I am looking for advice on this circuit. input source will be 500ohm, connected to portable recorder with 22kOhm input impedance. Unbalanced in/out
Everything will be smd and I am not sure of which capacitor ratings to use, anything over 10V ok? Also if anyone has advice on better low noise transistors...
Everything will be smd and I am not sure of which capacitor ratings to use, anything over 10V ok? Also if anyone has advice on better low noise transistors...
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Power / size constraints? 9V block battery, I assume? No bias --> it's for a dynamic mic?
Should work alright, I guess. Not super-low distortion and super-high output levels (without an Isource in the output stage) or near-theoretical noise levels, but fairly serviceable and thrifty on power. Do give it a few hundred ohms of output series R. C3/7 also strike me as a bit small.
BC849s/850s are some of the better xistors for ~50 µA bias levels actually, so I'm not sure what sort of upgrade you are looking for. If you want lower noise, you'd need to upgrade current driving ability first so you can get lower feedback network values, but these 10µ caps are on the lower end already.
As-is, I'd estimate an e_n of 7-ish nV/sqrt(Hz) (+/- 3 dB) for most of the audio bandwidth, limited by feedback network and current levels. Quite workable, if still about 6 dB short of diminishing returns level and 9 dB short of source noise limit. Low-voltage solutions can be a lot noisier than this; I found 20-ish nV/sqrt(Hz) on Realtek HDA mic inputs (and even that would still be drowned out by bias supply noise on lesser chips).
Should work alright, I guess. Not super-low distortion and super-high output levels (without an Isource in the output stage) or near-theoretical noise levels, but fairly serviceable and thrifty on power. Do give it a few hundred ohms of output series R. C3/7 also strike me as a bit small.
BC849s/850s are some of the better xistors for ~50 µA bias levels actually, so I'm not sure what sort of upgrade you are looking for. If you want lower noise, you'd need to upgrade current driving ability first so you can get lower feedback network values, but these 10µ caps are on the lower end already.
As-is, I'd estimate an e_n of 7-ish nV/sqrt(Hz) (+/- 3 dB) for most of the audio bandwidth, limited by feedback network and current levels. Quite workable, if still about 6 dB short of diminishing returns level and 9 dB short of source noise limit. Low-voltage solutions can be a lot noisier than this; I found 20-ish nV/sqrt(Hz) on Realtek HDA mic inputs (and even that would still be drowned out by bias supply noise on lesser chips).
You should have a load resistor at each input before the coupling caps, value from 1k-3k.
Also resistors should be at the outputs after the coupling caps, values 50k-100k.
Capacitor voltage ratings should be well over 9V, at least 16-25V.
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Thanks. I am obviously a beginner so barely able to keep up, but to answer:
This is going to be a stereo mic pre, 9Vpp battery. The mics are battery powered, no need for biasing.
I took this circuit from talkingelectronics.com, in the original the gain was set at 100, I just changed one resistor to 1k to set gain at 10 - which is what I'm looking for here, fixed gain. Next project I'll do a variable gain.
I want to keep the size to about 2.2 x 1.2 inches for one enclosure scenario
Or use a 2.5" hdd external case in which case it could be fairly larger.
A few more questions:
Is it possible to just use a single transistor amplifier for this since I only need a gain of 10?
To match the channel gains tightly, is something like a mat-02/03 in order? this will also save some board space
For grounding, would ground plane or star be best? I'll be making a pcb.
For biasing the transistors, I sort of get the math for the half rail bias, but as the battery discharges it will not bias correctly..?? Would I need regulation ?
The low fixed gain is mainly because that's about all I need for most situations. Secondary benefit is that I want to test the idea that a discrete first stage will sound better than IC, so if I use some line level gain from the recorder, post mic amplifier, I will be able to hear if this is true.
This is going to be a stereo mic pre, 9Vpp battery. The mics are battery powered, no need for biasing.
I took this circuit from talkingelectronics.com, in the original the gain was set at 100, I just changed one resistor to 1k to set gain at 10 - which is what I'm looking for here, fixed gain. Next project I'll do a variable gain.
I want to keep the size to about 2.2 x 1.2 inches for one enclosure scenario
Or use a 2.5" hdd external case in which case it could be fairly larger.
A few more questions:
Is it possible to just use a single transistor amplifier for this since I only need a gain of 10?
To match the channel gains tightly, is something like a mat-02/03 in order? this will also save some board space
For grounding, would ground plane or star be best? I'll be making a pcb.
For biasing the transistors, I sort of get the math for the half rail bias, but as the battery discharges it will not bias correctly..?? Would I need regulation ?
The low fixed gain is mainly because that's about all I need for most situations. Secondary benefit is that I want to test the idea that a discrete first stage will sound better than IC, so if I use some line level gain from the recorder, post mic amplifier, I will be able to hear if this is true.
Yes.
"To match the channel gains tightly, is something like a mat-02/03 in order? this will also save some board space. "
MAT12 (current version) is low noise, and should be ok.
"For grounding, would ground plane or star be best? I'll be making a pcb."
Either if properly done, but a ground plane is best for low signal levels.
"For biasing the transistors, I sort of get the math for the half rail bias, but as the battery discharges it will not bias correctly. Would I need regulation ?"
You have to design for the worst case, about 7V for a 9V battery. With a regulator's additional drop, you'd have to design for a 5V supply.
Use a large electrolytic (10uF-100uF) at the battery input node for decoupling.
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Thanks.
I'm deciding which way to go now, two transistor discrete or op amp (thanks for the ina217 help rayma).
Does anyone have opinion on what's the better way to go?
I was reading more tonight about the 2 transistor feedback configuration having much lower distortion than a single.
Mat12 is npn, and this circuit also uses pnp, but I did find another on the rod elliot site with 2 npn's.
I'm deciding which way to go now, two transistor discrete or op amp (thanks for the ina217 help rayma).
Does anyone have opinion on what's the better way to go?
I was reading more tonight about the 2 transistor feedback configuration having much lower distortion than a single.
Mat12 is npn, and this circuit also uses pnp, but I did find another on the rod elliot site with 2 npn's.
Bear in mind that some devices need a minimum voltage, and the INA217 needs 9V or more. Yes, feedback over two stages will lower the distortion.
You can use either NPN or PNP by just reversing the battery and electrolytic capacitor connections.
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How would I calculate current draw for this one...
ECM Mic Preamplifier
(delete R1 and connection to R2 since I'm not using an electret)
ECM Mic Preamplifier
(delete R1 and connection to R2 since I'm not using an electret)
That's a rather messy iterative affair. You assume some halfway realistic BE voltage drop (like 0.6-0.65 V), neglect Ib(Q1) for the time being and then create a system of equations involving the remaining unknowns Vc(Q1), Ve(Q1), Ic(Q1), Ve(Q2) and Ic(Q2), which you then proceed to solve. Once Ics are known, you can improve upon your Vbe drops and Ib(Q1) estimate and thus the other values. Should be covered in any decent transistor circuitry textbook.
Nature solves this using the transistor equations, which would be a real mess to do analytically.
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