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Phono Preamp Pt2: MC
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Fatmangolf View Drop Down
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Points: 8998 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jul 2013 at 7:31am
Thank you for sharing this Graham.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Points: 2020 		Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jul 2013 at 7:56am
Such insights are fascinating and informative, particularly with Grahams explanations of the why and how.
There was a time when it seemed everything was built with BC107, 108 & 109s. I see 109s are still in production, but at over £1 each for small quants.
Looking forward to the next episode, before I warm up the soldering iron.
Many Thanks Graham
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Points: 16298 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Jul 2013 at 12:40am
Chucked the circuit onto a sliver of scrap strip board earlier: awful soldering; ridiculously long component leads, no voltage regulator - long length of "fig.8" wire from a bench power supply, and no on-board decoupling whatsoever... I should be ashamed of myself, but it's been a busy day and I only gave it half an hour's work.

Also, the biasing was way out at 7V DC on the output stage instead of the 9V DC approx. Must try harder!

It measured thus:

THD+N: 0.02%

EIN: -110dB

Forgot to measure frequency response Embarrassed

Gain: 23.5dB

The noise is the worry. -120dB and I'd have been happy because lead trim and a proper voltage regulator plus proper shielding would have taken it closer to -130dB (which is b***** excellent), but I'm not sure it will go down 20dB from -110dB.

Will have another play....
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Points: 16298 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Jul 2013 at 1:42am
I'm back with the missing measurement...

Frequency response: ? to 120kHz

The ? stands for don't know at the bass end... the Audio Precision only goes down to 9Hz and it was still flat so I expect the lowest frequency is the 1Hz designed value.

120kHz for the high frequency end was exactly as predicted by design.

Back to noise and if you flick it into A weighting you get -121dB equivalent input noise. I definitely want -130dB if it's A weighted. I'm happy to accept -122dB measured 22Hz to 22kHz quasi-peak because that is a professional measurement and no other domestic audio manufacturer I know uses that - they all tend to go for the "boast factor" A weighting (which often hides the truth).

I have another 11dB to find somehow and I don't think shielding will get me there. It's obviously circuit noise and it could be down to the low frequency extension - noise increases rapidly at very low frequencies.

I'll be back when I get the noise down.

Graham

PS: forgot to mention I gave it 470uF of power supply decoupling this time around.


Edited by Graham Slee - 18 Jul 2013 at 1:43am
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Points: 16298 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Jul 2013 at 3:48am
Latest update:

Awful soldering apart the circuit works and works well. There's only one real problem, the noise.

Having been here many times before I cannot believe substituting a lower noise transistor will bring any improvement, nor will any amount of shielding. The circuit does at best -121dB A wtd. equivalent input noise and that puts the S/N ratio at 54dB for a 0.35mV MC cartridge - 2dB short of the target 56dB. Remember, this is a medium output MC - for the lower outputs in the 0.1 - 0.2mV region it will be noisy enough to hear.

This circuit, like many others I've tried from around the internet, is a non-starter.

Remember that this circuit has 40dB of negative feedback. Without it the noise would be incredibly loud. I'm reminding you here about those claims of no negative feedback so you'll know that it's all lies.

I'm sure the noise would be as Mr Elliot claims for its original 30 volt supply simply because that ups the open-loop gain - increasing negative feedback which reduces noise.

We can't do 30V here because the Genera supply is 18V regulated. We'd need a more complicated circuit to achieve what we want - possibly a better current source than the simple capacitor bootstrap.

I will update this topic when I figure out a design which does what's required.

Graham
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Points: 16298 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 18 Jul 2013 at 11:13am
Looking back at the circuit diagram above there is a way to increase gain further and with the existing negative feedback (NFB) it may take the noise lower. It's done by bypassing R3 with a capacitor so there's no NFB to Q1 base via R5. But will it make any difference? I doubt it - after all, it's not going to make Q1's open loop gain any larger than it already is.

The job probably needs both transistors operating with gain - Q2 is just a voltage follower or "buffer" - it has no voltage gain.
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Points: 8998 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 18 Jul 2013 at 10:16pm
When you're happy Graham I'll be breadboarding the circuit and trying it with my DL-103.

This article explains a lot, thank you, and I think it shows the technical and musical excellence of the Elevator EXP.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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