[David Ginsberg]

I'm now just starting to set up my new system. I have a pair of NuForm
'ribbons' which I think have an impedance of 6 ohms, or so. I have a
pair of Marantz 8b amps strapped as monoblocks and am wondering how to
determine the best output taps to use. I'm thinking of measuring the
driver to get exact dc resistance, take an equivalant power resistor
and test the amp taps for highest output before clipping. Other notes:
the amplifiers will be directly connected to the drivers with 6
inches, or so of fat copper litz wire soldered at both ends. The amp
will have some coupling caps changed to have the bottom end roll at
6db/octave below 1.5 kHz. I'm pretty sure I'll run the amps in UL
since I'm not sure I can spare the watts when running triode, but once
it's all running, I'll hang a scope across the driver and see how far
below clipping I'm running when playing dynamic stuff at the 85 dB
level to see how much headroom I have. Comments anyone?

[tubegarden]

On Feb 12, 7:26?am, David Ginsberg
wrote:
I'm now just starting to set up my new system. I have a pair of NuForm
'ribbons' which I think have an impedance of 6 ohms, or so. I have a
pair of Marantz 8b amps strapped as monoblocks and am wondering how to
determine the best output taps to use. I'm thinking of measuring the
driver to get exact dc resistance, take an equivalant power resistor
and test the amp taps for highest output before clipping. Other notes:
the amplifiers will be directly connected to the drivers with 6
inches, or so of fat copper litz wire soldered at both ends. The amp
will have some coupling caps changed to have the bottom end roll at
6db/octave below 1.5 kHz. I'm pretty sure I'll run the amps in UL
since I'm not sure I can spare the watts when running triode, but once
it's all running, I'll hang a scope across the driver and see how far
below clipping I'm running when playing dynamic stuff at the 85 dB
level to see how much headroom I have. Comments anyone?

Hi RATs!

Start with any set of output taps. Listen. Switch to another set,
listen again.

Repeat until you are sure you are getting the best sound.

Meters and scopes are great tools, but, they ain't ears

Happy Ears!
Al

[Patrick Turner]

David Ginsberg wrote:

I'm now just starting to set up my new system. I have a pair of NuForm
'ribbons' which I think have an impedance of 6 ohms, or so. I have a
pair of Marantz 8b amps strapped as monoblocks and am wondering how to
determine the best output taps to use. I'm thinking of measuring the
driver to get exact dc resistance, take an equivalant power resistor
and test the amp taps for highest output before clipping. Other notes:
the amplifiers will be directly connected to the drivers with 6
inches, or so of fat copper litz wire soldered at both ends. The amp
will have some coupling caps changed to have the bottom end roll at
6db/octave below 1.5 kHz. I'm pretty sure I'll run the amps in UL
since I'm not sure I can spare the watts when running triode, but once
it's all running, I'll hang a scope across the driver and see how far
below clipping I'm running when playing dynamic stuff at the 85 dB
level to see how much headroom I have. Comments anyone?

You have only to supply power to the ribbons above 1.kkHz, yes?

Therefore the average signal levels with music will be at about 1/4 of
the levels
used below 1.5kHz.

85dB average isn't very loud but let's suppose you need one watt for
85dB at 1M,
or about 2.45Vrms into 6 ohms.
Most ppl would find an 85dB level average for all their music is loud
enough,
and if there are two speakers, there is 83dB from each speaker, so only
a 1/2 watt is needed from each amp and most of the level is produced by
speakers
operating below 1.5kHz, so perhaps only 1/4 of a watt or less is needed
to the
speakers operating over 1.5kHz.

if you made your tweeter ribbons clip, you may find the sound deafening,
or unpleasant at the level, and don't push your luck with ribbons; they
are fragile.
But almost any kind of amp should give enough power.
I would only use the 4 ohm outlet if there is one, to make sure the
power
is going to be maximally class A, and there probably will be enough
power
whether using triode or UL. I am not familiar with the Marantz 8B, but
if they have EL34 or 6L6
then triode PP would be fine with or without NFB.
But if they only have 6V6 or EL84 then in triode PP you only get 5 watts
max, so peaks might clip.
So I prefer UL with the little tubes so you get 10 watts max and there
is no worry about clipping.
I know of nobody who can tell the difference between UL class A and
triode class A
as long as the response is the same the amount of NFB keeps the amp Rout
the same.

With a 10 watt amp use the 4 ohm tap, sit back and enjoy.
The headroom would be fine if you have all the bass frequencies filtered
away
because less signal is need than at bass F, which always clip first in
rull range amplifiers.


BTW, don't change ALL the coupling caps in the amp to reduce the
response.
Perhaps you may make the amp unstable and it will oscillate at some LF.

For simplest passive filtering use a single C&R filter at the input
only, and leave all the
other internal coupling caps alone.

The R can be 22k, so for a 1.5kHz pole the C would be only 0.0047uF from
input to the top
of the R, where the signal then goes to your amp input grid.
The R needs to be a lowish value, ie, not say 220k, because then the
noise of the resistor
could be a problem.

Patrick Turner.