Hi All
Iam looking to find out about diy solid state guitar power amplifiers that when driven pass their SOA will clip soft instead of the nasty ss clipping. I know there are some HI-FI units that use limiter-circuits at the input of the power amplifiers , that are preset to reduce the gain, once it pass a set input voltage(anyone have a simple schematic for one?),but iam looking for something like the Randy Slone Optimos . This Optimos power amplifier has a vas stage that is dynamically bias,that is it has drivers for the output mosfets that once they are overloaded will operate in their knee, non-linear area(iam not a amplifier Guru......i just try to play guitar)
and send this output to the output mosfets, henc......soft-clipping. :)
You could try i.e. LDR or FET based limiter (VCA) in the input and sense the output signal. All differential input power amps are essentially just high power opamps so you could try similar tricks than with them. For example, soft clipping in Vox Busker: http://www.schematicheaven.com/voxamps/busker.pdf
Usually it's more effective to deal with signal limiting in the input stage since it operates on lower currents. I never experimented with limiting the voltage amplifier stage.
Hi Teemuk
Is this Vox circuit the whole power amplifier,or just the input stage of thepower amplifier? also if it is just input can it be use with any any amplifier? will this circuit add noise to the amplifier output :)
The power amp is the part in the upper left. The IC2 is type TDA2030 so it is just an opamp that is capable for high output currents = "chip amp". As so, the circuit would work to higher loads as well but why bother? The gain of this circuit is about 100 and the diode resistor network works only by limiting the gain when the output voltage is getting near the rail voltages. It's pretty similar to typical "soft" limiter configuration that use two parallel diodes in the feedback loop - except that in this configuration the input can swing higher than the forward voltage of the diodes.
I have never tried this circuit - except from simulating it with pspice. Surely requires some tweaking to get it running from higher supply rails. Works as a preamp limiter as well but naturally the circuit has to be attenuated to limits of power amp's input sensitivity. If attenuated the SNR will increase - as is the case with every passive circuit.
Another form of limiting using feedback is shown in:
http://www.fender.com/support/amp_schematics/pdfs/Steel_King.pdf
It's a large schematic but see block B-1 on the first page. The circuit is labeled as "limiter".
Personally I try to avoid fiddling with impedances of the feedback loop since this might introduce DC offset. This is perfectly acceptable in the input stages but not in the power amp. Rectifying an attenuated portion of the output signal and using the outcome to drive either a led in LDR circuit or a gate of a FET acting as resistor seems less "risky".
I have designed some conceptual schmatics of LDR applications that detect the power amp output signal but they are quite crude. The problem is that all circuits like this have to be designed for a certain application: A circuit for amp with output swing of +-10V requires different component values than circuit with +-30 output swing. Basically, the detector is the heart of such circuits so better the "detector" is the better the circuit performs. Using a crude circuit is not very "HiFi".
Here is one example of more "HiFi" solution of power amplifier's detector/limiter circuit:
http://www.fender.com/support/amp_schematics/pdfs/KXR_60_Schematic.pdf (bottom right, first page).
I hope I was able to provide some ideas.
Do you happen to have a schematic for that Slone circuit? I'm always interested in solutions I'm unfamiliar with...
Hi
Thanks for the info.....i have Slone's book " The Audiophole's Project Sourcebook"....but i don't think i can post it up here because it is copyrighted.....sorry :)
.....on second thought i think someone has put one on the diy audio for fanatics......just do a search on Optimos........iam just trying not to break any ones rules.
I think the limiter-LDR type gain reducer would be best because it could be set for a gain level at the input of the power amplifier,that won't let overdrive happen.....iam no amplifier GURU.....but do you know where i can find a simple LDR/DIY circuit......THANKS SO MUCH :)
First hit on google "ldr limiter": http://sound.westhost.com/project53.htm
Edit: Found the schem: Pretty nice idea... the VAS stage is not just a typical common emitter but a cascode (Slone calls it a "pseudo cascode") that varies it's transconductance according to difference between output and rails. In other words, the transistor's gain gets lower when the output is swinging close to rails. Works with BJT output devices too.
The only setback is that this requires symmetric VAS stages (thus a symmetric "cascoded" input stage as well) since one VAS can detect difference to only one of the rails. The designs tend to get pretty complex with that stuff included... Anyway, the "soft limit" effect is pretty subtle (did a pspice simulation), it's closer to soft clipping than lowered gain effect of a dedicated opto/FET limiters circuit. Interesting solution anyway.
Ok
This one.....seem that you have to keep making ajustment for different output wattage andspeaker ohm impeadance.......is there one that just reduces the input gain.... without reguard to speaker impedance or output wattage......iam more of a guitar player than amplifier GURU.......THANKS for all the info
ps i have noticed that more Bass (ss amplifier) use limiter..why is that?..seem like guitar amplifier (ss guitar amplifier would need this limiter more?
againTHANKS
Well, you can't get away simple here: You can move the detector circuit to input side but it still has to work according to a specific signal amplitude and the problem is that among different amplifier circuits the amplitudes are never completely identical. As I said before it depends on the power output/signal amplitude. If the detector is on the input side, it has to work according to input sensitivity of the power amp (max input signal before power amp clips). You have to tweak those circuits, that's the point.
I have noticed the same thing as you: Bass amplifiers have more of the stuff that should be in guitar amplifiers as well: Limiters, clipping indicators, graphic EQs, FX loops, line out sends etc. They are generally designed better than guitar amps too. I really don't know why though... maybe an average guitar player settles for a more mediocre amplifier than an average bass player.
One of my guesses for the reason is that many guitar amplifiers are stuffed with FX circuits like distortion, nowadays all that modeling stuff too. Sometimes it's good but I'd rather pay for those limiters and graphic EQs and leave FX stuff outboard. Usually higher priced amps tend to have all those things that bass amps do so it is a price issue as well - and people that buy cheaper amplifiers think they need those "bells and whistles" more than they need an output stage limiter.
I personally don't like amps that have too many effects because this usually means that they lack features that are more important. Anyway, spring reverbs are nice.
Guitarists also tend to be more narrow-minded: Why do you think that so much of the stuff still uses inventions from 50's and 60's. Why so many tonestack circuits are still based on the same passive topology that Fender used in 60's while active topologies utilizing series resonances (i.e. graphic equalizer) perform so much better: You can dial in the same response with the latter and it has better SNR and less interaction between controls. Why no biamping guitar amplifier circuits that would use a digital amplifier for low frequencies? Why using different cabinet structures as a method to shape guitar sound was "invented" around 90's when the theories involved have been known since 1940's and used widely in HiFi/PA/Bass applications? Why so many guitarists are still uneducated about speakers system vs. output power theory? Why not that stuff used in bass amplifiers? Narrow-mindness, that's why.
A simple amplifier might be good when speaking about tubes but that is not a case with SS stuff. One common factor to nearly all famous SS amplifiers is that their design is somewhat "unorthodox" and complex when compared to typical stuff out there. Check Acoustic Control Corps's old amplifiers or Norlin's Lab series for example. Lot's of innovative stuff is used in them.
Yes
I see you havedone your homework on this ....i alway hear about how ss amplifier don't compare with tube amplifier as far as overdrive sound. I am always wondering way not eliminate the bad overdrive in ss units by using limiter and using tube pre-amplifiers to get the so-called tube overdrive...i see Marshall has an the Mode Four this is hyrid amplifier tube/ss power amplifier 350watt i have heard it and too me it sound good imho,but people say it's not power tube overdrive.....beats me? by having a Chip amplifier for an output you save on power tube replacment.
The Acoustic amplifiers you mention i know that the great Albert King used them in the 70's and80's and to he sounds great..a non-amplifier GURU
PS
i Know that you could use a variable pot to set the input level on limiter on the input of the power amplifier....you would not have to worry about different ohm speaker or wattage levels......Thank
I believe that a great part of that "tube power amplifier" tone relies in the interaction of high output impedance tranformer coupled output stages and the speaker. Solid state amplifiers generally have a low output impedance. If you want to play a bit with circuit theory you can think of the output stage as two series resistors fed by a power supply: The bottom one represents the speaker and the top one the output impedance. Speakers do not have a constant resistance throughout the audio band; the resistance rises (sometimes as high as 100 ohms) at their resonant frequency and towards the higher frequencies. Pick any value for output voltage and consider what happens to current, voltage and power over bottom resistor (Rspeaker) in following situations:
Routput = 0,1 and Rspeaker = 4 and 20 (latter in resonance and HF), this is for SS amps. Then Routput = 4 and Rspeaker = 4 and 20, this is for tube amps.
This is rough generalization of SS and tube amps and not all behave like this. Don't believe all tube hype. In most cases a great part of creating a decent overdrive tone is the shaping of frequency response - both post and pre clipping. Keep in mind that the tone is never created by the active device alone - it is always the product of the active device and the surrounding circuitry.
By the way, that ESP circuit is pretty simple: It's just controlling a transistor according to rectified sample of output signal. The transistor uses leds as it's collector load. Here is another example: http://www.picks.force9.co.uk/circuits.htm
and here:
http://www.silonex.com/s_images/audio/limit/fig1.gif
Here's example of a very crude circuit:
http://machines.hyperreal.org/categories/DIY/info/compressor
Here's another compressor idea based on varying FET resistance:
http://freespace.virgin.net/ljmayes.mal/comp/comp.htm
As you see, the detector circuit in this one is very similar to one in LDR circuits.
Well....mmm
I noticed that in the http://www.picks.force9.co.uk that the max input is 3mvrms and max is 2.6rms,now this unit would be placed between the output of the preamplifier and theinput of the power amplifier....ok, with it's input andoutput rms voltage would the limiter componet have to be changed....remember ....iam not an Amplifier Guru
also i have seen adiscreet transistor rig to the output of a ss (guitar)amplifier. It was tired to the input differential somehow to mimic a high impedance output i think i saw a ESP web site on a 100watt (diy)guitar amplifier...don't really know how it worked
as alway THANKS
THANKS
What do you mean by having limiter component changed? The circuit at http://www.picks.force9.co.uk/circuits.htm controls the gain of the opamp circuit by using a light dependent feedback resistor. This reacts to light emitted by led.
The variables you need to focus on these kind of circuits are the signal level required to start illuminating the led and the resistance of a particular LDR in dark and lighted conditions. For example, you can build the circuit but it will not work ideally unless you use LDR with same resistance figures. For different LDR models the gain set of the opamp stage would have to be altered.
You don't have to be an amplifier guru to deal with this stuff - I certainly am not - but it helps A LOT to know basic circuit theory.
Regarding the other topic, the ESP guitar amplifier mkII uses a variable output impedance circuit (known also as mixed-mode feedback) and so do many other guitar amplifiers - it's a quite common trick these days. The concept is also used in driver amplifiers of spring reverb circuits. The site (ESP) contains a very good article about the idea.
......Oh
I just reread your reply on the Slone Optimos amplifier.I don't have any Pspice simulation...i would not know how to even use it......but you say it shows that the Optimos is closer to soft-clipping than the Opto/fet limiter circuit....mmmh maybe i should just get a pc board of the amplifier and make one.....you say it really round thepos and neg output....and you can see this with that simulation you have? I only use tube preamplifier with my power amplifiers,i have a Hafler pro-500watt power amplifier/250per side....i know this is overkill but the Hafler is the same type of circuit symmetrical differential input but not the same VAS cascode that Slone use
Yes it's closer to soft clipping since the circuit clips to rails. The decreased gain just backs off the worst effect of this a little bit causing the clipped signal to have a softer "knee". The LDR limiters usually stay cleaner since they attenuate the input signal and the amplifier never really has to push it up to the clipping point. The only noticeable distortion in them is due to excessive "overdrive", which compresses the wave shape so much that it gets audible. There are also some speed issues with those circuits that may cause a little distortion. If considering "softness" of - can't talk about clipping here - then a properly working LDR limiters are much softer. The difference: Slone's circuit clips but does it fairly softly, Limiter: prevents clipping by attenuating waveform peaks.
I have tried to acquire Slone's book but haven't yet bumped into it anywhere. I don't want to order it via internet - I'm a bit sceptic about that stuff. Anyway, I have D. Self's book about power amplifiers and it's pretty good. Recommended reading for anyone interested about the topic. At least some of the Slone's circuits are based on the topologies presented in that book.
Just curious - since a SS amp is great at just doing what it's told at anywhere less than clipping, why wouldn't you do an entire soft clipping circuit ahead of the power amp and then just feed that to the accurate power amp?
The soft clipper only has to soft clip the input at a voltage less than would drive the power amp to clipping. As long as that happens, the power amp is completely unaware what's happening to it.
As long as a SS amp never clips, it never sounds ugly, just accurate. It's possible to follow a soft clipped signal accurately.
Quote from: R.G. on February 13, 2007, 06:35:39 PM
Just curious - since a SS amp is great at just doing what it's told at anywhere less than clipping, why wouldn't you do an entire soft clipping circuit ahead of the power amp and then just feed that to the accurate power amp?
The soft clipper only has to soft clip the input at a voltage less than would drive the power amp to clipping. As long as that happens, the power amp is completely unaware what's happening to it.
As long as a SS amp never clips, it never sounds ugly, just accurate. It's possible to follow a soft clipped signal accurately.
Thanks for the words of wisdom R.G., it couldn't have been said better.
A good ss power amp circuit will not color the sound, will not alter it in any way other than making it louder. So what you do is make your preamp do the clipping, soft clipping or whatever you want. Or, run a pedal into it. This is part of the reason why tube/ss amp combos are so popular. If you can get a tube preamp to sound really amazing, then hook it to a good ss power amp, it sounds great. Keep in mind thats not the best example however, because a real tube amp has a tube preamp as well as tube power amp and tube power amps DO color the sound. So to get a tube/ss hybrid to sound the same as a tube amp, you need to get the preamp to sound the same as a tube preamp and a tube power amp!
Regardless, listen to what R.G. said and you'll be good to go.
Limiting the preamp signal only is practically the easiest way but it also has some problems:
What will happen if the voltage gain of the power amplifier is not consistent throughout the audio band - this is true with some specific feedback configurations. It is quite hard to build a limiter that has a compensation for the odd frequency response in the detector; it is much easier to build a limiter that has a detector monitoring the difference between output and rail voltage. For example, limiter kicks in when output has 1V difference to Vrail. This way the operation is not tied to constant signal amplitude threshold and can even tolerate supply sag etc.
Most limiters are also quite slow, which means that they cannot react to initial transient fast enough. In such cases it is a merit if the power amp can also soft clip as clipping circuits are not troubled by the delay issue.
LM3886 chips are anything but soft clipping, and I've never had an issue with mine. I drive it hard and loud too.
QuoteWhat will happen if the voltage gain of the power amplifier is not consistent throughout the audio band - this is true with some specific feedback configurations. It is quite hard to build a limiter that has a compensation for the odd frequency response in the detector; it is much easier to build a limiter that has a detector monitoring the difference between output and rail voltage. For example, limiter kicks in when output has 1V difference to Vrail. This way the operation is not tied to constant signal amplitude threshold and can even tolerate supply sag etc.
As I noted, SS power amps are notorious for being accurate. With the LM3886 particularly, the voltage gain of the power amplifier is dead flat across the audio range unless you work at making it not flat. Non-flatness of the power amp voltage gain is simply not an issue with modern power amps. They're easier to make flat than not-flat.
As to limiters with odd frequency responses - well, don't do that. It is far easier to control limiter response at a line-level stage than by tinkering with a power amp. You have the same tools available in terms of circuit responses and components, and the advantage that a mistake will not vaporize your power stage. You have the same problems and solutions available, and lower downside risk of failure.
And for limiters kicking in with 1V to the rail - don't do that. Soft limiting doesn't happen right up at the power rail. That's hard clipping. Soft limiting starts kicking in maybe 20db down from max power and gradually increases as power approaches max. A good soft limiter starts clipping ever-so-gradually way down at lower powers. You may want a last-resort limiter to keep the power amp from ever clipping that hard limits at a fixed voltage down from the rail, but it is a last resort. It's not very musical.
This is not an argue R.G. I stated my opinions accompanied with what I consider as pretty valid arguments. Why persist that there is something wrong with them? Both methods - limiting based on PA output signal and preamp signal - can work fine. Both methods have been used in numerous applications. If you combine these methods you can get even better results.
Guitar power amplifiers do not have to have a dead flat frequency response but can be made to react to speaker impedance similarly as tube amplifiers do. It is very common that an amp like this exhibits higher voltage gain at speaker's resonant frequency and on higher frequencies. You can even make LM3886 exhibit the same character if you want to: It's all about the feedback configuration. Some amplifier models even have active tone control circuits in the feedback loop of the power amplifier. A reason why to make PA circuit's frequency response flat is as valid as the reason why not to. It's all about what the designer wishes to do. If the frequency response of PA is not flat then it's tough luck for limiters that detect only the preamplifier's signal level.
Anyway, a limiter that has a detector hooked to PA output and that attenuates the signal fed to power amp: Is that a preamp-based limiter or power amp-based... Well, we have a semantics issue here and it's quite hard to determine where to draw a line in this. It is easier to make a division between limiters that control the power amplifier's gain or power amplifier's input signal amplitude.
Vaporizing the power amp is a bit far-fetched but I guess it's valid to assume something like that would happen if someone doesn't quite know what he/she is doing. For example, I would not recommend tampering with resistance values in power amp's feedback loop (gain control) as it may severily disturb the DC offset and this something I already mentioned somewhere in this thread. If one does not know how to deal with the PA section safely then one should not tamper with it. Simple.
In my opinion, having the detector at the amplifier output is more accurate way of guessing when PA is approaching clipping levels than determing a certain preamp signal voltage level threshold where the limiter kicks in - at least in cases when power amplifier's gain is dictated by impedance (valid assumption) or when supply sag has to be considered (another valid assumption). It's the very same idea as in clipping detectors: Sampling PA's output gives more accurate indication of what's happening than sampling preamp's output. That's why you put clipping detectors to PA output not to preamp. The 1V-Vrail was just an example and naturally real values would depend on the circuit.
I do know that two limiter types are practically needed: One rectifying the sampled signal and controlling the limiting level gradually according to (usually filtered) DC potential of the rectified signal and one (usually) comparator-based that detects potential differences and acts faster but on a constant limiting level. Both serve their purpose: One cannot trust the first type to operate fast enough to protect the amplifier (or more importantly - the speakers) from the initial clipping transient. It is also quite difficult to make this limiter type to "hard limit" at a certain signal level so if the input signal amplitudes are high overdriving the power amplifier is always a possibilty with them. The latter type has at least a moderate chance for preventing both - although it's "not very musical".
If power amp has dead flat frequency response and doesn't exhibit a notable sag then there is really no need to make things any harder than needed. In practice, many amplifiers work fine even without limiters. ...But they also would work better with them - especially with all kinds of them. Keeping mind open for alternative methods is always beneficial.
QuoteThis is not an argue R.G. I stated my opinions accompanied with what I consider as pretty valid arguments. Why persist that there is something wrong with them?
I agree - this is not an argument. I have different opinions, and I consider the exploration of them to more than one ply of depth to be a discussion. No argument was intended.
Quote
Guitar power amplifiers do not have to have a dead flat frequency response but can be made to react to speaker impedance similarly as tube amplifiers do.
That's true in general; however, this forum was started with high consideration for the LM3886 amps, and that continues to be a major consideration. All of the "chip amp" style amps do have ruler-flat response unless you do something to change that, as they are all high-feedback, low output impedance amps unless you go to some lengths to make that not true.
QuoteYou can even make LM3886 exhibit the same character if you want to: It's all about the feedback configuration.
This being one way to make that not true.
QuoteSome amplifier models even have active tone control circuits in the feedback loop of the power amplifier.
And this being another. Note that this method still leaves a low impedance output with a tight "grip" on the speaker.
QuoteA reason why to make PA circuit's frequency response flat is as valid as the reason why not to. It's all about what the designer wishes to do. If the frequency response of PA is not flat then it's tough luck for limiters that detect only the preamplifier's signal level.
That's another version of what I said. We agree there.
QuoteAnyway, a limiter that has a detector hooked to PA output and that attenuates the signal fed to power amp: Is that a preamp-based limiter or power amp-based... Well, we have a semantics issue here and it's quite hard to determine where to draw a line in this. It is easier to make a division between limiters that control the power amplifier's gain or power amplifier's input signal amplitude.
I may not have been clear.
You can do the limiting entirely feed-forward by limiting at the preamp output or feed-back by watching the output of the power amp. There are multiple methods for each one.
Power amps are inherently high power opamps these days (neglecting tube amps, since this is a solid state guitar amp forum), and since they are, unless you are forcing them into some nonlinearity, the output follows the input as dictated by the feedback network. Whether you use a low power opamp or a high power opamp to do your limiting makes little difference if you make the assumption that you are not going to let the amp doing the limiting go into clipping - which is the goal you're after in the first place.
That being the case, it is a lower-risk technique to limit before you go into a high power stage, and to do the limiting in a way that whatever you feed the power amp will never force the power amp into clipping under any condition. The down side of doing your limiting in a low power stage ahead of the power amp itself is that (1) you have to use an additional US$0.15 opamp to do it instead of the power amp you would otherwise use (2) you have to do your power limiting at lower voltage, which is more of a design problem than it sounds like, and (3) you may give up a few watts of potential power out in the real power amp by designing not to drive it to the very edge of clipping.
It could be a problem to some people, but I don't consider 1 to be a problem, especially since it removes the risk of getting something wrong in the power amp stage and blowing it. Power amp chips are not as stable as general opamps, and high power oscillation will kill them. The LM3886, for instance, is not stable for gains under five. The TL072 is unity gain stable. You can also put in an RF decoupler and keep any mistakes from the limiting stage from getting into the power amp.
2 is an issue when you are depending on using semiconductor junctions as references for where you do your limiting. The smaller voltages in an opamp limiter as opposed to a power amp limiter make this a bit trickier and require more ability on the part of the designer.
3 is something you've already signed up for by doing soft limiting. The logarithmic response of human hearing make this almost inaudible anyway.
QuoteVaporizing the power amp is a bit far-fetched but I guess it's valid to assume something like that would happen if someone doesn't quite know what he/she is doing.
Yep. I can promise you that it can happen. I've done it. I guess I'm one of those people who didn't know what they were doing at one time in the past. Not all of us can always know what they're doing. ;)
QuoteIn my opinion, having the detector at the amplifier output is more accurate way of guessing when PA is approaching clipping levels than determing a certain preamp signal voltage level threshold where the limiter kicks in - at least in cases when power amplifier's gain is dictated by impedance (valid assumption) or when supply sag has to be considered (another valid assumption).
It is in my opinion too. But my opinion also is that there are other design reasons for making not having to make this determination accurately. If you don't need to make a razor-sharp determination of where something clips, so much the better.
QuoteIf power amp has dead flat frequency response and doesn't exhibit a notable sag then there is really no need to make things any harder than needed. In practice, many amplifiers work fine even without limiters. ...But they also would work better with them - especially with all kinds of them.
I believe that. Of course, I'm after soft-clipping for musical sound, not for protection of the power amp or speakers. IMHO, SS amps should never be let clip. Period. It's too nasty sounding. It's the sound I'm after. I'd like to control how the clipping happens so it's musical. I think this is easier to do in a low power stage than a high power stage. It's not the only way, just the one I like.
QuoteKeeping mind open for alternative methods is always beneficial.
I could not agree with you more. I think that keeping an open mind is essential, maybe crucial, to improving one's design abilities. :)