old altec amps

[Rob 515]

I will gently disagree with bfish on a couple points;

Line level for professional Altec Lansing stuff was 0 dBu (0.775v).
This points out that you need to always check the gear's specs

Impedance matching was critical a couple of decades ago. "Matching Transformers" were needed once upon a time. Today we select an input or an output transformer. They will usually be 1:1 (15k:15k, 10k:10k, 600:600). This 1:1 ratio means that what you connect across the secondary windings is pretty much what you see on the primary windings. Because transformers have no impedance of their own, the input/output selection is needed simply to handle a little more current on the output.

Voltage transfer is the goal for most of the gear you are messing with. Good voltage transfer needs better than a 1:10 impedance ratio. Your 2280 inputs are 15k ohms with or without the input transformers. A good source impedance would be less than 1.5k ohms. The source impedance of most pro gear is listed as "600 ohm". This is not really the impedance, it is the maximum rated load. Actual source impedance of pro gear tends to be at or below 100 ohms.

"This 1:1 ratio means that what you connect across the secondary windings is pretty much what you see on the primary windings. Because transformers have no impedance of their own, the input/output selection is needed simply to handle a little more current on the output"

So I really don't need the 1:1 transformer and could just stap that socket like the others in the amp? Correct? or should I leave the 15K:15K (1:1) transformer installed?

"Voltage transfer is the goal for most of the gear you are messing with. Good voltage transfer needs better than a 1:10 impedance ratio. Your 2280 inputs are 15k ohms with or without the input transformers. A good source impedance would be less than 1.5k ohms."

So just about any preamp should work? correct? Would'nt most consumer grade preamp be less than 1.5k ohms? What about vintage tube preamps while I'm asking? are those also below 1.5k ohms?

"Line level for professional Altec Lansing stuff was 0 dBu (0.775v).
This points out that you need to always check the gear's specs"

"The source impedance of most pro gear is listed as "600 ohm". This is not really the impedance, it is the maximum rated load. Actual source impedance of pro gear tends to be at or below 100 ohms."

Just about time I read a certain spec on a piece of gear, someone comes along and tells me that what it says it not what it really means! ha ha!

My problem with checking specs is about the same as my problem with reading Japanese.

Sorry about so many questions.... I figure a light bulb will come on one day and I'll "get it" but I'm 50 and it hasn't even slightly flickered yet.

thanks
rob

[whitebroncoii]

Rob 515: So I really don't need the 1:1 transformer and could just stap that socket like the others in the amp?
whitebroncoii: Yes. the input of the 2280 is unbalanced 15k ohms without the transformer

Rob 515: or should I leave the 15K:15K (1:1) transformer installed?
whitebroncoii: I would leave the transformer(s) installed and take advantage of the balanced input isolation.

Rob 515: So just about any preamp should work?
whitebroncoii: Yes

Rob 515: Would'nt most consumer grade preamp be less than 1.5k ohms?
whitebroncoii: Yes

Rob 515: What about vintage tube preamps while I'm asking? are those also below 1.5k ohms?
whitebroncoii: Check the source impedance spec of the preamp

[bfish]

Rob 515: or should I leave the 15K:15K (1:1) transformer installed?
whitebroncoii: I would leave the transformer(s) installed and take advantage of the balanced input isolation....

Well...

It's not really balanced if the other end comes from an unbalanced pre...

You could try it strapped first, and if you find no need for the added isolation (indicated by an acceptable noise floor), you could eliminate the +4 to -28 degree phase shift inherent to the xfmr. For matching applications, I always try strapped first, roughly half the time I get away with it.


Thanks for the correction on Altec's 0db(v) voltage Gary. In my defense, I did say "generic", as 1V is correct for a broad range of pro gear.

[Rob 515]

Well...

It's not really balanced if the other end comes from an unbalanced pre...

You could try it strapped first, and if you find no need for the added isolation (indicated by an acceptable noise floor), you could eliminate the +4 to -28 degree phase shift inherent to the xfmr. For matching applications, I always try strapped first, roughly half the time I get away with it.


Thanks for the correction on Altec's 0db(v) voltage Gary. In my defense, I did say "generic", as 1V is correct for a broad range of pro gear.

Thanks! Strapped it is and then transformer if needed.