Where to get a choke for a class-A single-ended MOSFET power

Morris Slutsky wrote:
On Feb 5, 12:30 pm, Lord Valve <detri...@ix.netcom.com> wrote:
Morris Slutsky wrote:
Thinking of a circuit like this. Simple and elegant-looking:
http://img524.imageshack.us/img524/4038/simplefet.jpg
Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.
What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like. No need
for expensive output transformer. A cheap FET like a IRF510, compare
to a 300B, yeah. No need for heater supply! And the choke . . . the
choke doesn't need to be NEARLY as large. A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range. Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys. Should be easy to find one I think.
But I'm not sure where. Any ideas where to get a 50-100 mH choke for
this? Needs to take an amp or three of current without saturating.
That's the hard part there I think. Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?
If I could find a choke this would be a nice cheap thing to build
though. Easy enough.
Wind your own.

Use 16G magnet wire. For 50-100 mH, it won't be all that large.
If you have an LCR meter and a variable speed drill, you'll be
done in less than 15 minutes.

LV

Hi LV,

Do you have any core suggestions?

Wouldn't it have to be iron? It's audio band...

Besides my previously mentioned
pieces of an old Gibson power transformer, which might do, hopefully I
could squeeze a pair of chokes out of them. I thought about doing it
air-core but basically it'd be like 500 feet of wire to do it that way
and DC resistance would become a problem with any reasonable gauge for
that.

On Feb 5, 6:36 am, legg <l...@nospam.magma.ca> wrote:
On Thu, 4 Feb 2010 20:48:37 -0800 (PST), Morris Slutsky



morris.slut...@gmail.com> wrote:
Thinking of a circuit like this.  Simple and elegant-looking:

http://img524.imageshack.us/img524/4038/simplefet.jpg

Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.

What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like.  No need
for expensive output transformer.  A cheap FET like a IRF510, compare
to a 300B, yeah.  No need for heater supply!  And the choke . . . the
choke doesn't need to be NEARLY as large.  A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range.  Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys.  Should be easy to find one I think.

But I'm not sure where.  Any ideas where to get a 50-100 mH choke for
this?  Needs to take an amp or three of current without saturating.
That's the hard part there I think.  Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?

If I could find a choke this would be a nice cheap thing to build
though.  Easy enough.

Up until today, I would have said Hammond, but their catalog seems to
have been chopped. You're looking at 2.5lbs of part, measuring 4 x 2.5
x 2.25 inches, for the low end of the inductance range quoted, at 2A.
Double the weight for the upper end - 4 x 4 x 3 inches.

RL

Huh. Hammond quit making chokes?

I have the laminations from a burned power transformer originally
belonging to a little Gibson guitar amp. Provided something like 40
VA I'd guess. Wonder if that's enough iron to make a choke, or better
yet even two. It would have to be air-gapped, right? Or not?

On Fri, 5 Feb 2010 07:30:18 -0800 (PST), Morris Slutsky



morris.slut...@gmail.com> wrote:
On Feb 5, 6:36 am, legg <l...@nospam.magma.ca> wrote:
On Thu, 4 Feb 2010 20:48:37 -0800 (PST), Morris Slutsky

morris.slut...@gmail.com> wrote:
Thinking of a circuit like this. Simple and elegant-looking:

http://img524.imageshack.us/img524/4038/simplefet.jpg

Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.

What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like. No need
for expensive output transformer. A cheap FET like a IRF510, compare
to a 300B, yeah. No need for heater supply! And the choke . . . the
choke doesn't need to be NEARLY as large. A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range. Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys. Should be easy to find one I think.

But I'm not sure where. Any ideas where to get a 50-100 mH choke for
this? Needs to take an amp or three of current without saturating.
That's the hard part there I think. Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?

If I could find a choke this would be a nice cheap thing to build
though. Easy enough.

Up until today, I would have said Hammond, but their catalog seems to
have been chopped. You're looking at 2.5lbs of part, measuring 4 x 2.5
x 2.25 inches, for the low end of the inductance range quoted, at 2A.
Double the weight for the upper end - 4 x 4 x 3 inches.

RL

Huh.  Hammond quit making chokes?

I have the laminations from a burned power transformer originally
belonging to a little Gibson guitar amp.  Provided something like 40
VA I'd guess.  Wonder if that's enough iron to make a choke, or better
yet even two.  It would have to be air-gapped, right?  Or not?

Burnt, it might be easier to dissassemble than otherwise, but a 40W
isolation transformer is physically too small (by about 25%). If it
was for a 40W PA, then it would be oversized by about the same amount
and allow for the scrapping of material that was distorted in the
effort. A lot of work.....but a choke has fewer things that can go
wrong than other types of structures, and the non-overlapping
laminations allow for fairly easy iterations.

Depends on what you want it to end up looking like. End covers and a
good paint job can cover a multitude of sins.

I'm surprised that a topology requiring a capacitively coupled output
stage is given serious consideration - there must be a thousand
reasons why not. If all the parts fell into your lap, it might make
sense. Thump at turn-on/off might be avoided here by loading the
speaker to the same rail as the choke, but then it would need to be a
non-polarized part. Like I said, there's no end of headaches being
embraced here.

RL

On Fri, 5 Feb 2010 07:30:18 -0800 (PST), Morris Slutsky



morris.slut...@gmail.com> wrote:
On Feb 5, 6:36 am, legg <l...@nospam.magma.ca> wrote:
On Thu, 4 Feb 2010 20:48:37 -0800 (PST), Morris Slutsky

morris.slut...@gmail.com> wrote:
Thinking of a circuit like this. Simple and elegant-looking:

http://img524.imageshack.us/img524/4038/simplefet.jpg

Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.

What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like. No need
for expensive output transformer. A cheap FET like a IRF510, compare
to a 300B, yeah. No need for heater supply! And the choke . . . the
choke doesn't need to be NEARLY as large. A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range. Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys. Should be easy to find one I think.

But I'm not sure where. Any ideas where to get a 50-100 mH choke for
this? Needs to take an amp or three of current without saturating.
That's the hard part there I think. Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?

If I could find a choke this would be a nice cheap thing to build
though. Easy enough.

Up until today, I would have said Hammond, but their catalog seems to
have been chopped. You're looking at 2.5lbs of part, measuring 4 x 2.5
x 2.25 inches, for the low end of the inductance range quoted, at 2A.
Double the weight for the upper end - 4 x 4 x 3 inches.

RL

Huh.  Hammond quit making chokes?

I have the laminations from a burned power transformer originally
belonging to a little Gibson guitar amp.  Provided something like 40
VA I'd guess.  Wonder if that's enough iron to make a choke, or better
yet even two.  It would have to be air-gapped, right?  Or not?

Burnt, it might be easier to dissassemble than otherwise, but a 40W
isolation transformer is physically too small (by about 25%). If it
was for a 40W PA, then it would be oversized by about the same amount
and allow for the scrapping of material that was distorted in the
effort. A lot of work.....but a choke has fewer things that can go
wrong than other types of structures, and the non-overlapping
laminations allow for fairly easy iterations.

On Feb 6, 12:02 am, legg <l...@nospam.magma.ca> wrote:
snip
Burnt, it might be easier to dissassemble than otherwise, but a 40W
isolation transformer is physically too small (by about 25%). If it
was for a 40W PA, then it would be oversized by about the same amount
and allow for the scrapping of material that was distorted in the
effort. A lot of work.....but a choke has fewer things that can go
wrong than other types of structures, and the non-overlapping
laminations allow for fairly easy iterations.

It's already disassembled. I took it apart out of curiosity as much
as anything, especially as the owner of the amplifier for some reason
didn't actually want his blown old parts back.

Anyway I just took the stack of E's, wrapped 64 turns of ordinary 22
gauge wire (with plastic insulation) around it, wired it up to a
doorbell transformer with a power resistor in series and tried to
measure the voltage drop to guess the reactance at 60 Hz.
Unfortunately, I don't own a LC meter. As it turns out, the
inductance varies wildly with how tightly the lams are squeezed
together, although definitely a millihenry or 10 is not impossible
here. Holding the I pieces in position was kind of tricky as the
whole assembly was vibrating. Probably these measurements don't
normally involve half an amp of current? I imagine that trying to do
a precision 'air gap' here is going to be a nightmare.

Laminations are laminated for a reason - don't let them short out at

Thinking of a circuit like this. Simple and elegant-looking:

http://img524.imageshack.us/img524/4038/simplefet.jpg

Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.

What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like. No need
for expensive output transformer. A cheap FET like a IRF510, compare
to a 300B, yeah. No need for heater supply! And the choke . . . the
choke doesn't need to be NEARLY as large. A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range. Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys. Should be easy to find one I think.

But I'm not sure where. Any ideas where to get a 50-100 mH choke for
this? Needs to take an amp or three of current without saturating.
That's the hard part there I think. Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?

If I could find a choke this would be a nice cheap thing to build
though. Easy enough.

"Morris Slutsky" <morris.slut...@gmail.com> wrote in message

news:9360f4f1-c838-41fc-bb63-df93800d1dfc_at_o26g2000vbd.googlegroups.com...



Thinking of a circuit like this.  Simple and elegant-looking:

http://img524.imageshack.us/img524/4038/simplefet.jpg

Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.

What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like.  No need
for expensive output transformer.  A cheap FET like a IRF510, compare
to a 300B, yeah.  No need for heater supply!  And the choke . . . the
choke doesn't need to be NEARLY as large.  A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range.  Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys.  Should be easy to find one I think.

But I'm not sure where.  Any ideas where to get a 50-100 mH choke for
this?  Needs to take an amp or three of current without saturating.
That's the hard part there I think.  Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?

If I could find a choke this would be a nice cheap thing to build
though.  Easy enough.

Just got back on-line after being without power for a couple days due to
that nasty snowstorm that left over two feet of snow sticking to trees and
power lines...

Signal transformer makes a choke (CH-1) that is 100 mH at 1 amp, 1.5 ohms..
It weighs 2.3 lb and is about 3" cube.

I tried an LTSpice of the circuit and I could not come up with a
combination of resistors that resulted in any better than about 10%
efficiency. I don't see any advantage to this design except that the output
voltage exceeds the rail voltage when the MOSFET is off. Following is my
LTSpice ASCII file. I'm just curious if there is a real reason to use this
design.

Paul

--------------------------------------------------------------------

Version 4
SHEET 1 880 680
WIRE 192 -80 -80 -80
WIRE 192 -64 192 -80
WIRE 192 16 64 16
WIRE 192 32 192 16
WIRE 288 32 192 32
WIRE 384 32 352 32
WIRE 192 48 192 32
WIRE -80 80 -80 -80
WIRE 64 128 64 96
WIRE 144 128 64 128
WIRE 384 128 384 32
WIRE 64 224 64 208
WIRE 64 224 -16 224
WIRE 64 240 64 224
WIRE -80 320 -80 160
WIRE -16 320 -16 304
WIRE -16 320 -80 320
WIRE 64 320 -16 320
WIRE 192 320 192 144
WIRE 192 320 64 320
WIRE 304 320 192 320
WIRE 384 320 384 208
WIRE 384 320 304 320
WIRE 304 336 304 320
FLAG 304 336 0
SYMBOL nmos 144 48 R0
SYMATTR InstName M1
SYMATTR Value IRF510
SYMBOL res 48 0 R0
SYMATTR InstName R1
SYMATTR Value 400
SYMBOL res 48 112 R0
SYMATTR InstName R2
SYMATTR Value 100
SYMBOL res 48 224 R0
SYMATTR InstName R3
SYMATTR Value 10k
SYMBOL ind 176 -80 R0
SYMATTR InstName L1
SYMATTR Value 100m
SYMATTR SpiceLine Rser=.1
SYMBOL cap 352 16 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1000µ
SYMBOL res 368 112 R0
SYMATTR InstName R4
SYMATTR Value 8
SYMBOL voltage -80 64 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 24
SYMBOL voltage -16 208 R0
WINDOW 3 -14 133 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(0 2 200 0 0 0 100)
TEXT -18 378 Left 0 !.tran .5 startup

"Morris Slutsky" <morris.slut...@gmail.com> wrote in message

news:9360f4f1-c838-41fc-bb63-df93800d1dfc_at_o26g2000vbd.googlegroups.com...



Thinking of a circuit like this.  Simple and elegant-looking:

http://img524.imageshack.us/img524/4038/simplefet.jpg

Really like a very cheap version of a single-ended triode stage.
Basic 'parafeed' arrangement except that no transformer is necessary.
Resistors provide bias, negative feedback, and give the FET a bit of
'plate resistance' like a triode to lower output impedance.

What's great about this cheap FET circuit is that it's cheaper than
the tube equivalent, much, and might sound something like.  No need
for expensive output transformer.  A cheap FET like a IRF510, compare
to a 300B, yeah.  No need for heater supply!  And the choke . . . the
choke doesn't need to be NEARLY as large.  A parafeed triode amp, to
get decent bass response, might need a choke in the tens or hundreds
of Henrys range.  Here, the current is much greater, the voltage is
much lower, impedance is much much lower, so you can get by with a
choke in the millihenrys.  Should be easy to find one I think.

But I'm not sure where.  Any ideas where to get a 50-100 mH choke for
this?  Needs to take an amp or three of current without saturating.
That's the hard part there I think.  Lots of dual "common mode" 5-30
mH chokes for sale but I imagine those don't take much DC current at
all before saturating, they're meant for AC circuits aren't they?

If I could find a choke this would be a nice cheap thing to build
though.  Easy enough.

Just got back on-line after being without power for a couple days due to
that nasty snowstorm that left over two feet of snow sticking to trees and
power lines...

Signal transformer makes a choke (CH-1) that is 100 mH at 1 amp, 1.5 ohms..
It weighs 2.3 lb and is about 3" cube.

I tried an LTSpice of the circuit and I could not come up with a
combination of resistors that resulted in any better than about 10%
efficiency. I don't see any advantage to this design except that the output
voltage exceeds the rail voltage when the MOSFET is off. Following is my
LTSpice ASCII file. I'm just curious if there is a real reason to use this
design.

Paul

But a choke load is much more efficient than a resistor load.

And twice as efficient as an active constant current source.

Only the choke load can actually give back the idle current
as necessary.

But a choke load is much more efficient than a resistor load.

Yes, but more hum and magnetic interference.

And twice as efficient as an active constant current source.

I'd bet you don't know how to design CCS(s) that
interact with the circuit. Didn't Mr. Pass teach
you anything? Maybe it was over your head. Was for
me till I built a few. Once you see what happens
with SE designs, and how current is used, you get the
picture. I'm not talking single device CCS devices,
which are little better then resistors, I'm talking
controlled transistor/Mosfet devices that read the amp
and adjust to the need. A choke just sits there doing
the best it can. Fighting what the PS feeds it.

Only the choke load can actually give back the idle current
as necessary.

See what I said above. A choke is a one trick pony, often
done well, but a interactive CCS reads the circuit and does
what is needed to keep the current flowing full tilt.

Yeah, more parts. But better operation.

Yeah, more parts. But better operation.

JJTj

But a choke load is much more efficient than a resistor load.

Yes, but more hum and magnetic interference.

And twice as efficient as an active constant current source.

I'd bet you don't know how to design CCS(s) that
interact with the circuit.  Didn't Mr. Pass teach
you anything?  Maybe it was over your head.  

I'm not talking single device CCS devices,
which are little better then resistors, I'm talking
controlled transistor/Mosfet devices that read the amp
and adjust to the need.  

"JJTj" <up yers.con> wrote in message

news:5u43n55kd1gd8k7k1v7fhjgmdlvcg9ggt5_at_4ax.com...





But a choke load is much more efficient than a resistor load.

Yes, but more hum and magnetic interference.

And twice as efficient as an active constant current source.

I'd bet you don't know how to design CCS(s) that
interact with the circuit.  Didn't Mr. Pass teach
you anything?  Maybe it was over your head.  Was for
me till I built a few.  Once you see what happens
with SE designs, and how current is used, you get the
picture.  I'm not talking single device CCS devices,
which are little better then resistors, I'm talking
controlled transistor/Mosfet devices that read the amp
and adjust to the need.  A choke just sits there doing
the best it can.  Fighting what the PS feeds it.

Only the choke load can actually give back the idle current
as necessary.

See what I said above.  A choke is a one trick pony, often
done well, but a interactive CCS reads the circuit and does
what is needed to keep the current flowing full tilt.

Yeah, more parts.  But better operation.

I was able to make an LTSpice simulation that gives about 37% maximum
efficiency with a 6 VDC power supply, 1.94 VAC input, and 3.13 VAC output
into an 8 ohm load, at 200 Hz. It appears to work from 20 Hz to 20 kHz. But
the biasing is very critical, and probably will be unstable with power
supply, temperature and device variations. I used an STD30NF06L MOSFET
which has a low turn-on voltage. R3 in the schematic is not needed, but the
V2 signal source should be low impedance.

I have some 100 mH chokes rated at 10 amps that I found at a Hamfest in a
pile of freebies. But they are probably 30 lb each.

I agree that this is not really a very practical design, and much better
alternatives exist.

Paul