OPT resistance for EL84 PP Class A

[pengboon]

Need some help on the above, could not manage to decipher from the datasheets and class A PP information, or am I missing something? How should I be going about figuring this out?

Basically wish to have flexibility on selecting triode, UL, Pentode in class A PP, would the Hammond 1650F @ 7.6k a good choice? How about a Lundahl?

thanks in advance for any help!

[SY]

Take a look at a loudspeaker impedance curve. Remember that the transformer reflects this impedance back to the plates. It's not usually worth sweating the details, pick one that looks good for your preferred operation and it will work at least acceptably with other topologies. Reputable manufacturers (and Lundahl is way better than reputable!) will be an excellent source of advice as to which of their transformers will be the best for you.

I would, though, look for something with 43% ultralinear taps.

[pengboon]

My preferred would by triode class A, however, I can only find the data for triode single end, which is Ra=3.5k, so is PP 7k? I don't see any Lundahl with that impedance from their website, any advice...? Hammond?

BTW, my speaker's impedance curve look like this:
http://www.stereophile.com/budgetcom...ty/index3.html

[Stuben]

I just finished a PP EL84 project using IT coupling/invertering. I used the Lundahl LL1682 OPT. This OPT has a 5k primary inpedance. I have had very good sonic results. The LL1663 will also work.

Stuben

[SY]

Originally posted by pengboon
My preferred would by triode class A, however, I can only find the data for triode single end, which is Ra=3.5k, so is PP 7k? I don't see any Lundahl with that impedance from their website, any advice...? Hammond?

BTW, my speaker's impedance curve look like this:
http://www.stereophile.com/budgetcom...ty/index3.html

Bingo! The impedance of your speakers varies 2 to 1 just in the midrange and more than 3 to 1 if we include the bass.

My advice still stands: contact Lundahl, tell them what you want to do, and ask them for a recommendation.

[pengboon]

Originally posted by SY

Bingo! The impedance of your speakers varies 2 to 1 just in the midrange and more than 3 to 1 if we include the bass.

My advice still stands: contact Lundahl, tell them what you want to do, and ask them for a recommendation.

OK, just did that. Can you share how to relate the speaker impedance curve with OPT? what do I look out for?

thx!

[SY]

A transformer is a dumb thing. All it can do is transform one impedance into another in a fixed ratio. If the impedance transformation is, say, 100:1 (same as a 10:1 turns ratio), then an 8 ohm impedance transforms to 800 ohms. A 16 ohm impedance transforms to 1600 ohms. And so forth.

So if your speaker's impedance fluctuates by a factor of two, the impedance reflected by the transformer also fluctuates by a factor of two. A transformer with a rating of 8000 ohms primary with an 8 ohm load will have an impedance varying between 6000 and 12000 ohms if your speaker's impedance varies from 6 to 12 ohms (the idea of an "8 ohm" speaker is marketing nonsense).

The moral is that sweating the difference between a 7000 ohm primary and an 8000 ohm primary is just not worthwhile unless you're planning on running your amp exclusively into test resistors. I assume you want to hook it to real speakers and listen to music...

Go with Lundahl's recommendation.

[pengboon]

Thanks SY, that helps!

[hacknet]

anyone ever tried using a resistor to damp the impedance fluctuations? i know it burns power but it seems like a sensible solution to cut down on "impedance" related problems.

[pengboon]

Erm Lundahl replied me he "can not suggest transformer type based on tube type or amplifier topology."

Anyway, I think the 5.5ohm at 5k model seems widely used... Prob I will get that one...