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Author Topic: Thinking about building a SS amp  (Read 1277 times)

edvard

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Thinking about building a SS amp
« on: June 09, 2018, 04:27:10 PM »

Yep, just the thinking part for now, though I already have most of it worked out.

Here are my first thoughts:
1 - Start with the speaker and power section.  I get the nagging feeling that designing a SS amp is akin to pairing up a distortion box with a 3-band EQ and power amp, so we start with the limitations (power watts and budget) and work down from there.  I'm thinking 50 watts is a good compromise between "I might have to gig with it someday" and cranking it up to a dull roar while the wife is out shopping.  I want the best speaker I can afford that doesn't have "British Voicing" (to my ears that means "sounds like a box"... sorry, that's just me).  I am also torn on the issue of the power section; basically I have 3 choices: Discrete, Chip amp, and MOSFET.  They all have their pros and cons, but that's for further discussion...

2 - Single channel.  In the '80s I cared about channel switching.  These days, I don't need to go back and forth; I have re-discovered my guitar's volume control so I can go from "Really Crunchy" to "Not So Crunchy" without having to set up two different tones. 
However, just writing this paragraph is making me think I may change my mind.  More about this later...

3 - Don't try too hard to emulate tubes, except for soft limiting in all clean gain stages.  An elegantly designed overdrive stage doesn't do much good if your signal hits the rails before it even gets there.

Thoughts?  Opinions?  I've read a good part of TeemuK's excellent book, so that's given me some good groundwork.  I'll lay out details in following posts, and give a complete schematic at the end for any other intrepid solder junkies.
« Last Edit: June 09, 2018, 04:29:59 PM by edvard »
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PMc

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Re: Thinking about building a SS amp
« Reply #1 on: June 10, 2018, 06:11:26 PM »

Try reading Randy Sloane as well - High Power Audio Amplifier Construction is a good one for discrete components, and his thoughts on FETs are at least worth considering.  For chip amps, Texas Instruments has published application notes for its ICs, including for power amps . Will see if I dig up a link.
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edvard

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Re: Thinking about building a SS amp
« Reply #2 on: June 11, 2018, 04:44:00 AM »

I've heard of Sloane's book, but haven't picked up a copy yet.  I've also amassed a large collection of datasheets for TI and other chipamps, as well as many Application Notes from TI, National Semi and others.  I've also pored over collections of schematics and have built quite a few stompboxes, so I'm not exactly new to the subject, I just have never before assembled the required parts into a cohesive unit, AKA a solid-state amplifier that is tweaked to MY tastes and preferences.

My first hurdle is going to be power, as in how am I to feed current into this collection of parts.  That means starting with the power amp.  In that regard, we have a few choices:
-Chipamp
Pros: Simple to design for, and can be inexpensive if you shop around.
Cons: Many good chips are now obsolete, and current production will someday be obsolete as well.  Not sure I want to chase that dragon, so if I settle on one, I should probably buy 3 or 4 more as backups. Or perhaps make the power section a separate module, so I can simply replace it with whatever the cool kids are using that month.
-Discrete
Pros: LOTS of designs freely available on the web to suit many tastes and power output needs.
Cons: Can be complex, which can lead to expensive, depending on what bells and whistles you want.  Speaker protection? Power-on muting? Even more complexity.
-MOSFET
Pros: Most common circuits are less complex than Discrete designs overall. 
Cons: Lateral MOSFETs (that means the GOOD kind) are designed for audio, and are VERY rare/expensive.  Cheaper vertical MOSFETs are not designed for audio, so they can be more "touchy" to work with, but see International Rectifier's Application Note AN-948 "Linear Power Amplifier Using Complementary HEXFET Power MOSFETs".  Very informative.  Also, heat... so much heat.
-Class D
Pros: Same pros as chipamps, but with vastly reduced heat concerns.
Cons: REALLY not targeted to guitar, and need to be protected from transient input spikes.  Overdriving a Class D chip is not pleasant.  At all.

I happen to have a few different chipamps, so I'll roll with that for this first project.  For the TDA7294 I have, ±30V should give me ~50 watts.  The Triad Magnetics F8-24 should get me there, is within budget, and has a 100VA rating, which Rod Elliott might argue is a bit under-powered for a 50-watt guitar amp, but anything more than that gets spendy pretty quick.  Besides, I don't plan on gigging with it any time soon; I'm not THAT good...  Alternatively, if I can find a good source of TDA2050s, I suppose I could live with the 35 watts it can put out with a ±18V supply and a 4-ohm speaker load.
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phatt

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Re: Thinking about building a SS amp
« Reply #3 on: June 11, 2018, 07:58:27 AM »

Hey Ed,,
Those TDA2050's are a 28Watts @ 4Ohms Watt chip,,Depending on load and PSU. 35Watts if you want to push 4 Ohms with 10% Distortion.
The 50Watt crap is music power which is a fancy fan-dangled way of making it look more powerful. That is what happens when marketing geeks use maths. :duh
Back at a less stressful 8 Ohms and it's somewhere between 18 ~22Watts RMS in the real world.
So I'd call that what it actually is,, about 20Watt Chip.

Frankly you sound like you are over thinking this whole thing. 8|

A SS power amp of any type normally used is a world away from a Valve equivalent. The rare circuits that might be able to compete will cost $$$.

With SS all your tricks are easier and a lot $$Cheaper$$ to do in the preamp.

That TDA7294 is likely the best bet and just follow the data sheets.
Though LM3886 has less pins and likely a bit easier to make a PCB.

Chip amps can run just fine for years if used as intended.
i.e. a proper heat sink and conservative loads.

I've built a few discrete pwr amps like ESP's P3A and they run fine. About as simple as it ever gets 8|. They have no Short protection and as Rod states in one of his pages the current protect circuits are not fool proof anyway and can fail.
So I just use a PTC on the output as they negate all the extra current protect circuitry which makes life easier for a home builder.  These are poly thermal switches and are sold as speaker protection devices BUT they can also protect the amp from shorts IF inserted right at the output of the power amp pcb. :dbtu:
They look like a disc cap.
https://www.jaycar.com.au/rxe075-ptc-fuses-speaker-protection/p/RN3460

I've built about a dozen amps using these for protection and had no trouble. <3)

Back to preamps,, Go purchase a bread board or 2 and start experimenting with a few simple preamp circuits.
I have a few circuits on here which are simple but quite effective, some are on my pedal board some tricks are in amps I have built.

My pedal chain is a fet preamp>> a couple of dirt pedals (never maxed out)>> mictesters (from FSB) compressor circuit (Brilliant) >> my own cab sim design>> my not Famous but very potent PhAbbtone and the amp is actually an old Laney keyboard amp. I have owned and built many amps, Valves as well and the above line up is still my go to rig. It just works.  <3)
I can plug my acoustic into this,, I can use my Strat and when needed I plug in keyboards as well.
I can pull country clean,, to ZZtop thick OD with ease.

As much as I always wanted to have  all this in one Amp I've had revert to pedals as it's just more versatile.
Most of my circuit schematics are on this site.
If it interests you I'll take some pictures?
Hope it helps ,, Regards.. Phil.
« Last Edit: June 11, 2018, 08:35:45 AM by phatt »
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edvard

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Re: Thinking about building a SS amp
« Reply #4 on: June 11, 2018, 02:36:26 PM »

Hey Ed,,
Those TDA2050's are a 28Watts @ 4Ohms Watt chip,,Depending on load and PSU. 35Watts if you want to push 4 Ohms with 10% Distortion.
The 50Watt crap is music power which is a fancy fan-dangled way of making it look more powerful. That is what happens when marketing geeks use maths. :duh
Back at a less stressful 8 Ohms and it's somewhere between 18 ~22Watts RMS in the real world.
So I'd call that what it actually is,, about 20Watt Chip.

Yep, I know the marketing-speak and I got the 35 watts at ±30V from the Volts-to-Power chart on the datasheet.  I know they are simple-but-tough little critters and you can push them to get better power, and bridging them is even funner, but I'm trying to keep it simple.  For the moment they are out of stock at Tayda, so I won't be trying them anytime soon.

Quote
Frankly you sound like you are over thinking this whole thing. 8|

HA! Ya think?  :dbtu:

Quote
A SS power amp of any type normally used is a world away from a Valve equivalent. The rare circuits that might be able to compete will cost $$$.

I stated in my first post I'm not trying to concoct a magic equivalent of tube amps, and I think that's a lost cause anyway, for reasons I am not interested in discussing.  Seems to me, the only things truly in common between tubes and silicon is they are both made from hot glass and they can both amplify.  After that, there's a lot of dead horses being beaten, and I want no part of that.

Quote
With SS all your tricks are easier and a lot $$Cheaper$$ to do in the preamp.

Pre-zactly.  The one thing I do like about tube amps is their innate habit of soft-clipping even when running clean, and that mostly holds for the pre-amp triodes.  Output pentodes can clip just as ugly as their 3-legged cousins.  Well, so can triodes if you abuse bias them properly, but I digress.  My pre-amp tricks aren't new, but hopefully pleasant as I try and avoid hard-clipping (I may say more about that later).

Quote
That TDA7294 is likely the best bet and just follow the data sheets.
Though LM3886 has less pins and likely a bit easier to make a PCB.

Chip amps can run just fine for years if used as intended.
i.e. a proper heat sink and conservative loads.

I got those because they were dead cheap, and my favorite supplier is currently out of stock of 3886s. I will probably pick up a few at some point, just trying to avoid fakes.

Quote
I've built a few discrete pwr amps like ESP's P3A and they run fine. About as simple as it ever gets 8|. They have no Short protection and as Rod states in one of his pages the current protect circuits are not fool proof anyway and can fail.
So I just use a PTC on the output as they negate all the extra current protect circuitry which makes life easier for a home builder.  These are poly thermal switches and are sold as speaker protection devices BUT they can also protect the amp from shorts IF inserted right at the output of the power amp pcb. :dbtu:
They look like a disc cap.
https://www.jaycar.com.au/rxe075-ptc-fuses-speaker-protection/p/RN3460

I've built about a dozen amps using these for protection and had no trouble. <3)

I guess it's kinda silly of me to complain about the complexity when I've built more than one Big Muff  :loco , but I can't help but get just a little freaked out looking at those glorious schematics.  I'll probably opt for Project 27 if/when I decide to do a discrete power section, and I am intrigued by the whole "current drive" thing anyway.  Thanks for the PTC suggestion, I'd wondered about that myself.  I've got a good handful of them in my parts buckets...

Quote
Back to preamps,, Go purchase a bread board or 2 and start experimenting with a few simple preamp circuits.
I have a few circuits on here which are simple but quite effective, some are on my pedal board some tricks are in amps I have built.

My pedal chain is a fet preamp>> a couple of dirt pedals (never maxed out)>> mictesters (from FSB) compressor circuit (Brilliant) >> my own cab sim design>> my not Famous but very potent PhAbbtone and the amp is actually an old Laney keyboard amp. I have owned and built many amps, Valves as well and the above line up is still my go to rig. It just works.  <3)
I can plug my acoustic into this,, I can use my Strat and when needed I plug in keyboards as well.
I can pull country clean,, to ZZtop thick OD with ease.

As much as I always wanted to have  all this in one Amp I've had revert to pedals as it's just more versatile.
Most of my circuit schematics are on this site.
If it interests you I'll take some pictures?
Hope it helps ,, Regards.. Phil.

I've got more breadboards than I can shake a stick at, so I'm good there.  Being a former pedal junkie, I got pre-amps pretty much covered, though if you have links to some of your favorites, I'm always interested, thanks.
I'm focusing on the power section at the moment because I know I can whip up a pre-amp practically in my sleep, and power has always been an afterthought; the job of whatever amplifier I'm unhappy with at the moment.  It's a challenge and I want to learn.
In fact, that's the whole point of this experiment, to make a standalone amp that I'm happy with because I put all my best ideas in one package.  Yes, a chain of pedals can be quite versatile, but if I wanted to fiddle with pedals, I would have kept the old Peavey I sold because I hated the distortion channel.  I want the simplicity of just plugging in and cranking up without having to worry if my batteries are fresh.  I built my own guitar because for the same reasons; I have two others that work just fine, but now that I've built one, it's my favorite, and I'm going to build another based on what I learned with the first one.

Yeah, I'm overthinking it, and I probably can't compete with what the "big guys" have already done and been doing for the last 50 years, but dangit, I'm having FUN...

P.S. My guitar project, if you're interested: https://imgur.com/a/7Vo5F
« Last Edit: June 11, 2018, 04:33:27 PM by edvard »
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flester

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Re: Thinking about building a SS amp
« Reply #5 on: June 16, 2018, 06:03:28 AM »

I like the guitar project. I also took inspiration from Ovation electrics using the Viper body shape.

Sent from my SM-G935F using Tapatalk

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edvard

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Re: Thinking about building a SS amp
« Reply #6 on: June 17, 2018, 03:13:16 AM »

I like the guitar project. I also took inspiration from Ovation electrics using the Viper body shape.

Those Vipers are pretty sweet, minus the pickguard; never liked those bubbly pickguards on the Viper or the Preacher, though I admit if I could own one of each, I probably would...

On to tone controls.
I've tested a few different takes on the Baxandall and the classic FMV tone stacks in my breadboarded preamp, and the one that ended up staying the longest is a slightly tweaked version of RunOffGroove's "Tonemender".  I wanted to like the Bax, I really did, but it's just too subtle for the duties I have in mind.  However, I've noticed a bit of high-end unpleasantness on higher gain settings of my preamp-currently-under-test, and now I'm wondering if there's something different that might work a bit better...
I'm thinking two gyrators for Bass and Mid, and a cut-only Treble.  The Bass control will be a 'shelving' control that cuts and boosts.  The Mid control will be centered at ~650 hz that only goes flat or notch. I may make it adjustable from ~350 to ~1200 hz because it's just only one more potentiometer, though I'm kinda questioning the usefulness.  Lastly, a cut-only Treble because the last thing most folks need is shriller highs from most amp topologies (mine included) that tend to end up on the sharp side of the spectrum just before tone controls anyway.
Thoughts?  Opinions?
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J M Fahey

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Re: Thinking about building a SS amp
« Reply #7 on: June 17, 2018, 03:33:58 AM »

Excellent suggestions by phatt, as always.
I´d like to add about:
Quote
I know they are simple-but-tough little critters and you can push them to get better power,
Sadly, not any more.
They were discontinued 2 years ago, last 5000 were *swept* from Mouser shelves in a couple hours, I got 50 myself ... and that was it.

As many as you want from China/Hong Kong, pity they are filled with wet sawdust or worse.

https://www.youtube.com/watch?v=R2J5xrSiNrI

Tayda can be trusted, and if they are out of a certain component, it´s because they can´t get the good ones.

LM1875 is still available, but it´s 20W tops .
In any case I prefer real 18/20W to unreliable 13W provided by fakes.

TDA7294 ia a good chipamp but Marketing Department advertised them as "100W amps"  whichb they are not, and die trying to comply.
Use them for 70W RMS or even better 50W RMS (which means rails about +/-30V tops or "whatever LM3886 uses") and they will work forever.

Whatever LM3886 puts out (50/8 or 60/4) is what can be dissipated by that tiny case, which amounts to two TO220 packages side by side, you can´t go against Physics Law and pretend it can safely dissipate more heat because it can not.

So just make that simple power amp and then experiment at leisure with Preamps; results can be very good.

And real 50/60W into a good 12" Guitar speaker is all it takes to play in a live Club situation, alongside a drummer and other Musicians.
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edvard

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Re: Thinking about building a SS amp
« Reply #8 on: June 17, 2018, 06:42:40 AM »

Thanks for the insight JM.  I knew the 2050s had gone out of production, but I didn't know it was that recent.  If I'd been paying attention, I would have snapped up some myself.  Either way, such is the fate of specialized devices, which is why I'm debating making this thing modular at the least.  I'll most likely build a proper discrete power section at some point, I'm leaning toward MOSFETs, but not sure if that's the best choice.
Until then I got these chipamps to make noise with, and that's what counts.

Back to the bench!...
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phatt

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Re: Thinking about building a SS amp
« Reply #9 on: June 17, 2018, 08:20:21 AM »

Good to see that JMFahey is still around, proly been busy makin good amps. :tu:
As to tone controls, be aware that "Tone mender" has a noise penalty but then most passive tone circuits do come with the old *Insertion loss* and as they are Hi Z they tend to pickup noise if your design and layout is a bit sloppy.

I spent many years bread boarding and building tone control circuits and the one that impressed me most was the HiWatt circuit. <3)

The most frustrating part was learning how to circumvent the noise problems. I doubt one will ever win the S/N battle but my PhAbbtone circuit is very quite when compared to Tonemender.

This will certainly work and (unlike fender and similar) has a midrange control that actually WORKS. ;)
This gives a big round bass as well as bright treble,, May even be too bright for your liking but you mentioned adding a top cut which is a good idea,, I just use a cab sim which has similar effect.
PhAbbtone can work as a pedal or split supply for amp circuits.
I've lost count of how many I have built for local folks.

The other circuit that impresses me is a simple compressor that a FSB
member *mictester* designed and posted.
Original post here BUT you will have to be a member to read it.

http://www.freestompboxes.org/viewtopic.php?f=13&t=8581&hilit=really+simple+compressor

With his permission I can post the schematic again here.
*****

Unlike some of the big name compressors that add a lot of hiss it's dead quite. The hardest part is to mak up the LDR and leds,, the rest is all basic opamp stuff.
I've just added a buffer and flipped the output to keep the signal in phase. not needed but I'm just fussy about that.  :P
As both of these circuits can easily be implemented into a preamp design I thought you might like to test them out.

If any interest I can post my Cab sim circuit as well?

Regarding Power Amp design.
In My limited experience (hey it's only a hobby for me)
I've researched and tested Current FB setups on SS power amps and although it does alter the outcome it's a not mind blowing difference.
I've played a few SS peavey amps with Sat control (I think that is what it's called) and it does alter the outcome a bit but I suspect that a SS power stage running an output transformer might reap far better results.  but that bumps up the cost :-\
At high volume CFB seems to help but for home use it may not be audible.
I have a massive Factory type PA amp which has 70/100 volt output transformer as well as an 8 Ohm tap which does a good job of replicating a cranked Valve power stage but same limitation, you don't notice it at low volume.
Phil.
Oh yes,,good looking job on that guitar. :dbtu: :dbtu:
« Last Edit: June 17, 2018, 08:24:17 AM by phatt »
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J M Fahey

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Re: Thinking about building a SS amp
« Reply #10 on: June 17, 2018, 11:10:38 AM »

Thanks Phatt.
Yes, since last October I have been very busy, revamping the new amp line, designing new models, etc.
Not much free time to visit my beloved SS Guitar Forum.  :-[

here´s some of the new stuff, which I introduced at the 2018 Convention of the Audio Engineering Society Buenos Aires branch:

« Last Edit: June 17, 2018, 11:14:33 AM by J M Fahey »
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R.G.

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Re: Thinking about building a SS amp
« Reply #11 on: June 17, 2018, 01:31:56 PM »

As usual, I have a mildly heretical viewpoint.   :)

Chip amps do have fundamental limitations on their ability to dissipate power. As a practical matter, the TO-220 packages like the LM1875 can only really do a 20W amp out at the edge of reliability. The LM3886 and similar packages can get to about twice that and be reliable without gigantic heat sinks.

My solution to this is to use the LM3886, which is an incredible deal at about US$5 each, and use more of them. There are circuits to parallel them up for higher currents and better power dissipation, and to run them bridged for higher voltage. However, I would not use them that way.

The LM3886 is quite reliable (given a decent heatsink) at 30-40W output. Most guitar amps run one 12" speaker on about 30-50W of amplifier power. When it gets over that most amps run two 12s or four 12s. Why fight the 30W per speaker practice? Why not make one highish quality amplifier per speaker. They're easier and more reliable that way, as well as more adaptable to situations and portability.

I just went through this in designing a replacement/repair power amp for the Thomas Vox amps. I opted for using an LM3886 per 30W speaker load. This covers the Buckingham and Viscount with 8 ohm loads with one amp each. The Royal Guardsman needs two modules to run two 12s at 60W, and the Beatle needs four for 120W.

It is far easier to get a 30W amp to run right than to get a 120W or higher to run right. And replicating several small things can be simpler than making one big one.

I did a PCB that is about 2" by 3" and runs one LM3886. I found a suitable heat sink for under $8 at heatsinksusa.com. Antek offers the AS-0522 toroidal transformer for $17.50, and this is a nearly ideal transformer to power the LM3886. You can fit the whole amplifier on one heatsink serving as a sub-chassis, power supply included.

And if one fails, you've thoughfully made one more power amp module than you really needed, so you swap in the spare while you fix the broken one.

Just sayin' ...
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edvard

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Re: Thinking about building a SS amp
« Reply #12 on: June 17, 2018, 03:57:20 PM »

...
As to tone controls, be aware that "Tone mender" has a noise penalty but then most passive tone circuits do come with the old *Insertion loss* and as they are Hi Z they tend to pickup noise if your design and layout is a bit sloppy.

I spent many years bread boarding and building tone control circuits and the one that impressed me most was the HiWatt circuit. <3)

The most frustrating part was learning how to circumvent the noise problems. I doubt one will ever win the S/N battle but my PhAbbtone circuit is very quite when compared to Tonemender.

This will certainly work and (unlike fender and similar) has a midrange control that actually WORKS. ;)
This gives a big round bass as well as bright treble,, May even be too bright for your liking but you mentioned adding a top cut which is a good idea,, I just use a cab sim which has similar effect.
I've recently run across your PhAbbtone circuit, and will be the next thing I test.  A usable midrange would be nice, though I've found a flat midrange is enough.  I haven't noticed much issue with noise in the ToneMender circuit, but then again, maybe I'm just used to it :P
What impressed me about the ToneMender (with my subtle tweaks) is that when I cranked a knob, I FELT it.  Most generic FMV and the Baxandall circuits I tested were way too subtle, and the ToneMender even had usable tones at extremes of the knob positions, which I found amusing.  Hopefully, I can give a good report on the PhAbbtone...

Quote
The other circuit that impresses me is a simple compressor that a FSB
member *mictester* designed and posted.
Original post here BUT you will have to be a member to read it.

http://www.freestompboxes.org/viewtopic.php?f=13&t=8581&hilit=really+simple+compressor
I've seen that one as well.  I like the simplicity.  I've thought about having a compressor/limiter on the end of the chain just before the power amp just to keep things tame going into the watt generator, but that might not work as well as I think.  I prefer a comp at the beginning of the chain, and even then mostly for clean stuff.  Come to think of it, maybe a good compressor circuit might work well as a second channel if I decide to make this a channel-switching amp.  Not that I do much clean playing...  8|

Quote
If any interest I can post my Cab sim circuit as well?
By all means.  That might make a good dedicated-out for headphone practice or in situations where I don't have access to a computer to play cab simulator (Yes, I know...).  Most cab sim circuits I've tried range from "is this thing on?" to "Good lord, what speaker is THAT supposed to be?".  Hopefully yours is different.

Quote
Regarding Power Amp design.
In My limited experience (hey it's only a hobby for me)
I've researched and tested Current FB setups on SS power amps and although it does alter the outcome it's a not mind blowing difference.
I've played a few SS peavey amps with Sat control (I think that is what it's called) and it does alter the outcome a bit but I suspect that a SS power stage running an output transformer might reap far better results.  but that bumps up the cost :-\
At high volume CFB seems to help but for home use it may not be audible.
I have a massive Factory type PA amp which has 70/100 volt output transformer as well as an 8 Ohm tap which does a good job of replicating a cranked Valve power stage but same limitation, you don't notice it at low volume.
Thank you for your insight on that.  When I first ran across the CFB idea, it simply made sense to me, even though I suspected it wasn't a huge difference in the sound (or else we'd ALL have CFB amplifiers in our home and car stereos).  That and Rod Elliott mentioned that CFB might add a little margin of safety for the speakers (emphasis on 'might' and 'little' :P).  I also have one of those 70 volt PA distribution transformers, which I bought a loooooooong time ago in anticipation of building a low-wattage tube amp.  Maybe I'll save that one for the eventual MOSFET experiments...

Quote
Oh yes,,good looking job on that guitar. :dbtu: :dbtu:
Thanks, I haven't been able to put it down since I built it, which makes things a bit inconvenient when I think about doing a few details like shielding the cavities, taming a few wily fret levels, or tweaking the tone control.  I've also noticed it really does need a belly cut.  Age and a wife who's an amazing kitchenista has not bode well for my midsection.  ;)

...
here´s some of the new stuff, which I introduced at the 2018 Convention of the Audio Engineering Society Buenos Aires branch:
Holy smokes JM, those are some fancy lookin' amplificators.   :dbtu: :dbtu:

As usual, I have a mildly heretical viewpoint.   :)
I remember you from the stompbox forums.  Your viewpoints have always been helpful, heresy or no  8|

Quote
Chip amps do have fundamental limitations on their ability to dissipate power. As a practical matter, the TO-220 packages like the LM1875 can only really do a 20W amp out at the edge of reliability. The LM3886 and similar packages can get to about twice that and be reliable without gigantic heat sinks.

My solution to this is to use the LM3886, which is an incredible deal at about US$5 each, and use more of them. There are circuits to parallel them up for higher currents and better power dissipation, and to run them bridged for higher voltage. However, I would not use them that way.

The LM3886 is quite reliable (given a decent heatsink) at 30-40W output. Most guitar amps run one 12" speaker on about 30-50W of amplifier power. When it gets over that most amps run two 12s or four 12s. Why fight the 30W per speaker practice? Why not make one highish quality amplifier per speaker. They're easier and more reliable that way, as well as more adaptable to situations and portability.

I just went through this in designing a replacement/repair power amp for the Thomas Vox amps. I opted for using an LM3886 per 30W speaker load. This covers the Buckingham and Viscount with 8 ohm loads with one amp each. The Royal Guardsman needs two modules to run two 12s at 60W, and the Beatle needs four for 120W.

It is far easier to get a 30W amp to run right than to get a 120W or higher to run right. And replicating several small things can be simpler than making one big one.

I did a PCB that is about 2" by 3" and runs one LM3886. I found a suitable heat sink for under $8 at heatsinksusa.com. Antek offers the AS-0522 toroidal transformer for $17.50, and this is a nearly ideal transformer to power the LM3886. You can fit the whole amplifier on one heatsink serving as a sub-chassis, power supply included.

And if one fails, you've thoughfully made one more power amp module than you really needed, so you swap in the spare while you fix the broken one.

Just sayin' ...
Thank you for the insight.  My current intention is to refill the unfortunate carcass of a late-80s Crate 'mini-stack' with my own design stuffed in the head and replacing the two 4x6" speaker arrangements with one 12" per cab.  Using one chipamp per speaker would fulfill my wattage desires, and have a little more beef than what was originally there (two TDA2030s, one for each speaker, running at 10 watts each).  Making these higher-powered chips run a little cooler would save me on power supply and heat sink costs.  Glad I wasn't the only one thinking on those lines. 
I'm really only intending to use the chipamps to get the thing making noise, and go discrete or MOSFET later.  I might even dip my toe in the Class D water someday, but today is not that day.
« Last Edit: June 17, 2018, 03:59:16 PM by edvard »
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J M Fahey

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Re: Thinking about building a SS amp
« Reply #13 on: June 17, 2018, 04:27:52 PM »

As usual, I have a mildly heretical viewpoint.   :)
Change that label to *creative*  and I´ll buy it :)

Quote
Chip amps do have fundamental limitations on their ability to dissipate power. As a practical matter, the TO-220 packages like the LM1875 can only really do a 20W amp out at the edge of reliability. The LM3886 and similar packages can get to about twice that and be reliable without gigantic heat sinks.
Just today I posted about the same on some Forum.

LM3886 have an impressive realiability record ... TDA729x not so much (and that´s an understatement).

Why?: both use the exact same case, one is expected to deliver 50W .... reaching 60W only at *reduced* +V rails, the other is happily touted as a "100W amp" and proudly says so on datasheet, also is happily touted as using +/-42V rails in Real World applications (as opposed to Lab tests) ... we all know results, Marshall had to make plug in modules so users can swap TDA729x themselves  :duh
Quote
My solution to this is to use the LM3886, which is an incredible deal at about US$5 each, and use more of them. There are circuits to parallel them up for higher currents and better power dissipation, and to run them bridged for higher voltage. However, I would not use them that way.
Fully agree: bridging is a nice way to turn 8 ohm loads into 4 ohm ones, and 4 ohm into 2 .... a mess and most nobody notices that.

Paralleling *voltage*  sources is a disaster waiting to happen.
The additional condition that now gain must track each other within 1% or less, any volume, any frequency, plus NEEDING cheesy series resistors to somehow help speaks volumes about what a poor Engineering choice it is.

Ultimate madness of course is bridging parallel TDAs , somebody at DIY Audio even proposed triple parallel bridged ones.  :loco :duh

Worst thing is that by then the amp, besides being way less reliable, has become more complex than a properly made discrete one !!!!  ::)

Quote
The LM3886 is quite reliable (given a decent heatsink) at 30-40W output. Most guitar amps run one 12" speaker on about 30-50W of amplifier power. When it gets over that most amps run two 12s or four 12s. Why fight the 30W per speaker practice? Why not make one highish quality amplifier per speaker. They're easier and more reliable that way, as well as more adaptable to situations and portability.
Excellent idea.

Quote
I just went through this in designing a replacement/repair power amp for the Thomas Vox amps. I opted for using an LM3886 per 30W speaker load. This covers the Buckingham and Viscount with 8 ohm loads with one amp each. The Royal Guardsman needs two modules to run two 12s at 60W, and the Beatle needs four for 120W.
Not sure I follow you: you use 4 x 3886 in parallel to drive a 2 ohm cabinet through a single cable pair or a special connector with 4 individual speaker cables?
Not problem in a combo, not so sure in a head+cabinet situation.
Quote
It is far easier to get a 30W amp to run right than to get a 120W or higher to run right. And replicating several small things can be simpler than making one big one.

I did a PCB that is about 2" by 3" and runs one LM3886. I found a suitable heat sink for under $8 at heatsinksusa.com. Antek offers the AS-0522 toroidal transformer for $17.50, and this is a nearly ideal transformer to power the LM3886. You can fit the whole amplifier on one heatsink serving as a sub-chassis, power supply included.

And if one fails, you've thoughfully made one more power amp module than you really needed, so you swap in the spare while you fix the broken one.

Just sayin' ...
:dbtu:
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R.G.

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Re: Thinking about building a SS amp
« Reply #14 on: June 17, 2018, 06:42:48 PM »

Quote
Paralleling *voltage*  sources is a disaster waiting to happen.
The additional condition that now gain must track each other within 1% or less, any volume, any frequency, plus NEEDING cheesy series resistors to somehow help speaks volumes about what a poor Engineering choice it is.
Well... maybe. Pretty much no really high power amplifiers in existence get by without emitter resistors. The emitter resistors are there to wash out the microscopic differences in Vbe. Engineering has pretty well accepted that sometimes you can't get big enough power devices and you're going to have to parallel them sometimes.

In a big high power amp, you're going to have one 0.1 ohm resistor as an emitter ballast per output device. It's only a trivial amount more waste to use one 0.1 ohm resistor per amplifier to be able to gang up N amplifiers.

The other issue is the 0.1% resistors. That used to be an impossible hurdle, but Mouser Electronics (sorry - I know this won't help you much) lists a few thousand 0.1% resistors in stock in just it's film resistor category. The 1K 0.1% and 22K 0.1% resistors that would be used for a common setup on the LM3886 and most discrete power amps in 1/2W are from US$0.25 to US$0.50 each, although you can buy much more expensive ones.

So ballasting and precision resistors are a pain, but it's a pain you can deal with.

As a side note, I am at least daily amazed at the tech items that I thought impossible in my youth that are available in days for reasonable prices. Sometimes I'm stunned at this topic.

Quote
Not sure I follow you: you use 4 x 3886 in parallel to drive a 2 ohm cabinet through a single cable pair or a special connector with 4 individual speaker cables? Not problem in a combo, not so sure in a head+cabinet situation.
Yeah, that's how it looks at first blush. Combo, no problem. Head and cab, you get into funny speaker cables or replacing speaker cables to run many outputs.

Neither of those is what I really foresaw. I actually wanted to use powered speaker cabs. This is beyond the Thomas amps, but it's a decent solution for a gigging musician. For practice, you need only a few watts, maybe only one. For band practice, you only need maybe 30-60W. For a small bar gig, 60W is plenty and 30 may be enough. For bigger halls, and especially bigger spaces, you might need 100+ watts. If your unit of power was one 12" speaker and a 30W amp driving it, you can lift the speaker enclosure and internal amp with one hand, like a small combo. Need 60W? Pick up one powered speaker in each hand. For 120W, you need to make two trips to the van. If you're playing a soccer stadium, you might need six or eight. The nice thing about this idea is that it scales nicely. It's similar to the way that all PAs are set up today. And you don't need to replace your preamp setup. Just glom on more power amps and speakers.

But before I get off into the ether, yes, you would probably have to do special purpose connectors and speaker cables.
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