Snail Phono Pre

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little eddy
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#166

Post by little eddy »

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.
little eddy
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#167

Post by little eddy »

So the good news:

1. Finished the amp with some temporary load resistors on the phono stage
2. DC voltages seem to be in the expected range
3. When put in my system, intially sounded promising

Then:
1. Both channels seemd to fade in and out cyclically every 3 to 4 seconds
2. When I lifeted the needle, a low level 'thud' or pulse could be heard, probably on the same timebase as the channel going silent
3. When I switched the input selector to another input, no sound of the 'pulse'
4. If I then returned to phono after a while, the 'thud had gone and the phono stage seemed to be working again fine for say another 2 to 5 minutes
5. Back to item 1

Please, please, please, please can one of you kind gentlement help and enlighten me on the probable cause and possible remedy?.
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Phono stage 1.jpg
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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#168

Post by little eddy »

Latest schematice with DC voltages attached.

So reading MJ, I guess this is blocking and it is caused by the input signal to the second stage rising to more than Vgk????

MJ doesn't go into how to solve it but I have some ideas:

1. Increase Vgk to 1.5V from 1.2V by increasing the current (will be done once CCS gets installed)
2. Increase the Rk of stage 2, or a combination of 1 & 2.
3. Try a 5751 with lower gain in the 1st stage but this would throw out the RIAA
4. Alter the RIAA configuration so I get some attenualtion through the series resistor
5. If I have sufficient supply voltage (which I may not have), what about DC coupling. If I use a 5751 in place of the current ecc83 then Va could fall by 20V.

Any other ideas?

Might this be caused by my choice of temporary load resistors and if so, might when I get the CCSs installed this should/might go away?
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11-11-27 Snail 2-stage with single RIAA 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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pre65
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#169

Post by pre65 »

Have you still got the volume pot in the "wrong" location ?
The only thing necessary for the triumph of evil is for good men to do nothing.

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little eddy
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#170

Post by little eddy »

Wrong! It's still after the line stage if that's what you mean but no different to what was heard at Owston without any issues. I am assuming that the CD input level then was higher there than I have with the phono stage connected.
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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#171

Post by Mike H »

Sounds like it wants to oscillate, and playing a record kicks it off. Or could be mains Voltage fluctuation.

Due to the low frequency, suspect it's feedback in the phono stages via the supply line, add in the overall high gain of both phono stages plus possibly the line amp, and off it goes.

Further suspect it's because you don't have a RC filter in the phono supply line between the 1st stage (ECC83) 47k and 2nd stage (6072) 36k anode resistors, and that's where it's occuring. Personally I might consider making the cap of the RC 100uF, e.g. my one uses 6.8k & 100uF, which both front-end stages share (just by the way). And just for good measure 250V of Zeners is used as well so the output doesn't wobble up and down too much when mains fluctuations occur.

HTH?
 
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little eddy
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#172

Post by little eddy »

OK so Phil's got me thinking as to which stage might be blocking (if that's what happening), as I had simply assumed the second.

With the second 6072a stage having a mu of 12.5, it may be that it could be the line stage that is sufferning from blocking because the 4V Vk is lower by proportion to the 1.2V on the 6072a itself.

What I might try is that with there being a phono out, wire this via one of the line stage inputs that has a series 910k resistor. If the amp 'blocks' less then my issue is with the line stage.

If it turns out that this is what is happening, obviously it would have been an advantage of having the pot before the gain stage. Maybe amore extensive rewire might have to be considered if there's no other way round it.
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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#173

Post by little eddy »

Phew.

Having a series 910k resistor between phono and line stage seems to have sorted it. Listened for best part of 2 hours without it exhibiting any glitches with some fairly loud pops on some older records.
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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#174

Post by Mike H »

Define "blocking" ?
 
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Mike H
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#175

Post by Mike H »

Oh wait you mean it's driving the line amp to clipping? Well I guess it could.

So you could increase the value of what is now the 65k resistor to compensate?
 
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little eddy
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#176

Post by little eddy »

Mike H wrote:Define "blocking" ?
Not clipping.

The input voltage swing or negative peak level to the valve is larger than the cathode voltage. I think it is page 190 in my version of MJ who maybe explains it better but it is then the interreaction with the dc-decoupling cap.

I thought I understood it until I tried to write an explanation!

In my instance, the newly inserted series 910k resistor must limit the input signal peaks so that a bias voltage always appears across the valve gate/cathode. That said, with Rg of 910k, I probably am getting less than a half of the phono output signal getting to the line stage.
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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#177

Post by Mike H »

Oh right gotcha.

So really amounts to the same problem, input signal is too large :D
 
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little eddy
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#178

Post by little eddy »

Mike H wrote:So really amounts to the same problem, input signal is too large :D
Yes but I am surprised that I have enough gain from the phono section to cause a problem. I had anticipated that I might be able to attenuate the CD input so the two inputs were fairly balanced. This may still be possible but the CCSs on the phono will only increase the gain further. Hopefully the 910k attenuator is OTT and I won't have to increase it further.

If I can't balance the overall amp input levels then a rewire of the POT might be the best solution. Let's see how it goes. Parts arrived for the phono CCSs today so should be in circuit for the weekend together with a PSU choke allowing me a little more supply voltage.
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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#179

Post by Mike H »

Ah well there you go, does the phono amp really need any more CCS's :D
 
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little eddy
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#180

Post by little eddy »

So now fitted the phono stage CCSs and been up and running with the chance of listening for a few hours. Initial impressions are favourable.

On some recordings the balance is good but on others, there would seem to be too much bass both with the original Mullard ECC83 and now GE 5751 in the first position.

EDIT: The 296k R1 of the RIAA was calculated with ECC83 Ra of 65k and CCS Rl of 300k. This also seemed to work OK when MikeH kindly simulated it.

But if I assume an Ra of 80k and 1M for the CCS in the calculations then R1 would fall to circa 224k. This should give an additional 2dB or greater above say 500Hz so this is what I will initially try and see whether directionally it helps in terms of the overall balance.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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