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From: Morris Slutsky on 5 Feb 2010 15:12
On Feb 5, 12:30 pm, Lord Valve <detri...(a)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? 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.
From: Les Cargill on 5 Feb 2010 16:47
Morris Slutsky wrote:
> On Feb 5, 12:30 pm, Lord Valve <detri...(a)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.

--
Les Cargill
From: Jim Thompson on 5 Feb 2010 17:36
On Fri, 05 Feb 2010 16:47:22 -0500, Les Cargill
<lcargill99(a)comcast.net> wrote:

>Morris Slutsky wrote:
>> On Feb 5, 12:30 pm, Lord Valve <detri...(a)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.

Years ago I bought from these people. Good prices, good service and
good advice...

http://www.mcdavisco.com/

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
From: Paul E. Schoen on 8 Feb 2010 18:20

"Morris Slutsky" <morris.slutsky(a)gmail.com> wrote in message
news:9360f4f1-c838-41fc-bb63-df93800d1dfc(a)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 1000�
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


From: Morris Slutsky on 11 Feb 2010 00:17
On Feb 9, 12:05 pm, JJTj <up yers.con> wrote:

> Yeah, more parts. But better operation.
>
> JJTj

Probably more parts would be better in every measurable way! You
don't build a single-ended class A amplifier for any objective reason
anyway. It's something you do for subjective, not objective reasons.
Like, say, building a guitar amplifier that uses 2 EL34 tubes with 300
V of plate supply and a 5K load. Not ideal in any design sense. I do
like the way it sounds, though.

> >But a choke load is much more efficient than a resistor load.
>
> Yes, but more hum and magnetic interference.

I understand that in general all coils will pick up interference. But
seriously, a choke that runs into an 8 ohm speaker?

> >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.  

Maybe it was over my head, maybe it wasn't, you decide I guess. Sure
I read his website already. And I guess I see his Aleph-type current
sources as not really single ended amplifiers. In fact, I know
someone with a Pass power amp and it sounds very very good. I could
almost see switching from a choke design to an Aleph type thing. But
that's really, really push-pull from my point of view. Nothing wrong
with that, I could have more than 1 transistor per channel - it'd be
so hilariously cool to do it with 1, though, you can understand that.

Efficiency is something that single ended designs often suck at. Push-
pull stages - ideal class B is 78.5% efficient, ideal class A push-
pull is 50% efficient, class AB lies in between. Ideal single ended
class A with an ideal choke or transformer is also 50% efficient.
With an ideal CCS load, 25%, and with a resistor load 12.5%.

Pass has a few designs up on his DIY site that lie between these
situations. He has, for example, resistor-loaded single ended amps
(with horribly low efficiency). He also suggests use of incandescent
lamps as loads, which have a strong temperature coefficient and could
perhaps even be seen as something BETWEEN a resistor and a CCS load.
He has CCS loaded single ended class A, the "Zen" amplifier.

And the Aleph is also sort of an in between thing, between CCS load
and push-pull class A.

> 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.  

What the Aleph current source does, as far as I can tell, is sense the
current into the load and adjust itself accordingly. It's responding
to the bottom FET that actually is providing the transconductance
which drives the load. The amount of current sensing/adjustment seems
to be variable - from pure CCS on one side, to fully compensating load
current swings on the other - at that point, what you've got is a
pretty clever way to do push-pull without an explicit phase inverter
stage. A lot like an SRPP or some push-pull pentode stages that don't
require a phase inverter - works so long as you stay in class A. And
so you can push it up to the same efficiency level as a choke loaded
stage if you want to.

I will consider such designs. I suppose chokes being expensive and
heavy and all. But there's something kind of elegant and brute force
about a single MOSFET just sitting there heating away.

Assuming I can keep a stable bias on the damned thing and get a decent
choke in the first place, which might be pretty hard actually.
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