----- Original Message -----
From: <[email protected]>
To: <[email protected]>; <[email protected]>
Sent: Friday, April 01, 2011 4:26 PM
Subject: Re: [Drakelist] My R-4B audio?
Given the same basic design of the speaker (and quality) the
4 ohm one
will be louder that 8 ohm. Same for headphones. I have 8 ohm
headphones
that are lots louder than the 600 ohm ones.
Yes, I tried 2 different 4 ohm MS-4 speakers...no
difference.
Not sure which vintage Collins you speak of but all mine are
4 ohm
speakers.
I even tried one of those...no diffference.
I think I have a issue in the R-4B audio chain...so off to
find it..
73,
Lee
The Collins catalogue, to be found on the Collins
Collector's site lists the speaker as being 8 ohms. I don't
have one so can't measure it. The speakers I have were
measured on a General Radio 1603-A Z-Y bridge at both 1 khz
and 400 hz. 400 hz is the standard frequency for
measurement.
There are several things of importance here; first, the
impedance of a direct radiator loudspeaker is not constant.
It tens to rise at a resonance at the lowe and and also
tends to rise at higher frequencies, the two beign for
different reasons. The low end rise can be to many times the
400 hz value and will depend on the efficiency of the motor
and degree of coupling of the cone. In any case, its
typically well below the frequencies of interest for voice
communication. The mid-range efficiency also depends on the
motor and cone coupling but usually at mid speech
frequencies the cone sees a pretty good acoustical impedance
match to the overall efficiency in terms of loudness is due
mostly to the motor. The more powerful the magnetic field in
the gap the louder the speaker will be. This variation can
be quite large when comparing cheap, starved magnet speakers
with high quality speakers with good magnets. By large I
mean ten db.
The amplifier in a Drake 4B is a very simple
single-ended Class-A pentode stage with no feedback. Such
amplifiers tend to have very poor voltage regulation (low
damping factor), they are essentially constant current
sources. That means the output will actually rise with
rising impedance. One result of this is the accentuation of
bass resonance in simple enclosures and the tendency to make
some loudspeakers sound strident because of their rise in
impedance at high frequencies. The mismatch can also
increase distortion if excessive. While the total power
available may be lower with a mismatch the voltage across
the speaker may become higher. The practical effect is that
the distortion may become greater at some level.
Nonetheless the difference between 4 and 8 ohms should
not make a great difference and an 8 ohm speaker may
actually be louder.
Now, the difference between 4 ohms and 600 ohms is 150
times, that is a lot so one would expect the difference in
level to be very great. In fact, I observe the same thing on
both my R4 and R-388, which also has a 4 ohm headphone
output. If you put a 600 ohm speaker across the line it
would also be very weak.
In any case, the very large difference in gain between
what your radio is doing and what is "normal" does suggest
its due to some problem in the audio chain and not just
speaker impedance mis-match. If its not the switch in the
headphone jack I think you are stuck with the usual routine
of checking tube socket voltages and resistances, etc. The
output tube is running wide open all the time which may be
helpful in tracking down the problem. Someone (Garey?) may
be able to supply numbers for normal audio levels in the
amp, if so a scope will be helpful in finding the source of
trouble.
If you don't have Garey's discs I urge you to get them.
There is much information there that is not in the Drake
literature, such as good photos of the works, plus it gives
you clean handbooks and schematics.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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