High voltage capacitors in audio

sheets:

(I sent this reply before but it got lost I guess)

I am also unable to find any complete datasheet for 2SD2400A,

2SB1569A. They only show some tabular data, curves are not included.

safe zone, but for hFE curve, I am unable to make a comparison with TIP29, 30.

Is it normal that these curves are not shown in the datasheet? Or is there another way to check this?

Thanks, Wimpie. If you are right I'll have to be extra careful also when I finished this circuit & go to test it. After that... the connection must never be broken (that is, take extra care connecting those pot wires).

By the way,

is it true that for transistors (like TIP29) there can be more than one manufacturer, and that their specifications may differ?

Because for TIP29 I found a datasheet from Fairchild, and from Power Innovations Limited. There seem to be small differences.

If so, then when buying a transistor, one should be careful to consider which manufacturer?

I just thought I could use the 5532 instead of 5534, because I need two 5534's (left + right channel), and 5532 is a 'two in one'.

Then the compensation capacitor (at pins 8 and 5) is not required because compensation is internally set in the 5532.

So far so good.

As a side issue, I came across following page:

Quote:

"DECOUPLING & STABILITY. 5532 and 5534 type opamps require careful supply-decoupling if they are to remain stable; otherwise they appear to be subject to some sort of internal oscillation that degrades linearity without being visible on a normal oscilloscope. The essential requirement is that the +ve and -ve rails should be decoupled with a 100nF capacitor between them, at a distance of not more than 2 inches. It is NOT necessary, and often not desirable to have two capacitors going to ground; every capacitor between a supply rail and ground carries the risk of injecting rail noise into the ground. The main rail decouple electrolytics can be used to do the job for several 5532/4 packages nearby, and this cost saving is an important layout point. Likewise, it is not normally necessary to decouple each package individually. One capacitor every few inches is sufficient if the power tracks are of reasonable thickness. (ie 50 thou)"

So according to the author there must be a 100nF (electrolytic, he says) capacitor between the Vcc and gnd pins of the 5532 nearby the

5532.

Has anyone ever heard of this requirement and is it a necessity? I am just surprised it is missing in the headphone amp's circuit (there are

100nF's in the power supply).

I heard, about 10 years ago, that one manufacturer of large audio consoles, stuffed with 553xs was supprised to see noticable noise from finished consoles, in the 3GHz region. This was not noticable on single channel strips, but only on the complete console, of say 64 input channels. IIRC

snip

martin

ectoplasm wrote: (snip)

Right.

This is a bit muddled, I think. For stability, the chip may need bypass capacitance, both rail to rail, and from each rail to signal common. It all depends on where signal current is going.

If you unhook the opamp output from everything but its intended feedback paths (minimal load) then there is little output current, though there may still be significant changes in supply current through the part during some parts of the signal waveform. If those supply current changes cause changes in the supply voltage (due to distribution trace inductance) and that supply voltage variation leaks into the signal stream, either through the internals of the chip, or through some external bias arrangement, then the signal purity is compromised. If the effect is large enough and the phase shift right, the whole circuit may achieve oscillation.

Then hook the output back to its intended load. Now, you have all the previous potential problems (pun intended) but also new currents from the supply rails through the output load to ground and/or back to the supply rails, depending on how the load is connected (see output stage that drives signal into a rail supplied bias network). So, now, the signal swing causes bounce in the ground rail, as well as new causes of bounce in the supply rails. All these new bounces might contaminate the signal purity or even cause oscillations.

I don't think it is a good idea to just bypass rail to rail and ignore the load current and its coupling between the power rails and the ground rail. I prefer to use two capacitors in series connected to form the smallest possible loop between the power pins of the chip, and their common node, then connected to the local signal ground through a common path. Alternatively, grounding the two capacitors to two different parts of the ground rail, but adding a jumper directly between their grounded ends works, too.

I hate to see designs with a chip between two ground rails that are connected together at some distance point/s, with one bypass capacitor going to one of them and the other bypass capacitor going to the other of them. This puts an unnecessarily large inductance in the path of the rail to rail current changes.

If you are worried that this bypass arrangement is going to inject noise from the supply rails into the signal ground, then you need to think about where that noise is coming from, and how to reduce it. The power rails must inject current into the ground rails through any grounded load, so you must figure out how to contain that supply to ground current, locally, with bypass capacitors, so it doesn't wonder all around the system. Having clean supply rails is a separate but related problem.

I disagree with this, for the above stated reasons. I think large supply electrolytics have one function... to store energy during the rectifier peaks, so that they can supply power between them. And the charge paths have to be separated from the discharge paths, so that they don't inject ripple frequency voltages into the ground.

It may not be strictly necessary, but, unless you are willing to determine the effects, experimentally, why risk it?

Length matters more than thickness, in this case, since we are dealing with inductive effects, mostly. Sharing bypass capacitors across several inches of trace length ( in precision analog circuits, not TTL logic) is a recipe for cross talk and instability. The whole concept of bypassing is to contain the highest frequencies of supply and load currents to the smallest possible (lowest impedance at the highest frequencies) part of the system, so that the DC rails do not act as high frequency signal paths.

I think you misunderstand. The electrolytic part of the discussion referred to the large capacitors (main rail decoupling caps), not the individual chip decoupling capacitors, which are not shown, at all. The 100 nF caps should be low ESR ceramic or film capacitors. I avoid Z5U and Y5V ceramics, and use X5R or X7R types as surface mount bypass, because, for the same capacitance and voltage rating, they generally have lower ESR. I like the Panasonic V series of stacked film capacitors for film bypass in through hole applications, though there are lots of good multilayer ceramic caps too.

Schematics gives no guidance (or only hints) as to exactly where any of the bypass capacitors go. The power supply schematic for this project hints that there should be a pair of 100 nF caps closely wrapped around the voltage regulators. Take that hint.

I would add a pair of 100 nF caps for each opamp chip, and mount the pair of 10 k resistors that bias the output stage so that they connect to the supply rails right where the bypass caps for the last opamp connect to those rails.

In addition, I would use the pair of 1000 uF storage caps, for the 22 volt rails in the supply, as bypass capacitors for the output stage (since that stage is their main load), connecting them directly between each of the output transistor collectors and the ground return point for the headphones. This means that the current from rectifiers to these capacitors must have separate traces than the load current leaving them, especially on the grounded side. That headphone ground return point then becomes the star ground reference point for the rest of the circuit, since all other ground currents are tiny, compared to the headphone currents. Route one extra ground trace for 2 grounded points in the output driver stage, (and their bypass capacitors) and another trace back to the first stage for the 6 places ground is needed there, and to its bypass capacitors. If you want to limit any possible noise injected into this branch via the bypass capacitors, add resistance between each of the supply rails and the opamps and their bypasses. 100 ohms should do it. Precise voltage regulation is not at all important, there, but injected signal voltage from the output stage, at high frequencies, where the supply rejection ratio of the opamp is not high, is important.

It is also quite possible the the circuit would work acceptably, if you did most of this sub optimally. I just went through the concepts for your education. Lots of ratty stuff works well enough that nobody notices the imperfections.

Thanks for explaining so clearly. That's a very good suggestion to have the 1000uF caps next to the output stage.

The rectifier diodes might just as well be there, too, then. All transformer leads will go there directly (15V - 0 - 15V).

Do I understand correctly that the 1000uF pair would be the only one? I.e. the regulators (7815/7915, not to be omitted I think) would be fed from these two storage caps, too (through separate traces for their minor load current).

The 1000uF caps' ground would be the main star point. The only down stream connection is the one to the transformer center tap.

Anyway, thanks for the help.

No, directly to the +-22 volt supply, the unregulated one. I think it makes like difference whether the diodes are close to the output pair or remote. The main difference might be capacitive coupling between the secondary AC and the opamp circuits. Having the diodes more remote keeps all the traces near the opamps having only DC or slightly rippling DC on them, lowering the possibilities for hum injection. If done this way, the positive and negative rectifier output and the common to the center tap should be brought over through a close triple conductor (either 3 parallel traces, or better, 3 twisted wires, so that the pulsed currents in them cancel their magnetic fields, which can produce a magnetic hum coupling mechanism.

For the 22 volt supply, yes.

Right. Keep those 100 nF pairs for each regulator right up against the regulator pins, for stability.

Right. Except, since the power comes from the transformer, I would call that an upstream path.

Are you starting to feel like you understand what is going on in this circuit, including the behind-the-scenes stuff?

That was supposed to be "little difference"

I was thinking downstream, as in flowing back to the source. The biggest return flows should be nearer to the source. You view the other way: the supply current, coming from the power supply to the circuit, down stream.

I got a much better understanding, yes. I see the circuit different than in the beginning, and I mean mostly in relatation to ground and supply lines. Thanks to everyone by the way...

"like" changed to "little"

I disagree with this idea. There can be a highish charging followed by a recovery current spike. These have a lot of energy right in the middle of the audio band. If would be much better to put this stuff elsewhere.

If the bridge and the filter are on the PCB, you need to be careful of how the ground and supply wires go.

22V Z ------- Z ! Z ------ AC+ ------+---! !----+------+-+----+---! 7815 !---+----- 15V ! ! bridge! ! 1000! ! ------ ! --- ! ! --- --- --- ! --- 0.01--- 0.01--- --- ---0.1 ! ---100u ! ! ! ! ! ! Z ! ! Z ! -------+------+----- GND AC0 ------+----------------+------+--------------+------------ GND --- mirror image for AC----

The ground trace hits the points in the order shown. It is a wide trace not a plane until you get to the right side of the drawing.

The points I put the "Z"s on are places where ideally, a small lossy impedance will be in series. The 0.01uFs on the bridge are to keep RF noise out of the system. They should be right at the legs of the diodes.

The 1000uF capacitor does the bulk of the filtering. The 0.1 and 100u are right on the legs of the LM7815. If the lead from the 22V to the LM7815 is very long, more capacitance at the input of the LM7815 should be used.

I show two ground connections at the output because the layout may actually be sort of like that. The return current of the output should not flow through the ground of the preamp stage to get to the capacitors.

[....]

I will add stress to the above. One inch of wire is too much between the LM7815 and the capacitor.

I think you need a few more Zs in there. The main load on the 1000 uF cap is not the regulator, but the output transistor. Where does the headphone ground return make connection to that schematic, and where is the Z in that path? I would want the regulator to have its ground reference connection be connected to ground at the point where the two channels of headphone grounds first connect together, not at some distant end of their common path back to the transformer center tap. I like what you show from transformer to storage capacitor, but not to the right of that.

All good.

I would keep the regulators close to the storage, if possible. If this is not possible for thermal or other reasons, than the 100 nF input cap bypasses quite a bit of path inductance at the frequencies where it matters. That is its purpose.

As I spoke about.

Okay. I stress that! ;-)

Found some good transistors...

2SA1930, 2SC5171: Toshiba isolated driver 180V, 2A, 200MHz, hFE ~ 230 (over 10-600mA)

This page shows a good selection of transistors for audio use:

(AmpsLab)

What is it with Z's? Do you mean Z as the symbol for impedance?

As MooseFET said "places where ideally, a small lossy impedance will be in series". These would be resistors, for dampening purposes?

You obviously need some more brain damage.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

What part of high-futility don't you understand?

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

Neither you nor i can guarantee that without testing.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

Where did this 0.55 Hz thing come from?

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

33.3 rpm vinyl artifacts

martin