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AMPEX 601 Dual Mono Altec Rack This design offers 2 matched preamp channels. Each has the full original circuit path of the 601 and my favorite mod: the addition of an Altec 15095 output transformer tapped off after the microphone preamp section of the 601.
The tape electronics section has been converted to a second mic/line preamp channel. Old carbon composition resistors have been replaced with Dale/Vishay 1% metal film resistors on both channels so they will match for use as a stereo unit. This has been completely recapped with Nichicon PW series electrolytics, Auricaps, polystyrene replacements for ceramic discs, and a few orange drops. Here's a sample recording of 2 classical guitars recorded through this preamp. The only processing applied is a little reverb and a limiter on the main buss. No EQ, noise reduction or other trickery. In the past, I have tried to use some of the old caps to try to preserve more of the "vintage sound." Even desirable caps reported to last (Budroc, Bumble bees, and ceramic caps) are failing these days and finding caps originally spec'd at 5% tolerance which are 100% or more out of spec has lead me to start replacing all the important components with high quality modern components. For those that are looking for that "vintage sound," please consider: when these units were fairly new, these components would not have been so out of spec. What you get with a deteriorated vintage unit is not the same performance that was achieved when they were new. To me, what is most important about the "vintage sound" comes from the tubes, the tube circuitry, and the transformers (The Iron!) Supporting those components with high quality caps and resistors provides excellent performance. Features:
All The important controls and I/O have been brought to the front for rack mounting. The hum balance pot and the fuse are the only things not available from the front. 1) Metering - One of the things I really dislike is when companies try to hide the drawbacks to their product through misdirection and misleading ads. I will tell you straight out about the things that are not ideal about a design. In this design, the metering isn't great. With 4 outputs (L/R Altec and L/R CF) and only one meter, one has to accept that the metering is limited. I felt it was very important to be able to check levels for all the outputs, so I designed the metering options to allow for this. There is a rotary switch (Output Meter Select) which selects the output to be metered. Then one can flip the L/R meter select switch between L and R. According to specs, the meter adds up to .3% THD. Some may wish to turn off the meter after setting levels. 2) Mic / Line switches - Individual switches means that one may use the mic input on one channel and the line input on the other. Flip the switches up for Mic and down for Line. (the left switch is for the left channel, the right switch is for the right.) To help remember: When the switches are in the MIC position (up) they are more out of the way of the XLR inputs.
3) Monitor selector switches (Output Select) - These switches select the output for each channel. In the left position, the Altec output is active. In the right position, the cathode follower output is active. There is also a middle position which makes both outputs active. So we have (A)ltec, (B)oth and (C)athode follower; from left to right. I chose to try to make the design logical enough so I would not have to slap labels all over it. There are some strange things that can happen with certain combinations of these switches, what the outputs are feeding, and the termination switches. Nothing harmful, but you will have some loss of gain from the extra loading of the circuits. Rule of thumb: When using both outputs, turn off the termination for the Altec outputs for best results. In fact, I prefer the sound of the Altecs when unterminated in most situations. 4) Line inputs - (unbal. 1/4" TS 250k impedance) The new channel uses a Neutrik combo jack for both the XLR and line inputs. These inputs are suitable for electric guitars as well as line level outputs from a CD player for example. These preamps work beautifully as the front end for a listening system. I listen to music played on a CD player and run into the line inputs, with the outputs of the preamp feeding my power amps and speakers - nothing else in the signal chain. The sound is very good. 5) L/R Unbal C/F Outputs - These are good for use as a Hi-Z, unbalanced output to feed a mixer, sound card, or similar device. This is an important feature for those that like different colors because these outputs do not go through an output transformer and they have a slightly different sound compared to the 600 Ohm balanced outputs. 6) Termination switches - These are the switches located next to the line level input controls. Up is 600 Ohm termination on, down is off. These switches allow one to turn off the termination which is usually desired when feeding modern gear with a vintage 600 Ohm output. When feeding modern gear, the termination allows for proper frequency response from the circuit. However, there may be times when a different flavor is desired or even more gain, so I have found that there are no rules when it comes to whether you use the termination or not. (well, ... actually one strong suggestion: When using both sets of outputs at the same time, I recommend turning off the termination on the Altec outputs.) 7) Cathode Follower "Rec Cal Pot" - I have replaced the original screwdriver adjusted "rec cal pots" with these nice new pots, knobs and faceplates. These controls serve as master level controls for the cathode follower outputs. One uses these to control the amount of "tubeyness," "color," or distortion - take your pick. If one turns these controls down and turns up the input levels, more color is the result. Everything from a filthy, nasty grunge to a very clear sound is possible. These controls only work with the cathode follower outputs. The Altec outputs provide the cleanest sound and simplest signal path, but don't have the overdrive capability of the cathode follower sections. -------------------------------------------------------------------------------------- The following shows the components for the 2 channels. This will help when tube swapping. The red shaded components are the left channel and the blue ones are the right. (sorry for breaking convention here. Red should have been right.) There are painted stripes on the chassis to indicate the channel paths.
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