Devil's Staircase

[John Larkin]

I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Fri, 31 May 2019 16:26:01 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> wrote:

>Why would you add that output decoupling capacitor when it is not needed?

To maybe avoid saturating the transformer. A good toroid has low DCR.
I could easily circulate a few amps DC in the primary.

None of the designs I have seen has a capacitor. Dig into it and find that you are chasing ghosts and just making a bad design by add a highly unlinear device

Notice that Class D is all about removing or compensating non linearities of the output stage.

Cheers

Klaus

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Klaus Kragelund]

You have 15mV offset on the output

See page 8 of the datasheet:

http://www.ti.com/lit/ds/slasea8a/slasea8a.pdf

For a 4 ohm coil that is only 4mA

You have no problem, get rid of the caps

Cheers

Klaus

 

[Cursitor Doom]

On Fri, 31 May 2019 15:36:39 -0700, John Larkin wrote:

I want to make a class-D audio amp, 150 watts or so, using a TI TPA3255
maybe. It's good for 600 watts mono!

Why class D? Doesn't seem like an obvious first choice for audio unless
it's for a PA system or something of that sort.



--
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[Klaus Kragelund]

Why would you add that output decoupling capacitor when it is not needed?

None of the designs I have seen has a capacitor. Dig into it and find that you are chasing ghosts and just making a bad design by add a highly unlinear device

Notice that Class D is all about removing or compensating non linearities of the output stage.

Cheers

Klaus

 

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote:

I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?

There's nothing special about audio:

https://www.pes-publications.ee.ethz.ch/uploads/tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Fri, 31 May 2019 17:01:21 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote:
I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?
Huh, I have more questions than ideas. (How can you use a polarized
cap.?)

Most polarized caps don't mind a bit of back bias. Polymers don't mind
a lot. A couple of power diodes would guarantee that a cap would never
see more than +-1 volt.

I should run a lot of AC current through some supercaps and see what
happens.

but some sort of neg feedback that sense's DC current?
George H.

Sure, but a cap is a lot less thinking.



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Fri, 31 May 2019 17:15:14 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

lřrdag den 1. juni 2019 kl. 01.55.43 UTC+2 skrev John Larkin:
On Fri, 31 May 2019 23:38:58 -0000 (UTC), Cursitor Doom
curd@notformail.com> wrote:

On Fri, 31 May 2019 15:36:39 -0700, John Larkin wrote:

I want to make a class-D audio amp, 150 watts or so, using a TI TPA3255
maybe. It's good for 600 watts mono!

Why class D? Doesn't seem like an obvious first choice for audio unless
it's for a PA system or something of that sort.

We plan to do three channels, 150 watts each, in a 2U rackmount
chassis.

It will simulate a 3-phase PM alternator hung on the gearbox of a jet
engine. We'll be able to program frequency, voltage, and complex
output impedance.


why do you need a transformer?

For isolation, to make 3-phase and other configurations, and to get
various output voltages.

Some of the downstream voltage regulators are weird. People like to
short out PM alternators for regulation. Our box will have to tolerate
that.

and why not some COTS amplifier, you can a 4 channel several 100watt amplifier in 2U for a few hundred $

We'll probably do our own class-D modules. We could use one of the TI
eval boards, but they are big and have a zillion jumpers and goofy
connectors.

We can get a 48V, 600 watt MeanWell power supply for $75!

We were thinking about putting the TPA3255 on the bottom of the board
and heat sinking it to the bottom of the box, which avoids a fan. It
has a huge, grounded power pad on the top of the chip.

Isn't most audio power class D now?


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Lasse Langwadt Christense]

lørdag den 1. juni 2019 kl. 01.55.43 UTC+2 skrev John Larkin:

On Fri, 31 May 2019 23:38:58 -0000 (UTC), Cursitor Doom
curd@notformail.com> wrote:

On Fri, 31 May 2019 15:36:39 -0700, John Larkin wrote:

I want to make a class-D audio amp, 150 watts or so, using a TI TPA3255
maybe. It's good for 600 watts mono!

Why class D? Doesn't seem like an obvious first choice for audio unless
it's for a PA system or something of that sort.

We plan to do three channels, 150 watts each, in a 2U rackmount
chassis.

It will simulate a 3-phase PM alternator hung on the gearbox of a jet
engine. We'll be able to program frequency, voltage, and complex
output impedance.

why do you need a transformer?

and why not some COTS amplifier, you can a 4 channel several 100watt amplifier in 2U for a few hundred $

 

[George Herold]

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote:

I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?

Huh, I have more questions than ideas. (How can you use a polarized
cap.?)

but some sort of neg feedback that sense's DC current?
George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Fri, 31 May 2019 23:38:58 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Fri, 31 May 2019 15:36:39 -0700, John Larkin wrote:

I want to make a class-D audio amp, 150 watts or so, using a TI TPA3255
maybe. It's good for 600 watts mono!

Why class D? Doesn't seem like an obvious first choice for audio unless
it's for a PA system or something of that sort.

We plan to do three channels, 150 watts each, in a 2U rackmount
chassis.

It will simulate a 3-phase PM alternator hung on the gearbox of a jet
engine. We'll be able to program frequency, voltage, and complex
output impedance.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Lasse Langwadt Christense]

lørdag den 1. juni 2019 kl. 01.39.03 UTC+2 skrev Cursitor Doom:

On Fri, 31 May 2019 15:36:39 -0700, John Larkin wrote:

I want to make a class-D audio amp, 150 watts or so, using a TI TPA3255
maybe. It's good for 600 watts mono!

Why class D? Doesn't seem like an obvious first choice for audio unless
it's for a PA system or something of that sort.

he wants an amplifier not a space heater

 

[John Larkin]

On Fri, 31 May 2019 16:00:33 -0700 (PDT),
bloggs.fredbloggs.fred@gmail.com wrote:

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote:
I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?

There's nothing special about audio:

https://www.pes-publications.ee.ethz.ch/uploads/tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf

A capacitor looks easier.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Lasse Langwadt Christense]

lørdag den 1. juni 2019 kl. 03.07.13 UTC+2 skrev Winfield Hill:

John Larkin wrote...

To maybe avoid saturating the transformer. A good toroid has
low DCR. I could easily circulate a few amps DC in the primary.

Doesn't sound so bad. Add a little series R.

if it needs to support shunt regulation it would probably need some to measure current any way

 

[Winfield Hill]

John Larkin wrote...

To maybe avoid saturating the transformer. A good toroid has
low DCR. I could easily circulate a few amps DC in the primary.

Doesn't sound so bad. Add a little series R.


--
Thanks,
- Win

 

[George Herold]

On Friday, May 31, 2019 at 8:38:34 PM UTC-4, John Larkin wrote:

On Fri, 31 May 2019 17:01:21 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote:
I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?
Huh, I have more questions than ideas. (How can you use a polarized
cap.?)

Most polarized caps don't mind a bit of back bias. Polymers don't mind
a lot. A couple of power diodes would guarantee that a cap would never
see more than +-1 volt.

I should run a lot of AC current through some supercaps and see what
happens.


but some sort of neg feedback that sense's DC current?
George H.

Sure, but a cap is a lot less thinking.
OK, just my only 'other' idea.

I was going to report an almost first time pcb spin today.
(Silk screen was a little off in places.*)
I then I blew up the TCA3072... I forgot bypass caps,
and had some hanging things... that shorted or something.

Otherwise it all looked good.

George H.
* I also forgot the pads for the output connector.
I did say almost, :^)

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Winfield Hill]

bloggs.fredbloggs.fred@gmail.com wrote...

There's nothing special about audio:

https://www.pes-publications.ee.ethz.ch/uploads/tx_ethpublications/08_Flux_Balancing_of_Isolation_Ortiz_01.pdf

That looks rather special!


--
Thanks,
- Win

 

[John Larkin]

On Fri, 31 May 2019 17:56:27 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Friday, May 31, 2019 at 8:38:34 PM UTC-4, John Larkin wrote:
On Fri, 31 May 2019 17:01:21 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote:
I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?
Huh, I have more questions than ideas. (How can you use a polarized
cap.?)

Most polarized caps don't mind a bit of back bias. Polymers don't mind
a lot. A couple of power diodes would guarantee that a cap would never
see more than +-1 volt.

I should run a lot of AC current through some supercaps and see what
happens.


but some sort of neg feedback that sense's DC current?
George H.

Sure, but a cap is a lot less thinking.
OK, just my only 'other' idea.

I was going to report an almost first time pcb spin today.
(Silk screen was a little off in places.*)
I then I blew up the TCA3072... I forgot bypass caps,
and had some hanging things... that shorted or something.

Otherwise it all looked good.

George H.
* I also forgot the pads for the output connector.
I did say almost, :^)

TCA0372 is an amazing amp at a shocking price. They seem to be pretty
tough.

Silk and solder mask are often misaligned. Which is why I don't do
"mask defined" pads.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On 31 May 2019 18:06:55 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

John Larkin wrote...

To maybe avoid saturating the transformer. A good toroid has
low DCR. I could easily circulate a few amps DC in the primary.

Doesn't sound so bad. Add a little series R.

I could have my transformer guy make the primary DCR a bit higher than
normal. That would soften up the source a little, but I am simulating
an alternator, which is highish impedance already.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

On Fri, 31 May 2019 17:38:07 -0700, John Larkin
<jjlarkin@highland_snip_technology.com> wrote:

On Fri, 31 May 2019 17:01:21 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Friday, May 31, 2019 at 6:37:06 PM UTC-4, John Larkin wrote:
I want to make a class-D audio amp, 150 watts or so, using a TI
TPA3255 maybe. It's good for 600 watts mono!

I'll use it full-bridge to drive a step-up transformer, probably a
custom toroid. But toroids are especially unhappy with any DC drive,
and the class D part will surely have some DC offset. The TI spec is
60 mV max output offset, which could be a problem into a good
transformer. Speakers don't mind a little DC, but transformers do. DC
can cause stairstepped increase in circulating current, the Devil's
Staircase, until they saturate.

So I'm thinking I'll add a series blocking cap so I can ignore any DC
problems. It will have to be big, 10s of millifarads at least. Biggest
thing on the board. Maybe use a low voltage electrolytic with
antiparallel power diodes, or a shorted bridge, to protect it from
accidental forward or backwards over-voltage.

Lytics will be big, and supercaps don't seem to like ripple current. I
think.

Any other ideas about driving a transformer from an audio amp?
Huh, I have more questions than ideas. (How can you use a polarized
cap.?)

Most polarized caps don't mind a bit of back bias. Polymers don't mind
a lot. A couple of power diodes would guarantee that a cap would never
see more than +-1 volt.

I should run a lot of AC current through some supercaps and see what
happens.

What's their ESR? High temperature is death to capacitors.

but some sort of neg feedback that sense's DC current?
George H.

Sure, but a cap is a lot less thinking.

But I thought electronics was fun?!