From: David Satz (DSatz@msn.com)
Subject: Re: Blue Tube Phantom problem
Newsgroups: rec.audio.pro
View: Complete Thread (4 articles) | Original Format
Date: 2001-01-15 07:14:27 PST
<roccoflores@my-deja.com> wrote:
> [ ... ] It´s incredible they can sell a product with such
> a big problem.
Unfortunately, U.S.-designed mike preamps in the lower price
category often have circuitry that can't possibly work well with
high-quality, transformerless condenser microphones. The
phantom powering circuit is often just plain wrong, and the
preamps overload far too easily.
I always check these two parameters plus the function of the
"clip" indicator, before trying out a preamp on live music.
Just for fun, here are some notes that I've kept over the past
few months. The issues here are:
[1] Input overload voltage (0 dBu = 775 mV, which would be a
decent spec to aim for; 6-10 dB higher would be better, but 0 dBu
is OK _if there is a reliable overload indicator_)
[2] adequacy of the phantom power supply for a microphone
that needs ca. 4 to 5 mA (for two such mikes, if the preamp is
a stereo unit)
[3] if an "overload" indicator is present, at what level relative
to actual overload does it light? (Note: I measure this only as
a steady-state voltage, not as an impulse--but both behaviors
are quite important.)
==============================================
Siemens V 72 overloads at -15 dBu in (no phantom powering,
no overload indicator)
Symetrix SX202 overloads at +2 dBu in (4 above LED on);
phantom powering OK
dbx 760X overloads at around +3 dBu in (5 above LED on);
phantom powering OK if extraneous resistor feeding the two
6.8 kOhm phantom resistors per channel is reduced in value
(modification approved by dbx technician)
Beyer MV 100 overloads at ca. -5 dBu in (6 or 7 above LED on);
phantom powering OK; overload point decreases as batteries
wear down, but "clip" LED seems to track this behavior.
PreSonus "Blue Tube" (fails 48 V phantom test) overloads ca.
-14 dBu in
Midiman "Audio Buddy" (fails 48 V phantom test) overloads ca.
-17 dBu in (LED lights exactly where output distortion is audible,
but can miss input overload)
JoeMeek VC3 (fails 48V phantom test) does NOT overload with
+6 dBu in! (original model, not the current model VC3Q)
Aphex 107 "Tubessence" (marginally fails 48V phantom test with
odd results; Aphex tech support described a fairly strange circuit
with non-standard voltages and resistor values, claiming that it
met the "AES" standard for phantom powering, which does not
exist) overloads ca. +4 dBu (ca. 7 dB above LED on, but the LED
interacts w/low cut filter also-try -4 dBu in at 500 Hz,)
Event EMP-1 does NOT clip at + 6 dBu in (though clip LED comes
on 9 dB before!); phantom power OK
ART Dual MP (squeaks by on 48V phantom test) normal setting does
NOT clip at + 6 dBu in; LED lights exactly where input distortion is
audible at higher gains. At +20 input gain, clips at about -8 or -7 dBu.
Nakamichi MX-100 "mixer" (3 in, 2 out): No phantom power; does
NOT clip at +6 dBu in; no overload indicator
M Audio DMP2 (original model): Phantom power OK if only one
channel loaded (open circuit voltage on other channel drops to +44.6 V);
input clips ca. 75 mA (ca. -20 dBu); LED reflects output clipping only
(lights ca. 2 dB below that).