Post by aamicrophones on May 21, 2017 17:44:29 GMT -6
Hi thanks guys, Yes I noticed the increased noise floor as did Joe the engineer. I also was expecting to hear a bigger difference in sound. I actually double checked the files that Joe sent me to make sure I hadn't copied the same file twice on first listen.
I did listen to the tube noise of that microphone before it went out and it seemed as quiet as our other tube microphones or fet microphones. So, not considering I sent it out with a slightly noisy tube I did not catch or one that went prematurely noisy:
1) The BV18 has an 8:1 turns ratio while the original BV8 and our BV8 have a 6.5:1 ratio. The 8:1 will have 2db more loss so the microphone will be 2db noisier than one with a 6.5:1 ratio.
2)
A] One of the interesting facts of the original BV8 is that it has a built-in HP roll off.
By enlarging the value of the transformer coupling capacitor with the original BV8 the low end actually gets less and not more. The four windings of the original BV8 are not wound equally and the two outer windings have fewer turns than the inner two.
This and the unique "stuffing" or stacking method of the transformer laminations in conjuction with a 1ufd capacitors cause the low frequencies to roll out quite quickly after 40hz.
Note, that nearly all 47s that came from radio stations have a different lamination stacking pattern, reducing the low end all the way up to 120Hz at about 10 to 12dB an octave. For studio use, these lamination should be re-stacked in broadcast versions of the U47.
However, the U47 meant for the studio with its original BV8-08 transformer is meant to roll out at 40hz and you are looking at the same 10-12db/octave roll-off.
B] Our BV8 transformer is flat down to 20hz and then rolls out 6db/octave, typical of a standard transformer but the original BV8 is no "standard" transformer.
The U47 will roll out 10db per octave starting at 40hz. There is one octave between 20hz and 40hz which means the CM48T would have at least 10 db more room noise down at 20hz where the rumble and flutter frequencies are located.
The CM48T has a built in HP filter that rolls the low end out 6db/octave starting at 60hz. This was not engaged for the shootout recording. This is one of the reason we use "A" weighting for measuring the signal to noise of microphone circuits.
When the microphone is in a 9' Bosendorfer piano whith the extra 1/2 octave on the low end I might want a response that is flat down to 20hz.
I am just building a station for measuring the internal noise of the GE/JAN 5654w tubes after we burn them in for a good period of time. We now have the room in the new shop/storage area that we can run all 100 tubes through a more rigorous testing process.
However, I believe the majority of the extra noise we hear is because the U47 more vigourously rolls out the low end below 40hz.
The difference between our CM47 and CM47ve is the capsule. The CM47 uses our AK67 capsule which is very smooth in the 3-5khz area and in the CM47 headgrill rise up 4db at 12khz.
This makes it sound like a mellow U47 but with 3db of Pultec like EQ at 12khz. I found in the days of tape machines that we often added +3db at 12khz to the U47 to get it to pop out in a busy mix.
The CM47ve has our AK47 capsule that we use in the CM48T and there is about a 3db lift between 3-5khz and barely a 2db lift at 10khz.
The CM47 and CM47VE also both have a 2-stage dual triode class "A" CCDA circuit which has 6db more headroom than the U47 circuit and an output impedance 20 times lower.
The CM47 and CM47ve is my humble way of improving the venerable VF14 circuit and distortion figures because of improved tube circuitry that came along after the U47 was designed.
The LA2 compressors for example use a similar method to drive its output transformer from a much lower drive impedance. The lower the source resistance the lower the transformer distortion.
The CM48T was designed to give a bit more of the single stage pentode strapped as a triode U47 type circuit sound where the low frequency (musical) transformer distortion is increased. This will give folks with budget preamps a bit more vintage warmth to start with.
The VF14 tube was designed to run at 105v. It was specially designed by Telefunken as an "high end" audio tube but was based on a series of tubes used in a 2nd World War German tank transmitter which worked from 14-6v batteries in series.
Circuits that use a EF14 or UF14 will not have the same plate output impedance as they were designed to run at a higher plate voltage not batteries. Klause Heynes, states that a U47 that has been converted to an EF14 or UF14 never sound the same as the original.
The GE/JAN 5654w is much closer as it was designed to run or an average operating voltage of 125 volts much closer to the VF14 than many other tubes plus its one hell of a lot cheaper as they are still used today in some critical avionic circuits.
Cheers, Dave
I did listen to the tube noise of that microphone before it went out and it seemed as quiet as our other tube microphones or fet microphones. So, not considering I sent it out with a slightly noisy tube I did not catch or one that went prematurely noisy:
1) The BV18 has an 8:1 turns ratio while the original BV8 and our BV8 have a 6.5:1 ratio. The 8:1 will have 2db more loss so the microphone will be 2db noisier than one with a 6.5:1 ratio.
2)
A] One of the interesting facts of the original BV8 is that it has a built-in HP roll off.
By enlarging the value of the transformer coupling capacitor with the original BV8 the low end actually gets less and not more. The four windings of the original BV8 are not wound equally and the two outer windings have fewer turns than the inner two.
This and the unique "stuffing" or stacking method of the transformer laminations in conjuction with a 1ufd capacitors cause the low frequencies to roll out quite quickly after 40hz.
Note, that nearly all 47s that came from radio stations have a different lamination stacking pattern, reducing the low end all the way up to 120Hz at about 10 to 12dB an octave. For studio use, these lamination should be re-stacked in broadcast versions of the U47.
However, the U47 meant for the studio with its original BV8-08 transformer is meant to roll out at 40hz and you are looking at the same 10-12db/octave roll-off.
B] Our BV8 transformer is flat down to 20hz and then rolls out 6db/octave, typical of a standard transformer but the original BV8 is no "standard" transformer.
The U47 will roll out 10db per octave starting at 40hz. There is one octave between 20hz and 40hz which means the CM48T would have at least 10 db more room noise down at 20hz where the rumble and flutter frequencies are located.
The CM48T has a built in HP filter that rolls the low end out 6db/octave starting at 60hz. This was not engaged for the shootout recording. This is one of the reason we use "A" weighting for measuring the signal to noise of microphone circuits.
When the microphone is in a 9' Bosendorfer piano whith the extra 1/2 octave on the low end I might want a response that is flat down to 20hz.
I am just building a station for measuring the internal noise of the GE/JAN 5654w tubes after we burn them in for a good period of time. We now have the room in the new shop/storage area that we can run all 100 tubes through a more rigorous testing process.
However, I believe the majority of the extra noise we hear is because the U47 more vigourously rolls out the low end below 40hz.
The difference between our CM47 and CM47ve is the capsule. The CM47 uses our AK67 capsule which is very smooth in the 3-5khz area and in the CM47 headgrill rise up 4db at 12khz.
This makes it sound like a mellow U47 but with 3db of Pultec like EQ at 12khz. I found in the days of tape machines that we often added +3db at 12khz to the U47 to get it to pop out in a busy mix.
The CM47ve has our AK47 capsule that we use in the CM48T and there is about a 3db lift between 3-5khz and barely a 2db lift at 10khz.
The CM47 and CM47VE also both have a 2-stage dual triode class "A" CCDA circuit which has 6db more headroom than the U47 circuit and an output impedance 20 times lower.
The CM47 and CM47ve is my humble way of improving the venerable VF14 circuit and distortion figures because of improved tube circuitry that came along after the U47 was designed.
The LA2 compressors for example use a similar method to drive its output transformer from a much lower drive impedance. The lower the source resistance the lower the transformer distortion.
The CM48T was designed to give a bit more of the single stage pentode strapped as a triode U47 type circuit sound where the low frequency (musical) transformer distortion is increased. This will give folks with budget preamps a bit more vintage warmth to start with.
The VF14 tube was designed to run at 105v. It was specially designed by Telefunken as an "high end" audio tube but was based on a series of tubes used in a 2nd World War German tank transmitter which worked from 14-6v batteries in series.
Circuits that use a EF14 or UF14 will not have the same plate output impedance as they were designed to run at a higher plate voltage not batteries. Klause Heynes, states that a U47 that has been converted to an EF14 or UF14 never sound the same as the original.
The GE/JAN 5654w is much closer as it was designed to run or an average operating voltage of 125 volts much closer to the VF14 than many other tubes plus its one hell of a lot cheaper as they are still used today in some critical avionic circuits.
Cheers, Dave