving the level shouldn't be a problem.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Mike H
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Post by Mike H » Sat Nov 12, 2011 4:28 pm

Yes resistor dividers will work, assuming no changes of expected input levels in the future.
 
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little eddy
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Post by little eddy » Sat Nov 12, 2011 5:20 pm

Must have done the trick. Vin reduced from 2.34 to 1.06V and Vac on the gain stage output is 21.2V so based on 143Vdc and increased supply to 209V plenty of space.

Power across regulator should be .005*66=0.33W. Should be fine as the K&K cascode regulators are stated as being finr up to 1.7W without a heatsink. The power dissipation of the CF load is slightly higher at 0.725W.

The cathode of the gain stage is at 4.2V so if the CD input attenuator remains, this can be reduced, reducing Ra, the power of the CF CCS and also allow me to reduce the supply voltages.

EDIT - Gain is 20 using the old RCA ECC82s. Now that everything is set, I can try some other valve options.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

little eddy
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Post by little eddy » Sun Nov 13, 2011 7:48 pm

So finally got round to having a listen today to the line stage while the rest were out swimming, (all this after Nick pointed me in the right direction in terms of getting the valve voltage regulator working better).

I was not expecting much on the basis that it another device in the signal chain but 1st impressions were not too bad. The one thing that stood out was the amount of bass - not sure why this is but perhaps the line amp is a better load for my CD or maybe the CF is better at driving my Rocky power amp. Also seems very authorative and I'm sure some detail that I'd not previously noticed.

I'm thinking the Clarily caps may take a while to settle in so back on the bench now for a few hours runnig prior to Owston.
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Mike H
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Post by Mike H » Sun Nov 13, 2011 8:45 pm

Never noticed ClarityCaps needing "running in". That might just be me of course. :D
 
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little eddy
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Post by little eddy » Sun Nov 13, 2011 9:04 pm

Mike H wrote:Never noticed ClarityCaps needing "running in". That might just be me of course. :D
Just me wondering if they might be the reason for their being more apparent bass. Won't do any harm.
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Mike H
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Post by Mike H » Sun Nov 13, 2011 9:09 pm

Well if they're doing it better than what was before, then that would seem reasonable. Could any one of many things though :D
 
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little eddy
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Post by little eddy » Wed Nov 16, 2011 8:34 am

Tried some different valves last night and the apparent exagerated base I think is a feature of the RCA 5814a black plates. I found a comment onthe we saying 'good bass' but to be it was too much. I can't recall this imbalance when used in I think my WAD KiT88.

Tried a pair of Amperex ECC82s and these seemed to be more balanced but not quite the 'presence' of the RCAs.

One question however. When powering up the Amperexs, a few second later there seems to be horizontal arcing on the top of the upper mica insulator. At the same time there is an audible crackle through the left speaker but things then settle down and it seems to play fine thereafter. Is this a terminal issue or something I need to replace?
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Mike H
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Post by Mike H » Wed Nov 16, 2011 3:03 pm

Doesn't sound good! Never seen an 82 do that before. Gassy?
 
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Post by little eddy » Fri Nov 18, 2011 12:04 am

Another question.

Once I get the amp up and running, for with the BSP and IXYZ CCSs, what sort of minimum voltage should I have acrooss them? I tested them with a 9V battery so would anything ver 10 or 20V yield any sonc benefits?
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Mike H
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Post by Mike H » Fri Nov 18, 2011 12:40 am

Datasheet suggests minimum is 10V :shock:

Below that may not have reached its plateau current yet so you may be getting a bit more current than you tested for with the 9V batt, if the Voltage is higher in actual use.

As for sonic benefits, haven't a clue, only one way to find out :D

EDIT: that was for the 10M45S, not sure about the other
 
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Post by little eddy » Fri Nov 18, 2011 2:32 pm

Mike H wrote:Datasheet suggests minimum is 10V :shock:
Thanks.

When installed up to 140V, the output currents are spot on where they were with the 9V battery so it was definitely worth testing each pair. Also there are two cascaded so will probably settle on 20V total although I believe the lower regulator will do most of the work.

Also I doubt if the smaller BSP129 would require any more voltage than the 10m45s.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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