Attenuators, Pots and Classes

[Echo North]

Hi all,

Been looking around all day and having a little problem with this topic. My question relates to 1176 A-E front ends vs. F-H...but this applies to everything.

I understand the basic difference between Class A/B/AB/C...in regards to how much of the waveform it amplifies. But I have a lot of questions.

1. Does the use of an attenuator vs potentiometer effect the classing of a signal? Is that classing done at transformer level...or something deeper in the circuit?

2. Why is it that we use an attenuator with the early revision UTC 0-12 tranny's but a potentiometer with lundahls and OEP's? Could you use an attenuator with an Mnats/Gyraf board? Would it make a difference? Would it cause worlds to collide? I'm guessing this has something to do with impedance?

Well that's a good start!

If you choose to take a shot at answering...keep in mind I'm beginner

Thanks,

Mike

 

[David Kulka]

There is no relationship between the type of input pot used in a circuit, and the design of the output stage. The Class of an amplifier amp has to do with biasing and circuit design, and there are many considerations that can be taken into account when deciding whether a given circuit should employ Class A, A/B, etc. If you want to learn more about this, there's tons of info on the web.

There's really no relationship between attenuator or pot vs. input transformer either, except maybe a monetary one. The original UREI 1176LN used a bridged 600 ohm attenuator and a UTC input transformer, both relatively expensive parts. Many DIYers with money to spend will opt for original parts, and I guess that people who are willing to spend the bucks for an input attenuator may also be prepared to spring for an original input transformer. But one could use the balanced attenuator with a cheaper transformer, or no transformer at all. One could also use a standard pot (or no pot at all) with a UTC transformer.

Your hunch about impedance is correct. Generally, if an input pot (or balanced attenuator) is feeding an input transformer (like in the 1176LN) the pot needs to be low impedance to minimize loading effects of the transformer.

 

[Echo North]

David,

Thank you very much for the info!

I'll continue my reading on the subject. It's frustrating for me to be at this point where I can assemble things...but have no idea how they work really.

I'm going to start my steady diet of minimal building and heavy reading now!

Thanks,

Mike