Except that a transformer does NOT provide noise free gain at all!
Consider a source, say 1mV @150 ohms (Fairly typical of most mics out there), now if I use say a 1:4 voltage ratio transformer the output will be 4mV, but the impedance seen by the gain stage input will now be a factor of 16 times higher, thus the noise contribution from the input current noise has gone up by a factor of 4 (relative to the now increased output voltage) even as the voltage noise contribution has fallen by a factor of 4....
Now if the current noise started out very much smaller then the voltage noise (Say because you were using a jfet input opamp with a low source impedance) then the transformer might result in lower overall noise, but that is not a given if the input was say a large geometry bipolar with low voltage noise but appreciable current noise.
It is this tradeoff between current and voltage noise that made the input transformer more or less ubiquitous in valve preamps (Short of grounded grid, never popular at audio) they could not get the noise impedance to match reasonably well any other way.
All of this stuff becomes a far more pressing concern at RF where sometimes a preamps input refered noise is no more then a fraction of a dB above the theoretical noise floor.
Regards, Dan.
Consider a source, say 1mV @150 ohms (Fairly typical of most mics out there), now if I use say a 1:4 voltage ratio transformer the output will be 4mV, but the impedance seen by the gain stage input will now be a factor of 16 times higher, thus the noise contribution from the input current noise has gone up by a factor of 4 (relative to the now increased output voltage) even as the voltage noise contribution has fallen by a factor of 4....
Now if the current noise started out very much smaller then the voltage noise (Say because you were using a jfet input opamp with a low source impedance) then the transformer might result in lower overall noise, but that is not a given if the input was say a large geometry bipolar with low voltage noise but appreciable current noise.
It is this tradeoff between current and voltage noise that made the input transformer more or less ubiquitous in valve preamps (Short of grounded grid, never popular at audio) they could not get the noise impedance to match reasonably well any other way.
All of this stuff becomes a far more pressing concern at RF where sometimes a preamps input refered noise is no more then a fraction of a dB above the theoretical noise floor.
Regards, Dan.
Yeah, what ever.
http://www.analog.com/library/analogdialogue/archives/39-05/web_ch6_final_i.pdf
page 6.5 seems to be relevant.
This is interesting and useful info. Thanks.
I have two projects I'm trying to reign in to the point of feasibility.
I understand the consideration of rn = en/in for op amps and how the AD797 is on the short list for solid state op amp mic preamps.
I had no idea how to begin thinking about this for tubes. There are a lot fewer choices there, and it seems like circuit topology is where the options might lie.
1) Is there a particular tube mic preamp design philosophy that tends toward lower noise, and is it safe to assume the preamp plays a larger role than the power supply for minimizing noise? I look at tube preamps as being less sensitive to power supplies for noise than solid state, but I'm not sure I could win any bets about this.
2) A op amp charge preamp for a sensor with <20Mohm impedance. Is the rn = en/in factor meaningful here as it is for matching op amps with input impedance? If I look at JFET op amps, there are some with low en (density), and many with extremely low in (density), leading to rn on the order of 0.5-5 Megohms...does it matter for such large impedance sources?
Thank you
Murray
Holland, MI
1) Assuming the tubes are working as single ended Class A devices, and nothing extraordinary has been done to improve their PSRR, then no, they have terrible PSRR compared to solid state opamps. Also, single ended transistors are just as bad when it comes to PSRR. You have to do some fancy things to make single ended Class A circuits have a better PSRR.
As for low noise tube preamps, noise is typically proportional to gain, so it's tough to make a low noise tube preamp with high gain. Thus, input transformers with high turns ratio were used to lower noise. The ones available now don't usually work well into a vacuum tube, but Sowter makes some that do:
SOWTER VINTAGE AUDIO TRANSFORMERS
Other manufacturers may also make compatible input transformers.
Even worse than the proverbial free lunch...I don't even know what to order!
What constitutes 'extraordinary' in your first comment about Single-ended class A tubes?
I'm looking at the TubeCad Aikido Cascode circuit configured as a mic preamp. It seems to accomplish some reasonable enhancement of PSRR without drastic complexity.
Thank you
What constitutes 'extraordinary' in your first comment about Single-ended class A tubes?
I'm looking at the TubeCad Aikido Cascode circuit configured as a mic preamp. It seems to accomplish some reasonable enhancement of PSRR without drastic complexity.
Thank you
+24 won't be a problem for your balanced output but will give you grief in unbalanced as that level is just shy of 35 Vp-p.
I've been using a homebuilt mic preamp since '82. With my AKG C451 condensers I find +46dB of gain rarely causes problems. The ambient noise is well over the noise floor of the preamp and you'd have to be recording a brass band in a small room to overload it (did that).
If you're using it with a PC you will also want an output level control in addition to the gain control. They are NOT the same thing. PC inputs overload at fairly low levels and can be damaged by load inputs (did that too). I would also be wise to put hard limiting on the output so that regardless of setting you can not damage the PC input. A small flock of 1N4148s stacked and both polarities would do the trick
What I use is similar to Rod Elliots project 66.
Low Noise Balanced Microphone Preamp
G²
I've been using a homebuilt mic preamp since '82. With my AKG C451 condensers I find +46dB of gain rarely causes problems. The ambient noise is well over the noise floor of the preamp and you'd have to be recording a brass band in a small room to overload it (did that).
If you're using it with a PC you will also want an output level control in addition to the gain control. They are NOT the same thing. PC inputs overload at fairly low levels and can be damaged by load inputs (did that too). I would also be wise to put hard limiting on the output so that regardless of setting you can not damage the PC input. A small flock of 1N4148s stacked and both polarities would do the trick
What I use is similar to Rod Elliots project 66.
Low Noise Balanced Microphone Preamp
G²
I would recommend that you test it, either in real life or in a simulator. Without some numbers, it's hard to say if the PSRR is any good for this cascode. I don't generally use cascodes so I can't speak to their PSRR directly, but the 30k resistor at the top of the one shown wouldn't do much to prevent power supply ripple from invading the tube's plate.
"Extraordinary" for me means extra RC filters and CCS devices to isolate the power supply from the circuit.
Hi,
Not really.
The lowest noise tube devices I know of are often relatively high gain and, more importantly, high gm ones.
As an example, the D3A penthode sports about the lowest Req of any valve I know of and pretty high gm too.
The problem is that most of the data given by manufacturers about intrinsic noise (if we can call it that) is derived from VHF/UHF measurements.
Going from my experience I'd say this is a good indication of how noisy a valve would be at LF frequencies but much depends.
In general, if low noise is your main target, dive in a series of TFK Lab books.
Although most nuggets have been dug up, you'll find that the lowest noise triodes (single or twin) are invariably of the frame grid VHF type.
Makes sense, doesn't it?
BTW, the ubiquitous over used 6DJ8 also falls into that category and is reasonably noise free.
It's what I'd use for a mike and that's what I'll use for a MC headamp. Anything else is PS supply related and noise cancellation trickery....
So, can I count on all of you guys to help me design the Ultimate Phono
MC preamp?
Not even a need for 48VDC phantom supplies there......
Ciao,
Not really.
The lowest noise tube devices I know of are often relatively high gain and, more importantly, high gm ones.
As an example, the D3A penthode sports about the lowest Req of any valve I know of and pretty high gm too.
The problem is that most of the data given by manufacturers about intrinsic noise (if we can call it that) is derived from VHF/UHF measurements.
Going from my experience I'd say this is a good indication of how noisy a valve would be at LF frequencies but much depends.
In general, if low noise is your main target, dive in a series of TFK Lab books.
Although most nuggets have been dug up, you'll find that the lowest noise triodes (single or twin) are invariably of the frame grid VHF type.
Makes sense, doesn't it?
BTW, the ubiquitous over used 6DJ8 also falls into that category and is reasonably noise free.
It's what I'd use for a mike and that's what I'll use for a MC headamp. Anything else is PS supply related and noise cancellation trickery....
So, can I count on all of you guys to help me design the Ultimate Phono
MC preamp?
Not even a need for 48VDC phantom supplies there......
Ciao,
Thanks all. I think I need to build some things and try them...
TFK? I figured out this is something related to MIT Radiation Lab Series...not sure what TFK stands for.
I have one volume of MRLS somewhere in the basement...one that has some oft-referenced circuits, but I need to do some review...it's not exactly the Mims Vacuum-Tube Cookbook for readability if one hasn't kept the thermionic theory skills sharp.
TFK? I figured out this is something related to MIT Radiation Lab Series...not sure what TFK stands for.
I have one volume of MRLS somewhere in the basement...one that has some oft-referenced circuits, but I need to do some review...it's not exactly the Mims Vacuum-Tube Cookbook for readability if one hasn't kept the thermionic theory skills sharp.
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Hi,
@Dirkwright: Have a look in the "Articles" section of the forum and search for SY's preamp. It uses a D3a.
It's not that rare in Europe but it ain't exactly cheap either.
Regarding the MC stage, here's a link to where it started, got high-jacked by the SS brigade and subsequently ended up a tiny bit orphaned:
http://www.diyaudio.com/forums/analogue-source/252588-mc-head-amp-stage-s-using-tubes.html
I'll describe my ideas in there later tonight together with was is needed to upgrade the existing circuit etc.
Ciao and thanks for offering assistance,
@Dirkwright: Have a look in the "Articles" section of the forum and search for SY's preamp. It uses a D3a.
It's not that rare in Europe but it ain't exactly cheap either.
Regarding the MC stage, here's a link to where it started, got high-jacked by the SS brigade and subsequently ended up a tiny bit orphaned:
http://www.diyaudio.com/forums/analogue-source/252588-mc-head-amp-stage-s-using-tubes.html
I'll describe my ideas in there later tonight together with was is needed to upgrade the existing circuit etc.
Ciao and thanks for offering assistance,
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