Idea of an simple mic pre.

[Chandra]

There was a 5 dollar mic preamp circuit in the internet.
That site is lost. If you search, there may be other diy places where you can find it.
It was said to beat a $1500 mic pre
One chip, some resistors and caps.
Regards.

 

[Newmarket]

There was a 5 dollar mic preamp circuit in the internet.
That site is lost. If you search, there may be other diy places where you can find it.
It was said to beat a $1500 mic pre
One chip, some resistors and caps.
Regards.

The famous $5 preamp - everything you need to know

 

[ct.waveform]

In my experience in the mixer business (decades ago) "impedance balanced" was the terminology used to describe that very specific topology where you add one equal value resistor in series with the - output lead connected to ground to convert an unbalanced output to (cough) balanced.

There have been voluminous discussions about the different flavors of "balanced" right here. I recall years ago digging out my copy of the IEEE S-100 standards dictionary. That S-100 search was not very rewarding (I expect a search here could reveal some of our past discourse on this topic).

JR

I'd call that balanced, strictly speaking. It's the impedances that decide whether it's balanced or not (and going to reap the benefits), whether or not you've got signal on it. It's also an invaluable mod for audio sources. I much prefer it with signal on the cold, though, and it's not going to cut it if you need distortion cancellation, which could happen as late as the speakers. But anything that gets rid of the hateful RCA phono plug is good in my book. It's an appalling bit of cheapskatery that has done the hi fi industry decades of harm.

That may well be from a Doug Self book. I seem to remember an output stage which avoided one of the op amps struggling if it got connected to ground. There were some limitations but I can't remember what they were. I should probably have a look again, not least because I think the same book has what I remember as unimpeachable tone controls, and I can't remember what he did there either.

Incidentally, Bill Whitlock is the person to read on balanced interfaces. He writes nicely and is totally in command of his subject matter. It's just a shame that the execution of the THAT chips didn't quite cut it on noise and distortion, even back then. (And that they were rather too expensive.)

 

[Chandra]

The famous $5 preamp - everything you need to know

Thank you.
That was the one.
Regards.

 

[Newmarket]

I must say that quite a lot of times I just prefer the "quasi-balanced" option of adding one resistor in series with the output for stability and an identical one from the cold terminal to ground.

+1. Some also advocate replicating any coupling cap'. I've done both options on DIY bits. Doubtful wrt cap tolerance matching but may be close if from same batch? And then only one sees any signal voltage and that should be small. Maybe overthinking it

 

[Newmarket]

I'd call that balanced, strictly speaking. It's the impedances that decide whether it's balanced or not (and going to reap the benefits), whether or not you've got signal on it. It's also an invaluable mod for audio sources. I much prefer it with signal on the cold, though, and it's not going to cut it if you need distortion cancellation, which could happen as late as the speakers. But anything that gets rid of the hateful RCA phono plug is good in my book. It's an appalling bit of cheapskatery that has done the hi fi industry decades of harm.

That may well be from a Doug Self book. I seem to remember an output stage which avoided one of the op amps struggling if it got connected to ground. There were some limitations but I can't remember what they were. I should probably have a look again, not least because I think the same book has what I remember as unimpeachable tone controls, and I can't remember what he did there either.

Incidentally, Bill Whitlock is the person to read on balanced interfaces. He writes nicely and is totally in command of his subject matter. It's just a shame that the execution of the THAT chips didn't quite cut it on noise and distortion, even back then. (And that they were rather too expensive.)

Why prefer signal on the cold leg. If I understand you correctly that would ground the signal into an unbalanced input?
Wrt output stage coping with an opamp output connected to ground. I think you are referring to the cross coupled output configuration. Used by, among others, DDA in the form of their EBOS module. And similar in IC form eg SSM2142 and better implementation in THAT Outsmart devices. Said to offer transformer like output in that if one leg grounded then signal on other leg gets +6dB lift.

 

[user 37518]

+1. Some also advocate replicating any coupling cap'. I've done both options on DIY bits. Doubtful wrt cap tolerance matching but may be close if from same batch? And then only one sees any signal voltage and that should be small. Maybe overthinking it

yes the double cap is good idea if one is looking for exactly the same impedance to ground, still, I think it is one of the things you can get away with if necessary. I always felt like it was a complete waste to get a high-valued expensive electrolytic just to parallel the resistor to ground, but it does work.

 

[Newmarket]

Yeah - and Soundcraft (referencing them as I am familiar with) didn't feel the need. There's a short piece from Graham Blyth on it somewhere. imo it kind of depends on whether the potential problem is "Ground Noise" or induced rf interference.
But for DIY stuff I'd tend to include it now as cost is relatively minor. Commercial product = different considerations esp if multi channel.

 

[user 37518]

There was a 5 dollar mic preamp circuit in the internet.
That site is lost. If you search, there may be other diy places where you can find it.
It was said to beat a $1500 mic pre
One chip, some resistors and caps.
Regards.

I don't know if it beats a $1,500 pre, but it is simply an INA217. I used to give that as a project to my students back when I taught at an audio school. They didn't find assembling it as enjoyable as I thought they would. Also, as far as I've seen, the INA217 is no longer available in DIP8, but who cares? I prefer the THAT 1510 anyway.

If I am feeling eclectic, I will use a couple low-noise transistors plus an AD797. Sam Groner has some good design in his webpage.

To be honest, I think we "are there" when it comes to mic pre's, I mean, sure, you can design something which has some sound that will be appealing to some crowd, but in terms of mere performance and specs, the Cohen topology is basically as good as it gets for a transformless pre, which is the same topology inside the INA217, THAT1510, and basically any inexpensive console, interface, etc...

If I were to focus my efforts on a mic pre, I would try to create a topology with transformers which cancels as much as possible transformer distortion, I guess that something like the AP SYS-1 comes to mind. Transformers are basically unmatched when it comes to CMRR, but they still distort at low frequencies much more than I would like. Jensen and Lundahl are also as good as it gets when it comes to transformers, and I don't know how much more significant improvement can be done in terms purely of magnetics, so focusing on distortion cancelation of transformer distortion/coil resistance is probably the way to go.

 

[Chandra]

It is about the simplicity and economy of the circuit and the opamp can be replaced by many other types.
This explains the hype in those expensive marketed units.

There are so many simple and good circuits from the past and the specified semiconductors are no longer manufactured.
People still use those circuits and find alternatives to those obsolete parts with modern equivalents.
We obviously know from all the electronics forums, more than half of the talks is about finding some equivalent parts.

Thank you,
Best regards.

 

[user 37518]

It is about the simplicity and economy of the circuit and the opamp can be replaced by many other types.
This explains the hype in those expensive marketed units.

Those are not really op-amps in the INA217 or the THAT1510, they are complete integrated preamps, you can consider them Instrumentation Amplifiers (actually that is what the INA in the INA217 stands for: INstrumentation Amplifier).

 

[Chandra]

3 opamps inside or Whatever.
This is a component for professional mic preamp.
Regards.

 

[user 37518]

3 opamps inside or Whatever.
This is a component for professional mic preamp.
Regards.

No, it is not just 3 op-amps inside or whatever, everything inside is in integrated form at the transistor level and I am sure there are at least 70-100 of them, it has interconnections, integrated resistors and so on, the op-amps are also not discrete, but integrated at the transistor level ; we don't say that a Microprocessor is 3 billion transistors or whatever. It can't be replaced with just three op-amp's. You can make a discrete version of it, but it will not be exactly the same.

 

[Chandra]

Thank you. You are a qualified teacher. You will twist anything that I would say.
I saw the representative block diagram from TI showing 3 opamps.
Regards

 

[user 37518]

Thank you. You are a qualified teacher. You will twist anything that I would say.
I saw the representative block diagram from TI showing 3 opamps.
Regards

So you know there are actually not just 3 op-amps in it as the diagram shows. Anyway, the point is that these are not just parts that can be swapped with op-amps as you say. Or am I missing something that you are trying to convey? perhaps you can be clearer.

 

[Chandra]

This circuit is for a simple mic pre as I see it suitable for the OP. Nothing more.
Contains zillion transistors FETs or opamps? Not important.
Just a simple 5 dollar mic pre, which I refereed.
Highly appreciate your experience as a teacher.
Best regards.

 

[ct.waveform]

Why prefer signal on the cold leg. If I understand you correctly that would ground the signal into an unbalanced input?
Wrt output stage coping with an opamp output connected to ground. I think you are referring to the cross coupled output configuration. Used by, among others, DDA in the form of their EBOS module. And similar in IC form eg SSM2142 and better implementation in THAT Outsmart devices. Said to offer transformer like output in that if one leg grounded then signal on other leg gets +6dB lift.

Yes, it would ground the signal. Why do I prefer it? In short, it gave me the biggest single audio improvement in an amplifier I have ever experienced. And it was a direct A/B. About 15 years ago I was working on a small, single supply chip amp which used a LM3875 (now discontinued) in a differential topology. The preamp part was 4 op amps whose functions were, Differential Input, Volume Control, LP filter and Output, and then Inverter (and output). Though there was always a place for the 4th op amp, when I built the prototype I omitted the 4th amp as I already had far more gain than I needed and just used a second 27R resistor. This seemed to be confirmed as a good decision when the volume control (which I wouldn't do the same way now) ended up with a lovely taper that felt very classy. (It was a linear taper pot with a resistor in parallel to mimic a log taper and it seemed I had chosen the right values.) Anyway, it sat that way for about 18 months until one night I decided on a whim to put the final opamp in, to squeeze out the last bit of S/N. As I have said, the difference was quite stunning.

I don't think the improvement could have been just S/N, though 6dB is substantial. There could have been some vagaries in the chip but most likely is that it overcame some deficiency in the grounding scheme. That alone is a good reason to do it, especially as grounding and layout must be at the top of the list of perfectly good circuits failing to perform sonically. (Or, if you like, failing to meet distortion specs.) On this particular amp, where pre and power shared an SMPS, though each part had its own regulation inside the amp, a pointer might be that I remember having problems getting the sometimes very expensive pre amp regulators to perform to spec. So it's not just about CMR, but getting the right reference to its destination, which I suspect I failed to do across just 5" of wire! Having a signal on the Cold could improve the probablity of success. I'd have to think this through more carefully, but maybe if the inversion happens in the second half of a dual op amp, you've virtually guaranteed it.

So, that's why I prefer it. Mainly my personal experience. But some of the stuff I'm working on at the moment actually needs a signal as the two halves have separate paths right through the box and don't meet anywhere to do their CMR thing or any distortion cancellation. Besides, who in audio electronics doesn't love symmetry?

 

[Newmarket]

Yes, it would ground the signal. Why do I prefer it? In short, it gave me the biggest single audio improvement in an amplifier I have ever experienced. And it was a direct A/B. About 15 years ago I was working on a small, single supply chip amp which used a LM3875 (now discontinued) in a differential topology. The preamp part was 4 op amps whose functions were, Differential Input, Volume Control, LP filter and Output, and then Inverter (and output). Though there was always a place for the 4th op amp, when I built the prototype I omitted the 4th amp as I already had far more gain than I needed and just used a second 27R resistor. This seemed to be confirmed as a good decision when the volume control (which I wouldn't do the same way now) ended up with a lovely taper that felt very classy. (It was a linear taper pot with a resistor in parallel to mimic a log taper and it seemed I had chosen the right values.) Anyway, it sat that way for about 18 months until one night I decided on a whim to put the final opamp in, to squeeze out the last bit of S/N. As I have said, the difference was quite stunning.

I don't think the improvement could have been just S/N, though 6dB is substantial. There could have been some vagaries in the chip but most likely is that it overcame some deficiency in the grounding scheme. That alone is a good reason to do it, especially as grounding and layout must be at the top of the list of perfectly good circuits failing to perform sonically. (Or, if you like, failing to meet distortion specs.) On this particular amp, where pre and power shared an SMPS, though each part had its own regulation inside the amp, a pointer might be that I remember having problems getting the sometimes very expensive pre amp regulators to perform to spec. So it's not just about CMR, but getting the right reference to its destination, which I suspect I failed to do across just 5" of wire! Having a signal on the Cold could improve the probablity of success. I'd have to think this through more carefully, but maybe if the inversion happens in the second half of a dual op amp, you've virtually guaranteed it.

So, that's why I prefer it. Mainly my personal experience. But some of the stuff I'm working on at the moment actually needs a signal as the two halves have separate paths right through the box and don't meet anywhere to do their CMR thing or any distortion cancellation. Besides, who in audio electronics doesn't love symmetry? in

Not making a lot of sense to me tbh. Unless you are saying you prefer conventionally balanced / symmetrical outputs. In which case you wouldn't use this circuit type

 

[ct.waveform]

Not making a lot of sense to me tbh. Unless you are saying you prefer conventionally balanced / symmetrical outputs. In which case you wouldn't use this circuit type

Wouldn't use what circuit type? You asked why I preferred signal on the cold leg. Those were my reasons, though there are more reasons than just those, especially with the possibility of distortion components cancelling in the loudspeakers. And obviously some of those distortions are amplitude related so you need a signal for that,

 

[Newmarket]

I was referring to the "Ground Sensing" configurations. I think we might be talking at cross purposes. Cheers.

 

[Matador]

I can confirm that the INA217, built with just the standard reference design in Figure 16, performs flawlessly, and you can make a 8-channel preamp for like $100 in parts (even with PCB's). The standard reference design includes a DC servo, which I eliminated and didn't hear any audible difference. A quick check with a scope confirms less than 10mV on the output, and you can just send the output directly to an XLR with an impedance-balanced output, and get excellent results. If the possibility of DC offset gives you heartburn, you can cap-couple the output.

It's also good to add a build-out resistor to the output: 47 ohms works, to buffer the output from long cable runs. If you use their driver IC (DRV134), then you get a fully-balanced and buffered output as well for about $10 in additional parts, however in my experimenting I thought it was overkill for the typical studio in a controlled environment.

 

[user 37518]

I can confirm that the INA217, built with just the standard reference design in Figure 16, performs flawlessly, and you can make a 8-channel preamp for like $100 in parts (even with PCB's). The standard reference design includes a DC servo, which I eliminated and didn't hear any audible difference. A quick check with a scope confirms less than 10mV on the output, and you can just send the output directly to an XLR with an impedance-balanced output, and get excellent results. If the possibility of DC offset gives you heartburn, you can cap-couple the output.

It's also good to add a build-out resistor to the output: 47 ohms works, to buffer the output from long cable runs. If you use their driver IC (DRV134), then you get a fully-balanced and buffered output as well for about $10 in additional parts, however in my experimenting I thought it was overkill for the typical studio in a controlled environment.

I would strongly advise to include at least a capacitor to block DC. As I mentioned previously, my students built many of these, a student built one with no caps and claimed to have fried one of the inputs of his DAW. Of course, one can't be sure this was the real reason, or whether a secondary issue in the preamp was the cause. But I still would recommend to include a cap just in case.

 

[Newmarket]

Yes. I would ac couple if only to avoid potential damage in case of a fault causing output to go to one of the voltage rails.

 

[user 37518]

Yes. I would ac couple if only to avoid potential damage in case of a fault causing output to go to one of the voltage rails.

Also, I would advice to use at least 50 V rated caps, it is not uncommon that people plug the outputs of these preamps to prosumer audio interfaces, which have the same input for line and mic signals. If for some reason phantom power is engaged, a cap rated for less than 50 V will snap. Which is yet another reason why it is wise to add a cap at the output of the pre. In that case, it is also a good idea to either use bipolar or two series electrolytics back-to-back.

 

[Newmarket]

Yes. On paper I'd spec' min 100Vdc min. But for DIY tempered by fact I can repair stuff relatively easily I'll likely go with whatever I have best available over 25Vdc.

 

[Matt Syson]

With all this 'to and fro' I have not seen much hrere about 'outside world proofing' of a design. things like the arrival of hf noise into the OUTPUT and input of any amplifier that needs proper RF filtering. The speaker cables of an amifier can and do bring received RF into a poorly filteres power amp circuit which then demodulates it and because of the inability of nFB etc to handle and correctly apply feedback at RF then demodulates it. The application notes for any chips are the starting point for a circuit that will function in a small environment of a test bench NOT in the real world outside. Stuffing a stray DC or a few volts of AC at powerline or RF frequencies into or out of a piece of gear is to be expected. Star earthing of a studio is great for protection against electric shock hazards but poor at RF noise 'control' The metal casing of any mains powered gear has to be grounded by law but RF and hum can't read so will sneak into gear by any path possible. Look at the shielding necessary to contain RF radiation from computer motherboards.

 

[ruffrecords]

With all this 'to and fro' I have not seen much hrere about 'outside world proofing' of a design. things like the arrival of hf noise into the OUTPUT and input of any amplifier that needs proper RF filtering. The speaker cables of an amifier can and do bring received RF into a poorly filteres power amp circuit which then demodulates it and because of the inability of nFB etc to handle and correctly apply feedback at RF then demodulates it. The application notes for any chips are the starting point for a circuit that will function in a small environment of a test bench NOT in the real world outside. Stuffing a stray DC or a few volts of AC at powerline or RF frequencies into or out of a piece of gear is to be expected. Star earthing of a studio is great for protection against electric shock hazards but poor at RF noise 'control' The metal casing of any mains powered gear has to be grounded by law but RF and hum can't read so will sneak into gear by any path possible. Look at the shielding necessary to contain RF radiation from computer motherboards.

Excellent points. It is sadly true that today we live in a rfi cesspool what with wifi, GSM, and bluetooth by the truckload. Back in the 70s the worst you had to contend with was a dodgy taxi parked outside the studio.

Cheers

Ian