There's hundreds of audio preamps around that just demonstrate the contrary. Telefunken V72, Neve 1073 are among those. You are presenting a situation where changing feedback pushes the closed-loop system into unstability or makes it run out of juice. Between these extremes areas there is a nice 30-40dB space for varying gain.Svart said:
Gain structure in a cascaded-gain microphone preamplifier
- Thread starter jdbakker
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Since I had not seen this preliminary schematic, I was speaking generally. Anyway, this is just confirming I'm right. You need 26dB gain to achieve 1.8dB NF.jdbakker said:
I was just assuming that you were aiming at superlative performance in every aspect. You have put so much importance in THD/operating level/sonic signature, that I had assumed you were looking at the same level of excellence on what is, according to most audio designers, the paramount performance aspect of a mic pre, noise. Yet, I agree with you, in practical terms, what is 0.9dB? But then, what is the 0.001% difference in THD or IMD? .
Again, since I had not seen the preliminary diagram, I was speaking generally. My comments are based on the block diagram you posted. And you seem so concerned about second-order of magnitude phenomena that I cannot help mentioning that the variation in frequency response between a 10k purely resistive load and a 600 ohm reactive load shall be more significant than some secondary effects of varying NFB.
jdbakker
Well-known member
Svart said:
A very good question.
I want a mic pre with better low-gain noise performance than the other transformerless designs around here. As Abbey Rd and others have pointed out, that means (amongst other things) having a low gain-setting resistor. However, if the gain resistor is 33R at 26dB gain it would have to be <1R at higher gains, and that is hard to do well, especially when the gain resistor is off-board. This was the driving force to look for different plans than the well-known Green/Cohen/Transamp/Valley People configuration.
I also want it to be able to handle high signal levels without clipping. From what I see in my own recordings (and from what others have posted here) I would like it to handle at least 1VRMS input signals. (This of course only applies at the lower gain settings).
I want it to be useful for others. I realize that you can't please everyone, though.
As a project I want it to be easy to build. No mandatory hard-to-get parts, preferably nothing that's single-sourced, no required tools other than a simple soldering iron.
I also want people to have flexibility in how much they spend on it. If you don't need top notch common-mode rejection, just use a simple dual ganged pot as the attenuator, or one of those $3 dual deck rotary switches. If you want to spend more, by all means have the poly film input capacitors, MAT-03 input transistors, non-inductive wirewound resistors and relay-switched attenuators.
This is intended to be more than just a thought experiment, offering a project that's a step up from the Green and the SSL9K.
JDB.
jdbakker said:
jdbakker
Well-known member
abbey road d enfer said:
Initially I was, but I went looking for other architectures for the reasons I mentioned.
JDB.
Svart
Well-known member
It looks like we are just talking about different ideas, sorry to have misunderstood.
I was speaking against such a thing. For some reason I kept reading "feedback" and instantly thought of the closed loop for the gain stage itself, not for the whole design. I suppose it's just a matter of how you like to do things. I personally like to have a gain stage that does not change feedback over gain changes. Others find no problem with it. I suppose it's all in what you want to get out of the circuit because there is always a way to get it to work!
Svart
Well-known member
I asked it because when a group of well educated and smart individuals get together to brainstorm and idea, we have information overload. Everyone has an opinion. Some have opinions that compliment other opinions and some have opinions that contradict. Because of this we tend to need to work towards a goal or else we toil in the details for much too long and lose sight of the big picture.. ;D
Svart
Well-known member
I meant that in a CFB design, the feedback resistor, Rf (in most equations), determines the closed loop, not the noise gain equation, 1+Rf/Rg, as you would see in a VFB setup. This means that you want your Rf to stay at an optimal value and not change. You could still change your Rg to change gain BUT as gain changes, the relationship between the INV input Z and Rf will change due to shifting poles. So at higher gains your optimal Rf will shift to a different value which is determined by the design of the CFB.
Ok, so, since you want to retain this fixed-gain architecture AND you want a better EIN than others, you'll have to find a way to improve the EIN without changing the gain. Looks to me like the only solution is to decrease the value of the FB and gain resistors by a factor of 3 or 4. Then you'll need serious output drive capability. Looks like you're slowly approaching power-amp territoryjdbakker said:
jdbakker
Well-known member
jdbakker said:
Doing some noise calculations (more on that later) it appears the difference is bigger than I first assumed.
Consider the proposed plan:
at its lowest gain, 20dB. For now let's ignore the noise of the gain blocks and pencil in 600R for the attenuators. For an input (mic) resistance of 150R, an ideal noiseless amplifier would hiss like 150R*10(20/10)=15k.
As drawn, the output of stage 1 hisses like 150R*10(26/10)=60k.
The first attenuator is set for 20dB, so after this attenuator the input contributes 60k/10(20/10) or 600R. Add to this the resistance of the attenuator (also 600R) and you get 1200R at the input of gain block 2.
Gain block 2 amplifies this by 20dB, so at its output we see 1200R*10(20/10)=120k worth of hiss.
Second attenuator is also set for 20dB and has a resistance of 600R, so at its output we see 1800R.
Third gain block amplifies this to 180k.
The third attenuator cuts this by 6dB for a final noise resistance of 45k, or a 4.8dB noise figure.
Now if the output attenuator is removed and its attenuation is moved to the second attenuator (now 26dB), the input of the second attenuator still sees 120k, reduces this by 26dB to 300R, add its own resistance to get 900R at the input of gain block 3.
Gain block 3 amplifies this by 20dB, to arrive at an output-referred noise resistance of 90k, or a 7.8dB noise figure.
On the other hand, if you keep the output attenuator and switch out the second attenuator and gain block for lower gain settings, the noise resistance is 30k, for a 3dB noise figure.
PRR said:
Not at all, and what you suggest is intriguing, and definitely better than an input pad. I'll ponder this a bit more, and will go looking for appropriate iron.
As a matter of fact I would prefer a 2400:600 output transformer (plus some output load isolators) over a resistive 6dB output pad, but even a cheap specimen will cost a significant fraction of the BOM for the entire pre, especially if you factor in shipping.
JDB.
I believe you have not taken the noise current into account; you have considered only the thermal noise of source resistances. I don't know for which impedance you have optimised the bias of the input transistors. Supposing you have optimised the first stage for 150/200ohms and the subsequent for 600, the actual noise build-up will be ca. 3 dB higher, making the NF more like 7dB.
Anyway, a NF of 7dB for 20dB gain is not so bad. That will leave you with 102dB S/N; not bad...
The PGA2500 has a similar loss of performance and the THAT 1512 sees an increase of its NF of almost 15dB at 20dB gain compared to 60db, and it is recognised as a serious improvement in that respect over the 2017/2019.
Anyway, a NF of 7dB for 20dB gain is not so bad. That will leave you with 102dB S/N; not bad...
The PGA2500 has a similar loss of performance and the THAT 1512 sees an increase of its NF of almost 15dB at 20dB gain compared to 60db, and it is recognised as a serious improvement in that respect over the 2017/2019.
Svart
Well-known member
Which is why I said:
But I also read you wrong when I said that, I thought you said that you were putting the attenuator after the last stage, when you actually said after the second stage.
Svart
Well-known member
you know, I've always wanted to see what would happen if I used something like:
http://www.hittite.com/products/view.html/view/HMC580ST89
As a front end gain stage in a mic pre. I suppose a transformer is needed to match the front end and it would be much too high of a ratio to be practical. :-\
http://www.hittite.com/products/view.html/view/HMC580ST89
As a front end gain stage in a mic pre. I suppose a transformer is needed to match the front end and it would be much too high of a ratio to be practical. :-\
jdbakker
Well-known member
Svart said:
...not to mention its 1/f noise corner.
JD 'probably measured in MHz' B.
Svart
Well-known member
For current or voltage? ;D
Honestly I can't find information on the current or voltage noise for these parts..
But I did find an interesting paper that claims to null out pink noise in a LNA.
Either way, I was going to suggest looking into some of the CMOS amplifier parts. They have low current noise and low voltage noise.
Honestly I can't find information on the current or voltage noise for these parts..
But I did find an interesting paper that claims to null out pink noise in a LNA.
Either way, I was going to suggest looking into some of the CMOS amplifier parts. They have low current noise and low voltage noise.
