Adcom GFA 5500 fet amp help

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Look at the schematic for the 5500.....you well see the two transform secondary centaps.....that run two the TH1 TH2...cl-40 thermistors... then to the amps chassis ....there is Not a three-pronged plugs on this amp...no ground wire from the wall coming in the amp.....only outside ground coming in to the amp is from the preamp output.

As i look at the Schematic ......it dose NOT show any grounds for each channel run from the center taps before the thermistors???.....
In the amps layout... looks to me as they are there...for each channel.. before the thermistors.....

So as the amp sets.. the only place the two amp channels center tap are connect together in this amp... are thought the two th1-th2-cl40s thermistors...that puts some resistance.....between the two channels.

So now when we put the preamps outputs.. to RCAs input with there com ground ..in the two amps channels...that we now have setup a differents betweent the two amps channel at the center taps?
I find this type of amps setup Can be noise, unstable for good sound.


SO here my question.....forget the chassis, adding to the center taps... I have never thought adding it... makes the sound better for any amp, pream....
Cutting the thermistors out so the two centertaps....are not connected....So this lets the amps run as two mono channels?
But well there be a problem?...noise-buzz because theres only one primary on the power transourmer?

How would Mr Pass setup his amps, preamps grounding, for the best sound?

Thanks to all for years of help to us Diyers.....an have the Happiest of New Years....




Some info off the web ....some my not have seen

Note: Many of the design features in the GFA-5500 are virtually identical to the GFA-5800 power amplifier. The primary difference between these amplifiers is power output. The circuit topology and design concepts are virtually identical.

POWER SUPPLY

Completely Separate Secondary Main Power Systems.
The GFA-5500 has a specially wound high power toroidal transformer which completely isolates the two channel secondary systems, not even sharing the ground connection. This eliminates ground loops and greatly reduces the possible interactions between power channels.

High Capacitance Storage
Each channel has nearly 120 Joules of energy storage, adequate for 14 kilowatts of power at 10% ripple. Under normal operation, the power supply ripple voltage is less than 1%.

Separate High Voltage Front End Supplies
The front end circuitry of each channel is supplied power by its own high voltage system - including separate transformer windings, bridge rectifiers and filter capacitors for each channel. This eliminates interaction between the early gain stages of the amplifier and the high power occurring in the output stages. The front end system is operated at a higher voltage than the output stage. This not only improves the distortion characteristic of the front end, but allows the front end to drive the output stage to its supply rails - obtaining every bit of available power to the load.

In Rush Limiting
The power supplies feature inrush limiting which prevents the high current surges which would normally accompany the charge cycle of the transformer and the power supply capacitors on turn-on.

CIRCUITRY

100% MOSFET Construction

Looking at the
 

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I posted Questions here.... because in the past...like you guys, I have diyed minny of the great... Zen an Alpha amps an preamps in the past..as far as sand amps go Mr Pass are one of the best ever....well to me anyway...an you can see most of the same parts in the 5500-5400... an medel work an sinks alone... I match parts, an spent hours on.. an more money ... than these amp cost used.
I got two 5500s like new for $300 in the boxs....
Could anyone give info on the input setup....if this is not Mr Pass work its a copy....for sure...
One thing I did was just run a wire from one rca at the input to the other rca ..ground -shield call it what you well....no earth in my preamp ...only in my krell 32 D to A....
Wow made the amp sound diff-better maybe....bias drop sinks not as hot...gofig
Got be room for some improvement yes...
My preamp is all tube...my speakers are the Acoustat M4s...2+2...an maggys....ML CLS...
thanks for any an all input.........
 

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The one and only
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5500.....you well see the two transform secondary centaps.....that run two the TH1 TH2...cl-40 thermistors... then to the amps chassis ....there is Not a three-pronged plugs on this amp...no ground wire from the wall coming in the amp.....only outside ground coming in to the amp is from the preamp output.

Yes. TH1 and TH2 each connect the center tap grounds of each channel to
the chassis. We note that the secondary power supplies of each channel
are otherwise independent. If you bypass TH1 and TH2 you will generally
find higher noise in the amplifier as you have closes the loop between the
two channels ground and the two ground lines going to the preamp, and
this ground loop gives noise. By having this resistance, the loop is largely
defeated, and the chassis still has some ground connection to the channels.

I recall that originally the intent was to Earth ground the chassis, and the
use of power thermistors was to have some resistance that became low
enough with high current to blow the fuse. This idea came from the tech
at Adcom, but I think was a good one.

In the Citation 12, a similar connection was made between one of the two
supplies and the chassis, consisting of a single resistor. It too had no
AC Earth connection.
 
Thanks for your time Mr Pass
Yes.
If you bypass TH1 and TH2 you will generally
find higher noise in the amplifier as you have closes the loop between the
two channels ground and the two ground lines going to the preamp, and
this ground loop gives noise. .

In the 5500 amp.. having this resistance between the two secondary in the power supplies center taps....We well get less noise buy closing the loop only at one place...at the preamps outputs common ground point?

Would we get lower noise if the two center taps were independent of each other? Like mono amps?
With out the chassis... as here with out the earth ground.. being in the setup?
 
One thing I have to ask?.....With the 5500-5400 center taps setup the stock way.... The preamp- or what ever is feeding or speakers hook up to the amp is in the loop?..
There has to be a good ground setup some where?...If the preamp or what ever power supply was setup the same way as these amps.. are you get a bad RCA cabel or a brake in the speaker crossover wire ...there all in the loop?... so it wont be closed?....
An you well-can get a Big squeal -buzzzz though you speakers that not only can blow a fuse...kill your speakers HF driver.......with the diff between the two center taps...this is what got me here....it seems it can be.. a unstable setup?
I get we like to get all things not needed in the loop out.. for better sound...like the earth an the chassis...that's why TH1-2 look like the way to do this.

Thanks to all... for any an all input on the best type of grounding setup for all the Diy
amps-preamps we may Diy....I have found this can-dose have affect on the sound of any single ended input-output circuit.
 
There has to be a good ground setup some where?...If the preamp or what ever power supply was setup the same way as these amps.. are you get a bad RCA cabel or a brake in the speaker crossover wire ...there all in the loop?... so it wont be closed?....
An you well-can get a Big squeal -buzzzz though you speakers that not only can blow a fuse...kill your speakers HF driver.......with the diff between the two center taps...this is what got me here....it seems it can be.. a unstable setup?
I get we like to get all things not needed in the loop out.. for better sound...like the earth an the chassis...that's why TH1-2 look like the way to do this.

My fix....
The cause of the noise,hum, sqiel ....in the GFA 5500 mosfet amp......is that the two Thermistor 1an2... always have some resistance between the to secdairy centertaps.....this can-dose setup a ground loop.....A Difference beween the two...an the chassis!
I ran a wire from the right RCA input ground.... to the other left RCAs ground.point...noise gone!

Also.... I use Apogee Stages an Minors....Acoustat 2+2s an ML CLS with new panels...
I had the Adcom 5500 for years...Why..... I have krell an tube amps...but being into diy for all the years....of Mr Pass helping all get more out of sound than I for one could have ever dream of....I had to have a amp that had all the parts like the inr 244- an others......but with the noise the amp could make I never put the 5500 in my setup an ran the amp for a week....only a day mabe....but with this fix....after being used to drive my Apogee stages for a week now..this amp now has that rich, full,almost that creamy class A sound that only MosFets can give.....
An lot still use today to make fet diy amps....

So I just would Say thanks to you Mr Pass.... for being there for us....Happy Holidays to All.............

All just one mans findings
 
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