Ampex 351 input distortion / clipping

[waterfreddy]

Hey guys, I've been checking out a couple of Ampex 351 pre's and noticed that they produce a pretty nasty clipping transistor-like distortion on the input when you send them a hot mic level (say from a guitar amp through a 57, or a loud vocal from a tube mic). When I put a 20dB pad on the mic cable just before it goes into the pre it sounds great. I've been searching up and down the web for answers to why exactly this is happening and haven't been able to come up with much. I'm assuming that the input transformer in the 351 isn't exactly equipped for modern high output mics and are getting over saturated. Does anybody have any insight into what's going on here? And, if so, is there a common mod to fix this?

Thanks!!!

[loujudson]

Umm, I think you are overloading the input. The solution is to not feed it such hot signal. You answered your own question!

If you are talking about old Ampex tape recorder electronics, well, they were built a long time ago and not desigend for today's loud sounds. It just doesn't like it. Het, I'm old and don't like loud sounds either!

Horses for courses, as they say, which means use it appropriately and it will be fine. Don't try to run the Indy 500 in a VW or Model T.

I'll bet it will make a fine preamp for ribbon mics on orchestras or acoustic music!

Lou

Oh, and re: "I'm assuming that the input transformer in the 351 isn't exactly equipped for modern high output mics and are getting over saturated." - No, if the transformer is saturated it will sound iron, if it sounds like transistor overload it is probably input stage overload. Same solution!

L

[Goreski]

Usually if your are hearing clipping - severe distortion - all you have to do is pad the input down and this is easy to do - Switchcraft makes a 10/20 db pad as I recall and you can just plug this in and fix the problem.

Remember those old Ampex Pre-amps are.... well... OLD! They were designed to work with old dynamic mics. So buy a good mic pad and see if that solves your problems.... bet it does!

Regards,

Goreski

Analog Rules!!! Pro-Audio Parts and Service Tons of info on Analog Recording !!

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[waterfreddy]

Thanks guys. I tried a pad and it definitely cleared things up. Sounds Great!! I probably wouldn't be using these for loud signal a lot, but I just wanted to try it on everything I could before I made a decision to buy or not.

The addition of an in-line pad raises another question for me....If I were to use a modern condenser (say a 414 or U87) do you think it would be a good idea to design or buy an in-line XLR barrel pad that would have a higher input impedance, say around 1500 ohms? (and keep the output impedance down at 150 ohms) The 351 input impedance is about 250 ohms, which is great for ribbons and such, but I thought a removable pad/impedance transformer would give me a nice option for higher output modern mics. This way, I wouldn't touch anything inside the pre itself.

Any thoughts?


Thanks again
Jeff

[FFTT]

I'm having a minor problem on the other end with my Ampex 403-P where the signal out of the Ampex is too hot for my M-Audio PMIO.

For now the solution is to take the output from the headphone jack.

Eric at Viking Amps modded mine slightly to produce a dual mono signal to normal stereo
headphones.

[Jay Pemberton]

Originally Posted by waterfreddy

The addition of an in-line pad raises another question for me....If I were to use a modern condenser (say a 414 or U87) do you think it would be a good idea to design or buy an in-line XLR barrel pad that would have a higher input impedance, say around 1500 ohms?

Jeff

One thing you will definitely need, to use mikes like those with that 351, is a source of phantom power--which didn't exist back when the 351 valve machines were made (c.1957--1965).

[waterfreddy]

yeah, i was checking it out using a rolls 48V supply. I was thinking I could then put the pad after that just before i get to the pre.

Jeff