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Messages - phatt

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Hi Lex it's ok ,, i've got time at the mo as I'm waiting for a bathroom reno to be finished so can't do much else cept wait. :'(

Although some parts might be in a slightly different position you will find it is electrically the same thing.

Pic2 just shows how the signal swing gets bigger and smaller as it passes through each section.
in this case the ,,gain 1 pot (R6) is set at 70% rotation, while gain pot 2 (R8) is set at 40% rotation.
The input voltage is 100mV and the output at U4 (Yellow trace) is still under 200mV so the preamp is basically clean until gain2 (R8) passes 60% rotation.
Which is why I'm fairly sure the preamp is working mostly clean so if you have distortion at low level then  I would look at the schematic of power amp and see that R18 which sets the gain around the powerchip is 130k and my guess is that resistor is way too large and hence the amp distorts long before the diodes conduct in the preamp. Normally R18 would be about 20k~ 50k max.
the other screen shot is the tone response curve at different points on the schematic which gives you a clue as to what frequencies are being amplified.
The tone controls are set with Bass Full up Mid full off and Treb Full up.
so the output at U4 (top of R17) is the purple trace of the frequency curve from the preamp. 

The Newcomer's Forum / Re: JLH 1969
« on: February 22, 2020, 07:07:28 AM »

I Agree with *Loudthud* :tu:
I would simply add that the term Class A is kinda meaningless for Guitar amps,, Leave that silly stuff for the HiFi cork sniffers.
Most of all the greatest guitar sounds you have ever heard were played on mainly Class A/B amps,,, NOT Class A.

With SS Poweramps there is only a small benefit to be gained by trying to emulate Tubey sound. Yes there are tricks like current FB which help. Peavey uses the term TransTube,,,, But I must be the worlds worse Cork sniffer  :lmao: 
Cause I've never heard anything that even comes close to tube sound from the Peaveys I've played. xP
Transtube only seems to work at loud SPL but at lower levels it's snake oils to my ears.

The best you can do with a SS rig is trick up the preamps and just build a simple SS Poweramp.
Chip amps are dead easy to assemble and work very well,, especially if you are new to this amp building fun game.  Spend time reading up on the subject before you commit to building.

Plenty here to wet your appetite;

Well written and with the novice in mind.
You can purchase pcbs from him with backup help if needed.

If this is your first build attempt then I'd go with Project 19, Single Chip 50W Power Amplifier.
It's for LM3876 or LM3886.

Tons of good info on this site.
Rod E has helped me with a few issues even though I have not purchased any projects. :tu:

Amplifier Discussion / Re: marshall 5212 channel bleed fix?
« on: February 19, 2020, 01:23:24 AM »
As to your suggestion; No because VR1 is Not A voltage divider (aka plian old volume control) it's a gain control for IC1b.

Rather than all the work why not just split the input into 2 separate input sockets, (if you have room on the panel).
now just use an A/B box to select channels. That will cancel bleed over.
But you would need to kill the switching setup

As for the imbalance of the 2 channels I doubt there is a simple solution.
Best I can think of is to lift the output of R10 (clean out) and mirror IC1a section and mix back into send out at output of R27. but again a lot of work,, which may or may not work.

You are correct in thinking these circuits like most amp circuits now are NOT 2 separate channels they are either channel switching or multi channel rigs and I'm yet to meet the owner of one of these multi channel rigs who is happy with all channels,, lol.
Most end up using one section (often the clean) and resort to pedals for all the other sounds. Also I've found single channel rigs are less likely to have problems, Or like the older rigs 2 separate channels with maybe common reverb. The amount of times I've had to trouble shoot circuits only to find the audio path actually works fine,,but it's all the crappy switching stuff that fails.

I'm working on a new pedal for that.  It's called the Degrader.  :cheesy:

Good idea,,Keep us posted :tu:

If it helps I've built the cab sim from Marshall JTM 60 schematic, the one I used on the Quadraverb recording I mentioned.
I built a few of those for local muso's. good feedback from them.
I've also built one loosely based on the cab sim from an old Nobels SST1 preamp box.
(looks like a rockman belt pak gizmo)
That is the one built into my pedal board now.

If you want to experiment, I've found some of those old telephone 1/1 Line Isolation Tx's do roll of excess top end, small and only rated to about 3~4khZ.
Some work ok some not so well as they tend to roll off bass too much.
I figure if you found the right one it would work.

Hey Joe,
I hazzard a guess that Digi models just have way too much bandwidth,, = Fizz at high gain.
Well that is what my ears tell me every time I have to repair one of those all in one COSM roland pedal gizmos. yuk
I have a crappy digiteck modeling pedal thing here that I got for nothing and it's the same issue.

Years back I went nuts trying to resolve the 20/20 bandwidth on an old Quadraverb. For their day they were top shelf gear.
while trying to record some guitar tracks with it I ended up just inserting a simple ANALOG Marshall cab sim between the quad and deck,,,, huge improvement.
From what I can gather not much has changed.
So as you can probably guess I'm not a fan of modeling setups.
A young lad asked me only last week why can't I get the sound of the great players from years ago?
Answer, TOO MUCH friggin bandwidth in modern amps and gear lad.  8|

regards to sims,, FANTASTIC learning/ teaching tool for noobs like me who wish to understand what the hell goes on inside amplifiers. :dbtu: :dbtu: :dbtu:
And you can't  blow them up if wired wrong or burn yourself with hot irons  :lmao:
Your preamp circuit looks like this with Circuitmaker,, it's not top shelf sim but it's all you need to come to grips with basic amp circuits. (even does Valve circuits if you want)
Cheers Phil.

The Newcomer's Forum / Re: Laptop charger 3 core cable
« on: February 18, 2020, 06:39:51 AM »
Good on you for collecting them as they are often sold very cheap. I pick them up from op shops for $2~$5. :tu:

As to the 3rd wire,, well if it powers up and you have DC voltage at the other 2 wires then obviously you won't need to connect that 3rd wire. (others here might have more experience with those)

Some come with 2 wire mains input but some may have a 3 wire IEC Mains input socket. With those you may find the Mains Earth is connected to the Negative of the secondary wire, in which case you already have a ground return path to Mains Earth which in my experience has helped with interference.

Hi Lex, this might be helpful.
Basic opamp designs explained,,minimum maths.

It's beyond the limits for most of us to give a blow by blow description of how it all works but briefly;
You signal which is an AC voltage floats on a DC voltage potential. (your DC supply Rails)
Each stage amplifies that AC audio signal.

From left to right,
U1 is the input pre,,,U2 is the clipping stage,, u3 makes up for the losses created by having a passive tone control.
U4 is likely not needed but hey they don't make a chip with 3 opamps so why not use the 4th as another buffer. :lmao:
the 4th chip on the right is the power chip which is just a much higher current opamp to drive the speaker. this is the one that bolts to a heat sink. In some small cheap amps it's just a slice of alloy.

The 130k fb resistor on the pwr chip sets the gain of the power amp along with other parts. If that is what is in your amp then no wonder you can't get a clean sound.

As the schematic is likely drawn wrong, i.e. pin 1 of power chip connects to both Pos & Neg of the speaker terminals so as drawn it can't work. :duh
Of course it obviously does work so I have no way of being able to work out how the circuit is actually wired up. :loco

The resistor I mention looks like it reads R19- 130k but who knows what is actually in your circuit. looks like it joins pin 1 and pin 8 on the power chip.
(I think)???

I've added this pic to show what I think is a mistake on the drawing.
If the red marked link was removed then the circuit makes more sense.(R19 is just to the left of that red cross.

The Newcomer's Forum / Re: Laptop charger 3 core cable
« on: February 17, 2020, 07:41:01 PM »
Just a warning , some of those lappy supplies are noisy due to the switchmode circuits used.
it's a lottery but you can get lucky so don't commit until you have tested in the real world.
I use a SMode 19Volt laptop PSU and it's noise free, I added a 9volt regulator circuit for the lower voltage pedals.
Ground return paths can cause issues so I have added a ground back to the mains plug which is built into the pedal board.
If you don't understand how to do that then get advice when working on a mains setup.

The cases of those Lappy supplies are plastic so they can radiate a lot of noise/switching hash if close to audio circuits so some shielding maybe needed.

Well you don't have to turn all the diodes around, just turn the two-pin connector around.

Ha Ha very good. :P
Thanks Enzo trust you to see the simple way to do it. :tu:

BTW I've just been studying the Schematic for this Amp and it's dawned on me that the output of power chip (pin1) connects to both +&- terminals of the speaker.  xP
Maybe I've missed something but might be a mistake.
Also regards to the amp distorting even at low volume;
I just simulated the preamp and a 100mV signal at input does not distort until you turn up past halfway.
But I notice the FB resistor around power chip is 130k. :loco
surely that is rather high and I'm guessing would distort even at low gain. (And that was the main reason for *Lex's* initial post)
The schematic is not very clear and I can't read the part number on the power chip.
Or maybe I'm just not tecky enough to know what I'm doin,. :lmao:

Oh I forgot the obvious page,
Here is a good explanation of the TS9 circuit by R G Keen of Geofex;
It explains how BB diodes work in the fb loop of an opamp which is the type your circuit uses. :tu:

I've come to the conclusion that I'd rather work with a single CLEAN channel amplifier and just use pedals for all the other FX.
My main working amp is not even a guitar amp,, it's an old Keyboard amp with treble bass and volume, but does have a real spring reverb. All the fancy tricks are on my pedal board and forgive me for blowing my horn here but half of those pedals are my own designs or modified brand name units. Took many years to work out what I wanted,, an sure many of those designs ended up as land fill but in the end I got exactly what I wanted.

WHY pedals?
Cause I'm yet to meet a player who uses multi channel amps and totally happy with all channels. :lmao:
Unless you are rich and can afford to have someone custom build exactly what you want then it's likely most of the off the shelf shop rigs will leave you less than impressed. :(

I spent years trying to get an ALL in ONE amp to work for me but in the end I resorted to pedals. I don't like pedals but for me I'm playing many different styles of music and the pedals cover most of what I need.
Sadly pedals are an evil necessity for me,, it's a love hate thing. :-X

Reason 2/
 It's a heck of a lot easier to swap out pedals than it is to have to rebuild amplifiers all the time until you find the sound you want.
My rant over for now,, hope it helps

Yes much better :tu:

There are 2 basic ways to do this Distortion trick.
1/ across the signal path and 2/ in the feedback loop.
Although your setup is in the Fb loop this page might help you understand that it's not just about what fancy pants diodes are used.

This explains the finer details of how diodes work across the signal, often driven by an opamp stage.

In your case you might do well to back off the gain of the input stage as that will slow down how early the amp goes into distortion.
That may well be easier to do that muck around with a whole lot of diodes.

Yes,, what Joe said.  You wire them back to back in that loop.
lay them side by side and they should look like this;
  + -
<     >
  - +
Just be aware that there is no magic diode involved here it's the combination of distortion (Dirt) or whatever diodes you use. The secret to great OD/Distortion/Fuzz is in the tone shaping of the circuits NOT so much the diode type.

So Leds might work for one circuit but sound crap in another amp circuit.
There are just so many combinations of how to do these tricks and then add to that; Each player might want something quite different.
Hence you see Utube clips where someone raves about how good the latest Dirt pedal sounds but you get it home and it sounds nothing like what you heard.
Because your guitar-Amp- speaker- style of music you play- talent level-blah blah.

As mentioned before, a lot of small bedroom amp designs are off the shelf circuits and they don't tend to put much effort into tone shaping to suit the speakers used.

I'm not trying to put you off ,, by all means experiment as that will slowly teach you how to interpret sound/tone.
I've been experimenting for years and it takes a while to find what works and what is a waste of time.

Amplifier Discussion / Re: marshall G30R CD clean channel volume problem
« on: February 16, 2020, 01:42:52 AM »
Oh yeah,, just re reading to check what I may have missed.
You mentioned the input and switch sockets had some issues.
up shot is the control voltage that runs IC2&3 may not be working correctly which would likely cause the issue you have.
I'd be checking that before replacing chips. Control is around TR1.
hope it helps,

Amplifier Discussion / Re: marshall G30R CD clean channel volume problem
« on: February 15, 2020, 11:55:26 PM »
Sorry but it's the wrong forum for anger management issues.

We try to help sometimes we get it wrong,, but we don't have the amp in front of us. xP

If you are well versed in Valve circuits then you would know how important ground return paths are.
Yes every Common point may well be at Ground/Zero **BUT it's the path each section takes back to Main Common point that can make or break a design.**

Tiss why I suggest you take a wire from Volume Common back to to central PSU (where the 2 main filter caps join) and see if it resolves the issue. As you have a new chip coming I hope it works out for you if not then I'd go hunting Ground Paths.

I've worked on enough gear to know that many problems are ground related.
Just as wire layout is important in Valve gear it's no different with SS PCB layout. Unlike lead layout in p-p valve circuits which can be easy rewired,, PCB is Fixed once printed and resolving ground return path issues becomes a lot harder.

I fixed a mates Pro junior Valve amp,, The most sensitive audio trace ran right along side the screen supply trace. :duh
Hence the damn thing hummed like crazy. :trouble
I has to cut a few tracks and hand wire to shut the thing up.
So that was a Fender design layout stuff up.
Without a deep understanding of ground plane layout even the big name gear is often full of suspect design issues.
I wish you well and hope you resolve the issue.

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