In every configuration you can think of, a transformer is the ideal low noise "amplifier". It happens that I don't like them, in particular for cost reasons
If your preamp is suitable for MC heads moreover (low output impedance), then because you have configured the input stage as follower (hi-imp), indeed of a common base input stage (lo-imp)?
Francesco.
Cost is surely an issue. Flexibility is another issue. A transformer for MC cartridge is a bandpass filter, the high pass cutoff frequency is usually located very close to the resonant peak of the cantilever / stylus damper system. Getting a linear frequency response from the transformer / cartridge system can be challenging.
But if this preamp is not for MM neither for MC, anybody can explain me clearly for what other purpose?
Ciao a tutti, Francesco.
syn08,
I`ve seen your approach as a lesson in how not to design a MC amp. Just looking at it makes me feel bad. Since nothing is done right, discard the whole idea and start from scratch.
I`ve already suggested Hiraga`s design to you. I assure you, it would not be easy for anybody to come up with something better. No ugly ICs, grounded base input topology, simplicity. Battery supply is a must, otherwise talking about noise is pretty meaningless.
I`ve seen your approach as a lesson in how not to design a MC amp. Just looking at it makes me feel bad. Since nothing is done right, discard the whole idea and start from scratch.
I`ve already suggested Hiraga`s design to you. I assure you, it would not be easy for anybody to come up with something better. No ugly ICs, grounded base input topology, simplicity. Battery supply is a must, otherwise talking about noise is pretty meaningless.
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Grapes are sour, aren't they?
You did not show over the years on this forum any finalized substantial constructions, but rather only a bunch of unsupported (if not flat wrong) assumptions and some half baked ideas without any analysis or context.
You also did not present on this forum any theoretical contributions of any value, other than anecdotic, and based on an obvious lack of understanding of physics and EE fundamentals.
Under these circumstances, you are not qualified for anything other than simple opinions. If you appreciate that ancient chunk of circuit, just build it and share your results.
"The Howland current pump noise is anything but low, and it's not about the opamp noise, but about the matched resistors network."
Dont write this off. If you are feeding it from well regulated supplies and using a good low noise op-amp the noise performance is very good - I've used this in low noise instrumentation circuits where I had to measure down to uV - th e HCP was thew least of my worries. Wrt the resistors . . . 4 x 0.01% is easy - just take a look in the digi-key catalog. Better still, you can probably get some as samples.
Dont write this off. If you are feeding it from well regulated supplies and using a good low noise op-amp the noise performance is very good - I've used this in low noise instrumentation circuits where I had to measure down to uV - th e HCP was thew least of my worries. Wrt the resistors . . . 4 x 0.01% is easy - just take a look in the digi-key catalog. Better still, you can probably get some as samples.
"I`ve seen your approach as a lesson in how not to design a MC amp. Just looking at it makes me feel bad. Since nothing is done right, discard the whole idea and start from scratch."
Lumbar, show us some of your stuff - pictures and measurements, and then let us have an open discussion about it.
Lumbar, show us some of your stuff - pictures and measurements, and then let us have an open discussion about it.
Actually one idea that I am further contemplating is a floating input stage, similar to prof. Leach circuit. I would certainly use common base in such a circuit. The big advantage of a floating configuration is that you can get zero MC cartridge current, even with unmatched bipolars, since the MC cartridge is not referred to ground. It's not that simple, though
Here's an alpha (that is, in the idea/exploration stage) version example of such a circuit.
Purists can be happy; it's open loop and hence has some 0.1% distortions for a typical MC input
What I like in this approach:
- Cartridge current is zero, independent of the bias currents.
- Because of this, the devices currents can be optimally chosen, for minimum noise Ic=Vt*SQRT(Beta)/Rsource. Therefore, noise can be made extremely low, without paralleling devices and balancing currents.
- Gain can be adjusted via R7, making the MC/MM switching very simple.
What I don't like:
- Large electrolytics in the signal paths. Not because their "sound" but because I am not sure how their noise (due to natural vibrations) is going to impact the overall performance.
- Large distortions. I am not happy with 0.1%
- Low input impedance (here, about 300ohm). I was never able to find an authoritative reference about the effect of input impedance on MC/MM cartridges. All I know is that if the input impedance is low, you won't be able to always follow the cartridge manufacturer's recommendations (some still recommend 47k in parallel with some cap). You can make a high impedance low without impacting the noise, but not the other way around.
Lots of work to optimize such an approach...
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X type circuits can free you from Ib considerations but I think they will make you compromise many aspects of the approaches you usually follow. Then again if it is to be a capacitor, better use a teflon one at the input if afraid of an input stage failure frying some treasured cart, while retaining your current design approaches. With 300 Ohm Rin you will not be happy with higher than 15 Ohm Rs carts also. In my experience, the load resistors act as current termination dampers at the end of the cable transmission. Beyond the obvious signal level retaining 10:1-10:20 ratio base they just work less and less towards high frequencies and the carts come across as thinner. Manufacturers keep it at 47k because of transformers for MC to MM specified there, and for keeping the carts voltage output at max if directly connected MM with around 1k and more Rs. If you use a 100k trimmer Rin and a test disc with pink noise bands you will be very interested on what you will see for optimum load and capacitance on FR RTA at the phono out. Riaa tolerance considerations are dwarfed before what just typical non optimized loading can do on MM. Averagely needed loading is much higher than 47k and capacitance much lower than average 300pF for MM. For low inductance MC carts I recommend you to check out listening with a 16:1 Rl/Rs ratio and no extra capacitance than that of the TT cable's own. Easy to instantly compare loading with the dip switches you got there. Nice to see your V4 phono. Textbook electronics work as always. Congratulations.
I do not want to interfer with your thread but my MPP topology answers a lot of questions i have seen here. It is balanced, parallel symmetric and has a theoretical noise of 0.5nVqHz with MAT03/MAT02. It dumps very little current into the cardridge because PNP and NPN transistors are used on both inputs in equal measure. It uses a common base, transimpedance stage but is much more advanced then the Hiraga or Leach in terms of distortion, speed and powersupply rejection. I have a lot of experience with transimpedance stages and their much lower input impedance dating back to the early 80th where i worked directly with Hiraga and also produced my own transimpedance headamp. If anybody is interested i can supply some answers based on long experience. One little thing : i do not think that you can damp the movement of the cardridge even when you shortcircuit it. There is only microcurrent flowing that is simply not strong enough to clamp the cardridge.
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