High voltage capacitors in audio

[martin griffith]

Where did this 0.55 Hz thing come from?

33.3 rpm vinyl artifacts


martin

 

[MooseFET]

I recommended taking the rectifier output directly to the
two storage caps by a twisted triple conductor, so that no
current pulses passed through any ground or supply traces.
The storage caps should be right at the output pair, since
their load current is the single largest single load on
these capacitors, and proximity keeps those large output
currents out of the rest of the supply and ground traces.

The output circuits currents have a mcuh lower peak value than the
charging current of the storage capacitors. The frequencies in the
charging current are harmonics of the mains frequency and not really
related to the content of the sound bing produced. A small cross talk
from the output section's current will be a small distortion of the
signal and thus much harder to detect.

 

[joseph2k]

I made my own amplifier electronics and put it in an old (1970's I
think) amplifier box. The original front label has "solid state
amplifier". It still has the original rumble filter switch which I
think is the same thing..
D from BC

70's? That is not vintage nor is it up to current capability, I had a '70s
amp, i upgraded it, twice. Threw it away almost 15 years ago. It was / is
uneconomic to try to build better, though i easily could. I still can
beat consumer equipment, but i can't see money in it.

 

[MooseFET]

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.

I thing misunderstood the purpose of the "Z"s. They mark locations
where the impedance will be non-zero.

Where does the headphone ground return make
connection to that schematic, and where is the Z in that
path?

The headphones hook to the GND on the right. There should be no "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.

The only ground in the output section is the return path of the
headphones. What you suggest here is effectively what I've done. The
regulators may be bolted to the chassis forcing the issue somewhat on
exactly how the wires go.

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.

For mechanical reasons, the storage capacitors are likely to be some
distance from the regulators. In a lot of ways, it would be best if
they were nearer the driver stage than the output pair, but
mechanically this doesn't make sense.

As I spoke about.

Yes, I was just making it clear.

[....]

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

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

Okay. I stress that! ;-)

 

[MooseFET]

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 could use resistances if you had to. I would suggest lossy
inductors, if cost is no object.

Working left to right.

The ones at the left side of the drawing, reduce the peak current in
the bridge, and block RF. It is a serious bummer when your new
amplifier circuit starts picking up the nearby radio transmitters.
One path that radio signals can use to get into the circuit is up the
power cord. For this reason, you want a more impedance at 1MHz than
at DC.

The impedances after the bridge further suppress RF and also help to
spread the current pulses from the bridge out. They force the RF to
take the path through the capacitors on the bridge. The ground side
of the bulk capacitors is likely hooked to the chassis ground and is
certainly hooked to your signal source. You want to keep RF from
going that way.

The impedances on the power to the regulator is to keep the high
frequency currents of the output section's connections confined to the
bulk capacitors. Since the two capacitors on the regulator are
physically right at its body, there is a path from the input of the
regulator through its 0.1u to its ground leg and then back up onto the
regulated power by the 100u. Without the impedance in the way, this
high frequency stuff could get to the front end power supply.

 

[Eeyore]

Where did this 0.55 Hz thing come from?

33rpm.

Graham

 

[Eeyore]

You could use resistances if you had to. I would suggest lossy
inductors, if cost is no object.

Working left to right.

The ones at the left side of the drawing, reduce the peak current in
the bridge, and block RF. It is a serious bummer when your new
amplifier circuit starts picking up the nearby radio transmitters.

Oh for heaven's sake !

Through the power supply ? Are you being even remotely serious ?

Graham

 

[MooseFET]

Oh for heaven's sake !

Through the power supply ? Are you being even remotely serious ?

Yes absolutely. Don't think of it as a power supply. Think of it as
the connection to the antenna. The detector would be some diode
junction in the first op-amp.

It looks like this:

Cable House wiring
Audio ============->!================ Mains
Diode

The capacitance of the transformer makes it disappear from
consideration by time you get to the AM band.

 

[Eeyore]

Yes absolutely. Don't think of it as a power supply. Think of it as
the connection to the antenna. The detector would be some diode
junction in the first op-amp.

It looks like this:

Cable House wiring
Audio ============->!================ Mains
Diode

The capacitance of the transformer makes it disappear from
consideration by time you get to the AM band.

You're blowing out of your backside.

Stop inventing non-existent problems for the poor OP.

Graham

 

[John Popelish]

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?

The Zs are just a way to consciously remember that every
piece of conductor has resistance and inductance, and so,
will produce voltage drop in response to current and changes
in current, respectively.

 

[MooseFET]

You're blowing out of your backside.

Stop inventing non-existent problems for the poor OP.

Oh shut up. I gave him good advice. I didn't suggest a problem I
haven't seen in real life.

 

[MooseFET]

The Zs are just a way to consciously remember that every
piece of conductor has resistance and inductance, and so,
will produce voltage drop in response to current and changes
in current, respectively.

Note that I said "ideally". I was suggesting "Z"s be placed on
purpose. This can be just the way the wires go, small value resistors
or even lossy inductors.

It doesn't matter much how they are done, it only matters that
something like that be the case.

The ones over at the left are to keep RF out as well as to spread out
the current in the bridge. They are likely to be actually resistors.

 

[Eeyore]

Oh shut up.

Shut up yourself.

Are you a professional; audio designer ?

I gave him good advice. I didn't suggest a problem I
haven't seen in real life.

There's got to be something seriously wrong if the PSU is causing RF pick up.


Graham

 

[John Popelish]

Note that I said "ideally". I was suggesting "Z"s be placed on
purpose. This can be just the way the wires go, small value resistors
or even lossy inductors.

It doesn't matter much how they are done, it only matters that
something like that be the case.

The ones over at the left are to keep RF out as well as to spread out
the current in the bridge. They are likely to be actually resistors.

Sorry I misunderstood you. I thought you were just labeling
the unavoidable impedance of those current paths. Have you
designed audio or other equipment in which you actually
added those components?

I have added a capacitor across the secondary of
transformers to lower the resonant ringing frequency that
occurs when the rectifiers snap off, but not small series
impedances.

 

[MooseFET]

Shut up yourself.

Are you a professional; audio designer ?

I have designed a great many things that work at audio frequencies
some of which were intended to deliver sound to the user and I got
paid for doing this job.

I have learned from experience not to ignore the RF coming in on the
power cable.

There's got to be something seriously wrong if the PSU is causing RF pick up.

Obviously you didn't understand what I said. I will try again.

Imagine the big long wire that is the power wiring of the house. It
picks of RF power as a common mode signal. This connects to the plug
which connects to the transformer. The capacitance from the primary
to the secondaryy of the transformer has a low impedance at RF
frequencies so it goes through the transformer onto the secondary
winding. From there the RF current flows through the ground of the
amplifier to the cable that goes to the signal source.

Now that you can see the path the RF is passing through consider that
the input section of the amplifier and the audio cable may not be very
good at RF frequencies. The op-amp can rectify this RF.

 

[MooseFET]

Sorry I misunderstood you. I thought you were just labeling
the unavoidable impedance of those current paths. Have you
designed audio or other equipment in which you actually
added those components?

Yes, partly.

In one case: The impedances at the transformer connections where 0.22
Ohm resistors. The impedances in front of the regulators were type 77
- 0.25 inch toroids. The others were just the wiring.

The amplifier in question had a gain of about a million over the 850Hz
to 4KHz band. Without the 0.22 Ohms in the transformer winding, the
harmonics near 2KHz caused troubles. Spreading the current pulse out
decreased them enough.

In another case, we had several LM78XX regulators. The transformer
and bridge were in another chassis. Here in the SanFransisco area,
the AM radio station KGO seems to like to get into stuff. Keeping it,
the mains harmonics, the noise from the data computer and clatter from
other parts of the system out of the high gain electronics required
that there be EMI filters on everything coming and going and several
1.0 Ohm 2W resistors in the unregulated power distribution circuits.

I have added a capacitor across the secondary of
transformers to lower the resonant ringing frequency that
occurs when the rectifiers snap off, but not small series
impedances.

The recovery time of the rectifiers is the main way that the bridge
invents new frequencies.

 

[ectoplasm]

http://sound.westhost.com/project24.htm

Could someone tell me the function of the 22uF and 10uF capacitors
around the gain potmeter? (After the first opamp)

I want to replace the gain potmeter by a gain switch, with 3 positions
(that is, two resistors in series where the potmeter is now; the
switch selects one of the points).

There should be a capacitor to reduce switching clicks (150nF or so),
but where should it be placed?

I want to add a volume potmeter at the input of the amplifier, after
the 10uF. But it would affect the filter (with the 1nF), wouldn't it?

 

[MooseFET]

http://sound.westhost.com/project24.htm

Could someone tell me the function of the 22uF and 10uF capacitors
around the gain potmeter? (After the first opamp)

As I suggested, the one feeding the pot serves no real purpose. The
one after the pot makes it so that the 100K to ground is the only DC
path to the second op-amp.

I want to replace the gain potmeter by a gain switch, with 3 positions
(that is, two resistors in series where the potmeter is now; the
switch selects one of the points).

Keep the capacitor and 100K to ground after the switch. This way the
volume goes to zero if the switch connections go open. Letting the
input of an opamp float is a bad thing to do.

BTW: Why only 3 positions?

There should be a capacitor to reduce switching clicks (150nF or so),
but where should it be placed?

If this is a real mechanical switch, you are doomed to have switch
clicks. If there is a signal on one side of a switch and none on the
other, you get a click when it is operated just because the voltage
steps from zero to where the waveform currently is.

I want to add a volume potmeter at the input of the amplifier, after
the 10uF. But it would affect the filter (with the 1nF), wouldn't it?

Why not leave the volume control where it is?

 

[ectoplasm]

Keep the capacitor and 100K to ground after the switch. This way the
volume goes to zero if the switch connections go open. Letting the
input of an opamp float is a bad thing to do.
Yes.

BTW: Why only 3 positions?
[...]
Why not leave the volume control where it is?

The 'volume' control is actually a gain control, it doesn't offer
volume 0%-100%. As the author says "The gain control being used to pre-
set the gain so that the pre-amp's gain control is normally used for
setting the listening level.". Gain might start from 100%, maybe less,
but not 0% (silence).

I don't use a preamp, so I have to add a volume potmeter. I don't see
sense in having two potmeters (one volume, one gain) do about the same
thing, so I figure gain could be a 3-position switch which makes a lot
more sense (I could also use a 2 position or 4 or 5... but 3 I think
will be enough).

I think the volume should be in the first stage before the first
opamp. However it would certainly affect the filter around the 1nF in
some way, I'm afraid.

 

[ectoplasm]

Keep the capacitor and 100K to ground after the switch. This way the
volume goes to zero if the switch connections go open. Letting the
input of an opamp float is a bad thing to do.
Yes.

BTW: Why only 3 positions?
[...]
Why not leave the volume control where it is?

The 'volume' control is actually a gain control, it doesn't offer
volume 0%-100%. As the author says "The gain control being used to pre-
set the gain so that the pre-amp's gain control is normally used for
setting the listening level.". Gain might start from 100%, maybe less,
but not 0% (silence).

I don't use a preamp, so I have to add a volume potmeter. I don't see
sense in having two potmeters (one volume, one gain) do about the same
thing, so I figure gain could be a 3-position switch which makes a lot
more sense (I could also use a 2 position or 4 or 5... but 3 I think
will be enough).

I think the volume should be in the first stage before the first
opamp. However it would certainly affect the filter around the 1nF in
some way, I'm afraid.