There is nothing that substitutes for allowing lead time.  You have known that your Line 6 isn't working for how long?  I have a sign up in my workshop that says;

"Lack of foresight on your part
does not constitute an emergency on my part"

...and as someone who has been fixing musician's, theater, and show gear, sound and light, for many years, there is a good reason for that sign.  I will give you all the help I can, but no sympathy at all for putting yourself in this jam.


Find and post the Line 6 circuit, then we can get down to details.  I'll check back in a few hours. 

Until then, very generally - you should be able to pick up the effected signal in the Line 6 from the wiper of the master volume control.  This can then be taken to the least sensitive input on your Vox.

You will need a length of screened cable long enough to get from inside the Line 6 to the input socket on your Vox, and a plug that matches your Vox input.  If the Vox has an Fx Loop then Fx Return is another possible place to insert the Line 6 signal.  You need to start gathering these bits now.

Quote from: leadplayer on July 10, 2012, 10:30:49 PM
if the amp is stereo you can blow the side that you disconnect the speaker on

OP specifically said it was a solid state amp, and operating a solid state amp without a load will not harm it.  A valve/tube amp will be damaged by being driven without a load.

{this drawing seems to double in size every time you upload it. ()}

You need to be specific about what voltages you are seeing where - you are our eyes here.

You appear to have added two voltages at the ends of R30 which are most interesting; +5.2V and +5.4V.

Now if we start at the half-rail (point (E)), and we apparently have no current flowing in any of the output emitter resistors R33-R38, then we have no voltage drop across them.

The typical voltage drop across the base-emitter junction of a transistor is around 600mV or 0.6V, so this is what the bases of Q13-Q15 should be (similarly for the negative side).

Q11 adds another 0.6V so the voltage at the base of Q11 should be;

2*0.6=1.2V.
3*0.6=1.8V

TP6 and TP7 are given as 1.45V+/-20mV

1.45/3=0.48333333

So let's compromise and estimate around 0.5V across each of the three EB junctions in series.

If your voltages are right then there has to be an open Base-Emitter junction in both the upper and lower path, e.g. Q11 and Q12.

We have two choices; mistaken measurements, or some dead silicon between Q11 and Q18, and I'll bet on Q11 and Q12.

What is a serious concern here is that the TP table says you should have only +/-1.45V at TP6 and TP7 (i.e. across Q6), and until you determine why there is so much voltage across Q6, almost 12 volts when it should be closer to 3V, simply replacing dead drivers Q11 and Q12 will only fry the new ones.

Also, note that the voltage at TP3 cannot be corrected if the loop is open, and if there are transistors with open BE junctions in the loop then TP3 could be just about anything, so don't worry about that for the moment.

Important point; can the voltage between TP6 and TP7 be brought down to +/-1.45V by adjusting AR1, and if not we need to find out why and fix that before anything else.


BTW, you said "I also tried to short the speaker jack" - I hope I misunderstand because that is something absolutely NOT to do at any time.

So,
* post the table of DC voltages for all your Test Points.
* Does adjusting AV1 bring TP6 and TP7 to about 3V apart, or a bit less?

Impedance matching is a rather vexed question - it depends a great deal on the situation.

Strictly speaking we match the impedance of the load to the source to achieve maximum power transfer.

In the case of valve (tube) amps the high output impedance of the power amplifier valves is matched to the much lower speaker impedance using a transformer, and in that case something close to an impedance match is sought.

With the line voltage level connection between preamp and main amp it is voltage not power that we are trying to transfer and therefore no attempt is made to match impedances.  The output impedance of a preamp will be low, hundreds of ohms to ohms, and the input impedance of the main or power amp will be very much higher, 10's of k ohms up.

These two impedances are effectively in series and divide the actual output voltage source, so making the ratio between them very high drops the least voltage.

Similarly where the output of a solid state amp drives a speaker the output impedance of the power amplifier will be a small fraction of an ohm while the impedance of the speaker(s) will be typically 8 ohms (but maybe 16 down to 4).

If we tried to actually match the impedance of the solid-state output stage for maximum power transfer, half the power would be dissipated (uselessly) in the output stage electronics as heat, and the other half in the speaker(s).  Typically this would also grossly exceed the current capacity of the output stage and destroy it.

Impedance mismatch is actually used all the time to limit the available power, and no attempt is make to match the impedance of your toaster or electric jug to the power station because, if it were, you would get half the power station capacity appearing in your kitchen - both spectacular and unhealthy.

So these appliances are constructed with a deliberate and gross impedance mismatch to the power station so that they only dissipate a few hundred watts of the megawatts the power station is churning out.

In radio systems, such as CB antennas, the object is maximum power transfer, voltage and current, so the antenna is matched to the impedance of the transmitter output stage.

@Onewatts - this is another case again, where the multi-effect is used mainly for signal conditioning, and PA system inputs actually have much greater voltage sensitivity than your typical guitar amp input.

You would consider this a preamp in the guitar amp sense of the term of lifting signal voltage level if it only fed a power output stage, say via Fx Return, but very generally most Fx units won't produce the sort of voltages required by most main amps for full output (e.g. 100mV v 1 volt).

Quote from: Rutger"
example #1: when you want to know the input impedance of this circuit, for example to match it with the output impedance of a preamp,  is the input impedance 1M (R1) or 22k and 1M in parallel (R1llR2)?

The first thing to note is that the reactance "Xc", or effective resistance, of a capacitor depends on its value and the applied frequency;

Xc = 1 / (2 {pi} f C)

where;
Xc is the reactance in Ohms
f is the frequency in Hz
C is the capacity in Farads

At the frequencies of interest the reactance of the input coupling capacitor is low compared to the 1Meg and the 22k, so the effective input impedance is 1M||22k, which is roughly 22k for all frequencies of interest.

When we couple the output of a preamp to the input of a power amp our concern is voltage transfer, not power transfer, therefore an impedance match is not required; rather the load impedance, the power amp input, must be much greater than the source impedance, the output of the preamp.

In this case the input impedance of the main amp is about 22k and the source impedance of most preamps will be low to very low, from hundreds of ohms down to perhaps fractions of an ohm.

We can see that when the reactance of C1 is equal to the resistance R2 a 2:1 divider is formed giving half voltage or a 6dB drop.  By using the relationship above we can find the frequency that this 6dB point will occur at;

Xc = 1 / (2 {pi} f C)

make f the subject; multiply both sides by f, divide BS by Xc;

f = 1 / (2 {pi} Xc C)

f = 1 / (2 * pi * 22*10^3 * 2.2*10^-6) = 3.28832527Hz or 3.3Hz

We should not forget that the input impedance of the chip amp is also in parallel with the 22k resistor so the actual input impedance at this point may be lower again.

Quote from: Rutger
example #2: when you want to know the behavior of the 22k pot, do you need to take the 1k resistor into account (1k+22k)?

The object of the 1k appears to be to limit the available output drive should the Send Level be set at maximum.

The signal level available at the Send output will be 22/(22+1) of the signal level at the cathode.  22/23=0.95652174 or about 5% reduction which is quite a small loss for the benefit of providing some protection for external equipment connected to Send.  This stage may be capable of delivering signal levels in the tens of volts, but may be connected to solid-state devices that would be damaged by those voltages if unlimited current were allowed to flow.

Again we can find the low cutoff frequency, as above, from the 470nF and 22k.

f = 1 / (2 * pi * 22*10^3 * 470*10^-9) = 15.39216084Hz or 15.4Hz

HTH

I'd be checking that you have good +/-16V rails before you plug the op-amps in.

{I think we've lost @abe}

"shattering it against a wall" brought back some memories.   

Some time ago I was in a forum trying to explain why kits cost as much as the built item, that the designer has no control over the construction and testing, and making that bit idiot-proof can take a great deal of work.  Even done all in-house it is amazing what mistakes can sometimes slip all the way through production and testing and out the door to the unsuspecting client.

And what is one to make of a specification that says a conveyor brake energiser must have a "linear logarithmic ramp" output?  Of course it turned out that the young engineer who wrote the specification didn't have a clue  what sort of output was required.   

Sorry, I mis-read the TP table; the DC voltage at TP6 and TP7 should be +1.45V and -1.45V, not +/-40 as I previously said (that's the fully driven AC voltage at these points)

Quote from:
Ha! I SWEAR I checked the continuity of that switch jack... in fact, checking it again out of circuit, the old jacks test fine. They just didn't work... can the metal corrode or something where it starts to act like a resistor? All I know is this is the second time I had this same mysterious problem with old Cliff jacks... test fine, but don't work in circuit.

Yes, all jack contacts seem to be prone to this.  One trick is to give them a good scrub with the edge of a bit of newspaper to clean any crud off, then they work fine again, for a while...

+16V rail needs to be 16 volts or very close to it.  6V implies it is being heavily loaded by something. The output stage depends on the +/-16V rails being about right, so this won't help at all and you need to find out why it's so low.

D20:  presuming you didn't get your fingers into the act, and you measured it with one end disconnected, this is very wrong - should measure about 600mV one way and open circuit the other.  You can't get a reliable reading in circuit (because of C10).  But these measurements won't give you 6 volts on the supply either.

I'm inclined to think there is actually nothing wrong with D20 or R24.

Now previously you had about the correct voltages on TP4 and TP5, and more importantly TP10 and TP11.  If the +16V is now only +6V then something on the +16V rail is drawing too much current, and the most likely suspect is one of the op-amps powered off that rail.

Pull the one you have socketed (IC1?) and see if that restores the 16V supplies.  IC1 is not needed in place to measure the DC conditions of the output stage.

The only difference between the 2N3402 and 2N3403 is the gain range, 75 to 225 against 180 to 540, so they overlap.

I think we need a fresh set of DC voltage readings for all the Test Points.

Yeah, you are right, and the comment you reported is wrong.

The signal will start to be clipped when it gets to the turn-on voltage of the LED's, say about a couple of volts peak at that point.  The op-amp however won't start clipping until its output gets to the supply rails, more like +/-15 volts peak.

Logically there wouldn't be much point in putting the LED's in if the op-amp clipped first.

If there is one thing that you really have to watch out for these days it's the well-meaning but uninformed comments such as the one you saw.  Anyway, you did a bit of study, applied a bit of logic, and worked out that it wasn't right - good on ya.

As a general rule of thumb we expect a signal level of around a volt at the main volume control.

The point on the circuit marked "Pre In" is a bit confusing because it is actually Main In and would be driven by "Pre Out", above.

If you want to work out signal levels along the chain you can start with the maximum output voltage to the speaker, which is one supply rail (I can't read any voltages on my fuzzy copy) which is the clipping peak, divided by the power amp gain which is R? 33k / R30 1k or x33.  This gives the peak voltage at "Pre In" for maximum output, and you can continue working backwards in this manner to discover what peak voltages at each point will give full output (with the "Volume" right up).

You can also model this tonestack in Duncan's Tone Stack Calculator, which incidentally happens to be quite a bit of fun too.

HTH

Quote from: sewage666
I'm beginning to understand why, years ago, one tech I asked about working on this amp said it was on a short list of amps he refuses. Oi vey.

If they can build it and get it going, you can repair it.  The tech's attitude may have had more to do with the commercial aspect, being time consuming and unprofitable; but as it's your gear, and your time is your own, the economics are quite different, so don't get discouraged - okay?  You have already made very serious progress.

Quote from: sewage666
I accidentally made contact on the tip end of the line-in jack/switch. Noise. Big noise.

(AAARRRGGHH!  :grr )  YAY!  :dbtu: I'm sure JM and I would both have mentioned the jack switch contact as a very common cause of this sort of problem, were it not for the fact that it was marked on the circuit "ckd continuity"!

Quote from: sewage666
So, I swapped the jacks for new ones, and behold! My problem was mechanical all along (and I feel really dumb)!

Most of the time it is, and some of the time you do, but with time you learn to double check the stupid stuff first.

Now;

Quote from: sewage666
Now that the amp is amplifying, it's buzzing and distorting REALLY bad, and the signal cuts abruptly while the instrument's strings are still decaying.

Okay, now you are WAY ahead, but earlier you said...

Quote from: sewage666
I've measured 0V passing through R31 or R32.

...and what you are now observing is perfectly consistent with what you previously observed, the output stage is operating without any bias; there is no voltage across them because there is no current flowing through them (and stony cold) - and this most likely goes back to the burnt resistors R29 and R29.  We are very hot on the trail here, but now we (you) must be very careful because one wrong move around the output stage could produce quite a lot of damage.

I think that this is almost certainly a pre-existing problem, not something you have just caused.

The table says you should have +40 volts on TP6 and -40V on TP7.  My guess is that you won't have, that these voltages will be quite a bit lower.

At this point I normally revert to cold checking, and in this case I'd be pulling first Q6, the bias transistor, and Q7 and Q8 the pre-drivers to make sure they are good.

It could well be that somebody has simply fiddled with the bias setting pot (looks like "AR1" on my cct, in series with R23), but I'd want to be sure before I went changing it.  I'd also want to know the two protection transistors Q2 and Q3 are healthy.

In all these cases you must be extra careful to restore the right transistor to the right position, the right way around.

I doubt the op-amp has anything to do with this problem, but by all means restore IC1 to a 5532 if you feel happier about it.

While testing around the output stage it is best to disconnect your loudspeakers and check for near-zero volts between (K) and ground before you reconnect them each time. (Yeah, I know it has a speaker protector, but the amp also has faults and an unknown tech work history; better to be cautious...).  I would also be using a limiting lamp in the mains feed while I was working on the output stage.

You have made considerable progress and I think you are not far off finding and clearing the remaining fault(s)

Quote from: sewage666
I'm not getting any voltage at E... and I'm wondering about R39. I checked the Ohms in circuit, and it reads 0... I know that's not the proper way to check, but R40 is the same type and it reads 10 Ohm. I'll unsolder R39 tomorrow and double check. I'm not sure what you mean by read across the emitter resistors though... read each side of each resistor? Or from emitter to emitter?

Zero volts to ground on point (E) is good, it tells us that the whole DC-coupled output stage seems to be balancing itself correctly, which in turn means it is very unlikely to have any dead or dodgy devices.

I mean measure the voltage across each of R31 to R38, probe at each end of each resistor in turn to measure the voltage between the ends of each resistor.  R31 and 32 should have about a volt across each, while the rest should have about 10 to 20mV across each.  Basically this is to check that the two drivers, Q11 and 12 are passing about the right current through R31 and 32, and that each of the output transistors Q13 to Q18 are all passing about the expected current, and therefore can be presumed healthy.

The reason this is important is that the toasty resistor R28 and R29, and the relay contacts on T90 all suggest excessive current in the output stage at some time.  This may have been due to a fault since repaired, or simply that the resistors and relay contacts were a bit under-specified and are showing some age.  I strongly suspect the latter, that there is no serious problem around the output stage (which is a damn good thing 'coz these stages can often develop terrible cases of galloping silicon cancer).

R39 is generally only used as a former for the coil L1 but may also be a damper; shorted by the coil it will only read zero ohms.

Until you can see about a volts AC of signal at TP2 there isn't much else you can do, and I suspect that when IC1 is replaced you will restore the signal level and overall operation.

Just be certain you get the new op-amp in the right way around.

Well abe, look what has turned up!  Circuit, FET's, AND a PCB layout.  :tu:

http://www.albertkreuzer.com/preamp.htm

HTH

First up, thanks for being your own best friend and posting the circuit/schem complete with your measurements - that's a huge flying start.

I'm curious about how the club power situation could cause an amp to fry, but we'll leave that for the moment.

IC1 - you won't see any signal on pin 6 because pin 5 is connected to ground and that makes pin 6 a "virtual earth" point, but you should see signal on the output pin 7.

The readings at TP4 and TP5 say the +/-16V supplies are okay, despite the apparent reading at TP10, and I wouldn't expect any AC at TP4 or 5 (see table).

Like JM, if I had to guess between the opto and the op-amp I'd go for the op-amp every time.  These LDR optos hardly ever fail, and it would be very unlikely for it to fail shorted across the LDR without some external help in the form of a loose solder dag or such.  Op-amps however are a bit more fragile and not unknown to simply give up.

I feel quite strongly that you have two different, and quite possibly unrelated, situations here; the left hand side around the low level stages and op-amps, and the right hand side around the main power output stages, and I don't think they are actually related.

I think the reason the amp has stopped is around IC1, and that the damage around the output stage is an accumulation over time that had not yet got so bad as to do any serious damage - that TP13 in the speaker protector goes low and pulls in the relay T90 strongly suggests that there is nothing wrong with the power amp portion.

Again the voltages at TP8 and TP9 tell us that the main supplies are okay.

I find R28, R29 and the relay T90 being toasted particularly interesting.

TP13 should only be low and relay T90 pulled in if the output of the main amp ("half rail") is very close to ground, which it should be (the relay etc being there to protect the speakers from being burned up by DC).  So it's possible that R28 & 29 had only got very hot, and not actually failed, otherwise I would have expected to see some other damage (dead transistors?) or off voltages around the output stage.

That is, the primary problem in the output stage may have been the protection relay itself, or at least its contacts.

If you could provide a voltage reading between point (E) - the main output - and ground, and across each of R31 through R38, the 3.3 and 0.33 ohm emitter resistors (~1V, and ~10-20mV), that would help a lot in getting a picture of the condition of the output stage.

I would nominate IC1 as your most likely suspect simply because it is fairly exposed to the outside world via the 'Line In' connector, and dodgy club power could potentially have put a high voltage in via this connector if it happened to be being used at the time.

HTH

If the Peavey is now being over-driven even on its deaf input you are now well ahead and have a couple of options.  If it has an Fx loop you can try injecting your signal at Fx Return, but his means that all the preamp controls will be inactive.

Another option is to knock up an attenuator box to go in the lead that interconnects the amps.  You could make this a fixed attenuator with a couple of resistors in about a 10:1 to 100:1 ratio, or you could make it a bit more flexible by simply mounting a pot in any small container.

The input side would have a stereo plug and most of the lead, wired across the outside pot contacts, and given that this is at headphone impedance (very low) the cable can be just about anything to hand such as fine twinflex - it doesn't need to be screened or shielded.

The pot can also be just about anything to hand that is more than about 1k and less than about 100k, log/audio is ideal but linear would work too.

The output lead should be shorter, say just enough to allow the box to to sit on top of the Peavey and be caught under the amp handle, and ideally should be screened lead, 'tho if the pot is of a lowish value you might get away with more twin; the screen going to the end of the pot connected to the source amp ground (stereo plug body) and carried through to the mono plug body output going to the Peavey, and the screened inner going from the pot wiper to the mono plug tip.

The container can be just about anything, such as a plastic soap container from the $2 shop or whatever you can scrounge, but a small tin such as are used for mints is near ideal since if you ground the tin to the common it will provide shielding. In this case I'd consider finding some screened cable for the input side which will make the gizmo useful for other applications were the source impedance isn't as low as a headphone output, for example the Fx Send of another amp.

I have a bag full of gizmos like this to adapt things together when required on-stage or as part of the PA/sound system, and I use plastic keyrings with paper labels on the cables to record how each is wired.

HTH

abe,

[response to PM]

I'm sorry but I don't have a PCB layout for these circuits, and I don't intend to do one because It's a lot of work and I don't have a personal need; there are two more stages of circuit development before you get to anything like a final PCB layout; and you obviously intend to make commercial use of this design.

I don't have a problem with giving you a bit of help with developing something without reward that you are going to sell, but there must be a serious limit, and if I do it for you then you learn nothing.

So professional advice for free;

* you need to obtain some Veroboard, stripboard, or dabboard, some components, and build a first prototype stage by stage to prove the circuit works and to change any component values as required to get it to work as you wish, particularly the drive channel clipper.

This means drawing a physical layout of the components that takes into account their size and which lead is which on the transistors. 

This also means you will also have to build the prototype power output stage and cabinet/speaker box so you can hear what it does, and prove that circuit as well.

* the next stage is transferring the proven circuits and layouts to a PCB design (on paper) and then etching a second prototype onto actual PCB laminate, say using a waterproof felt tip pen for resist.

PCB Tutorial
http://www.guitargear.net.au/discussion/index.php?topic=32366.0

Once this stage is proven;

* you then need to create a proper PCB artwork so that a PCB manufacturer can make proper PCB's for your amp manufacturing.

If you are operating on your own and only intend to build a few amps a month then you may decide to stay with hand-drawn, home etched boards.