Mg100hfx hissing

[NORSOREX]

Hello,

I have a "new" MG100HFX from a deal.
It produce to much hssing when I turn pot volume & Master.
I have just opened and see to PCB (3 parts) AMPLIFIER - DSP+PREAMP - POT&SWITCH FRONT PCB
i have the necessary scope, generator etc. but I need the schematic.
It is more simple to follow the signal from input and try to find where is "the hiss generator".
BE CAREFULL I have found many schematic of MG100HDFX OR RCD etc. but they are not use discrete transistors but TDA7293/94 chip power amplifier for the final part of the amplifier.
On the MG100HFX we find 2SC5200 & 2SA1943 on the final stage.
Potentiometers on face just send command to DSP to adjust the settings There is no analog signal thrue the PCB.
All cables are not shielded from DSP to amplifier PCB ???? !!!!!!!!!!
If any body have a link or the shematic, It will be so great.

Thanks
Steven

[J M Fahey]

Looks they finally noticed TDA729x is NOT a 100W chip ... not a reliable one at least.

Good move going back to real transistors.

As of the hiss, if you get it running master and gain to full, lowers when lowering treble and almost disappears when setting gain/volume to 0, specially on the dirty channel, then it's normal, nothing "broken" to be "repaired".

*Maybe* somebody can suggest a lower noise IC for the IC1 position, but there's a catch: if it's a TL072, the industry standard which Marshall also uses all over the place, better ones with the same pinout are available, but if it's a switching type, such as M5201, then you are stuck with it.

[NORSOREX]

Thanks to your reply

I have investigated with my scope and I have found this hiss noise when all potentimeters are at 0.....
If I leave a generator in Input with 100mV and all potentiometers at 0 I find It on a pin in the Input connector on the Main Amplifier PCB beetween DSP PCB & Main Amplifier PCB.
I think there is a little problem in the first Input in the DSP PCB. Maybe effectively on the first IC1. I think that there was an error connection and the last owner before me have connect a very High output device in the Input.
That result an IC1 out of order.
I wil continue to investigate and trace on the DSP PCB but if I couold have schematic, It is really more simple to understand and trace.
I have found the schematic of MG30FX and It seem that they have ONLY the same DSP circuit (not sure, I have to verifiy).

About the "hiss", It is not a high frequency but like shshshshshshsh as a cat make when It is angry.

Looks they finally noticed TDA729x is NOT a 100W chip ... not a reliable one at least.

Good move going back to real transistors.

As of the hiss, if you get it running master and gain to full, lowers when lowering treble and almost disappears when setting gain/volume to 0, specially on the dirty channel, then it's normal, nothing "broken" to be "repaired".

*Maybe* somebody can suggest a lower noise IC for the IC1 position, but there's a catch: if it's a TL072, the industry standard which Marshall also uses all over the place, better ones with the same pinout are available, but if it's a switching type, such as M5201, then you are stuck with it.

[Jazz P Bass]

See if this helps match things up.

It's the Analog & Digital section of the MG15-30FX.

MG15_30FX.zip

[NORSOREX]

Hi,

Yes It is the same that I have found.

Thanks

Steven

See if this helps match things up.

It's the Analog & Digital section of the MG15-30FX.

[ATTACH]38471[/ATTACH]

[J M Fahey]

Sorry for insisting but does that unbearable hiss respond to volume pot setting or not?

What happens when you set volume to 0 ?

I mean, listening at the speaker a few feet away.

Thanks.

[NORSOREX]

Hi,

Sorry, I am in investigation.

Found that I have NOT hiss if all potentiometers stay at 0.

If Gain is at 0 and make a variation on Volume or Master I have the hiss.
Hiss only exist if volume and Master different from 0.
Example : put volume to 1 and master to 10 make equal hiss like volume to 10 and master at 1.

Also discover that capacitor value to make filter on DC 3.3V and 5V is 1µF !!!!!!
3300µF on +43/-43V main power DC to the amplifier PCB and 2200µF on 7.5V/-7.5V to AOP on Main power AMPPCB and DSP/PREAMP PCB. Sure it is correct value.
I Put 1000 µF on 3.3V & 5.5V and the "hiss/noise" is dividing by 10.
But there is also 5 five different floating ground !!!!!! and not shielded cable.
But almost sure that It is on this way that I have t work to make better.
Now I have a question :Marshall make and error to assemble 1µF or Not. If not, that is a very bad amplifier with hiss and noise and only If you play guitar very louder, the hiss and noise will not audible.

I continue

Steven

Sorry for insisting but does that unbearable hiss respond to volume pot setting or not?

What happens when you set volume to 0 ?

I mean, listening at the speaker a few feet away.

Thanks.

Sorry for insisting but does that unbearable hiss respond to volume pot setting or not?

What happens when you set volume to 0 ?

I mean, listening at the speaker a few feet away.

Thanks.

[nickb]

Hi,

Sorry, I am in investigation.

Found that I have NOT hiss if all potentiometers stay at 0.

If Gain is at 0 and make a variation on Volume or Master I have the hiss.
Hiss only exist if volume and Master different from 0.
Example : put volume to 1 and master to 10 make equal hiss like volume to 10 and master at 1.

Also discover that capacitor value to make filter on DC 3.3V and 5V is 1µF !!!!!!
3300µF on +43/-43V main power DC to the amplifier PCB and 2200µF on 7.5V/-7.5V to AOP on Main power AMPPCB and DSP/PREAMP PCB. Sure it is correct value.
I Put 1000 µF on 3.3V & 5.5V and the "hiss/noise" is dividing by 10.
But there is also 5 five different floating ground !!!!!! and not shielded cable.
But almost sure that It is on this way that I have t work to make better.
Now I have a question :Marshall make and error to assemble 1µF or Not. If not, that is a very bad amplifier with hiss and noise and only If you play guitar very louder, the hiss and noise will not audible.

I continue

Steven

I have some doubts about what you are doing.

It's not at all unusual to have small capacitors on power rails, especially ones that have linear regulators on them as the 3.3V and 5V almost certainly do. All the big ripple filtering has been done prior to the regulator. The regulator then rejects what little is left. Any caps put after the regulators are for stability and noise. Shoving a 1000uF where a 1uf should be may cause issues as the resonant frequency will be lower and so may not suppress high frequencies well at all. It is for this reason that you often see several different value of capacitors in parallel - they work together to make a single cap that is effective over a wide range.

5V and 3.3V are typical values for digital power supplies. Of course I'm only guessing in this case since we don't have a schematic. The point is that digital circuits are not going to ordinarily produce hiss so changing caps on the digital power rails should not change the hiss. I cannot explain why your cap change has altered the hiss, if it even has. We need a schematic to know more.

Also, I seriously doubt that Marshall messed up - these assemblies get checked pretty thoroughly as any production fault quickly hits the bottom line (profits).

Write to Marshall and get a schematic and don't do anything else to it until you do. I fear you are changing components without any real understanding of the whys or wherefores and you could quickly find yourself in a big unsolvable mess.

Also, I'm not even convinced that the amount of hiss you are hearing is abnormal. If your turn the gains up, hiss is what you get.

[NORSOREX]

You have a good idea but there are No regulators.

I have found 200mV RMS noise at output when the pot are at 10 (volume + master) after put any capacitors on power rail.
RSB at 43db Is It really a Marshall amplifier ?

I think that there also 50Hz harmonic turn in the five types of GND;
No ground plane etc...
Is It a beginner who design PCB ?

I have read on any post that users have the same problems.
In this forum, in an another post on 2010, someone have made the same way to discover the reason of this hiss. unsuccessfully.

After a long look to understand the PCB, i have found there are no regulators on 3.3V & 5V.
I have the firm conviction that the DSP PCB has been made by a real professionnal (ground plane -separate GND AGND & DGND with cap beetween them.
After that a beginner take the schematic of the amplifier part and added the power part without electronic knowledges of implement.

I am sure that if we can not find the schematic, there is really a big problem and Marshall do not want that we discover that.

I am not sure to resolve all of problems because I have spend yet 10 hours on It.

Without schematic It is more complicated to understand all of the problems.
Sure that is the amount of each noise who make this hiss not only one and they interfer in.

[[[ I am very sorry to my bad English]]]

Thanks

Steven

I have some doubts about what you are doing.

It's not at all unusual to have small capacitors on power rails, especially ones that have linear regulators on them as the 3.3V and 5V almost certainly do. All the big ripple filtering has been done prior to the regulator. The regulator then rejects what little is left. Any caps put after the regulators are for stability and noise. Shoving a 1000uF where a 1uf should be may cause issues as the resonant frequency will be lower and so may not suppress high frequencies well at all. It is for this reason that you often see several different value of capacitors in parallel - they work together to make a single cap that is effective over a wide range.

5V and 3.3V are typical values for digital power supplies. Of course I'm only guessing in this case since we don't have a schematic. The point is that digital circuits are not going to ordinarily produce hiss so changing caps on the digital power rails should not change the hiss. I cannot explain why your cap change has altered the hiss, if it even has. We need a schematic to know more.

Also, I seriously doubt that Marshall messed up - these assemblies get checked pretty thoroughly as any production fault quickly hits the bottom line (profits).

Write to Marshall and get a schematic and don't do anything else to it until you do. I fear you are changing components without any real understanding of the whys or wherefores and you could quickly find yourself in a big unsolvable mess.

Also, I'm not even convinced that the amount of hiss you are hearing is abnormal. If your turn the gains up, hiss is what you get.

[nickb]

These are designed to a price so they dumped the regulators. 200mV is 5mW into 8 ohms, or as your say -43dB S/N. Sounds about right to me with the gain cranked. If you want to lower the noise you should concentrate on the very first input stage as that has the biggest contribution, but I think that has already been said. Try to find an op-amp with lower noise. It would really help to trace that bit out to understand how the various components contribute to it's noise. Perhaps these are carbon composition resistors in that stage. Replace with film types.

You might want to measure the ripple at the cathode of those rectifiers - there might be a cap you missed. The ripple on the 5V output is reduced by the zener action and also the load current is crucial. It look like the load current might be quite small. Looks as though this might be powering the DSP.

The DSP board has to be designed to meet performance and EMC targets and that has a big effect on the PCB. The analog PCB is doesn't have quite the same issues. So long as it's cheap and meets a the not-so-high specs, it's good enough.

[J M Fahey]

1)I found the schematic of the TDA7293 version which I am prettey certain is the exact same amp, with a different power board so it should do very well.

2) it does have not 1 or 2 but 3 regulators: 7015 , 7915 and 7805.

The DSP card, whose circuit is not shown, might have its own on board 3.3V sub regulator, that does not change the situation substantially.

3) the amp is professionall designed and made by a market leader, with 53 years experience and hundreds thousand amplifiers delivered and factories around the World, I would accuse them of anything but lack of experience and cheesy production.

Yes, they have made many poor decisions along that long career, such as trusting TDA729x datasheets and factory suggested circuits (what in the past worked very well for them) but those have been the exceptions, not the rule (pure Darwinism) and they recovered or dropped them sooner or later ... this very amp is a living example

Marshall MG100DFX.pdf

looking at it, I see RC4558 Ics were used in the first stage.
They are good but can be improved, I'm certain Forum members can suggest a couple low noise ones, same pinout.

Mind you, hiss should go down appreciably, but not dissappear, hiss is a reality of life.

Regulated rails should be left with original factory designed capacitor values, this schematic shows a couple 4.7uF caps in parallel, which might be too much (although regulator datasheets sometimes suggest up to 10uF and these meet that) but as suggested above they are not for ripple filtering, decoupling is the main idea.
They are shown "all together" in the schematic, I bet they are placed at strategical points all over the PCB where local decoupling is needed.

[NORSOREX]

Hello,

I have made a big job and try to make a schematic capture of my MG100HFX.
After I have made simulation to understand the function of all switches and potentiometers.
I always need the schematic amplifier part.
If Jazz P Bass have the rest, I will very enjoy.
Just send the beginning of simulation.
Send the rest for preamp when I have finished.



This is the first stage.
See on frequency transfer (1 top & 2 bottom) switch is off (top ) and on (bottom ).
On scope the switch is on an signal is blue.
It seem to be the crunch function but not sure.
I will made all simulations and make a real control on the amplifier to verify who is switch by switch and theirs functions.
Compare scope simulations to real probe scope.

At least, I have change the TLC2274 and the new S/N ratio is 78dB.
There is another IC damaged, and i Will change TL072 TL074.

i think finally that the previous owner have push a very high level in the input when the amp is on OD1 or OD2 and make damages on TLC2274 and TL072/074 concerned by these functions when there are ON.
After these damages, the AOP are "leaked " and produce noises.

Sorry, you have understand, I have a very bad English.

When I have finished all of my work on Proteus to make schematic and simulation, I give you that.
May be it will help someone.

Also on the schematic capture, all references are issue by the MG30 NOT on the real MG100HFX.
The references are different on the MG100HFX but It seem that the first part of the premap is the same of MG30.
I have to verify but how far ?

See you

[J M Fahey]

Thanks a lot
The simulation is very useful; I noticed the input stage you show is not the same as the MG100HFX I posted, which I guess is the same as yours , just different power amp.
What is seen is what I expected, but it's good to show the actual curves so others may understand it better:

1) the stage has switchable gain
a) with switch open, it's ruler flat from end to end.
b) with switch closed, it adds 8dB gain (3k9/2k2)+1=2.8X but within a reduced range, so useful guitar sound is boosted but not noise outside the band, we see C32 470nF and R25 2k2 in series cutting bass below 120Hz and R26 3k9 in parallel with C33 1nF cutting highs above 20kHz, although -3dB point is staggering 50 kHz , no doubt because of the advanced Op Amp you used.

And this also shows something important: the modern Op Amp is good, too good , improved noise a lot (which is expected), but also extended range a lot, which *might* be a problem, simply because Marshall designer did not take that in consideration.

Don't expect problems here, but if the Op Amp is inside a power amp (many 70's designs used a 741) , any improvement may throw stability compensation down the drain and cause big trouble, I see that a lot in DIYAudio, where users try to improve 80's or 90's Hi Fi equipment.

Don't remember where I picked this (might even have been here at MEF) but this is eye opening:

Edit: your English is excellent, but please tell us where are you writing from, might help us understand some twist of language

[NORSOREX]

Hi,

At first I'm writing from France

The schematic that you have posted is not MG100HFX but MG100HDFX.
It was older than the MG100HFX.

I'am working...

Wait & See

Thanks a lot
The simulation is very useful; I noticed the input stage you show is not the same as the MG100HFX I posted, which I guess is the same as yours , just different power amp.
What is seen is what I expected, but it's good to show the actual curves so others may understand it better:

1) the stage has switchable gain
a) with switch open, it's ruler flat from end to end.
b) with switch closed, it adds 8dB gain (3k9/2k2)+1=2.8X but within a reduced range, so useful guitar sound is boosted but not noise outside the band, we see C32 470nF and R25 2k2 in series cutting bass below 120Hz and R26 3k9 in parallel with C33 1nF cutting highs above 20kHz, although -3dB point is staggering 50 kHz , no doubt because of the advanced Op Amp you used.

And this also shows something important: the modern Op Amp is good, too good , improved noise a lot (which is expected), but also extended range a lot, which *might* be a problem, simply because Marshall designer did not take that in consideration.

Don't expect problems here, but if the Op Amp is inside a power amp (many 70's designs used a 741) , any improvement may throw stability compensation down the drain and cause big trouble, I see that a lot in DIYAudio, where users try to improve 80's or 90's Hi Fi equipment.

Don't remember where I picked this (might even have been here at MEF) but this is eye opening:

Edit: your English is excellent, but please tell us where are you writing from, might help us understand some twist of language

[NORSOREX]

Hi,

Have a look and discover that TR2 is missing ???
From factory ????



Who know what is this TR1 ?



Thanks to every body