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#31
Amplifier Discussion / Re: Soft clipping power amp de...
Last post by Loudthud - February 22, 2024, 12:03:18 AM
The input is simply a dual differential pair, one NPN the other PNP. Each pair has it's own active current source. The outputs of the diff-pairs feed current mirror driver transistors where most of the Voltage amplification takes place. Bias for the output stage comes from a Vgs multiplier which matches the temp-co of the output MOSFETs much better than a Vbe multiplier. The source follower type output clips softly because the output Voltage can only swing to about 5V less than the rail Voltage. In addition, the zener diodes across the feedback resistor can cause soft clipping depending on the diode's characteristics.

Hard to tell what the open-loop gain is without modeling or building one, I don't speak German or trust any translation on the net.

Designs like this can be found on the diyaudio.com forum.
#32
Amplifier Discussion / Soft clipping power amp design...
Last post by ampetrosillo - February 21, 2024, 03:26:55 PM
I have found the following design on the 'net:

http://xipix.de/elektro/gimos50.html

It looks interesting, although very unfamiliar to me: it's a soft clipping power amp with, by what I read, low global feedback, current feedback and (therefore) high output impedance. It looks very well suited to guitar duties, although I'm not an expert (I can find my way around preamps and I can also design them, but I'm not familiar with power amps).

I've tried deciphering the schematic, but as I said, a lot is unfamiliar to me: the input stage isn't the common differential pair and kinda looks like an output stage? Also, it employs MOSFETs while most of the things I've read up on employ BJT transistors (there shouldn't be much difference though, MOSFETs are biased similarly to bipolar transistors, with the difference that the gate is basically electrically isolated).

Could you please walk me through it and explain to me if and where it can be improved upon?
#33
Amplifier Discussion / Re: Silvertone SS XL 40 WITH L...
Last post by Joe49 - February 21, 2024, 10:43:57 AM
Thanks for the info on Q1 and how to use the amp. Appreciate it and will replace Q1 first to hear if there's any improvement. 
#34
The Newcomer's Forum / Re: Super Basic Question for a...
Last post by phatt - February 20, 2024, 08:59:41 PM
hi Megatrav,

Wise not to focus on one aspect like Efficiency.
Heck, Valves waste a lot of energy as heat yet they are still considered the holy grail of tone for a lot of guitar players.
There are just so many rabbit holes in this field that are mostly a waste of time.
I have 3 Valve Amps but my main setup is a pedal board which delivers a great sound into an old 70's 80's era Keyboard Amp.

If you research how Valves actually work you will realise that a lot of the magic is due to a very poor Power supply.
Depending on the design With no signal the HT voltage might read 400VDC in a valve amp. Turn up the volume and hit a big power chord hard and that 400V will drop like a brick, You might see it drop by 100V or more, depends on The ability of the PSU design.
As that signal fades away that voltage rises back up.

This is effectively causing compression so the sound level actually drops and limits the absolute SPL and as the Voltage rises back up it gives the impression of more sustain.
But with SS amps the supply voltage is very stiff and hence no magic sustain.
A SS 40 volt supply might only sag 2 or 3 Volts and by then you get into hard clip which is ugly, so hence SS gear gets a bad reputation.

So to recreate that effect with SS gear you will need a compressor.
I do all of that with my pedal board and the Amp just makes it louder.
The Amp adds very little colour to the final sound although it does have a spring Reverb which adds a final touch. I use 3 OD dirt pedals into my Compressor.
the Gain of all 3 dirt pedals are set fairly low and as you turn each one on you get more drive. And the compressor does 2 jobs obviously it adds compression but it also keeps the absolute SPL in a set range so when I play leed parts nothing gets too loud.
I use an optical compressor as they tend to work very well for OD rock guitar sounds.

As for Class D stuff I've noticed that the freq response often goes down way too far and that can be a big problem if you are trying to reproduce the classic sounds.

Understanding and limiting the bandwidth of your gear will help refine your sound.
Too much Low and High freq will just frustrate you and drive you crazy.
There are many ways to great tone and even when you get there you will find that different venues and rooms can give different results. I have built many circuits and thought I had cracked the holy grail only to find that when I played live it sounded like crap.

If it interests you I posted a recording of my gear on here a while back.
This will give you some idea of what can be done with all SS gear.
https://www.ssguitar.com/index.php?topic=5309.msg41595#msg41595
Regards from an old bloke who has spent lots of time down these rabbit holes, Phil
#35
The Newcomer's Forum / Re: Super Basic Question for a...
Last post by megatrav - February 20, 2024, 01:12:56 PM
Quote from: phatt on January 13, 2024, 04:34:38 PMJust a note on Digi Amps and SMode supplies.
Ok they are cheap, light weight, and so so convenient.
But keep in mind, Unlike Smode supplies Iron and Copper Transformers have an indefinite life span, if used within spec.
Jezzus Not even god could make a more bullet proof design. 8)

 I doubt that in 50 years from now that these SMPS circuits will still be running as they are far more complex and hence far more prone to failure.
Notice how many Dead computer Power boxes fill bins in repair shops.

Now I'm not up to speed on Digi amps but the few I've had the displeasure of working with leaves me thinking that it's hard to beat a basic old school transistor poweramps.
Well designed unit go for years without issue. Of course Can caps dry out eventually but simple to replace.
with most things now surface mounted and hard to work with it's a throw away item. But by then you find out OH sorry they don't make them anymore. ::)
 and you are left with land fill. :'(  :'(

If it interests you Rod Elliot has a ton of info on Amp design, a lot of which is aimed at muso gear.
Start here;https://www.sound-au.com/

I have found him to be very helpful while trying to build Analog Power Amps.
Phil.

Hey Phil,

Thanks a ton for the response.

I agree with you on the idea that building things with the intention of them not lasting is never a good idea. It's wasteful and isn't good for anyone.

I have nothing against transistor amps other than the lack of efficiency.
The most appealing thing about Class D is that if someone wanted to build a high wattage bass amp they could keep the weight lower because they wouldn't need as large of a heat sink.
I've looked into Class H amplifiers but they seem even more complicated and with only slightly more benefit over Class AB.

I actually think that when it comes to building an amplifier, a power transformer is a better idea because it will last longer and probably have less failure (potentially?). This may be a silly question, but can one use a power transformer with a Class D amp?


Lastly, these small SMD modules were sort of my Segway into learning about Class D amps. If I were ever smart enough to build my own, I would want to use through hole components.
That's how I build everything I make now. 
#36
Amplifier Discussion / Re: Silvertone SS XL 40 WITH L...
Last post by Loudthud - February 20, 2024, 01:34:28 AM
I would suspect Q1. Any big negative Voltage hitting the input will slowly degrade Q1 (making it hiss) as it has no protection.

Always plug the guitar cord into the guitar first, then plug the other end into the amp. That loud buzz you hear when you do the opposite is what damages the input transistor.
#37
The Newcomer's Forum / Re: LA4282 in BTL bridge mode?
Last post by Loudthud - February 20, 2024, 01:18:40 AM
The LA4282 has a max load of 8 Ohms per channel so you'd have to use a 16 Ohm load in bridge mode. If you want something louder than 10W, use both channels each driving it's own speaker.
#38
Amplifier Discussion / Re: Silvertone SS XL 40 WITH L...
Last post by Joe49 - February 19, 2024, 07:04:18 PM
Thanks, I'll try this.   
#39
The Newcomer's Forum / Re: LA4282 in BTL bridge mode?
Last post by joecool85 - February 19, 2024, 02:53:34 PM
Quote from: Maddus on February 15, 2024, 03:10:24 AMI was given a LA4282 from my fathers workshop (he repaired lots of TVs in the 90s).
To use this for my guitar projects, I would like to know if I can bridge this like a LA4440 for example, and what external circuitry is needed.
The LA4282 already has input and feedback resistors for a gain of 101 integrated.


Datasheets:

LA4282:
https://www.mouser.de/datasheet/2/308/1/LA4282_D-2314539.pdf

LA4440:
https://www.electroschematics.com/wp-content/uploads/2012/07/la4440-datasheet.pdf

You can actually get a LOT of volume out of 10 watts.  I would consider using the chip as-is and hook it up to a quality 12" guitar speaker.
#40
Amplifier Discussion / Re: Silvertone SS XL 40 WITH L...
Last post by g1 - February 19, 2024, 02:01:21 PM
If the hiss is so loud that the amp in unusable, it is likely a noisy transistor is the cause.
If you understand the signal path, you can try a cap to ground at various points along the path to try to isolate the location.  Something like 0.1 or 0.47uF rated 100V or more should work.
If the cap to ground kills the hiss, you know the source is somewhere ahead of that in the signal path, so you move the cap further toward the input and try again.