Transformerless discrete balanced micamp
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clintrubber
Well-known member
from PRR:
Was it right at the start already named "Current Feedback" ?
Wondering, apart from that it's fully discrete
(ehh, another buzz-word but now from anyone but those chip-makers :wink: ), what are the advantages over the 'original' plan with the integrated opamp ?
Bye,
Peter
Thanks for the interesting read.
Yes, it has become a lame term. I'll get the Green-sch and have another look at it.
Was it right at the start already named "Current Feedback" ?
Nice circuit, thanks for drawing.
Wondering, apart from that it's fully discrete
(ehh, another buzz-word but now from anyone but those chip-makers :wink: ), what are the advantages over the 'original' plan with the integrated opamp ?
Bye,
Peter
henk
Well-known member
Looks great .
What are the specs of it , something i would like to know.
What are the specs of it , something i would like to know.
henk
Well-known member
Hey PRR
Have compared your schemo with the front of the crest v12 , here on the forum somewhere , en it looks almost the same only there are more components in use by the crest.
But it is a nice start for a (very) good mic preamp.
Have compared your schemo with the front of the crest v12 , here on the forum somewhere , en it looks almost the same only there are more components in use by the crest.
But it is a nice start for a (very) good mic preamp.
hitchhiker
Well-known member
[quote author="PRR"]>> The output impedance is quite high, like 4K4, so you want a couple emitter followers. .[/quote]
hi PRR,
This is the part I'd like to learn... to add a good common bjt and bias it and fine tune it.
I tried to learn this from a book once but when I tried to do one it didn't work.
I know to most of the forum this is childs play but not for me .
Lance
hi PRR,
This is the part I'd like to learn... to add a good common bjt and bias it and fine tune it.
I tried to learn this from a book once but when I tried to do one it didn't work.
I know to most of the forum this is childs play but not for me .
Lance
> Have compared your schemo with the front of the crest v12 ... it looks almost the same...
???
I stole that from Rod Elliot. Literally copy/paste, except paste twice and connect the lines.
I hadn't seen the V12. It is "almost the same" as Rod's #66, and about 100 other designs. There just are not that many ways to do something like this.
Rod's #66 uses cap-coupling between a BJT-compound and an IC opamp. The V12 uses DC coupling (aided by a servo) to an IC opamp. Mine uses two nearly identical stages, no opamp at all.
I don't claim it is original. Everything has been invented before. However it may be an interesting way to do full-discrete without falling into the trap that everything has to be an opamp. Opamp design glosses-over some major design headaches, but the cost in unnecessary complexity is high.
???
I stole that from Rod Elliot. Literally copy/paste, except paste twice and connect the lines.
I hadn't seen the V12. It is "almost the same" as Rod's #66, and about 100 other designs. There just are not that many ways to do something like this.
Rod's #66 uses cap-coupling between a BJT-compound and an IC opamp. The V12 uses DC coupling (aided by a servo) to an IC opamp. Mine uses two nearly identical stages, no opamp at all.
I don't claim it is original. Everything has been invented before. However it may be an interesting way to do full-discrete without falling into the trap that everything has to be an opamp. Opamp design glosses-over some major design headaches, but the cost in unnecessary complexity is high.
PRR, someday we'll meet and I'll have to buy the beer. While the schematic you posted isn't in line with what I'm working on, it caused a flash to happen, and possibly solve a problem I've been wrestling with for a while.
Thanks!
Thanks!
> someday we'll meet and I'll have to buy the beer.
Sure, except I'm not going to Minnesota this week (brrrr!). You drink it for me.
> While the schematic you posted isn't in line with what I'm working on
I hope not. It should work, but is clearly not optimized. The output swing (not even 15V peak) is rather lame for +/-15V supplies. The base resistors in the 2nd stage are a heavy load on the first stage and might be reconsidered. And no, Henk, I have not figured the complete specs, except I figure it will bring a mike up to "line" without any great flaw. (Hmmmm... the bandwidth might be MHz, which actually could be a "flaw" in a high-RF location.)
Also, everybody should note that Rod's design predates common use of Phantom Power, and has no provision or protection from Phantom. Use the usual scheme: a couple of 47uFd caps, + side to the XLR, with a couple 6K8 resistors to a 48V supply. Protection diodes are wise but perhaps not really needed with resistor bias and these sturdy switch-transistors (and no critical DC offset worries).
> it caused a flash to happen
That's what wacky un-optimized ideas are for. Back around 1970 some clever guys drew transistors every which way, in 1980 they added IC opamps, and ever since we have been basically using the SAME few topologies. Because they are simple and they WORK. But the drop in part prices and the rise in labor costs make other topologies interesting. Ten transistors was a lot in 1970, but today the 10 transistors are about a buck while in DIY your value-of-labor is going to be $100+: layout, PCB etching, stuffing, debugging. And in commercial work, parts like these have little effect on selling price. So what can we do with a dozen 10-cent transistors? And how many of our clients need to see "totally discrete amplification"?
(Not that there is anything wrong with ICs. The selection of parts is limited but the parts are super-cheap so we can mask-up schemes too messy to solder. Baked-in matching makes some nice things possible. The main drawback seems to be that ICs only make sense if you can sell 100K of the exact same thing, and the audio market is not that big, not at the high-end.)
So it is time to draw things every which way, throw them on top of each other, and see if anything sticks. Here I just threw two of the same thing down, and saw that they butted-together with a usable result. If this landed next to something fluttering in your brain and went FLASH, groovy.
The idea of using a dual-linear pot to control the gain of both stages is interesting. Dual-linear may be easier to find than reverse-audio. (Though either way, the value needed is low for a pot.) Instead of gain varying linearly as with a single-linear pot, it changes as square of gain. With a small value of R9 in each stage, gain increases rapidly in the high-gain end of rotation, but not as bad as simple linear control. However if you make each stage's R9 about 250Ω and each VR about 1K, you get about 22dB at one end, 36dB in the middle, and about 52dB at the other end. That's only 30dB of range, but the dBs are nicely spread around the rotation. An even better scheme would be to make first stage R9 very low (250Ω is really too high for great noise performance) and the second stage R9 maybe 250Ω or somewhat higher. Then at the high end, the second stage (and overall) gain doesn't soar as you near the end. My pencil isn't sharp enough to work out this peicewise-linear approximation to a linear-in-dB curve. I suspect that for a good 40dB or 50dB range without any sharp kinks you need a triple linear pot, not a common part. (There is much to be said for switches, if you don't need to trim gain on-air.)
> possibly solve a problem I've been wrestling with for a while.
Do what they do in the WWF. Dance around the problem, snarl, do an armlock and a headbutt, then if not pinned yet just whack it with a chair and throw it out of the ring.
Sure, except I'm not going to Minnesota this week (brrrr!). You drink it for me.
> While the schematic you posted isn't in line with what I'm working on
I hope not. It should work, but is clearly not optimized. The output swing (not even 15V peak) is rather lame for +/-15V supplies. The base resistors in the 2nd stage are a heavy load on the first stage and might be reconsidered. And no, Henk, I have not figured the complete specs, except I figure it will bring a mike up to "line" without any great flaw. (Hmmmm... the bandwidth might be MHz, which actually could be a "flaw" in a high-RF location.)
Also, everybody should note that Rod's design predates common use of Phantom Power, and has no provision or protection from Phantom. Use the usual scheme: a couple of 47uFd caps, + side to the XLR, with a couple 6K8 resistors to a 48V supply. Protection diodes are wise but perhaps not really needed with resistor bias and these sturdy switch-transistors (and no critical DC offset worries).
> it caused a flash to happen
That's what wacky un-optimized ideas are for. Back around 1970 some clever guys drew transistors every which way, in 1980 they added IC opamps, and ever since we have been basically using the SAME few topologies. Because they are simple and they WORK. But the drop in part prices and the rise in labor costs make other topologies interesting. Ten transistors was a lot in 1970, but today the 10 transistors are about a buck while in DIY your value-of-labor is going to be $100+: layout, PCB etching, stuffing, debugging. And in commercial work, parts like these have little effect on selling price. So what can we do with a dozen 10-cent transistors? And how many of our clients need to see "totally discrete amplification"?
(Not that there is anything wrong with ICs. The selection of parts is limited but the parts are super-cheap so we can mask-up schemes too messy to solder. Baked-in matching makes some nice things possible. The main drawback seems to be that ICs only make sense if you can sell 100K of the exact same thing, and the audio market is not that big, not at the high-end.)
So it is time to draw things every which way, throw them on top of each other, and see if anything sticks. Here I just threw two of the same thing down, and saw that they butted-together with a usable result. If this landed next to something fluttering in your brain and went FLASH, groovy.
The idea of using a dual-linear pot to control the gain of both stages is interesting. Dual-linear may be easier to find than reverse-audio. (Though either way, the value needed is low for a pot.) Instead of gain varying linearly as with a single-linear pot, it changes as square of gain. With a small value of R9 in each stage, gain increases rapidly in the high-gain end of rotation, but not as bad as simple linear control. However if you make each stage's R9 about 250Ω and each VR about 1K, you get about 22dB at one end, 36dB in the middle, and about 52dB at the other end. That's only 30dB of range, but the dBs are nicely spread around the rotation. An even better scheme would be to make first stage R9 very low (250Ω is really too high for great noise performance) and the second stage R9 maybe 250Ω or somewhat higher. Then at the high end, the second stage (and overall) gain doesn't soar as you near the end. My pencil isn't sharp enough to work out this peicewise-linear approximation to a linear-in-dB curve. I suspect that for a good 40dB or 50dB range without any sharp kinks you need a triple linear pot, not a common part. (There is much to be said for switches, if you don't need to trim gain on-air.)
> possibly solve a problem I've been wrestling with for a while.
Do what they do in the WWF. Dance around the problem, snarl, do an armlock and a headbutt, then if not pinned yet just whack it with a chair and throw it out of the ring.
Hey All,
PRR, I was thinking about your cascaded gain stage and what if instead of AC coupling them we DC coupled them and flip the second stages bjts to NPN and used the DC voltage at the collector of the first stage to bias stage 2. This could allow for a wider voltage swing on the outputs.
It does make keeping the gain setting resistor the same for each stage harder because to set the output DC level the collector resistors will be higher for stage 2, but a dual gang roatry switch could work. Also using resistors for current sources gets tricky because the low values on stage 2 due to lower emiter to rail voltage effect the gain to much so I switched to high imp. constant current sources. Maybe setting the idle current in the second stage higher could yield lower Rc's for stage 2 and equal up the gain setting value, but maybe there are negative effects to that?
Heres a schem of it (values are very 'roughed' just to get idea across)
Just thinking..... any thoughts on this out there? good?/bad?
Brian
PRR, I was thinking about your cascaded gain stage and what if instead of AC coupling them we DC coupled them and flip the second stages bjts to NPN and used the DC voltage at the collector of the first stage to bias stage 2. This could allow for a wider voltage swing on the outputs.
It does make keeping the gain setting resistor the same for each stage harder because to set the output DC level the collector resistors will be higher for stage 2, but a dual gang roatry switch could work. Also using resistors for current sources gets tricky because the low values on stage 2 due to lower emiter to rail voltage effect the gain to much so I switched to high imp. constant current sources. Maybe setting the idle current in the second stage higher could yield lower Rc's for stage 2 and equal up the gain setting value, but maybe there are negative effects to that?
Heres a schem of it (values are very 'roughed' just to get idea across)
Just thinking..... any thoughts on this out there? good?/bad?
Brian
Carl_Huff
Well-known member
Fellas,
I really like where this is headed. A real circuit is just over the horizon!
Go, go, go!!
I really like where this is headed. A real circuit is just over the horizon!
Go, go, go!!
> I was thinking...
Good!
> what if instead of AC coupling them we DC coupled them and flip the second stages bjts to NPN
Yeah, should be workable and is a neat trick.
A very obscure comment: you have drawn "perfect current sources". What happens when you make them "real", and check noise at low gain?
Fer example: I3 will probably be a PNP transistor, Base biased 2Vbe below the V+ rail, and 150Ω under its emitter.
For high gain, R7 is very small, I3's current leaks into both R3 and R4 (same for I4) and any noise is cancelled (assuming the output is taken differentially).
But we also need low-gain setting. Go to an extreme: R7 is infinite. I3's current appears only in R3. The PNP in I3 has a noise voltage due to Rb, say about 0.2uV. This is amplified by the ratio 1.5K/150Ω, so about 2uV of noise appears at R3. Likewise for I4 and R4. Output noise at R19 R20 is already 12uV, before we even consider 2nd-stage and other noise sources.
If R7 is infinite, with your plan, the signal gain is zero. That's not an interesting setting.
However, if R7 is more than about 150Ω, the output noise is more due to I3 I4 noise than Q1 Q2 noise!
We can also note that if R7 is more than about 150Ω, output noise is dominated by R7 noise, not mike resistance noise.
Nothing wrong with that. As near as I can tell, ALL low-noise variable-gain amps suffer from rising noise floor at low gains. In many case you can summarize noise as input noise times gain, plus an "output" noise that is fairly constant, and tends to be greater than input (or source) noise at low gains.
When the channel was wax or tape or radio, this was not a big deal. But I've recorded performances with 130dB SPL peaks, and pauses long enough for the ear to relax down to 14dB SPL room noise. I didn't then have a 116dB dynamic range recorder, but I could have run 20dB compression and smashed it into CD. Now I can buy 20+bit recorders. And I'd probably rather hear "room" rather than mike-amp hiss in the silences.
So I could want a mike amp that takes 1V signal with 1uV equivalent input noise at that gain. Your "rough" values give low-gain noise about 20dB higher than that. My attempt, single stage with 500Ω in the top of the gain-set network, was about the same.
Using resistors instead of current sources gives less noise gain, but won't allow very low signal gain or stupendous CMRR.
Using current mirrors seems to be worse than simple current sources, but I have not really followed through on that.
Good!
> what if instead of AC coupling them we DC coupled them and flip the second stages bjts to NPN
Yeah, should be workable and is a neat trick.
A very obscure comment: you have drawn "perfect current sources". What happens when you make them "real", and check noise at low gain?
Fer example: I3 will probably be a PNP transistor, Base biased 2Vbe below the V+ rail, and 150Ω under its emitter.
For high gain, R7 is very small, I3's current leaks into both R3 and R4 (same for I4) and any noise is cancelled (assuming the output is taken differentially).
But we also need low-gain setting. Go to an extreme: R7 is infinite. I3's current appears only in R3. The PNP in I3 has a noise voltage due to Rb, say about 0.2uV. This is amplified by the ratio 1.5K/150Ω, so about 2uV of noise appears at R3. Likewise for I4 and R4. Output noise at R19 R20 is already 12uV, before we even consider 2nd-stage and other noise sources.
If R7 is infinite, with your plan, the signal gain is zero. That's not an interesting setting.
However, if R7 is more than about 150Ω, the output noise is more due to I3 I4 noise than Q1 Q2 noise!
We can also note that if R7 is more than about 150Ω, output noise is dominated by R7 noise, not mike resistance noise.
Nothing wrong with that. As near as I can tell, ALL low-noise variable-gain amps suffer from rising noise floor at low gains. In many case you can summarize noise as input noise times gain, plus an "output" noise that is fairly constant, and tends to be greater than input (or source) noise at low gains.
When the channel was wax or tape or radio, this was not a big deal. But I've recorded performances with 130dB SPL peaks, and pauses long enough for the ear to relax down to 14dB SPL room noise. I didn't then have a 116dB dynamic range recorder, but I could have run 20dB compression and smashed it into CD. Now I can buy 20+bit recorders. And I'd probably rather hear "room" rather than mike-amp hiss in the silences.
So I could want a mike amp that takes 1V signal with 1uV equivalent input noise at that gain. Your "rough" values give low-gain noise about 20dB higher than that. My attempt, single stage with 500Ω in the top of the gain-set network, was about the same.
Using resistors instead of current sources gives less noise gain, but won't allow very low signal gain or stupendous CMRR.
Using current mirrors seems to be worse than simple current sources, but I have not really followed through on that.
henk
Well-known member
Hey PRR
You must be write to say that there over 130 different design of these transistor stages , but they are always nice to here instead of some ic's that i dont like.(ssm2017/2019for example)
The distorsion and sound throug a stage like this will sound ok , but if possible a dc trough stage 1 and 2 would make it even better.
hope you will work that out , so i will wait for the next design with dc.
thanks
You must be write to say that there over 130 different design of these transistor stages , but they are always nice to here instead of some ic's that i dont like.(ssm2017/2019for example)
The distorsion and sound throug a stage like this will sound ok , but if possible a dc trough stage 1 and 2 would make it even better.
hope you will work that out , so i will wait for the next design with dc.
thanks
A very obscure comment: you have drawn "perfect current sources". What happens when you make them "real", and check noise at low gain?
Thanks for the look over and comments...
In Spice, the ideal current sources yield better noise as compared to a PNP source by 12 nV/Hz(^.5) from about 30 to 42. I have not built this circuit and done any testing though.
Yet another circuit to fiddle with. Now if I could just get the bench clear of all the other crap....
Brian
Thanks for the look over and comments...
In Spice, the ideal current sources yield better noise as compared to a PNP source by 12 nV/Hz(^.5) from about 30 to 42. I have not built this circuit and done any testing though.
Yet another circuit to fiddle with. Now if I could just get the bench clear of all the other crap....
Brian
henk> a dc trough stage 1 and 2 would make it even better. ... i will wait for the next design with dc.
Someone with your deep understanding shouldn't have to wait for someone else to do your designs.
Buz> In Spice, the ideal current sources yield better noise as compared to a PNP source by 12 nV/Hz(^.5)
Yes, something like that.
It happens that it was SPICE who told me that. I don't trust SPICE any futher than I can throw it. I didn't trust that answer until I walked outside (away from stupid SPICE) and thought about it.
Our "current source" is really a current limiter, and based on a high-gain amplifier. (It amplifies a 150Ω resistor into about a 100K dynamic resistance!) And any high-gain real amplifier has noise. In this layout, reducing R7 (cross-emitter gain-set resistor) increases the gain at the signal inputs and also reduces gain from the current sources. For high signal gains, current source noise is negligible. But at low signal gains, I could picture (without SPICE's help) how the self-noise of the current source transistors was passing into the output, at higher gain than the signal-port noise sources.
The effect is reduced if you have a larger voltage on the current source emitter resistor. We use 0.6V because a diode is convenient and stable, and it takes away very little of the available supply voltage. After I saw the problem, I tried larger voltages, 4V or 6V. Noise is lower, but not really low. And input headroom was reduced. (Worse because I was staking everything vertically under one supply rail.) I could "fix" it by adding more supply voltage, but it takes nearly double voltage to really make this noise source negligible at very low gain.
Someone with your deep understanding shouldn't have to wait for someone else to do your designs.
Buz> In Spice, the ideal current sources yield better noise as compared to a PNP source by 12 nV/Hz(^.5)
Yes, something like that.
It happens that it was SPICE who told me that. I don't trust SPICE any futher than I can throw it. I didn't trust that answer until I walked outside (away from stupid SPICE) and thought about it.
Our "current source" is really a current limiter, and based on a high-gain amplifier. (It amplifies a 150Ω resistor into about a 100K dynamic resistance!) And any high-gain real amplifier has noise. In this layout, reducing R7 (cross-emitter gain-set resistor) increases the gain at the signal inputs and also reduces gain from the current sources. For high signal gains, current source noise is negligible. But at low signal gains, I could picture (without SPICE's help) how the self-noise of the current source transistors was passing into the output, at higher gain than the signal-port noise sources.
The effect is reduced if you have a larger voltage on the current source emitter resistor. We use 0.6V because a diode is convenient and stable, and it takes away very little of the available supply voltage. After I saw the problem, I tried larger voltages, 4V or 6V. Noise is lower, but not really low. And input headroom was reduced. (Worse because I was staking everything vertically under one supply rail.) I could "fix" it by adding more supply voltage, but it takes nearly double voltage to really make this noise source negligible at very low gain.
hitchhiker
Well-known member
hey PRR,
I bread boarded your bjt version of the ESP66 just as your schematic
specified but had to use bc559 instead of the 2n4403.
It sounds good, gives full mids but not too much. It comes to life with higher gains. Low gains its easy going and a bit gentle.
I'll try out some different mics when I can get to it again.
I'd like to try out a dual 1k gain pot as you describe but that'll have to wait
until I get a hold of one.
All in all a good pre that I will build for sure . Thanks! :thumb:
Lance
I bread boarded your bjt version of the ESP66 just as your schematic
specified but had to use bc559 instead of the 2n4403.
It sounds good, gives full mids but not too much. It comes to life with higher gains. Low gains its easy going and a bit gentle.
I'll try out some different mics when I can get to it again.
I'd like to try out a dual 1k gain pot as you describe but that'll have to wait
until I get a hold of one.
All in all a good pre that I will build for sure . Thanks! :thumb:
Lance
> had to use bc559 instead of the 2n4403.
That works, of course.
The 4401/4403-size "500mA" transistors will probably have lower noise than "100mA" BC559-size devices in this very low-Z circuit. At least on paper. Of course noise isn't everything, and in many real studio situations it is almost irrelevant.
Let's see: Rb of 2N440x is around 4Ω, for BB559 it is more like 30Ω. We have two input devices, so in one case we have about 8Ω of Rb, in the other about 60Ω of Rb.
Right-off, we note that the gain-set resistor adds to Rb noise. If gain-set is higher than about 100Ω, the difference in transistor Rb is utterly pointless. Use what you got handy.
In Rod's #66 the gain-set resistor can go as low as 22Ω, and we are using similar values in this initial cut-paste-paste derivative. So for 440x we have 8+22= 30Ω, for 559 we have 60+22= 82Ω of series noise resistance.
For a microphone with self-noise equvalent to a 150Ω resistor, for 2N440x we have 150+30= 180Ω of noise resistance, for BC559 we have 150+82= 232Ω of noise resistance. Noise voltage rises with square-root of noise resistance.
Taking 150Ω as "ideal", zero dB Noise Figure, then 2N440x gives NF=0.8dB, BC559 gives NF=1.9dB.
Both are very low, and the difference 0.8 to 1.9 is nearly inaudible. Also we have neglected several other noise sources that will add another dB or two of noise in either case. And this is at maximum gain (about 60dB); at any lesser gain the gain-set resistor swamps the difference between transistors.
So the difference between 2N440x and BC559 (between "500mA" and "100mA") devices may be utterly inaudible.
That works, of course.
The 4401/4403-size "500mA" transistors will probably have lower noise than "100mA" BC559-size devices in this very low-Z circuit. At least on paper. Of course noise isn't everything, and in many real studio situations it is almost irrelevant.
Let's see: Rb of 2N440x is around 4Ω, for BB559 it is more like 30Ω. We have two input devices, so in one case we have about 8Ω of Rb, in the other about 60Ω of Rb.
Right-off, we note that the gain-set resistor adds to Rb noise. If gain-set is higher than about 100Ω, the difference in transistor Rb is utterly pointless. Use what you got handy.
In Rod's #66 the gain-set resistor can go as low as 22Ω, and we are using similar values in this initial cut-paste-paste derivative. So for 440x we have 8+22= 30Ω, for 559 we have 60+22= 82Ω of series noise resistance.
For a microphone with self-noise equvalent to a 150Ω resistor, for 2N440x we have 150+30= 180Ω of noise resistance, for BC559 we have 150+82= 232Ω of noise resistance. Noise voltage rises with square-root of noise resistance.
Taking 150Ω as "ideal", zero dB Noise Figure, then 2N440x gives NF=0.8dB, BC559 gives NF=1.9dB.
Both are very low, and the difference 0.8 to 1.9 is nearly inaudible. Also we have neglected several other noise sources that will add another dB or two of noise in either case. And this is at maximum gain (about 60dB); at any lesser gain the gain-set resistor swamps the difference between transistors.
So the difference between 2N440x and BC559 (between "500mA" and "100mA") devices may be utterly inaudible.
hitchhiker
Well-known member
good stuff PRR,
I tried my u87ai through this preamp tonight and it was over driven by it.
I raised the gain resister on the second stage to 150r and it was much
better. I then set the input stage high and it would still over drive if
the source was loud. The preamp really has a good tone and presence
if you perform below the distortion threshold . An electric quality to
the acoustic guitar, but not harsh. I like this preamp.
Lance
I tried my u87ai through this preamp tonight and it was over driven by it.
I raised the gain resister on the second stage to 150r and it was much
better. I then set the input stage high and it would still over drive if
the source was loud. The preamp really has a good tone and presence
if you perform below the distortion threshold . An electric quality to
the acoustic guitar, but not harsh. I like this preamp.
Lance
> it would still over drive if the source was loud.
The output stage gain-set resistor can be 50Ω for high gain to 1KΩ when you want low gain.
The input stage gain-set resistor can be 22Ω for high gain to 1KΩ when you want low gain.
With both at 1K, total gain is somewhat less than 1:4 or 12dB. Input overload is over 1 volt. Yes, you can find situations where loud sources in hot mikes make more than a volt: that's what pads are for.
With 22Ω and 50Ω, overall gain is 60dB or 63dB, way too much for many modern studio sessions. More than I honestly need for dynamic mikes on choir.
With big condenser on fairly loud sources, start around 500Ω in the output-stage, 300Ω in the input stage. That should handle most singers a foot away, or many smaller guitar amps. It will do for piano if you are a few feet away. Do not be afraid to increase the input stage gain-set when using self-amplified mikes like big hot condensers: the large resistance gives higher mike-amp noise, but these mikes have self-noise like a 5K resistor.
The output stage gain-set resistor can be 50Ω for high gain to 1KΩ when you want low gain.
The input stage gain-set resistor can be 22Ω for high gain to 1KΩ when you want low gain.
With both at 1K, total gain is somewhat less than 1:4 or 12dB. Input overload is over 1 volt. Yes, you can find situations where loud sources in hot mikes make more than a volt: that's what pads are for.
With 22Ω and 50Ω, overall gain is 60dB or 63dB, way too much for many modern studio sessions. More than I honestly need for dynamic mikes on choir.
With big condenser on fairly loud sources, start around 500Ω in the output-stage, 300Ω in the input stage. That should handle most singers a foot away, or many smaller guitar amps. It will do for piano if you are a few feet away. Do not be afraid to increase the input stage gain-set when using self-amplified mikes like big hot condensers: the large resistance gives higher mike-amp noise, but these mikes have self-noise like a 5K resistor.
hitchhiker
Well-known member
I'm wondering what an unbal to bal input buffer with transistors would be for this cascaded circuit. I can't seem to find a discrete example anywhere.
I guess all the older stuff would be transformer based.
For gain I have a 1K pot on the first stage then 22r 75r 150r & 200r
with a 4 pos switch on the second stage. It's a smooth but fast mic amp.
Lance
I guess all the older stuff would be transformer based.
For gain I have a 1K pot on the first stage then 22r 75r 150r & 200r
with a 4 pos switch on the second stage. It's a smooth but fast mic amp.
Lance
