hasirish.blogg.se

1. AMPEX 351 INPUT TRANSFORMER GENERATOR
2. AMPEX 351 INPUT TRANSFORMER MOD
3. AMPEX 351 INPUT TRANSFORMER PRO

This testing was stand-alone, without feedback.I've recently begun restoring a classic Ampex 351-2 studio tape recorder that I've been collecting parts for over 10 years.

I tested the 351 output transformer years ago, and was shocked at how mediocre it was, given how good the 351 sounds. The early units had a VU meter on/off switch, while on the 351 and 354, you would switch the meter to BIAS or ERASE. Those in the know would turn the VU meters off when transcribing a tape. I can't remember exactly how much it went up, but it was in the order of 0.1 to 0.2%. This was due to the non-linearities of the built-in copper-oxide rectifier in the VU meter. Virtually all the loads at the Columbia-Princeton Electronic Music Center, where I worked, were 15K or higher.īTW, the distortion went up in all the old Ampex machines when the VU meter was turned-on. I know this, since I used to repair them when I was in college, and we usually never turned-on the 600 load, since the distortion went up. With feedback, it works well, and maintains good response, even into no load. The Ampex 351 line out stage has a fair amount of feedback from a tertiary winding (what He calls the 'headphone winding'), and it doesn't work very well without feedback.

AMPEX 351 INPUT TRANSFORMER PRO

Most line output transformers, at least in the pro audio world are pretty conservatively designed and hold frequency response, even with no load. He is correct in theory, but you mainly see frequency response anomalies when the transformer is driven by a high impedance source, such as an interstage transformer. Now for the ampex fans, here is some more info in the unit from an earlier email from john. Then it hit me, that 7dB loss of gain is the result of Rsource > Rload. This is the first time I have seen this behavior and it took me a few days to come to grips with it. BTW, the frequency response is virtually the same if the transformer secondary is connected unbalanced (Blk N/C, Grn grounded). In the unloaded case, there will be 7dB more feedback, thus flattening the response even more. It should be remembered that in the Ampex circuit, a fair amount of feedback is used, which will further flatten the frequency response in both cases. The frequency response isn't very good in either case, although the 600 ohm load flattens it out. The drop in mid-band level is due to the effect of the 600 ohm load with the 2 x 10K source impedance.

AMPEX 351 INPUT TRANSFORMER GENERATOR

The generator source impedance is 40 ohms, thus negligible. Looking at the 12AU7 data sheet, this gives an Rp of just about 10K, hence the 10K resistors driving the transformer. According to the Ampex 351 schematic, each half of the 12AU7 driving the transformer is drawing 6.5mA and has a plate-to-cathode voltage of about 275V. The second strike was John Atwood and I reeled that one in because he has a huge amount of knowledge / experience on the 351 and he not only had a sample of the transformer in question but the AP to measure it.SWEET!Īnd here is his commentary on the measurement method.Īttached is the result of my 351 line output transformer tests. To cut to the chase I cast out a line among my friends to try to score one and the first strike came from a tape friend with a carcass. It was insisted that loading the transformer improved the overall top end behavior considerably and I didn't buy it. Was in a discussion on another asylum about transformers and loading when this device came up and I was skeptical about the alleged behavior. 4) Existing transfer switch used to provide input pad and a DI function.

3) Stepped resistor ladder for 'record level' control. 2) A built-in phantom power supply (using the old 'DC heater' transformer winding).

AMPEX 351 INPUT TRANSFORMER MOD

I have been working on the preamp mod for the Ampex 351, but with certain modifications: 1) A regulated 12V heater supply.If the 351's output is feeding a device with a 600 Ohm INPUT (like a lot of old, transformer-input devices), you won't need the 600 Ohm output. One that has something higher than a 600 Ohm INPUT impedance). Technically, you're going to want to terminate the 351 with a 600 Ohm resister any time the 351 is plugged into a 'modern' device (i.e.

The usual transformer used in this installation is a UTC HA-100X. The numbers stamped on it are UTC P-1245 and B-6299. I'm looking for specifications, pinout and hookup details.