Darling amp using a 7.5K impedance transformer

[Sal Brisindi]

As some of you know, well maybe only Tom knows I like to buy things on the cheap. I can purchase 5 watt 7.5K transformers for $10.00 each. The Darling design calls for a 5K transformer.

How much would a 7.5 impedance transformer affect the Darling? Me thinks less output (which might be a bad thing as the Darling is only 3/4's of a watt as it is...)

Sal Brisindi

[EWBrown]

That would Probably drop the output from 750 mW to 600-650 mW (hardly noticeable) , but also decrease the THD by a significant figure. THe slight (1 or 2 dB) loss in max power would be offset by the lowered 2HD and 3HD content. Go for it!

You could probably increase the B+ voltage to 275-280VDC, assuming the usual 25 mA cathode current, and 5 to 7 VDC drop across the OPT primary, that would still allow for the 250VDC plate to cathode voltage. and the higher B+ and OPT primary impedance combination should be a win-win situation. Yellow_Light_Colorz_PDT_03

/ed B in NH

[Sal Brisindi]

Thanks for the info, I might just go for a pair.

Thanks,
Sal

[SDS-PAGE]

Sal,
I built a cap-coupled one with 8k Z OPTs before. I lost a bit of power as well as distortion. A good trade off. If you build a direct-coupled, you can always push the B+ higher and get that 750 mW back. Jeff Larson did something like it:
http://home.att.net/~abraxasaudio/projects/d3/d3.html
-Min

[Sal Brisindi]

Min,
Thanks for the info and link. Jef some nice amps on his site!

Sal

[Blooze]

So Sal, did you ever try the 7.5K OPT's?

I know from speaking to Jef from Abraxas many times that he prefered the 6.5K ones for his Darlings. He really liked the old RCA's especially.

[EWBrown]

The Darlings run at low enough 1626 plate current that you can get away with using old PP iron, without worrying about saturating the cores. No need for spendy "fancy" SET iron... There is plenty of old non-UL EL84 / 6V6 PP iron in existence, most of it can be obtained pretty inexpensively.

Gary Kaufman did up a DC (Direct Coupled)-Darling, using the power and output iron from a Heathkit AA-151 carcass, since the amp is direct-coupled, the 1626s sit well above ground potential, so that they still have only approx 250VDC P to K across them.
The grid is at the VA tube's plate voltage.

Go here: http://www.the-planet.org/audio.html and then scroll down through the page to about mid-way.


/ed B