Forssell Technologies SMP-2 User manual

F(t) Forssell Technologies Inc
SMP-2 Microphone Preamplifier
User Manual
Forssell Technologies Inc Sandpoint Idaho USA [phone]

Introduction
The Forssell Technologies Inc SMP-2 is a 2 channel, transformerless, Class A discrete JFET
microphone preamplifier housed in a single rack space chassis. Each channel on the SMP-2 has
front panel mounted controls for gain, output muting, polarity reverse, and phantom power
on/off. The mute, polarity, and phantom power switches each have LED indicators which
illuminate when the switch is pressed in to indicate that the switch is on. The gain control is
variable from minimum to maximum gain in 24 stepped positions. Bicolor LED level indicators
are also present for each channel. These LED indicators show green with the output signal level
is above –5 dBu, and red when the signal level reaches +22 dBu, which is 2-3 dB prior to
clipping. A power on/off switch and LED are also located on the front panel.
The rear panel contains the two input and one output XLR connectors per channel. All XLR
connectors are wired Pin #2 hot. There is an input XLR for phantom powered mics (labeled
P48), and one for non-phantom powered mics (DC) such as ribbon, dynamic, or externally
powered condenser mics. There is also an IEC AC power connector and fuse holder on the back
panel. The fuse holder is integrated into the IEC power connector and uses 5x25mm slow blow
fuses. There is a spare fuse located within the fuse holder. The fuse type is 500 ma time-delay
fuse. If your preamp is wired for 220 VAC, then please use a 250 ma time-delay fuse.
Operation
Operation of the SMP-2 is very simple and should be intuitive for most users. Microphones are
connected to the SMP-2 preamplifiers via female XLR connectors to the rear panel, and the
preamplifier outputs are connected to male XLR connectors. Each channels input and output

XLR connectors are located next to each other and correspond to the channels as laid out on the
front panel.
Phantom power is applied to the P48 input XLR when the phantom power switch is pressed in.
It is a good habit to apply phantom power AFTER a microphone is plugged into the XLR
connector on the SMP-2 to prevent a voltage spikes appearing at the input of the preamp
circuitry. The phantom power supply voltage on the SMP-2 is +48 VDC and is a high current
robust design to provide consistent phantom power voltage to all inputs regardless of the number
of phantom powered mics plugged into the SMP-2.
No phantom power is present on the direct coupled input (DC) XLR even if the phantom power
switch is pressed in on the front panel. PLEASE NOTE… only one of the two input XLR
connector can be used at any given time. The unused XLR should have no cable plugged into it.
Gain control on the SMP-2 variable from a minimum of +6 dB to a maximum of +68 dB (or
+74 dB if high gain option is installed) in 24 steps. The gain control use high quality Elma
switches for long life and good signal switching qualities. The gain setting for each position are
shown on the front panel graphics.
.
Ventilation
The SMP-2 circuitry uses Class A discrete JFET amplifiers. These amplifiers generate a lot of
heat because of their Class A biasing, but the SMP-2 should not require extra space above or
below the unit under most conditions. Should the SMP-2 be mounted above something that
generates a lot of heat (like vacuum tube preamps), you should leave a 1U air space between the

units. Further, if you have multiple SMP-2 preamps in a rack, it may be a good idea to place an
1U air space every 4 units.
Specifications
Minimum Gain = +6 dB
Maximum Gain = +68 dB or +74 dB
Input Impedance = 13k ohms
EIN (Max gain) = better than –128 dBu (22 Hz to 22 KHz, unweighted), typically –130 dBu
CMRR (measured at +45 dB gain) = better than 90 dB
Frequency Response = 5 Hz to 100 Khz +0, -0.5 dB
THD +N = better than .005%
Maximum Output Level >= +24 dBu
Recommended Load Impedance >= 600 ohms
Preamp Output Impedance = 49.9 ohms
Power consumption = appox 16 watts

SMP-2 Two Channel Discrete JFET Mic Preamp Test Data
Unless otherwise specified all tests into 10k ohm || .001 mfd load
Forssell Technologies Inc 05/31/07 14:29:03
A-A THD+N vs AMPLITUDE
Mike Pre THD+N vs AMP.at2c
0.0001
1
0.0002
0.0005
0.001
0.002
0.005
0.01
0.02
0.05
0.1
0.2
0.5
%
-0 +24+2 +4 +6 +8 +10 +12 +14 +16 +18 +20 +22
Output Level dBu
THD+N vs output voltage from 0 dBu to +25 dBu
Forssell Technologies Inc
05/31/07 14:34:58
A-A DIM IMD vs AMPLITUDE
MikePreDIMvsAMP.at2c
0.0006
1
0.001
0.002
0.005
0.01
0.02
0.05
0.1
0.2
0.5
%
-0 +24+2 +4 +6 +8 +10 +12 +14 +16 +18 +20 +22
Output Level dBu
DIM vs output voltage

ForssellTechnologiesInc
05/31/0714:38:05
A-A FFT SPECTRUM ANALYSIS
Mic Pre FFT.at2c
-150
+0
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
d
B
u
10 20k20 50 100 200 500 1k 2k 5k 10k
Hz
FFT plot 0 dBu output
ForssellTechnologiesInc
05/31/0714:39:56
A-A FFT SPECTRUM ANALYSIS
MicPre DualFreq FFT.at2c
-150
+0
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
d
B
u
10 20k20 50 100 200 500 1k 2k 5k 10k
Hz
Twin Tone (17KHz and 19KHz) FFT plot

ForssellTechnologiesInc
05/31/0714:42:46
InputNoise
A-AEIN.at2c
-135
+0
-130
-125
-120
-115
-110
-105
-100
-95
-90
-85
-80
-75
-70
-65
-60
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
E
I
N
d
B
u
20 20k50 100 200 500 1k 2k 5k 10k
Hz
EIN at max gain 20 Hz to 22 KHz
ForssellTechnologiesInc
05/31/0714:50:11
A-A FREQUENCY RESPONSE
Mike Pre Freq Lo.at2c
-3
+3
-2.75
-2.5
-2.25
-2
-1.75
-1.5
-1.25
-1
-0.75
-0.5
-0.25
-0
+0.25
+0.5
+0.75
+1
+1.25
+1.5
+1.75
+2
+2.25
+2.5
+2.75
d
B
r
A
20 200k50 100 200 500 1k 2k 5k 10k 20k 50k 100k
Hz
Freq Response at +6 dB gain (blue) and +60 dB gain (red)

Description
This microphone preamplifier is housed in a 1U rack mount chassis. It is a two channel mic
preamp with individual controls for each channel.
Front panel controls include; Mute switch (mutes output), polarity reverse switch (relay),
phantom power, 24 position rotary gain switch, and dual color LED signal level indicator (green
@ -5 dBu, Red @ +22 dBu).
There are two inputs per channel. One has +48 volt phantom power supplied to it (when
phantom power is switched on) and its signal passes through very high quality capacitors to
block the phantom power DC voltage from the input stage of the preamplifier. The other input is
connected directly to the input of the preamplifier, bypassing the phantom power blocking
capacitors. You can use this “direct coupled” input when you are using microphone that do not
require phantom power, such as vacuum tube mics, ribbon mics, and dynamic mics. You can
only use of these inputs at a time. Do not connect the unused input to anything cable of
microphone. The connector in the middle is the Direct Input. Input and Output connectors are
XLR-3 wired pin #2 hot.
The front panel mounted bicolor LED will light green when an output signal level of –5 dBu or
greater is present, and this LED will turn red when the output signal level reaches +22 dBu. This
is approximately 2-3 dB before output clipping on the 2 channel preamp.
Circuit description
Class A discrete JFET front-end coupled to three discrete JFET opamps. No IC opamps are used
in the direct signal path, but an IC opamp is used as a servo amp (DC offset control) on output.
The signal path is entirely DC coupled except for the phantom powered inputs. +48 VDC full
current phantom power supply. +/-22 VDC power supply rails.
Contact Information:
Forssell Technologies Inc
235 Wild Horse Trail
Sandpoint, Idaho 83864 USA
208-263-0286
www.forsselltech.com