Gamut M250i User manual

M250i
Mono Power Amplier
Userguide

Contents:
Safety instructions 3
Unpacking 4
Connecting the input cables 4
Warming up 4
Sensitivity (Gain) 4
Cleaning the cabinet 5
The protection circuitry 5
A second of silence 5
Connecting your loudspeakers 5
Loudspeaker impedance 6
A lifetime of enjoyment 6
Normal use 6
What you can check if there is no sound 7
The service 7
A few useful specications 8
Interesting trivial facts 8
Recommended CDs 9

SAFETY INSTRUCTIONS:
Never let children (or grownups) push anything into holes or slots in the case - this could result
in an electric shock and could cause damage of the set.
Never guess or take chances with electrical equipment of any kind - it is better to be safe than
sorry.
Warning: To prevent re or shock hazard do not expose this appliance to rain or moisture
No user serviceable parts inside. Refer servicing to a qualied technician. Never remove covers
unless qualied to do so. This unit contains dangerous voltages.
DO read the user’s guide before you attempt to use this equipment.
DO ensure that all electrical connections are properly set in accordance with the manufacturer’s
instructions (this includes the mains plug, extension leads and interconnections between pieces
of equipment).
DO check that the mains fuse rating is set correct for your local mains voltage.
DO NOT obstruct the necessary all-around ventilation: Especially DO NOT place the set close to
curtains or on soft furnishing such as carpets. Overheating can cause unnecessary damage and
shorten the life of the set.
DO NOT switch the amplier on, then o, then on again quickly. The blue light must disappear
completely before you switch on again. Only switch on and o using the power switch.
DO NOT unplug the mains cables while they carry current. This will corrode and damage the
surface of the metal of the mains plugs and sockets. This can cause reduced sound quality and
electrical res.
The mains switches have special metals in the contact points selected for handling high in-rush
currents, as well as to break large currents. Plugs and sockets are not made of such metals.
DO NOT use the ‘direct’ speaker output for tests with square waves or sine waves and a capacitor
across the output terminals, as you may damage the amplier.
DO NOT connect output ground to input ground. Do not connect any output grounds together.
DO NOT connect headphones to this amplier without using protecting resistors.
Make sure that you do not let any tiny part of the speaker cable going to a terminal touch the
other terminals. Only connect this amplier to loudspeakers, and only one loudspeaker on each
channel. Do not connect the output of this amplier to another ampliers outputs or inputs.
DO NOT connect a red terminal to another red terminal.

Unpacking
Open the inner box, remove the 2 top blue foam parts
Lift the amplier up and place it on its feet. Be careful not to damage the gold plated connectors
on the rear panel. Keep the box and the blue foam, it is specially made to protect the amplier well,
and you may need it some day, when you move or if you want to upgrade the amplier.
Connecting the input cables
The main input is the 3-pin XLR socket. This is electronically balanced, and it will
therefore reduce hum pickup in the input cable. So the best choice of cable is a balanced
XLR cable, coming from a balanced output on your preamplier.
In case you have a preamplier with only RCA outputs, you can use the RCA input. Remember to
plug in the XLR Terminator plug, when you use the RCA input. It connects the negative
input to ground, making sure that the input is not “oating”with the risk of noise or oscilliation
comming in to the amplier circuits.
You release the XLR connector by pressing on the release button next to the XLR socket, and
pulling the XLR plug out at the same time. Do not pull on the cable, only on the plug.
The XLR connector: always connect Pin 1 (the ground) rst and disconnect it last. This way,
XLR connectors can be plugged and unplugged while the amplier is switched on.
However, some DC from the preamplier may still cause loud clicks, so it is recommended to
wait 10 seconds and then switch the power amplier on as the last precaution. If you use a tube
preamplier, please wait several minutes the rst time you use it. Always switch o the power
amplier rst before switching o your preamplier or any source. Always remember to turn down
the volume before switching on of o the poweramplier, if you fail to do so, the output relay
will ware down faster.
DO NOT CONNECT LOUDSPEAKER WIRES OR OTHER HIGH VOLTAGE CABLE TO THE INPUT.
Warming up
The GamuT power amplier will play sound a few seconds after you turn it on. However, please
allow minimum 30 minutes before you start serious listening, as the sound quality will slowly
improve, because all the parts need to reach their optimum operating temperature.
You may also note a slight improvement in the rst months of use. This is caused by the
capacitors, which do improve with use.
A CD player in repeat mode can speed up the process while you are not at home.
Do not play too loud.
After this break-in period is over, simply switch the GamuT on when you get home, and it will be
ready to perform at its best after dinner.

Cleaning the cabinet
The front panel is made of aluminium. It can be cleaned by using normal mild solvents and clean
paper tissue or cloth. Do not use excessive amounts of solvents, and do not let any of the solvent
spill into the amplier.
The protection circuitry
The ideas for the protection circuit comes from our professional ampliers, where it has worked
well since 1982. Due to the rugged MOSFET, we can allow high peak currents , and we only need
to mute the input signal in case of longterm overload.
The protection detects DC errors, high level subsonic signals, long-term ultrasonic signals, high
temperature, and to low impedance loads or short circuits.
If needed, the protection will simply mute the input signal, and if necessary open the output relay.
The relay will normally open only after muting the input signal to avoid relay switching of strong
currents that could corrode the relay contacts.
So in case of i.e. a short circuit the input is muted for a second. Then a 1 millisecond long pulse is
sent out to test again. If the short circuit still is present, it waits a second and tries again, and so on.
One second after the user has removed the faulty cable, the system will quickly remove the muting.
The protection also detects too low impedance loads, and alerts the user by muting for a second.
So the user can be sure to hear only clean non-distorted audio. The normal current limiting and
momentary distortions of the music are avoided in the GamuT M250 power amplier.
A second of silence
You may experience a second of silence while the music is playing loudly. This is the protection
acting to protect your tweeters. Any amplier will start to clip the top of the music if you exceed
the maximum power level. Technically, clipping generates a lot of high frequency energy, which
can burn out the tweeters in your loudspeakers. The protection constantly checks the energy
above 20 kHz, which humans do not hear. The distortion is momentarily very high anyway,
because you have exceeded the maximum power, so just reduce the level a bit.
You can also provoke the subsonic protection by playing warped records without using a
subsonic lter. Please check your tonearm/cartridge resonance frequency, which should be about
8-12 Hz to reduce the eect of warped records.

Connecting your loudspeakers
Normal power ampliers have a coil in parallel with a resistor between the amplier and the
output connector. Its purpose is to protect the amplier against capacitive speaker and cable loads.
However, this will slightly reduce the delity of the sound in the treble range, when the amplier
is used with normal loudspeakers. So we have added a direct output in addition to the normal lter
protected output.
The red terminal closest to the heat sinks is the direct output. The other red terminal is the lter
output.
The white terminals are ground.
Biwiring: This also gives you the ability to connect biwiring nicely, using the lter output for the
bass and the direct output for the treble. However, you will achieve the best possible sound and
the most correct tonal balance when biwiring both bass and treble from the same pair of output
terminals, choosing either direct or normal.
NB: When biwiring an electrostatic loudspeaker, please connect both cables to the normal
(lter) output.
Inside the cabinet the ground terminals are kept biwired until they meet at the main star ground
point.
So please do not connect them together.
The red terminals go positive when there is a positive signal on the Pin 2 on the input XLR.
In other words, GamuT ampliers are non-inverting. Some other brands of ampliers do not follow
this standard. Keep this in mind if you want to bi-amp, with another amplier from another brand.
Loudspeaker impedance
Most loudspeaker are 8 or 4 Ohm, and they are specied so by the manufacturer.
Unfortunately, some loudspeakers have an impedance, which is much lower than specied.
They will not damage the GamuT power amplier, because it will simply mute.
In the GamuT M250i, we use a gem of a MOSFET transistor. It handles 500 Watts, and a peak
current of 400 Ampere and 100 Ampere long-term.
Such high currents will destroy any HiFi loudspeaker we know of.
The lower the impedance, the higher the current will be.
Somewhere, we have to draw a line between what is a loudspeaker load, and what is a short circuit.
We have set this line at 1.5 Ohm, which will draw about 47 Ampere peak.
The impedance of a few electrostatic loudspeakers does go down below 1.5 Ohm at high
frequencies. The protection circuit will not allow this longterm on test signals. However, it works
well with Quad ELS-63 and most Martin Logan electrostatic loudspeakers.
Please also note that a Quad ELS-63 electrostatic loudspeaker cannot handle more than 55V peak,
and the GamuT can give more than 70V peak. So be careful.

A lifetime of enjoyment
So far, our oldest ampliers from 1982 still work in professional recording studios, often doing
their job 16 or 24 hours a day. So we expect our GamuT ampliers to do likewise.
A more likely cause for change would be your wish to upgrade in some years time. Here our
modular approach serves to keep the upgrade cost low, and you avoid selling with a loss on
the second hand market.
Apart from being good economy, this long term approach is also better for the environment.
Most parts of the cabinet are made of aluminium, which is easy to recycle.
All parts are screwed together to enable easy disassembly. Your grandchildren will be pleased
with your choice.
Normal use
When you switch on, the blue light in the front will light up to conrm you have mains power on.
A moment later a hum sound and relay click will conrm the soft start delayed switch on function
of the powersupply, and a second blue LED will light up.
One second later a small click can be heard from inside the amplier; they conrm that the
amplier has connected the loudspeakers using the output relay.
One second later still, there will be another click in the loudspeaker.
This click is your conrmation that the protection system have found everything to be OK.
The input signal can now pass through to the loudspeaker.
What you can check if you there is no sound
Is there light on the front panel? If not, have you plugged in the mains cable correctly? Is the
fuse broken? Check both in the plug and on the back panel of the amplier.
Do you have mains power on? Check the mains socket by plugging in another working unit.
Only change a blown fuse once, and use the spare slow blow fuse located in the mains socket on
the back panel of GamuT M250i. Use only the special spare fuse in the fuse holder.
For 230-240 Volt Mains use sand lled 6.3 Ampere slow blow fuse type marked T6.3AH
For 115-120 Volt Mains use sand lled 10 Ampere slow blow fuse type marked T10AH
If the fuse blows again you must consult your dealer, the amplier is most likely defective, and it
cannot repair it self, so changing fuses many times will not help.

Changing the fuse more than once can increase the repair price, and you can cause a re or a deadly
electrical shock.
These sand-lled types can break up to 1000 Ampere current, a normal slow blow fuse can only
break 35 Ampere, so it may not stop the current!
If the amplier still is silent, but the blue power lights are on, you can check it in the following way:
Switch o power and wait 15 seconds and then switch it on and listen carefully. There should be
a quiet mechanical click coming from the inside of the amplier.
If not, disconnect the input cables and try again. Now listen closely to the loudspeaker.
A small click should come from the loudspeaker, just after the click from inside the amplier.
If not, check the loudspeaker cables.
If you hear clicks in the loudspeakers, and still do not get sound, the amplier is probably OK..
Try exchanging the input cables with another cable. and check that it transmits a signal, by connecting
it to antoher amplier.
If you only get the mechanical click (it comes from the relay closing inside) and you have checked
the loudspeaker cables and the loudspeaker, then you should call your dealer.
If you hear a small click every second from the loudspeaker, then you have a bad input signal,
or a short circuit on the output, which causes the protection circuit to take action.
Check that the input cable screen is OK and that all plugs are tightly and correctlt connected.
Check the loudspeaker cables. In case you cannot solve the fault by doing the above, call your
dealer.
The Service
Most of the parts are on the circuit-board, which can be changed at a low cost, should your amplier
ever need service.
You can contact your dealer, whom may solve your problems locally, or ship your unit to our
service department.
Please note your dealer’s telephone number here:

A few usefull specications
Shipping weight is 35 kg.
Dimensions are 164 mm high, 431 mm wide, and 470 mm deep.
Idle power consumption is 50 Watts.
The slow blow mains fuse limits power consumption at 1,150 Watts long-term.
Output power in 8 Ohms resistive load is 250 Watts.
Speaker loads below 1.5 Ohms are not allowed by the protection circuit.
Noise is at least 100 dB below 250 Watts in 8 Ohms. This depends on the gain selected, and it is
below the noise from a CD.
Strong external electromagnetic elds can increase the noise level. Use only shielded signal cables.
The balanced XLR input has a 40 kOhm input impedance. The 3-pin XLR socket has Pin 2 as plus,
Pin 3 as minus, and Pin 1 as ground. The RCA unbalanced input has a 20 kOhm input impedance.
Sensitivity can be selected internally by your dealer to be 0.775V, 1.55V, 3.1V or 3.9V for full power.
The output for a loudspeaker is single or biwiring, using 4 mm banana plugs, spade lugs or
stripped wire.
Interesting trivial facts
It may interest you to know that the output voltage is 55 times the input voltage at maximum gain.
However the input signal current in the 20kOhm RCA input is only 38.8uA at full power, and the
output current is 10.7 Ampere at full power in 4 Ohm, as current gain is 275,000.
Power is voltage multiplied by current, so output power is 15,100,000 times larger than the power
of the input signal.
Yes, a power amplier is a suitable name for what it does.
And note that this input signal power is still more than 10,000,000,000 times more powerful than
the residual noise power at the input.
So in 4 Ohm, full output power is more than 151,000,000,000,000,000 times larger than the input
noise power.
So do take care to place your signal cables apart from your loudspeaker cables. They should not lie
close together, and if they need to cross, they should cross at a 90 degree angle.
Do not use unscreened interconnect cables.
The current in the mains power cable is a very distorted version of the signal, and it contains
very high frequencies.
A special power cable can reduce radiation of interference, and it can improve your sound quality.

Recommended CDs
This is a list some of the recordings that have been used to evaluate the sound quality of this amplier.
The LP version can be superior, but it is dicult to obtain.
Most recording engineers today try to make the music sound ne on the cheapest stereo systems,
simply because they are in majority and thus brings more sales of CDs.
The following recordings will test the limits of a eigh-end system, from the subtle to the outrageous.
They combine this with being very enjoyable for the artistry of the musicians and composers.
Verve 521485-2
Verve Grammy Winners Stan Getz/Joao Gilberto track 9, "The Girl from Ipanema"
This recording gave Phil Ramone a Grammy for best sound in 1964.
Many years later, it still allows you to dim the lights, make a nice cool drink and imagine you are in
Brazil with the girl standing very close to you, singing her song quietly in your right ear.
This is what a high-end system is all about, to recreate the past so vividly, that the emotions of the
past performances can be experienced again and again.
It is interesting to hear how dierent the same track sounds on Verve 521 414-2, the digitally
re-mastered version of the album. And then try hearing the LP from 1963, Verve V6-8545.
Reference Recordings RR-70CD Stravinsky, Eiji Oue conducting the Minnesota Orchestra
The full dynamics of Stravinsky requires a HDCD recording. The quiet opening of the rst track lets
you hear the sound of the concert hall, and the big drum will surprise your friends or your speakers.
Almost visual three-dimensional sound.
"The rite of spring", well, Stravinsky wrote this based on the sacrice of young girls to please the old
gods. It is not pleasant for the humans involved, and it should not sound pleasant.
If you want to relax, choose the next CD instead.
dmp CD-457, Warren Bernhardt, “Hands On”
The rst track opens up with a very grand piano played in classical music style, which later transforms
into very relaxed jazz. Natural detail indeed; you hear the piano sound as felt hammers hitting metal
strings, and also the unique series of resonances from strings, metal frame and wood.
A ne Steinway or a Bösendorfer grand piano costs about EURO 100,000.
Is it reasonable to expect a stereo system costing much less to reproduce it perfectly?
Deutsche Grammophone 427 772-2, “Horowitz at home”
The poor studio technician had to come to the master’s home this time and make this recording.
Fascinating, very dicult recording to reproduce, but with a core of golden inner details..
Compare the two types of piano sound, this and dmp. A good stereo system must highlight the
dierences between dierent recordings and play both of them well.
Vertigo 800 051-2, Dire Straits,“Dire Straits”.
Track 4, "six-blade knife", is actually better than several of the later recordings.
This is due to the mixing console; the expensive console used for the later recordings is known for
its harsh sound, but it was used in many large studios due to its excellent automated mixing computer
system
Yes, there is tape hiss, but the detailed bass guitar and soft drums are wonderful to hear.

Warner Bros. 7599-27313-2, Fleetwood Mac,“Rumours”
Many excellent tracks, "Oh Daddy" being one of the best.
It was a huge best-seller, nice to see that quality is rewarded by the public.
It has some three-dimensional qualities, which normally is a non-existent feature in rock or pop
recordings. Compare depth to:
EMI CDS 7 54368 2 Georges Bizet“Carmen” with Maria Callas
The conclusion of Act II opens the second disc. You can hear the male singer walking across at the
rear of the hall while singing, and later walking towards stage center front.
The recording is a reference for three-dimensional sound.
On a ne system, you can measure the size of the hall with your ears, and on the best systems you
can hear that he walks deeper into the stage while walking across from right to left.
You can clearly hear the acoustics change while he is walking.
The recording is from 1964, it has some distortion at high levels, and Mrs Callas was always a bit
shrill sounding, but who cares when the drama unfolds like this. It is like "seeing" with your ears.
The new remastering EMI 7543 5 566281 2 1 fortunately retains most of the magic.
Telarc CD-80098, Erich Kunzel and Cincinnati Pops Orchestra,„Ein Straussfest“
Telarc show here that CD strings can be sweet sounding, and the rst track, "Explosions Polka", will
jolt everybody when it starts.
On a ne system, the explosion is extremely fast and splits in an air-blowing attack, a roar and a
decay of the hall, just like a real New Year’s celebrational explosion sounds.
Warner Bros. 7599-26656-2 “Fourplay”
Bob James, Lee Ritenour, Nathan East and Harvey Mason cooking together is an experience to
be savoured.
A dry studio recording, not much depth to speak of, but exceptionally clean.
The rst track builds slowly into a crescendo, which will test the system’s ability to resolve a
complex sound at high levels over a longer time. A power supply and heat sink test. When the
level goes down at the end, the sound should be the same as in the beginning.
This CD has been used at many shows, and it does not fail, many visitors ask for the label and
number, so that they can go out and buy it.

Every musical note…
Congratulations! You have just invested in one of the nest powerampliers available!
Many years ago, we chose the word “GamuT” to be our company name. This single phrase in Latin
means ‘a complete array’ or ‘an all-encompassing spectrum’. In our case, it refers to the entire range
of musical notes.
The GamuT name therefore ts our philosophy perfectly: Our goal is not missing out on one single
element among the myriads of tiny bits of information that create a realistic sound experience.
There are many more things to reproducing music than just reproducing the individual musical tones.
But it is the complex spectrum of multiple individual tones created by the musicians’instruments and
voices, and not least the reected signals thereof that create the sounds we detect with our ears and
send to our brain to give us the wonderful sensations only music is capable of. It is Gamut´s mission
to give to you these unique musical feelings.
As with every GamuT product, our ampliers are designed with the capability of giving you the best
possible musical experience, which is synonymous with reproducing all of the essential information
in the recording to create the illusion of “being there”.
To ensure this, we use only the best“raw materials”trimmed and crafted to perfection by the best
engineers and sound enthusiasts in the industry.
Following the guidance lines in this manual will assure you of an exquisite sound experience, so be
ready for “the entire range of musical notes”
We wish you years of proud ownership and happy listening. We feel condent that you will cherish
this amplier for both its amazingly clear reproduction as well as for its splendid craftsmanship and
beautiful appearance.
Should you have any questions or other matters that you would like to share with us, then please
do not hesitate to get in touch with us. We will be looking forward to hearing from you.
Best wishes
GamuT

Siggårdsvej 2
DK-6818 Årre
Denmark
Tel: [phone]
www.gamutaudio.com
[email protected]