ampex 351 channel modded for standalone mic pre attached
heres albinis page on it
Famous 1960s Ampex tape machine channel we rebuilt and modified to be used as a microphone preamp. Qty: 6 (4 in Studio B, 2 in Studio A)
www.electricalaudio.com
the B-17331 input transforers are popular MC SUTs.
I have a tool box with Albini's signature on it, from when he came to the Twin Cities to do a show. He put the LOMO logo on it too. By accident someone in the band left the purse that had all the payments from the shows they had done on that tour. They were pretty glad to get that back!
WRT the input of the schematic at the link: the B-17331s are the Ampex input transformers. You might want to run a square wave through them and look at the output. I used a pair of those in a mic preamp I designed just last week; there was a bit of overshoot on the square wave and the transformer began rolling off at 12KHz. To solve that, I had to load the output of the transformer (since I did not provide any input loading); that value was 60K which killed most of the overshoot and flattened out the bandwidth so it went well past 20KHz.
Douglas has experience with some rather out of the box circuit designs, so I'm confident he has a plan to make it function.
The consideration is that when driving balanced lines usually ground is ignored. There are a couple of reasons for this.
The first has to do with Common Mode Rejection Ratio (CMRR) and is the reason input and output transformers do not use a center tap. The center tap degrades the CMRR quite a lot. The second reason is by ignoring ground, ground loop buzz cannot be amplified. When you have two outputs, as in the case of a differential amplifier driving the outputs directly or via CF circuits, you have both problems, as the outputs are not exactly the same; tiny differences can result in fairly big changes in the CMRR, plus of course ground is involved so the shield of the interconnect cable, which is normally just a shield, is now part of the audio circuit and thus influences the 'sound' of the cable.
There is a standard for balanced operation known as AES48. It outlines the practices used for proper transmission and in particular how the circuit is to be grounded. Typically a balanced line connection is also low impedance; in the old days the termination standard for line level was 600 Ohms. If you look on the back of the Ampex 351 tape electronics, you'll find a switch that puts 600 Ohms across the output. Its there because the output transformer needs that load to have flat frequency response in case the electronics are not connected to a line level input (such as a mixer console). This would allow the electronics to drive a high impedance input with proper bandwidth.
It is on this account that balanced equipment output levels are specified in dBm rather than Voltage. 0dBm is 1 milliwatt into 600 Ohms; a VU meter looking at that will read 0VU. The output of the Ampex 351 is good to about +24dBm so its got plenty of output to drive any input with lots of headroom.
The circuit I proposed in my prior post has this capability plus it supports AES48. You don't have to worry about matching triodes or the like since the output is the secondary winding of the transformer. Any 12AT7 with reasonably close internal triodes (but not matched) will work fine- the only difference will be how much distortion is made, not whether the output is properly balanced.
The only issue with supporting AES48 is connecting to single-ended inputs. In some cases you can get a buzz if you don't hook it up right. That is why
Rane put up this page years ago. If the circuit isn't compliant, you don't have that problem because the non-compliant circuit is already referencing ground in the generation of either the pin 2 or pin 3 signal. If you use an output transformer though its no worries tying one side to ground; transformers are very good about converting from single-ended to balanced or vice versa.
It will need to drive a digital cross over.
