I'm looking at the James 6113HS OPT at www.euphoniaaudio.com for $129
a pair. Seemes like a decent price. There's full specs there, but
basically it's a potted 10W, 70ma, f/response -1db 35-29K. If I use
these with a SE UL 6L6 amp I want to build (similar to the Fi Primer &
a version by Chris Beck) will the useable frequency response of the amp
improve due to the use of a overall negative feedback loop that's used
on this amp or do I need go to Electra-Print or one of the other more
expesive OPT's with better low end f/response to get a decent full
range sound?

I might ad that the James 6113HS has a 3.5K Primary with a 43% screen
tap (for those wondering how I'll use it in UL) as well as a 2.5K
primary tap along with a 8 & 4 ohm secondary.

5u4 wrote:

I might ad that the James 6113HS has a 3.5K Primary with a 43% screen
tap (for those wondering how I'll use it in UL) as well as a 2.5K
primary tap along with a 8 & 4 ohm secondary.

Stick with the 3.5k primary and use the UL tap.

The Ia and Ea will have to be set up to suit the 3.5k load.

Just what the stability of the amp will be when
global NFB is added will depend on you application of
correct techniques and and some understanding of
unconditional stablity and how to arrange it, how to test it,
and the nature of the Cshunt and leakage inductance of the transformers
you buy,
regardless of what brand you select.

When using global NFB, you need high Lp, low Cshunt, and low LL.

Patrick Turner.

5u4" wrote in message
oups.com... I'm
looking at the James 6113HS OPT at www.euphoniaaudio.com for $129
a pair. Seemes like a decent price. There's full specs there, but
basically it's a potted 10W, 70ma, f/response -1db 35-29K. If I
use
these with a SE UL 6L6 amp I want to build (similar to the Fi
Primer &
a version by Chris Beck) will the useable frequency response of
the amp
improve due to the use of a overall negative feedback loop that's
used
on this amp or do I need go to Electra-Print or one of the other
more
expesive OPT's with better low end f/response to get a decent full
range sound?


There are two problems with using nfb to widen the bandwidth of an
OPT.

Firstly, the roll-off at each end is accompanied by phase shift, so
there is a limit to how far you can go without stability problems.
There are two poles at the top end, and from the info you have you
don't know where they are. One is caused by the primary capacitance,
and one by leakage inductance, and neither is stated. Consequently
you can't really predict the stability margin.

Secondly, there is the risk of saturation at the bottom end. The
quoted inductance is measured at 50Hz, so you don't know what it
will be at, say, 20Hz full power. Beyond the open loop bandwidth,
the nfb may just drive it into saturation.

The quoted bandwidth is presumably for the stated primary source
impedance and secondary load. Your output stage may present a
different impedance, and this will affect the frequency response.

The quoted max 70ma may be a clue. I don't know much about SE
stuff...is 70ma half-way to saturation? Does that mean it saturates
at 140ma? Is it possible to guess the full power lower frequency
limit from this info? Should be for someone who makes
transformers...

cheers, Ian


"