Hello Folks
First post - I joined 10 years ago and every so often I end up back here for an answer - but, this time I can't find an answer. Please bare in mind I can just about remember Ohms law and understand a capacitor but when it gets all wiggly I tend to get a bit confused...
I see this topic crops up across the net and here every so often: why can't I use a mic pre-amp with my turntable? I know there are reasons why not - RIAA, impedance mis-match, capacitance mis-match, level mis-match.
I've read some discussion about the merits of pre or post pre-amp RIAA correction - but, to use the mic-pre amps I will be using post-pre amp software RIAA correction.
Requirements:
1) Impedance match from 3.7KOhms to 47KOhms;
2) Capacitance (variable?) match to present between 200-500 uF to the cartridge;
3) Balanced;
4) Possible amplification (5 mV MM Cart to 10mV Mic - What's that in db...?);
5) If amplified, then phantom powered.
The mic inputs I'm trying to drive take a minimum input of -45dBu for max output, which my 4mV cartridge will just/ almost meet according to analog devices db conversion tool - but, it's not good to drive an amp at maximum - or is it?
I first looked at this:
Phono Transfer System Bare Flat Preamp PC Boards
Following a discussion on using a mic amp for a phono cart here:
Mic preamp as phono preamp? - Gearslutz
So I assumed it was for mic level input, but it's not. It's for line level input and so negates the mic pre-amps I'm trying to make use of, uses up a couple of live level inputs I want for other things and I suspect it's rather more amp than is necessary.
I then found these:
Groove Sleuth Adaptors
At the other end of the spectrum - they're very expensive and I suspect are nothing more than an XLR with a resistor inside...
So, does anyone know of a diy project that fits the requirements of 1-5 above? Or, any other comment... 🙂
(Also, have I posted this to the best forum - there's so many on this site!)
Thanks for listening.
Morgan
First post - I joined 10 years ago and every so often I end up back here for an answer - but, this time I can't find an answer. Please bare in mind I can just about remember Ohms law and understand a capacitor but when it gets all wiggly I tend to get a bit confused...
I see this topic crops up across the net and here every so often: why can't I use a mic pre-amp with my turntable? I know there are reasons why not - RIAA, impedance mis-match, capacitance mis-match, level mis-match.
I've read some discussion about the merits of pre or post pre-amp RIAA correction - but, to use the mic-pre amps I will be using post-pre amp software RIAA correction.
Requirements:
1) Impedance match from 3.7KOhms to 47KOhms;
2) Capacitance (variable?) match to present between 200-500 uF to the cartridge;
3) Balanced;
4) Possible amplification (5 mV MM Cart to 10mV Mic - What's that in db...?);
5) If amplified, then phantom powered.
The mic inputs I'm trying to drive take a minimum input of -45dBu for max output, which my 4mV cartridge will just/ almost meet according to analog devices db conversion tool - but, it's not good to drive an amp at maximum - or is it?
I first looked at this:
Phono Transfer System Bare Flat Preamp PC Boards
Following a discussion on using a mic amp for a phono cart here:
Mic preamp as phono preamp? - Gearslutz
So I assumed it was for mic level input, but it's not. It's for line level input and so negates the mic pre-amps I'm trying to make use of, uses up a couple of live level inputs I want for other things and I suspect it's rather more amp than is necessary.
I then found these:
Groove Sleuth Adaptors
At the other end of the spectrum - they're very expensive and I suspect are nothing more than an XLR with a resistor inside...
So, does anyone know of a diy project that fits the requirements of 1-5 above? Or, any other comment... 🙂
(Also, have I posted this to the best forum - there's so many on this site!)
Thanks for listening.
Morgan
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I think there is one thing missing from your list:
6) Noise matching. The equivalent input noise current of a microphone preamplifier will typically be a bit high for a MM cartridge.
A JFET source follower (or rather a pair of them, since you want it balanced) with fairly high transconductance and with an input termination circuit could do the job reasonably well. It won't give you any voltage gain, but I don't believe you need any. The 4 mV of the cartridge is a nominal rather than a maximum voltage, and a gain just below 1 will be sufficient to ensure the noise of the microphone amplifier doesn't dominate.
6) Noise matching. The equivalent input noise current of a microphone preamplifier will typically be a bit high for a MM cartridge.
A JFET source follower (or rather a pair of them, since you want it balanced) with fairly high transconductance and with an input termination circuit could do the job reasonably well. It won't give you any voltage gain, but I don't believe you need any. The 4 mV of the cartridge is a nominal rather than a maximum voltage, and a gain just below 1 will be sufficient to ensure the noise of the microphone amplifier doesn't dominate.
This works fine and can be run from the phantom power balanced or unbalanced. For MM/MI no gain is needed and no need for more than $10 or so per channel.
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cbdb, it's not quite that simple.
You can characterize the noise of an amplifier with two equivalent input noise sources, an equivalent input noise voltage source that is in series with the amplifier input and an equivalent input noise current source that is in parallel with it. The equivalent input noise voltage of a well-designed microphone preamplifier is indeed very small, but the equivalent input noise current need not be very small. That's because the impact of the equivalent input noise current is proportional to the source impedance, and a typical microphone has a much lower impedance than a moving-magnet cartridge, especially at high audio frequencies.
That's also why I suggested using JFET source followers. A JFET with a transconductance of 5 mS or more will reduce the impedance to a level that's of the order of what microphone preamplifiers are designed for or smaller.
You can characterize the noise of an amplifier with two equivalent input noise sources, an equivalent input noise voltage source that is in series with the amplifier input and an equivalent input noise current source that is in parallel with it. The equivalent input noise voltage of a well-designed microphone preamplifier is indeed very small, but the equivalent input noise current need not be very small. That's because the impact of the equivalent input noise current is proportional to the source impedance, and a typical microphone has a much lower impedance than a moving-magnet cartridge, especially at high audio frequencies.
That's also why I suggested using JFET source followers. A JFET with a transconductance of 5 mS or more will reduce the impedance to a level that's of the order of what microphone preamplifiers are designed for or smaller.
Thanks Marcel and Scott. I, like cbdb, thought mic amps were quiet and didn't realise the complications re noise. But, it's good to know the cost is low.
Re the noise, I'm a little confused. I understand noise to be what's left once you've taken away the signal you want. The amp and cartridge will both have a noise floor - is this a question of matching the noise floors? Perhaps I'm better to ask if there's some reference material suitable for a layman?
Are there any examples of this sort of thing, or am I doing as well to explore sites like this:
Common Drain Amplifier Circuit Diagram | Equivalent Circuit
FET Common Drain: Source Follower Circuit >> Electronics Notes
https://www.electronics-tutorials.ws/amplifier/amp_3.html
?
Thanks all
M
Re the noise, I'm a little confused. I understand noise to be what's left once you've taken away the signal you want. The amp and cartridge will both have a noise floor - is this a question of matching the noise floors? Perhaps I'm better to ask if there's some reference material suitable for a layman?
Are there any examples of this sort of thing, or am I doing as well to explore sites like this:
Common Drain Amplifier Circuit Diagram | Equivalent Circuit
FET Common Drain: Source Follower Circuit >> Electronics Notes
https://www.electronics-tutorials.ws/amplifier/amp_3.html
?
Thanks all
M
Hans and Scott bouced around a few ideas here Designing a universal diff-in/diff-out Head Amp . For MM there are single ended and diff designs that can be done. On my list for some time in the future when back log is lower.
Maybe I should have used the more generic and less confusing term noise optimisation instead of noise matching. Some of the measures you can take to reduce the equivalent input noise voltage lead to an increase of the equivalent input noise current and vice versa. The idea is to come up with a circuit that has a combination of equivalent input noise voltage and current that gives good performance given the characteristics (particularly the impedance) of the signal source.
Ah, a question of obtaining the best balance of the various parameters bearing in mind that noise is a slippery fish...
On a more macro level, if I were to find what I'm looking for - and thank you Bill and Scott for the pointers - putting it close to the cartridge (eg. using it to replace the turntable breakout box) would negate any consideration of cable capacitance for the cart? But, would that then introduce such a consideration for the active buffer or mic amp - I guess not for the mic amp as it has to deal with all sorts of cable length and I suspect not for the buffer as it's mechanically a different kettle of fish to the cartridge?
(Apologies for the fish; don't know where they came from.)
On a more macro level, if I were to find what I'm looking for - and thank you Bill and Scott for the pointers - putting it close to the cartridge (eg. using it to replace the turntable breakout box) would negate any consideration of cable capacitance for the cart? But, would that then introduce such a consideration for the active buffer or mic amp - I guess not for the mic amp as it has to deal with all sorts of cable length and I suspect not for the buffer as it's mechanically a different kettle of fish to the cartridge?
(Apologies for the fish; don't know where they came from.)
You still need to load the cart (MM) but now you have control. Having everything low impedance after the TT is good for noise incursion. Those little things built into and RCA/XLR adapter still are high impedance and the cable still loads the cart.
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That's low voltage noise. For a MM preamp you need low current noise as MM cartridges are high impedance (due to the massive inductance of the winding, upto 1H or so).
Microphones typically 200 to 600 ohms, low inductance, so mic preamps are designed for a noise impedance of a few hundred ohms, and thus low current noise is seldom a priority and high bias currents are often seen.
MM cartridge around 2k + 0.5H, which at 10kHz is 30k impedance, so current noise will typically dominate for bipolar circuits.
Current noise converts to noise voltage by multiplying with the source impedace (resistive or reactive). Current noise is normally
mainly determined by the bias current (the shot-noise formula applies in many devices).
So the noise from a source is calculated as the (power) sum of 3 components:
amp's voltage noise
amp's current noise times the source impedance
Johnson noise of the resistive part of the source impedance.
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Thanks Mark and Scott - it's going to take me a little while to digest all of this. With Mark in Cambridge, England and Scott (almost) in Cambridge, Massachusetts I suspect I'm standing between people very much more educated in this area than myself - thanks for indulging me 🙂