Quote from: Rutger
I tend to follow the same principle as many stompbox-manufactorers (so there is any consistency) and at least have an output cap in my builds. Further more I tend to isolate pots with coupling caps whenever possible. Would that be a good starting point?

Yes, if there is any chance of DC across them during normal operation then it's a good idea.  Generally speaking I include caps at input and output - Just In Case.

Quote from: Rutger
Speaking of this, it reminds me that I've build a marshall lead 12 preamp where there's no couplingcap existing at all, none in the whole design, and it works fine. Probably due to the fet-opamps in it.

Fully DC coupled designs are quite possible; it's just a case of "God help you" if you get some rogue DC on the input.

This seems to be mainly a fetish of the Hyper-Fi crowd who have a fairly irrational objection to capacitors in the signal path (yet are happy to include them in the power supply, forgetting that it is in the signal return path).  Some caps do indeed have some undesirable traits, disk ceramics for example can be microphonic, but generally speaking when it come to audio, concerns about caps are pretty grossly overcooked.

You will note that most solid-state power amps are DC coupled, and while they generally have a few caps for gain setting and bootstrapping there are usually none in the direct signal path except right at the input.  A major exception of course are amp that run on a single (rather than split) supply rail and have a thumping great electrolytic coupling the output to the speaker.

Fully DC coupled makes an amp harder to design (considering the accumulation of voltage offsets and thermal drift) and harder to service.

Quote from: Rutger
I was wondering about how designs with different supplies and reference voltages can match? Like for example a preamp with a dual power supply and a reference of 0V and stompboxes with a single supply and a reference of say 4,5V, or stompboxes with different reference voltages. It would seem to me that they would mess with each others reference voltages and could cause a decrease of headroom and unwanted rail clipping (asymmetrical).

Do couplingcaps also take care of this problem?

Yes and yes.

Your reference point is AC "ground" but if you have a single-ended supply, such as a 9V battery in a stomp, it may not be DC ground; then you have to have DC blocking caps somewhere near the input and output or you won't be able to bias input stages, and the standing DC offset from the output stage may well cause problems downstream.

When you use split rails, such as with op-amps, then the DC conditions can be arranged to center on ground the same as the AC conditions, so you can get away with not having any DC blocking caps (or more correctly, you can design them out).

You normally bias an amplifying stage so that its output is half way across the available supply, so as to maximise headroom.  If one side of the supply is ground than this "Q point" must be some non-zero voltage.  But when you have both +ve and -ve supplies the idle Q-point can be at ground or zero volts and still be half the available voltage swing.

{JM; You will notice how often I use the words "generally" and "typically"  }


CAUTION: @sewage666 - I want to hear specifically that you now have a limiting lamp in series with the mains feed to the amp.  With an amp this size it is not optional.

http://www.ssguitar.com/index.php?topic=2093.0

This is SOP for bringing up a faulty or (we hope) repaired solid-state amp.

The reason is that the output stage is connected directly across the power supply.  If, for any reason, both the top and bottom rank of output transistors should turn on together they would short the supply.  This normally results in the rapid demise of the output transistors, sometime the drivers and pre-drivers, followed a short time later by the power supply rectifier, and the power transformer will burn up next.

So it's really important to avoid this happening.  If there is such a problem then the input power will appear in the lamp, lighting it brightly, not the output stage (and lighting it brightly  ).

In your case it is almost certain that there are dead transistors (open circuit base-emitter) somewhere following the diodes you are about to replace; most likely the pre-drivers Q9 and Q10, or the drivers Q11 and Q12.

At this point I would remove the drivers Q11 and Q12, test them, and if okay put them to one side for the moment.  Note carefully that these are a NPN/PNP pair and that they must go back in the right places (all the MJ15022's are along the top, all the MJ15023's are along the bottom).  With these transistors removed there is no way that the output set can be driven - a safety factor while we get the voltages in the pre-driver right.

Once the voltage between TP6 and TP7 is something more reasonable (like about 3 volts), AND we have located and replaced the dead transistors that are lurking downstream somewhere at the moment, then we can try a power up with limiting lamp and no speaker, checking for reasonable voltage (and currents) before we let it fully loose.

I can't stress enough how important it is to have located and replaced ALL the dead silicon (and any other dead components) in a solid-state output stage before you let it rip on unrestrained power.

Even then you need to be certain that it is balancing the output to zero volts before you connect your speakers (this one does have a speaker protector, but most don't, and we can't be certain that even this one is working properly yet).

HTH

Why do F1 racing car drivers start out racing GoKarts?

If you want to build a nitro-fueled dragster it's a good idea to start out on something more modest like a lawnmower and work your way up.

There is no reason why you can't get to a 100 watt amp by, say, your forth build, but if you are going to learn by blowing stuff up (and in truth we all did a bit of that in our early efforts) then it's better to blow stuff up that isn't too expensive and won't go "bang" too violently.

Amps can be split into two parts, the preamp before the master volume control, and the main or power amp that follows.  As it happens it is around the master volume control that you can mix and match these two sections.

There is also something to be said for getting a large case and heatsink and starting with a limited build first, then enhancing and rebuilding by stages until you arrive at a very full-blooded guitar amp.  As you upgrade your preamp and power amp your ideas will change about what you want, and gradualism allows room to change your mind in the next stage.

The water analogy certainly isn't going to help much with something like conjugate impedance, but there seems to be a general need here to get the basic concept of Ohms Law across, the very basic relationship of a voltage driving a current through a resistance.

While I don't seem to encounter the sort of thing JM mentions, it comes up in a different way with things like the current capacity of plug packs, and the fear that a 12 volt 600mA pack will somehow force more current through a circuit (a device like a stomp) than one rated at 12 volts at 300mA. It's a bit of a worry when you find that this is the sort of wrong advice people have got from sales staff, who should know better, in electronics retailers.

I got razzed on another forum for posting a picto-graphic "circuit", and while I'm not keen on them it's sometimes hard to remember the initial struggle you had relating a circuit to the reality when you've been doing it for a long time.  It's a bit like reading music vs using guitar tabs.

No problems - I've been doing this stuff for the best part of fifty years now and had trainees most of that time, and I consider it giving back to my late mentor Theo van Bemmel who saw it as his duty to pass on what he had learned.  In retirement I now spend about half of every day on forums and replying to e-mails explaining stuff, and I find it stops my brain turning to mush.

These questions are a bit sticker.  I often look at some circuit arrangement and wonder what the designer had in mind (and sometimes what they had been sticking up their noses).

Q1
If there were some consistent law that said "Thou Shalt Always Put a Blocking Cap on the Output", and if designers and manufacturers actually followed it, then I guess you would never see a cap on an input.

But it is, as they say, a Free Country, (or more accurately "there are no rules") so manufacturers do exactly what suits them.

When you decide to include an input blocking cap, for example, this also means you need to include a resistor to ground ahead of it to make sure there is no DC offset when you plug something in, and cause a loud "splat" (if not actual damage).  Since this is in parallel with whatever resistance is tying the actual electronics input (say an op-amp input) to ground it appears in parallel with it and therefore lowers your input impedance - but you typically want to keep your input impedance at one meg or greater, so the struggle with trade-offs begins.

There is not a lot in electronics that is a one-way deal, more of X is good and there is no downside - there is nearly always a downside.  As you bump up your input resistance so you start to get problems with voltage offsets due to tiny leakage out of the op-amp input; you start to get noise due to very high value resistors, so you have to make compromises.

Q2
Good practice is to keep DC off pots, but not everyone follows good practice.  It may be because they have an amp design that passes hardly any current through the pot, such as a FET or FET input op-amp, or perhaps it won't be a problem until the amp is out of warranty or they don't care because at its "price point" they don't mind if it's a bit crappy compared to their better offerings (the cynical approach).  Maybe it's purely cost, or maybe their "designer" simply doesn't know any better.

On the service bench you frequently encounter situations where somebody has said "That'll work/be enough" and it hasn't and isn't.

In no particular order examples that come to mind are - the power rating for resistors feeding zener droppers for preamp supplies; heatsinks - often; transistor voltage and current ratings, electrolytics near hot things, just about every PCB mounting external connector, etc and so on.

My personal credo is "If you can't make it indestructible, make it easy to repair".  Where possible try to allow for failure modes, for where things go wrong (such as accidentally plugging a guitar amp output into a mike channel input), and as far as possible prevent or limit damage.  Not everybody thinks like that.

So why do many stomps have no DC blocking on their inputs?  Perhaps the assumption that they will be alone, or at least first in the chain, or that any up-stream box will have an output blocking cap.  And mostly they are right.

You actually remind me of an old Marshall valve/tube PA that I used to look after.  The grids of the input triodes went directly to shorting microphone jacks.  That's it; no DC blocking, and the assumption that whatever was plugged in would provide the required DC path to earth for the valve.  I consider that a very brave assumption by Jim.

And this is part of the reason you normally see a resistor as the very last thing across the output after the DC blocking cap; mainly to charge the cap to de-thump a plug in, but to also DC tie the output line back to ground JIC - Just In Case.

I know these answers aren't very definite, but as I said, I sometimes look at circuits and equipment and wonder what on earth they were thinking.  :duh

HTH

Yeah, what happened to the good old hydraulic analogy; the hose on the garden tap?

Voltage is pressure.

Current is flow.

Resistance is friction.

Well the first has to be Duncan's Tone Stack Calculator.  This is what it does, this is all it does, but it does it brilliantly.

In general circuit modeling Spice has been around for a long time but you either had to pay through the nose or put up with student cripple-ware.  Well now there is LTSpice which is full-featured and FREE.  Its only drawback is that as it comes it has component libraries limited to Linear Technology devices, however adding to those libraries is easy, and there is a very active Yahoo users group where some seriously beefed up libraries and models can be downloaded.

And as Phil says, all this, and you don't need any maths - but when you do need to do a bit of math another tool I make quite a bit of use of is SpeQ Math which is like very full-blooded "calculating paper", and does a lot of graphing functions too.

PS: I've yet to encounter a soundcard I would rate above "just average" and while I'm sure somebody somewhere makes a truly high quality sound card, when it comes to using computer-based instrumentation such as signal generators or CRO's the sound card is  the major weak point.  Mind you, if you write a high definition sound file you can always port it out to something that will do a better job.

Quote from: sewage666
There is no voltage across them either direction, so they're passing no current. They're shorted.

That's good because that makes sense with the voltage readings.  :tu:

Now you may think I'm being picky here, but unless you think about the properties of these components the right way in terms of resistance, voltage and current, you are never going to make sense of what is going on.

In this case they are shorted, like a length of wire, they have no voltage across them - but does a length of wire, a short, pass current?


D10 and D11 aren't in this particular part of the circuit to prevent current from flowing backwards, but rather because they have a fairly constant voltage drop independent of current; in other words they are acting here like very low voltage zeners to provide a fixed drop of 2x 0.6V (roughly).

Because one on each side is shorted the effect is to turn Q7 and Q8 on too much, giving too much current though Q6 and therefore excessive voltage between TP6 and TP7.

Hopefully when you get good diodes in those positions TP6 and TP7 will only be about 3 volts apart.

Yes, you do get +1 for methodically testing them all in order.  :dbtu:

This is one hell of an amplifier you are servicing, a bit like learning to drive in a tank, and you are gaining ground, but with an amp like this I would "hasten slowly" myself, so you are doing fine. 

In the first example just assume for a moment that the output wiper of the 100k volume control isn't connected to anything.  The CR circuit is 68nF and 100k no matter where the wiper is.

Now assume that the output goes to another stage which has an input resistance of 1 megohm.  With the wiper at minimum the CR is still 68nF and 100k, and with the wiper at maximum it's 68nF and 100k in parallel with 1 Meg, which is still pretty close to 100k.

Now if the load is only 100k then at maximum the CR would be 68nF and 100k||100k or 50k which would be double the cutoff frequency, but still quite low.

In either case with the volume pot set at 10% the R is still going to be very close to 100k.


The second example is rather more complex, but the answer is basically "yes" because the cutoff is still CR, but in this case it is the effective R of the following network (in this case the low pass formed by the two 10k's and two 2n2's, and the 100k volume pot).

Just by eyeballing this we can see the two series R's are each one-tenth the 100k volume pot, and similarly the two 2n2 in shunt are only one-tenth of the 22nF coupling cap, so the effect of the 10k's and 2n2's can be assumed to be fairly well removed from the CR formed by the 22nF coupling cap and 100k volume pot.

In the two LTSpice plots attached you can see that the low frequency rolloff in the second example is a bit higher than the first because the coupling cap is only one-third, but you can also see that the effect of the two 10k's and 2n2's is to provide some high frequency rolloff and doesn't really have any effect on the low frequency rolloff. {I've ignored the 1uF path in the second circuit for simplicity}

I suggest that you download Duncan's Tone Stack Calculator and fiddle about with the values in one of the simpler networks such as the Big Muff to get a feel for how changes in capacitance and resistance effect the frequency response.

HTH

The Class of an amplifier tells us over what portion of the signal wave the active device is working.

Class-A - conducting over the entire cycle.  Almost all preamps.

Class-AB - conducting less than the entire cycle but more than half a cycle.  Generally in guitar and similar amps it is not much more than half, just enough to overlap.  Implies two devices sharing the work in push-pull operation.

Class-B conducting for exactly half a cycle.  More efficient than Class-AB but prone to crossover distortion.  Implies two devices sharing the work in push-pull operation.

Class-C conduction for less than a half cycle.  Not used in audio but common in radio frequency applications such as transmitters.  Very efficient but very distorting.

Class-D switch mode between fully off and fully on at very high speed and where the time proportion of off-to-on is proportional to the signal voltage.  Very high efficiency but requires complex circuits and post filters to prevent the high switching frequency getting to speakers, tweeters, or following equipment.

More detail;

http://www.duncanamps.com/technical/ampclasses.html

http://en.wikipedia.org/wiki/Electronic_amplifier#Power_amplifier_classes

As you will see, a great deal of utter balderdash is talked about amp classes, particularly in Hi-Fi circles, but even in guitar circles for example the myth persists that the Vox AC30 is a Class-A amplifier.

As Phil and Enzo imply, class of operation isn't really all that significant against many other factors.

HTH

Quote from: QReuCk
all this process of trying things with an open mind led me to a wonderfull tone with the Peavey alone

:cheesy:  That's good.

I've had a look at the circuit of the Ibanez LF7 and I'm inclined to say that you may be better off with a graphic or parametric EQ.

Quote from: QReuCk
accoustic with a non-preamped piezzo

This is a whole other issue.  Piezo's must work into a high impedance or they sound thin and scratchy, and by "high" I mean around 2-3 megohms or more, which is a lot higher than most guitar amps.  To get anything like a full range from a piezo you really must have a Hi-Z FET buffer or something similar.  There are a number of designs of simple DIY FET buffers for piezo's on the web;

http://scotthelmke.com/Mint-box-buffer.html

http://www.diyguitarist.com/Guitars/PiezoBuffer.htm

http://www.diystompboxes.com/smfforum/index.php?topic=92274.0

www.youtube.com/watch?v=O53h5mW8mBk

&c&c&c.

In all cases these buffers must go as near to the piezo pickup as possible, ideally inside the guitar, at worst in a belt pack - on the floor with a long leading cable won't help nearly as much and pick up a lot of electrical noise.

HTH

I'm confused by your "D10 and D13 were passing no voltage".  Components pass current and have voltage across them, or between points.

Specifically did you find D10 and D13 open or shorted?

Shorted I hope because open wouldn't explain the symptoms.  The diode test range on a DMM displays millivolts, thus a good diode will look like an open circuit one way, over-range in reverse; and somewhere between 500 and 700mV the other, forward, way.  A shorted diode will generally read something low or quite low, less than 100mV, both ways, while an open diode will read over-range both ways.

Patience is something you must have if you are going to solve these sorts of problems - they simply don't yield to macho gust and bust but to thorough and methodical.

Okay if you still have cannabis-fiber money    but here in Oz-tralia we changed over to plastic a while ago and I think it's superglue-proof.  While PVA+paper is fine on the actual cone, when it comes to splits on the surround I use rubber-based glue to try and retain a bit of flexibility.

Well okay, but that doesn't really change anything much; you still have +/-5 -something volts where you should only have +/-3V (2x 1.45 = 2.90), and it just means that another couple of transistors are suspect.

You still need to establish if altering the bias control will get these TP's to a difference of 3 volts or a bit less, then you have to move down each line of B-E junctions getting lower voltages as you go.  The emitter of Q9 (base of Q11) should be about half a volt lower than TP6; the emitter of Q11 (bases of Q13/14/15) should be about half a volt lower again.

Similarly working along from TP7 on the negative side through Q10, Q12 and Q16/17/18.

At one of these transistors (on both the positive and negative sides) you will find a base-to-emitter voltage much larger than half a volt, and those devices are dead.

But as I said before, there is no point in just replacing them until you find out why TP6 and TP7 are so far apart, and getting the voltage between them back in spec - +/-1.45V.

The fuse is there to stop the amp catching fire if something goes wrong.  Blowing a replacement fuse clearly shows that something has gone wrong.  Simply bypassing the fuse, as you are doing, is not only personally dangerous to yourself, it is highly likely to do even more damage to the amplifier.

I have to say I agree with JM; you don't seem to realise you are dicing with death and you really should put it in the hands of somebody with a bit more experience.