Moving Coil preamp design

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#21
Paul, you are going to have a difficult time balancing your needs with a realistic solution. About 1/3 of a century ago, we used a pair of 4401-3's to make a moving coil circuit. In my patented approach, we did NOT need either input or output caps. However there were other problems, especially with higher output mc cartridges and I moved on to fets. Low noise Toshiba fets are at least as quiet as a 4401-3 pair, BUT they requre several Ma of current to work successfully. They also usually require more voltage as well.
Back in 1973, Mark Levinson and I paralleled 4 pairs of 4401-3 parts to make the input low noise enough for an Ortofon MC cartridge like yours. We also ran about 8 ma per channel, so we had to use D cell batteries, and even they ran down, all too soon. This AA battery stuff is a big compromise, because if you STARVE the transistors, the battery will last, but the noise will go up big time! Working fets in their 'triode' region will not get you the gain that you need. I wouldn't worry too much about the leakage of a cap, just use a good one. Try to use Japanese or German caps, if you can. Cap size matters! IF you use too small a value (like 10uf) the transistors will get VERY noisy below 1kHz. Not good. I would recommend a simple 2sk170 Toshiba fet running at a few ma and even up to Idss of the part. Common source or common gate will both work. I would probably use a 10 ohm resistor with a 2sk170 fet and direct couple to the gate. I would NOT bypass the 10 ohm resistor. I would set the load resistor, wherever I get the most voltage swing. I would probably start with 6V, but 12V would be better. Maybe a rechargable gel-cell battery pack. Either a 170 bl or gr will work.
 
#22
just for , sort of , reference


2SB737 and 2SD786
 

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#23
john curl said:
.....Low noise Toshiba fets are at least as quiet as a 4401-3 pair, BUT they requre several Ma of current to work successfully. They also usually require more voltage as well.
Back in 1973, Mark Levinson and I paralleled 4 pairs of 4401-3 parts to make the input low noise enough for an Ortofon MC cartridge like yours. We also ran about 8 ma per channel, so we had to use D cell batteries, and even they ran down, all too soon. This AA battery stuff is a big compromise, because if you STARVE the transistors, the battery will last, but the noise will go up big time! Working fets in their 'triode' region will not get you the gain that you need. I wouldn't worry too much about the leakage of a cap, just use a good one. Try to use Japanese or German caps, if you can. Cap size matters! IF you use too small a value (like 10uf) the transistors will get VERY noisy below 1kHz. Not good. I would recommend a simple 2sk170 Toshiba fet running at a few ma and even up to Idss of the part. Common source or common gate will both work. I would probably use a 10 ohm resistor with a 2sk170 fet and direct couple to the gate. I would NOT bypass the 10 ohm resistor. I would set the load resistor, wherever I get the most voltage swing. I would probably start with 6V, but 12V would be better. Maybe a rechargable gel-cell battery pack. Either a 170 bl or gr will work.
Click to expand...
Hi John, thanks for the advice. I do like the idea of using a 12V Gel-cell, particularly as my other vintage motorcycle hobby means I have a number of "battery tenders" kicking around. These do not boil the life out of the rechargeable batteries.
I can see from the 2sk170 data sheet that at 5mA the noise figure is leveling off at around 0.9nV/rtHz. The Yfs at this point being around 30mS woud give around 30 ohms input R in common source, so this would also suit the Ortofon (about the same as their 32dB transformers loaded with 47k).

Paul J
 
#24
Hi,
some while ago such a headamp as John describes was presented here along with photos (however, I do not find it yet).
It was said you need a huge cap across the batteries (around 500.000uF) for good sound. I used such a thing along with a le pacific phono stage.
In that situation, the bigger the batterie was, the better was the sound (tried 0.7, 1.2 and 7.2 Ah).
Rüdiger
 
#27
Hi,

I have often wondered what the effect would be by using several 2.3V Ultracaps in series, with balancing resistors, in a location like this.

Compared with some caps I buy, these are not too expensive at about £4 each for 10 Farad units.

Maybe I should try it sometime with a battery supply.

Regards,
 
#29
Architect and engineers' specifications for Moving Coil Cartridge Amplifier

Architect and engineers' specifications for Moving Coil Cartridge Amplifier (hereafter called pre-pre)

1) The pre-pre should have balanced inputs to minimise hum pick-up.
2) The pre-pre should have FET inputs for better sound.
3) The pre-pre should not have any input caps, this rules out bipolars for the most part.
4) Noise should be as low as possible. Not that the LP is a low-noise source, on the contrary, but with lifted stylus any hiss is annoying and objectionable.
5) Power supply should work also in the weekend or after long listening sessions. This rules out batteries and rechargeable lead-acid cells.
6) All precautions should be made to avoid pick-up of Russian transmitters and other RFI sources. If you still hear mumbling voices after the record has ended consult a psychiatrist or shrink.
7) LOL
😎 😎 😎
 
#31
Talk about confusing the issue: Elso has stated a list of conditions that are both impractical and problematic.
1. Balanced input may be a good solution, BUT you always make a noisier input stage with balanced, because 2 devices must be in series to make the input stage. You always lose 6 dB, compared to using the same input devices in parallel.
2. Batteries are good, because they are clean (mostly) and they prevent ground pick-up, due to the fact that they are floating.
3. If you do make a balanced input, it will hum, unless a balanced lead from the cartridge is used with an XLR or equivalent plug is used. Single RCA's will not work.
 
#32
Have you heard about hiraga's pre-pre? I never tried but some people are very happy with this simple design but with monster power supplies (near farad or more and with plomp battery in parallel with the supply).
 

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#33
pre-pre

john curl said:
Talk about confusing the issue: Elso has stated a list of conditions that are both impractical and problematic.
1. Balanced input may be a good solution, BUT you always make a noisier input stage with balanced, because 2 devices must be in series to make the input stage. You always lose 6 dB, compared to using the same input devices in parallel.
2. Batteries are good, because they are clean (mostly) and they prevent ground pick-up, due to the fact that they are floating.
3. If you do make a balanced input, it will hum, unless a balanced lead from the cartridge is used with an XLR or equivalent plug is used. Single RCA's will not work.
Click to expand...


Hahaha, of course the tonearm cable must be rewired and not with those stinking XLRs but 2-pin Lemos.
You prefer hum over noise John? Please don't treat me as a fool! :joker:

Actually I have built such a pre-pre as specifed and it works very well.
🙄 🙄 🙄 🙄
 
#34
HI HUGOBORS,

Hiraga's pre-pre amp has a low input impedance. See my post #19.
Rabeyrolles proposed an more elaborate version of Hiraga's circuit where a bit of negative feedback enhances the input impedance. Maybe I've got the full schematics somewhere.
Another problem with Hiraga's circuit is how it must be considered :
inverting or not ? If it is considered as having a current input, it is inverting. If it is considered as having a voltage input, it is not inverting.
This is a question typical of Hiraga's circuits ambiguities.

~~~~~~ Forr

§§§
 
#35
Elso, this balanced business is mostly a myth. My Vendetta phono input is one of the quietest preamps in the world, and it is single ended input. The only thing that I have to worry about is putting a power supply next to the preamp or the wires that come from the phono cartridge.
Of course, balanced may be necessary in some applications, such as running a low level signal from a soundstage 50 meters into another room, or a remote mixing board. Also, if you have light dimmers installed and running, balanced might help. However, balanced is overrated. This I know from personal experience.
Dave Wilson is using the pre-preamp version (SCP-1) in his home reference siystem. You know, $200,000 in cost, with BIG subwoofers. If it hummed, he would certainly tell me so, and I just spoke to him yesterday.
Hum is usually caused by no external shielding, or ground loops within the chassis.
 
#36
john curl said:
Elso, this balanced business is mostly a myth. My Vendetta phono input is one of the quietest preamps in the world, and it is single ended input. The only thing that I have to worry about is putting a power supply next to the preamp or the wires that come from the phono cartridge.
Of course, balanced may be necessary in some applications, such as running a low level signal from a soundstage 50 meters into another room, or a remote mixing board. Also, if you have light dimmers installed and running, balanced might help. However, balanced is overrated. This I know from personal experience.
Dave Wilson is using the pre-preamp version (SCP-1) in his home reference siystem. You know, $200,000 in cost, with BIG subwoofers. If it hummed, he would certainly tell me so, and I just spoke to him yesterday.
Hum is usually caused by no external shielding, or ground loops within the chassis.
Click to expand...


Dave Wilson?

🙄
 
#38
hugobors said:
Have you heard about hiraga's pre-pre? I never tried but some people are very happy with this simple design but with monster power supplies (near farad or more and with plomp battery in parallel with the supply).
Click to expand...


Wow, this thread I started has certainly touched a nerve in the DIY community. I think it illustrates the point that if you don't get the amplification right at the source, the rest of the signal chain is not going to improve it.
If you look back to my first post on this, one of my criteria was to not have significant DC current in the pickup coil. The Hiraga circuit has to have perfectly matched transistors and resistors to achieve this. My Ortofon cartridge has a 3 ohm coil. Just 10mV of offset will cause 3mA of current in the coil. Compare this to the normal signal level of 13uA peak - about 200x. This can't be a good thing.
The link from Herrmann to Erno Borbely's '99 article on the use of the 2SK170 was very good. Erno did a recent article in Audio Electronics on a RIAA preamp - he sells these as kits on his web site here:
http://www.borbelyaudio.com/products.asp
He describes his new design as "Low cost starter kits" and uses his favorite complementary input with four fets, together with a DC servo. I have to admit, I do not like DC servo's. It means you a turning a low feedback design into one that has essentially infinite feedback at DC. What happens if the system overloads (eg from pops on the vinyl)? The amplifer DC point will shift and it will take several seconds to recover. This is particularly a problem on some power amplifers, where the DC blocking cap in the feedback network can only recover slowly via the relatively large feedback resistor from the output. A servo is no different - in fact often worse due to the huge gain involved from an op-amp.

Paul J
 
#40
Paul, I find that servos work for me, but you can do what you want to.
As far as hum pick-up is concerned, this is a quote from a 'Stereophile' review of the Vendetta SCP-2 phono stage using an Ortofon 3000 input stage, that has about the same output as your Ortofon cartridge. "When no disc was playing, there was a very faint, muted hiss and not a trace of hum." So much for balanced input, because the SCP-2 is single ended. Gordon Holt June 1988 p.3 in 'Stereophile archives.
 
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