Hi Guys, the original U87 has a transformer with a 5:1 ratio (200 ohms) and can be strapped at 10:1 ratio (50 ohms)
The U87ai has a transformer with a 7:1 ratio. Here are the schematics for the U87 made before 1988 and the U87ai made after 1988.
Both microphones have the same single class "A" fet transformer coupled circuit running from the same supply voltage but the polarization circuit is quite different.
The original U87 has a polarization voltage of 46v while in the U87ai the capsule is polarized at 60v yielding more output from the capsule and a bit more upper midrange lift.
I have serviced many U87's and U87Ai's and to my old ears the original always sounded better, (once the issue it was sent in for is resolved).
The original U87 always seemed to sound warmer and smoother. The U87Ai just sounds brighter and more sizzly to me but some folks do like that sizzle.
You never have to brighten a U87ai or a U87 in my experience but it never sounded as good to me as adding 3db at 12khz with the Pultec to the U47.
Having said that, I like to stay away from EQ whenever possible!!!
Because the output of the capsule is a few db higher from the 60v polarization the single fet amplifer has to handle this extra output and the circuit is already at its limits.
So, the U87ai has less headroom than the original U87 which had a lower output from the capsule and the original U87 has 3db less transformer loss than the U87AI.
Even at 69 I can still hear at least a 3db difference in headroom between the two microphones.
I agree with one of my hero's the late great Roger Nichol's who stated on the liner notes of Katy Lied that, "Individual microphone EQ is frowned upon".
I have never had a problem hearing the vocals and background vocal parts on a Steely Dan recording and they are usually more complex than other rock & roll recordings.
The legendary Al Schmitt would first change the microphone or microphone position rather than resort to EQ.
When I worked once with Bill Porter he was very concerned about where the artist was positioned in front of the U47.
He moved the snare microphone rather than ask for it to be EQ'd.
At Ocean if a producer wanted a brighter microphone the C12 or C414eb would be pulled out first rather than a U87.
One very famous producer brought in a Sanken CU44 which is very bright and we had several other tube condenser microphones to choose from including 5-U87's.
They only had U87's at Little Mountain and Bob Rock would come over to Ocean Sound to cut vocals on one of our U47's.
Vincent, used my CM49 quite successfully and only rolled out the very low end and the very high end above 10khz to move it back into a more 50's soundscape.
He never had to EQ anything in the midrange frequencies.
Knowing, the REDD 47 circuit it would be difficult to bypass the built in negative feedback.
You design a negative feedback amplifer for maximum gain and then, as you add more negative feedback, the gain (output) level is decreased.
So, it is quite possible that when you gain the Redd down to +4dbu you are adding the maximum amount of negative feedback.
Someone mention the U87 sounded good through a Neve 1073 which has two gain stages utilizing negative feedback.
Our MT8016 transformer coupled preamp also uses two stages of negative feedback and it makes my TLM103 sound much more palatable to my old ears.
I have built up a Redd47 circuit and it does something really nice to whatever you put into it.
The one problem I see as a microphone builder is the more circuitry you put inside a microphone the more parts there are to fail.
Just one tube inside a microphone produces a fair bit of heat.
You can always patch into another preamp if one fails, so I am not sure about putting all my eggs in one basket?
However, I have spent most of my life with a single microphones connected into a single microphone preamps.
The track on the Redd site sounds really good and its a lovely sounding room.
However, someone building their first studio could buy two tube microphones and a 2 channel preamp from us for that kind of money.
Also, the microphone is also only competing with an acoustic guitar on the track I heard. I would want to hear a bit more "spark" when the bass and drums kick in.
The one advantage the Redd has over all others is you are bypassing 30' of microphone cable or more in some cases.
I would love to hear the Redd microphone directly patched into my IZ Technology A/D converters.
I will have to resurrect my Redd 47 project. It will work brilliantly with the 6:1 transformer we are currently using in our MT8016.
Cheers, Dave
aamicrophones.com