Moving Coil preamp design

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#41
Paul and others

Pauls request for minimal (almost zero) DC-current through the coils rules out a lot of circuits. I wonder how important this is in general for moving coil preamps. What are the consequences of a little DC-current through the coils? What levels would be critical?
DC-current seems not to have been an issue when the MC-part of the Threshold SL-10 preamp was designed. According to the schematic below quite a bit of current is injected to the MC-cartridge by the SL-10:

http://www.passlabs.com/np/SL10 SCH.tif

I have no knowledge on the subject but I am interested to know about this. I might be building an moving coil headamp myself some time. I tend toward a jfet circuit as suggested by John Curl to Paul but I would like to build one or two more circuits for comparison.

Torben Herrmann
 
#42
Originally posted by john curl



Elso, this balanced business is mostly a myth.

``````````````````````````````````````````````

HiFi news & Record review did their schtic on a balanced

output configuration turntable setup a few years back,

i suspect this to be an large improvment, as they reported.

````````````````````````````````````````
Dave Wilson is using ... , with BIG subwoofers.

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http://www.madisound.com/cgi-bin/index.cgi?cart_id=3950703.18665&pid=99
 
#43
tomtt said:
Originally posted by john curl



Elso, this balanced business is mostly a myth.

``````````````````````````````````````````````

HiFi news & Record review did their schtic on a balanced

output configuration turntable setup a few years back,

i suspect this to be an large improvment, as they reported.

````````````````````````````````````````
Dave Wilson is using ... , with BIG subwoofers.

````````````````````````````````````````

http://www.madisound.com/cgi-bin/index.cgi?cart_id=3950703.18665&pid=99
Click to expand...

Hi I am using Klipschorns.
I think hum-pickup is a serious issue with the very weak signal of the moving coil cartridge. I already moved the transformer of the player to the outher world.
I don't believe a word from those Journals whether it be Hifi News & records or Stereo Pile.

A good friend of mine has Wilson Grand Slams. I am not impressed! This friend likes my pre-pre combined with phonostage though. It outperforms the Krell KRC. Now I must offer this thingy to one of those journals for a in depth test, don't you think so?
:clown:
 
#44
Hum can indeed be a problem. But once you get it to work, there's nothing that can touch step-up transformers and a balanced circuit. (It has worked for low-output microphones for more than half a century.) And the transformers are crucial. You need something like the Telefunken Ela balancing transformer, as it's known in pro audio. In our world, it's simply a step-up transformer. It makes the Shindo Arome sound like dirt. It all comes down to the low noise-floor of vinyl.

When did the impossible ever stop a DIYer?
 
#46
Originally posted by Elso Kwak


Now I must offer this thingy to one of those journals for a in depth test, don't you think so?
``````````````````````````````````````

indeed i do!

pick an obscure latin word for the Name,

reviewers love this/that sort of thing
 
#47
Folks, I don't care whether you want to do balanced input or not. Lemos are expensive, and balanced input stages will generate more hiss than an equivalent single ended stage made of the same parts.
Here we have some person, who started this thread, trying to make a successful pre-preamp with an Ortofon, low output MC cartridge. All kinds of input stages have been discussed that are more noisy than any pre-preamp that I have designed for the last 1/3 century. Most of these designs will be unsuccessful with this phono cartridge, because they are so noisy.
I mention 1/3 of century, because that was when I seriously approached this problem with Mark Levinson, and guess what the cartridge we used was? YES, Ortofon! I had been using one since 1966. I was 'shown the door' at Ortofon headquarters in Denmark, in 1967, when I mentioned that I was going to work on a MC pre-preamp. They declared it impossible at the time.
So, in 1973, I was very familiar with the Ortofon, and we found that we had to parallel 8, yes 8! 2N4401'S and 3'S to get the hiss low enough. What are you folks avocating? 1-2 parts?
Now what about hum? Hum was NOT a problem with the Levinson JC-1 any more than the Vendetta SCP-2 designed 15 years later, and they are BOTH single ended input. What is the problem with single ended then? Well, bad layout (ground loops) and nearby transformers will cause hum. This is still not the fault of single ended input, except that balanced does tend to 'bulletproof' your design from external pickup. But then it will hiss more. You chose!
 
#48
system interaction may well be a base for different

results, and most likely is.

my own hearing changes, though significant resolution

gains are attained with caffeine.

now john has been correct, early and often, for

many years.

much like the 'bible codes', where significant people,

places, things, and events are found in the Bible,

http://www.google.com/search?hl=en&q=bible+code

john's name is found in Maxell's Equations-

http://stereophile.com/reference/1095/index6.html

you might have to type in the '/1095/index6.html'
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
none of this is to be taken with out a bucket of salt,

except for the part about caffeine
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
#49
john curl said:
Folks, I don't care whether you want to do balanced input or not. Lemos are expensive, and balanced input stages will generate more hiss than an equivalent single ended stage made of the same parts.
Here we have some person, who started this thread, trying to make a successful pre-preamp with an Ortofon, low output MC cartridge. All kinds of input stages have been discussed that are more noisy than any pre-preamp that I have designed for the last 1/3 century. Most of these designs will be unsuccessful with this phono cartridge, because they are so noisy....

Click to expand...

Hi John, funnily enough, what made me start this thread was that since moving house, I have been unable to find my Ortofon MCA76 moving coil preamp! Since this design is also 1/3 century old, I thought I would see if I could do better. Ortofon claim that the unit has a noise only 5dB higher than the theoretical minimum (it uses multiple transistors, each connected to the cartridge via 1,000uF caps!). The actual spec is -69dB rel to output at 10cm/sec. A typical marketing ploy using a reference twice the normal 5cm/sec. Still, that is impressive, equating to an equivalent input noise of around 64nV, or 0.45nV/rtHz in a 20kHz bandwidth. If I can find it I might test it and see if it is even close to this. I have also ordered 50 2sk170 transistors so I can select four to use in a paralled single ended preamp, along the lines of what you suggested, and in the Erno Borbely article, using a rechargeable battery. This arrangement should meet my requirements for noise, low bias current in the cartridge and no input coupling cap. It will still need a resistor on the input for damping, but at least I can choose the value, and not have it dictated by the amplifier topology.
Paul J
 
#50
Hum? Hiss?

john curl said:
Folks, I don't care whether you want to do balanced input or not. Lemos are expensive, and balanced input stages will generate more hiss than an equivalent single ended stage made of the same parts.
Here we have some person, who started this thread, trying to make a successful pre-preamp with an Ortofon, low output MC cartridge. All kinds of input stages have been discussed that are more noisy than any pre-preamp that I have designed for the last 1/3 century. Most of these designs will be unsuccessful with this phono cartridge, because they are so noisy.
I mention 1/3 of century, because that was when I seriously approached this problem with Mark Levinson, and guess what the cartridge we used was? YES, Ortofon! I had been using one since 1966. I was 'shown the door' at Ortofon headquarters in Denmark, in 1967, when I mentioned that I was going to work on a MC pre-preamp. They declared it impossible at the time.
So, in 1973, I was very familiar with the Ortofon, and we found that we had to parallel 8, yes 8! 2N4401'S and 3'S to get the hiss low enough. What are you folks avocating? 1-2 parts?
Now what about hum? Hum was NOT a problem with the Levinson JC-1 any more than the Vendetta SCP-2 designed 15 years later, and they are BOTH single ended input. What is the problem with single ended then? Well, bad layout (ground loops) and nearby transformers will cause hum. This is still not the fault of single ended input, except that balanced does tend to 'bulletproof' your design from external pickup. But then it will hiss more. You chose!
Click to expand...

John, exactly the hum problems with the Mark Levinson JC1-AC prompted me to try balanced input MC cartridge preamplifier. I don't have a hiss problem now. (heavy modified schematic from the MAT04 datasheet, cross-quad)


😎
 
#51
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...


I used the Hiraga pre-pre with an Ortofon SPU GME and was pleased with it. The problem is the potential for offset voltage at the emitter of the input transistor to flow through the cart.

I think I've figured out how to prevent it, but haven't tested the circuit yet. (involves changing the current mirror to bipolar voltage, and adding a large cap (not the best, but...) from the base of the input transistor to ground. This way, the supply floats, and current shouldn't flow through the cart).

I'm used LM394ch's, and the noise was extremely low. The sound was better than the T-20 stepup tranny I'm now using.
 
#52
Based on Hiraga's common base input, this is an elaborate version with negative feedback due to Serge Rabeyrolles.

~~~~~ Forr

§§§
 

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#55
I don't understand why people want to use common base, as it massively raises the input offset currents, but has no noise or linearity advantages in the limit of low impedance sources - which is the one that applies for MC.

The circuit also has a massive defect in its noise performance - the high values in the feedback chain (2K and 120) give masses of Johnson noise. With this circuit, there is no point using low rbb transistors, as the noise is dominated by the feedback network.

If you want half way respectable noise performance, you must use low impedances in the feedback net - which aren't possible in this design.
 
#57
It has been a while since I started this thread, so an update is in order!
The circuit I ended up building was published in AudioXpress magazine in Sep '06 by Paul Rossiter. They have a PCB available for it here:
http://www.audioxpress.com/bksprods/PCBCTB.htm

This design essentially created a lower distortion version of the Leach preamp, by using complementary transistors. I used two 12V gel rechargeable batteries, as unfortunately, the floating battery topology prevents the use of one battery to feed two channels. When not in use, the battery is kept topped off by a Battery Tender, with a 4 pole 2W switch to turn on the unit and remove the battery tender. The original unit ran off 9V so I had to modify the bias to run on 12V.
The noise level is lower than the (very expensive) Ortofon T30 transformer, both hum and hiss, when using my Ortofon MC20 Mk II cartridge.
The following RIAA preamp is a Hagerman Bugle and power supply:
http://www.hagtech.com/bugle.html
using AD8599 opamps in place of the OPA2130's originally specified.
I am pleased with the sound quality this combination makes.

Paul Joyce
 
#58
Well, since there's no PC boards available for the phonoclone (always a day late and $1 short here), perhaps the Paul Rossiter design might do...but I can't find any info on it. I'd love to see a schematic before I commit myself to building it by buying the boards.

Is there anywhere where I can check out the schematic without buying a year's worth of the magazine?
 
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