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#1
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Input transformer padding vs mic loading
How important is mic loading? In the below schematic the mike will see
150 ohm load during unpadded but see ~1300 ohms during -20 dB pad. The circuit seems more concerned with input transformer primary loading than mic loading. Am I missing something here? http://www.jensen-transformers.com/as/as016.pdf Thanks Ed |
#2
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Input transformer padding vs mic loading
Edvard Puskaric wrote:
How important is mic loading? Very important. Depending on the type of microphone, the consequences of improper loading can very well include: [a] decreased sensitivity [b] altered frequency response [c] increased harmonic distortion [d] diminished maximum SPL In the below schematic the mike will see 150 ohm load during unpadded but see ~1300 ohms during -20 dB pad. Unfortunately I don't have time to view the schematic, but a "150 Ohm" microphone should not be loaded with less than about 1000 Ohms. The circuit seems more concerned with input transformer primary loading than mic loading. Am I missing something here? http://www.jensen-transformers.com/as/as016.pdf Hmmm--Jensen's engineering is first-rate as a rule. I wish I could look into this further at the moment. I'll check back later--probably someone else here will have delved in by then. |
#3
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Input transformer padding vs mic loading
Edvard Puskaric wrote:
How important is mic loading? Important. I use the 'rule of thumb' of loading with 10 x source Z. For a 150-200 ohm source a load of 2k is appropriate and widely used by major console manufacturers. Proper loading ensures optimum voltage transfer to the following stage and therefore best signal/noise ratio and also the above loading should ensure flattest response from the mic. In the below schematic the mike will see 150 ohm load during unpadded No actually, the 'see other schematic' doesn't define the final load impedance. but see ~1300 ohms during -20 dB pad. No again for the same reason. Certainly a higher Z though. Very wrong, the pad should have both series and parallel resistors. Basic stuff actually. I also see that they suggest adding series resitance for Low-Z mics. This is idiotic since it will degrade the noise level ( higher source Z - i.e. thermal noise ) and lose signal voltage ( worse S/N ratio ) ! The circuit seems more concerned with input transformer primary loading than mic loading. Am I missing something here? Don't think so. Why do you want to use a transformer for a 150 ohm mic anyway with all its inherent response and distortion problems ? Graham |
#4
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Input transformer padding vs mic loading
On Sat, 11 Oct 2003 03:41:58 -0400, Edvard
wrote: How important is mic loading? In the below schematic the mike will see 150 ohm load during unpadded but see ~1300 ohms during -20 dB pad. The circuit seems more concerned with input transformer primary loading than mic loading. Am I missing something here? Hi Ed, The pad is out of circuit when switched to "unpadded". So the mic sees the preamp as load (and the phantom resistors in series with each other). Give it another look and you'll see it differently. Good fortune, Chris Hornbeck new email address |
#5
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Input transformer padding vs mic loading
On Sat, 11 Oct 2003 15:51:45 +0100, Pooh Bear
wrote: Why do you want to use a transformer for a 150 ohm mic anyway with all its inherent response and distortion problems ? I have built a pair of fixed gain tube mic pre's and was looking to pad the inputs. I ran across the jensen info while doing some design research. Ed |
#6
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Input transformer padding vs mic loading
How important is mic loading? In the below schematic the mike will see
150 ohm load during unpadded but see ~1300 ohms during -20 dB pad. The circuit seems more concerned with input transformer primary loading than mic loading. Am I missing something here? Yes: typically a Jensen input transformer, when properly terminated, presents a load of 1k or greater to the microphone. For example, the JT115K-E is a 1:10 transformer, nominally 150 ohms : 15k. But those are the equivalent *source* impedances; the recommended loading is 150k, which transforms down into a 1.5k load, at least at most audio frequencies (we won't get into that). As for the series resistors...as you note, the designers are concerned with input primary loading. Some transformers, when presented with a too-low source impedance, ring. As Graham notes, that will degrade input noise performance. Whether that matters or not depends on the source; if it's a low-Z ribbon microphone it'll make a real mess, whereas if it's a low-Z high-output condenser, like a TLM-103 or a Schoeps with loud stuff in front of it, the extra resistance won't matter at all, as it'll be swamped by the microphone's self-noise. Peace, Paul |
#7
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Input transformer padding vs mic loading
Edvard Puskaric wrote:
On Sat, 11 Oct 2003 15:51:45 +0100, Pooh Bear wrote: Why do you want to use a transformer for a 150 ohm mic anyway with all its inherent response and distortion problems ? I have built a pair of fixed gain tube mic pre's and was looking to pad the inputs. I ran across the jensen info while doing some design research. OK, so you want the 'valve sound' hence a transformer is a must as a voltage step up device. Do you know what load the primary of the transformer presents to the mic when directly connected ( will depend on your tube stage values ) ? Given that It'll take a couple of mins to give you a 'proper' pad design. Graham |
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