Folkcafe
Well-known member
What, not even some bare bone Millennia HV preamps? This test sucks already.
On a serious note, not sure what a comparison of stage vocal mics will tell us on unknown voices. Crowd seems into it so why not.
On a serious note, not sure what a comparison of stage vocal mics will tell us on unknown voices. Crowd seems into it so why not.
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drtechno
Active member
ISA 110 is an op amp mic preamp and the loss interaction is centered around its input transformer that terminates with a 2.2K resistor. But the termination impedance is not constant, so its higher input impedance at lower gain levels.
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drtechno
Active member
I would have to dig and find info on the 1820 but the omega is an instrumentation amp type like is in a lot of interfaces (INA217)
rob aylestone
Moderator
I think it's that we've all got a fixed perception if common mics, which might be wrong. We are looking at 'labels' - as in X is dull, Y is bright, Z is thin and W is bassy - DrTechno's comments made me wonder a bit if people can hear the things we give the labels to? So a pretty simple task - record 8 mics on the same subject, and it probably has to be in the same place, with the rest of the chain the same, on a 'normal' preamp, so it's more generic than specific. Then, the mic swaps make some sort of sense, if we listen carefully. If I number the mics, and make the recordings shortish and control distance, we can note the ones we hate, like or are just sort of OK on. If everyone hates one, likes another - then it's worked. If everyone has different likes and dislikes, then that too actually says something.
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drtechno
Active member
I imagine the character is the same, but slightly different from different mic pres due to the way they make the input circuit so they can provision a condenser mic. The Phantom power circuits as well as the coupling are losses the passive dynamic mic must pass through. In reality, the input impedance comes from the source impedance of dynamic mics and the input impedance for condenser mics is the ac load of the input termination. These combination condensor/dynamic mic preamp designs they settled on to support both types has always been a design compromise in one way or another. Which really questions why they continue into recording world with them in the first place. A PA console, I see the benefit of generically supporting both types, But one would think they would do something more audiospecific in the recording sector.
You've mentioned that your Midas M32 has an input impedance of 10K (schematic attached). Which is derived from 13.6K from the phantom power injecting resistors, 10 ohm plus the resistance of the capacitor at test fequency plus the 47K Rin resistor in series that is in parallel to the 13.6K (~96020) for the actual impedance of approx. 11,912.72 ohms. The dynamic mic signal has to pass through the 10 ohm resistors and the 47uf cap and the resistance at that frequency to the ideal infinite impedance of the gain device.
One thing I wondered is why recording people haven't looked at broadcasting equipment and how they make mic preamp circuits that are specific to dynamic mics. They don't have any of the phantom power, and the blocking capacitors. I do notice they limit the impedance to 200 ohms in the lowest design and their datasheet distortion rating is derived from shorting the input and measuring noise floor at 50, 80, and 100% gain. Its way different than how they measure but don't guarantee specifications in recording sector equipment.
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gianluca68
Member
You also have to consider it has been designed about 50 years ago! At that time thay haven't neodymium magnets for high level outputs (and also preamps were not as silent as nowadays)... Today, microphones don't need a transformer and the frequency response is simulated and adjusted by computer design.
rob aylestone
Moderator
Sorry, but that is soooo wrong.
This is all about impedance matching, and load - nothing to do with neodymium magnets. ECC mentioned it back a page or two. You have to consider a few things - the input impedance of the device you are connecting to, and the impedance of the coil. You can design a microphone with incredibly thin wire, so that it is more efficient and converting physical movement into electricity, or you can use much less delicate, thicker wire that can stand being inside a kick drum. The thicker and heavier coil and it's former will have more of a resistance to starting to move, and once moving to slow down at the other end of travel. The impact is mainly changes to frequency response, but also a different 'sound'. The lower impedance of one version compared to the higher resistance of the other also works against you by the mic trying to work the other way around - as in the connection to the preamp loads the coil, which actually dampens it. Typical preamps might well have an impedance of between say 150 Ohms up to maybe 1K or a bit more. In this range, the unwanted loading with some impedance capsules presents a good transfer and minimal 'back pressure' using descriptive, not science words. You can connect an SM57, with the transformer missing to a guitar amp - probably the worst combination, but if the amp has enough gain, it sort of works. It gets wrecked a bit by the capacitance of the cable, and the length, but apart from sounding a bit woolly, it works, sort of. You can take a loudspeaker output from an amp rated as perhaps 8 Ohms, and with care stuff it into a guitar amp expecting maybe a 50K pickup? It works. As an example of mismatches, I currently have the output of a Sennheiser IEM receiver, connected to the line input of a powered PA speaker. The mismatch means I have great quality, but the IEM is turned up nearly full and the volume on the speaker is high too. Turning it off makes a huge loud bang. The battery went flat, and I swapped it for a Trantec IEM 4000 - the output impedance of this is very clearly different. I need to have the volume knob way, way down. If I plug earphones into both of them - the volume is the same. The earpieces are 15 Ohm, the input impedance of the speaker is highish - not sure exactly, but probably 25K maybe?
This is what happens when you have impedance mismatches. If the mismatch works well, it may well turn a rather bright mic mellow, or just reduce it's output, or loads of other consequences. My advice is simple. try it and see. You cannot have a rule, because the transformer in a mic is there for a purpose - perhaps just to make it work on a wider range of kit without changing sound? You could design a mic that did not need a transformer, just by altering the windings - more/less winds and more/less thickness to the cable. Just don't allocate the 'better' sound to the mic mod - consider the mic AND your preamp, you have made it a combination. Your good mic could sound rubbish on a different preamp - something that does not happen very much with designed products.
This is all about impedance matching, and load - nothing to do with neodymium magnets. ECC mentioned it back a page or two. You have to consider a few things - the input impedance of the device you are connecting to, and the impedance of the coil. You can design a microphone with incredibly thin wire, so that it is more efficient and converting physical movement into electricity, or you can use much less delicate, thicker wire that can stand being inside a kick drum. The thicker and heavier coil and it's former will have more of a resistance to starting to move, and once moving to slow down at the other end of travel. The impact is mainly changes to frequency response, but also a different 'sound'. The lower impedance of one version compared to the higher resistance of the other also works against you by the mic trying to work the other way around - as in the connection to the preamp loads the coil, which actually dampens it. Typical preamps might well have an impedance of between say 150 Ohms up to maybe 1K or a bit more. In this range, the unwanted loading with some impedance capsules presents a good transfer and minimal 'back pressure' using descriptive, not science words. You can connect an SM57, with the transformer missing to a guitar amp - probably the worst combination, but if the amp has enough gain, it sort of works. It gets wrecked a bit by the capacitance of the cable, and the length, but apart from sounding a bit woolly, it works, sort of. You can take a loudspeaker output from an amp rated as perhaps 8 Ohms, and with care stuff it into a guitar amp expecting maybe a 50K pickup? It works. As an example of mismatches, I currently have the output of a Sennheiser IEM receiver, connected to the line input of a powered PA speaker. The mismatch means I have great quality, but the IEM is turned up nearly full and the volume on the speaker is high too. Turning it off makes a huge loud bang. The battery went flat, and I swapped it for a Trantec IEM 4000 - the output impedance of this is very clearly different. I need to have the volume knob way, way down. If I plug earphones into both of them - the volume is the same. The earpieces are 15 Ohm, the input impedance of the speaker is highish - not sure exactly, but probably 25K maybe?
This is what happens when you have impedance mismatches. If the mismatch works well, it may well turn a rather bright mic mellow, or just reduce it's output, or loads of other consequences. My advice is simple. try it and see. You cannot have a rule, because the transformer in a mic is there for a purpose - perhaps just to make it work on a wider range of kit without changing sound? You could design a mic that did not need a transformer, just by altering the windings - more/less winds and more/less thickness to the cable. Just don't allocate the 'better' sound to the mic mod - consider the mic AND your preamp, you have made it a combination. Your good mic could sound rubbish on a different preamp - something that does not happen very much with designed products.
E
ecc83
Well-known member
Not quite sure what drtechno is getting at but...let us examine pre amp input impedance WRT loss of signal?
The nominal output impedance of the SM58 is 150R. Feed that into the common average pre of 1k2 and you get a loss of ~0.9dB. Not something that is going to 'noise up' even the most critical recording? For a pre amp with a Z in of 10k the loss is 0.13dB. Sleep will not be lost over that!
There has been a "fashion" in the last 5 years or so to make mic pres with higher than the normal 1k2 input Z but most authorities agree that the effects are subtle and really only evident with dynamic mics, especially ribbons. Certainly the above figures show that sensitivity is not compromised with a 1k2 input. Of course, if you go as low as 150 Ohms then you lose 6dB or so but then if a 6dB loss of signal makes your noise level unacceptable, DON'T use an SM58 or buy a better pre amp!
Oh and the 47uF caps in that schematic insert an extra 136 Ohms at 50Hz giving a ~2dB loss but again, if a good low frequency response is important to you, a dynamic, directional mic is the wrong tool. Above 100Hz, nay bother.
Damping: I personally doubt that there is enough current flowing in the 1k2 circuit to produce any significant Electro-Magnetic damping? For sure there will be next to bugger all at 10k! I used to have a BBC hardback book about microphones, wish I had never sold it it but in it they had electronic analogues of microphones such as the STC "Ball&Biscuit". In these math analogues, mass is modeled by inductance, compliance* by capacitance and aperiodic loss by resistance....the "circuits" were incredibly complex as were the resultant equations. WAY out of my pay grade! So, AFAICT the interaction of a mic and pre amp is a complex thing beyond most of us and so we can only suck it and see.
One point about "broadcast" pre amps? These are/were generally configured for maximum headroom as against minimum noise, something to bear in mind if a 50yr old radio station desk comes up on Ebay for a bargain nifty! A good point though is that they usually used input transformers for high RFI immunity. Slap in a really low noise op amp such as the LM4562 and you might have a good deal.
*The reciprocal of stiffness.
Dave.
The nominal output impedance of the SM58 is 150R. Feed that into the common average pre of 1k2 and you get a loss of ~0.9dB. Not something that is going to 'noise up' even the most critical recording? For a pre amp with a Z in of 10k the loss is 0.13dB. Sleep will not be lost over that!
There has been a "fashion" in the last 5 years or so to make mic pres with higher than the normal 1k2 input Z but most authorities agree that the effects are subtle and really only evident with dynamic mics, especially ribbons. Certainly the above figures show that sensitivity is not compromised with a 1k2 input. Of course, if you go as low as 150 Ohms then you lose 6dB or so but then if a 6dB loss of signal makes your noise level unacceptable, DON'T use an SM58 or buy a better pre amp!
Oh and the 47uF caps in that schematic insert an extra 136 Ohms at 50Hz giving a ~2dB loss but again, if a good low frequency response is important to you, a dynamic, directional mic is the wrong tool. Above 100Hz, nay bother.
Damping: I personally doubt that there is enough current flowing in the 1k2 circuit to produce any significant Electro-Magnetic damping? For sure there will be next to bugger all at 10k! I used to have a BBC hardback book about microphones, wish I had never sold it it but in it they had electronic analogues of microphones such as the STC "Ball&Biscuit". In these math analogues, mass is modeled by inductance, compliance* by capacitance and aperiodic loss by resistance....the "circuits" were incredibly complex as were the resultant equations. WAY out of my pay grade! So, AFAICT the interaction of a mic and pre amp is a complex thing beyond most of us and so we can only suck it and see.
One point about "broadcast" pre amps? These are/were generally configured for maximum headroom as against minimum noise, something to bear in mind if a 50yr old radio station desk comes up on Ebay for a bargain nifty! A good point though is that they usually used input transformers for high RFI immunity. Slap in a really low noise op amp such as the LM4562 and you might have a good deal.
*The reciprocal of stiffness.
Dave.
