Yamaha DSP A2070
 

In article ,
Arny Krueger wrote:
"Dave Plowman (News)" wrote in
message
Just to say thanks to all for the tips - a couple of 10µF
electrolytics mounted on the PCB sorted it.

You're saying that it is now working 100% - none of the origional
symptomology?

Yup. I've had it on test for a day or so. No more crackling from the rear
amp and no more tripping. Never did find out the cause of the crackling -
but I re-flowed the entire thing anyway. Or it could just have been a
dirty connector I've disturbed.


--
*How do you tell when you run out of invisible ink? *

Dave Plowman London SW
To e-mail, change noise into sound.

Yamaha DSP A2070
 

On Sat, 21 Nov 2009 17:22:06 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Sat, 21 Nov 2009 13:54:53 +0000 (GMT), Jim Lesurf
wrote:

There may be swings and roundabouts. Many years ago I decided to avoid
using rail-ground-rail caps on the amp board as they tend to inject
distortion from half wave ripple into the amp via any ground
imperfections. But you then need to ensure the amplifier has an
inherently high ability to ignore rail variations.


That is where really well designed grounding is vital. Stars rule!

Ideally. The snag is ensuring that the stages well clear of the chosen
point are still happy with ground or rail impedances at ultrasonic/RF that
aren't zero. I used to deal with that by making as many stages as I could
either common mode differential, or hang via constant current or current
mirrors. Then the rails can flap about all over the show, but provided they
have voltages that are 'enough' the amp can ignore all the flapping. :-)

And I wonder how many died through oscillation. Warrantee repairs are
horribly expensive things to deal with.

Given the above was the cause of the problem I'm curious as to why the
oscillations waited until after warranty. :-) Gives me the feeling
that something else has degraded.


If oscillation started because of the inevitable degradation of a
component (a cap losing value somewhere, maybe?) then they didn't go
through the design centering stage of development. A "what happens
if..." scenario chase is really quite important.

Well, to be fair, you can always find failure modes once you accept that
eventually a component will fail or change.

Here's a little stability thought I've been pondering - and modelling in
Spice. Cdom goes around the voltage amplifier, then the output stage
follows. Now, if the output stage is simply a voltage follower with no
gain or inversion, why would I not connect Cdom to the output of that
stage, rather than just the one transistor's collector? Would that not
give an advantage for HF distortion suppression, since it would not
matter that the overall feedback was degraded by lost HF gain in the V
amp...

Not sure ATM. TBH I never used to approach design of power amps that way.
:-)

If you look back at the 700 amp design there isn't an explicit Cdom cap
shoved in at the end of the voltage gain sections. Instead I put a snubber
on the front long-tailed pair and got it to be stable with that.

Yes, I've used that technique myself, but I've always preferred to use
a specific pole on the voltage amp. I understand what I'm doing better
that way. Was there something specific to do with the FET front end
that prompted it?

Also, I'm guessing that by suing the FET inputs, the usual voltage
offset you see on the output of a power amp is pretty much eliminated?

That said, I also linked the drivers to the output so they could shunt past
the output devices if they weren't able to keep up with any HF. I also
though this would help to 'fill in' as a quasi-class-A stage driving the
output. Bit like current dumping.


I see you run two current sources (ok, voltage amplifiers really) in
series to bias the output stage - did you have any trouble with them
fighting each other? I've been investigating that with an idea for a
power amp that starts with a high speed op amp, but I wasn't at all
sure if it was really kosher. Obviously the op amp could mop up any
imbalance, but it just felt a little wrong.

I know Doug Self did a lot of models, etc, of various o/p arrangements. But
I found the above simply worked better even if his results indicated
otherwise. Presumably because this all depends on the specific details of
the devices, etc, etc. I also found that tiny movements of the wiring loom
altered the performance. Can't recall anyone in a book dealing with that.


Self does talk about dressing wiring looms. His concern is induction
of spiky power line currents into input stages. I seem to remember
that his conclusion is that the + and - rail wires should always be a
twisted pair with minimal loop area, and run well away from the input
stage.

IIRC I tended to dislike having large caps in the output area as it seemed
to just give problems with slew limiting. But I am trying to recall decades
ago, so I'm sure I've forgotten most of this! :-)

Using the conventional Cdom technique, slew rate limiting is easily
calculated from I = C dv/dt, which gives the necessary current for the
input pair. I never really considered it from the point of view of an
output stage.

One more thing - I see you degenerated the emitters of the PNP pair.
What was that about? Were you not just throwing away open loop gain?
That stage shouldn't really have any linearity issues to address.

d

Yamaha DSP A2070
 

In article , Don Pearce
wrote:
On Sat, 21 Nov 2009 17:22:06 +0000 (GMT), Jim Lesurf
wrote:


If you look back at the 700 amp design there isn't an explicit Cdom cap
shoved in at the end of the voltage gain sections. Instead I put a
snubber on the front long-tailed pair and got it to be stable with
that.

Yes, I've used that technique myself, but I've always preferred to use a
specific pole on the voltage amp. I understand what I'm doing better
that way. Was there something specific to do with the FET front end that
prompted it?

It was probably prompted by wanting to avoid leaving the voltage stages
having to current limit into a Cdom. I've never really liked that idea. My
own view is that it is just as likely to cause problems as cure them. I
know that that - and a tendency to use stabilised rails, have been popular.
But they aren't approaches I really liked. The nice thing about the snubber
is that you give the stage a defined resistive load at HF so control the
demands placed on the devices. In effect I just tailored the HF open loop
gain.

So far as I can recall this wasn't specific to using a FET pair at the
front. But it was decades ago so my memory is hazy. I think I did the same
where experimenting with BJT pairs at the front.

Also, I'm guessing that by suing the FET inputs, the usual voltage
offset you see on the output of a power amp is pretty much eliminated?

Yes. The FETs I chose were very expensive at the time as they were sold as
'instrumentation' devices specifically for very low dc offset and closely
matched parameters. Made as two FETs on one chip/substrate and then in the
same pack. Made at the time by Siliconix, although you can still buy
equivalents from other people. Still not cheap. IIRC I also had selected
ones which bumped up the price as I wanted minimal dc error even as things
warmed up, etc.

It also meant I didn't have to worry about needing any bias current though
the feedback and input dc defining resistors which can be another cause of
dc offset in BJT pairs even when matched.

That said, I also linked the drivers to the output so they could shunt
past the output devices if they weren't able to keep up with any HF. I
also though this would help to 'fill in' as a quasi-class-A stage
driving the output. Bit like current dumping.


I see you run two current sources (ok, voltage amplifiers really) in
series to bias the output stage - did you have any trouble with them
fighting each other?

Nope. They just behaved like current mirrors with doubled allowed max power
dissipation compared with one device. I wanted to use good HV devices and
decided doubling up was better than using higher-power devices than were
poorer in other ways.

I've been investigating that with an idea for a power amp that starts
with a high speed op amp, but I wasn't at all sure if it was really
kosher. Obviously the op amp could mop up any imbalance, but it just
felt a little wrong.

So far as I can recall it gave no problems.

I know Doug Self did a lot of models, etc, of various o/p arrangements.
But I found the above simply worked better even if his results
indicated otherwise. Presumably because this all depends on the
specific details of the devices, etc, etc. I also found that tiny
movements of the wiring loom altered the performance. Can't recall
anyone in a book dealing with that.


Self does talk about dressing wiring looms. His concern is induction of
spiky power line currents into input stages. I seem to remember that his
conclusion is that the + and - rail wires should always be a twisted
pair with minimal loop area, and run well away from the input stage.

I found that you had to have all the wiring in a very tight and well
defined loom. Waggling almost anything could alter the stability margin or
HF distortion performance.

IIRC I tended to dislike having large caps in the output area as it
seemed to just give problems with slew limiting. But I am trying to
recall decades ago, so I'm sure I've forgotten most of this! :-)

Using the conventional Cdom technique, slew rate limiting is easily
calculated from I = C dv/dt, which gives the necessary current for the
input pair. I never really considered it from the point of view of an
output stage.

One more thing - I see you degenerated the emitters of the PNP pair.
What was that about? Were you not just throwing away open loop gain?
That stage shouldn't really have any linearity issues to address.

Again, I can't recall for sure as it was so long ago. But I think this was
all about controlling things like the open loop behaviour, defining the
operating conditions of all the devices, symmetry, etc. In effect I wanted
a design that behaved well open loop and was stable (also in thermal
terms), gave low distortion and symmetric. Then have bags of gain to
feedback without problems.

Also, having the resistors means I could start with a current source
pulling the FET pair, then define the current levels though the following
stages up to the drivers. So in effect, control the limiting currents from
the start. The HF behaviour of that was then modified by the snubber at the
start.

I probably preferred losing gain in ways like having emitter resistors to
shoving a Cdom in at the end. Mind you, I don't think my design was as
weird as the Audiolab 8000. Wish I'd had their output devices back then,
though. :-)

In effect, I suppose it means I was mixing local and global gain control in
way that seemed best. But I can't now say why I ended up with any specific
details except that I experimented with a very large number of
permutations, layout, etc, etc, and ended up with the result shown. No
doubt I'd do thing differently now given newer devices and components. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Yamaha DSP A2070
 

On 22 Nov, wrote:
In article , Don Pearce
wrote:
On Sat, 21 Nov 2009 17:22:06 +0000 (GMT), Jim Lesurf
wrote:


One more thing - I see you degenerated the emitters of the PNP pair.
What was that about? Were you not just throwing away open loop gain?
That stage shouldn't really have any linearity issues to address.


Also, having the resistors means I could start with a current source
pulling the FET pair, then define the current levels though the
following stages up to the drivers. So in effect, control the limiting
currents from the start. The HF behaviour of that was then modified by
the snubber at the start.

One thing it occurs to me to add to that.

When making with production in mind I always felt that defining the gains
with resistors was advantageous. Bear in mind that although a bag of
transistors may be all the same 'type' and have the same numbers on them,
they aren't identical. A few stages with typical device variations and you
can easily end up with amplifiers whose detailed performances differ quite
a lot from one to the next on a production line.

That need not be a problem for home builds as you just need to get one or
two working reasonably well. But statistics can thown up problems if you
are making batches where they all need to be in spec and not have a
percentage show problems due to 'unlucky' combinations of individual
components.

But as I said previously, I can't now recall details so don't know if that
was a factor in the above - although my feeling is that I would have had it
in mind.

BTW whilst talking about emitter resistors I am reminded that Armstrong
used to use some flat 'thick film' resistors for the output devices. These
were little ceramic slabs with contacts at two edges. I used to regard this
as perfect for output devices. They were low-inductance, high power, and
shattered if over driven, so didn't go short. So acted as fuses in extemis.
Not seen anyone sell them for years. Any idea of a supplier?

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Yamaha DSP A2070
 

On Sun, 22 Nov 2009 11:59:01 +0000 (GMT), Jim Lesurf
wrote:

On 22 Nov, wrote:
In article , Don Pearce
wrote:
On Sat, 21 Nov 2009 17:22:06 +0000 (GMT), Jim Lesurf
wrote:


One more thing - I see you degenerated the emitters of the PNP pair.
What was that about? Were you not just throwing away open loop gain?
That stage shouldn't really have any linearity issues to address.


Also, having the resistors means I could start with a current source
pulling the FET pair, then define the current levels though the
following stages up to the drivers. So in effect, control the limiting
currents from the start. The HF behaviour of that was then modified by
the snubber at the start.

One thing it occurs to me to add to that.

When making with production in mind I always felt that defining the gains
with resistors was advantageous. Bear in mind that although a bag of
transistors may be all the same 'type' and have the same numbers on them,
they aren't identical. A few stages with typical device variations and you
can easily end up with amplifiers whose detailed performances differ quite
a lot from one to the next on a production line.

That need not be a problem for home builds as you just need to get one or
two working reasonably well. But statistics can thown up problems if you
are making batches where they all need to be in spec and not have a
percentage show problems due to 'unlucky' combinations of individual
components.

But as I said previously, I can't now recall details so don't know if that
was a factor in the above - although my feeling is that I would have had it
in mind.

BTW whilst talking about emitter resistors I am reminded that Armstrong
used to use some flat 'thick film' resistors for the output devices. These
were little ceramic slabs with contacts at two edges. I used to regard this
as perfect for output devices. They were low-inductance, high power, and
shattered if over driven, so didn't go short. So acted as fuses in extemis.
Not seen anyone sell them for years. Any idea of a supplier?

Slainte,

Jim

Ok, yes I can see how defining open loop gain makes HF compensation a
doddle. I know Spice has Monte Carlo analysis, but I'm not sure it can
be applied to the internals of a transistor model. Obviously placing a
tolerance on Rs and Cs is easy.

I have used high power chip resistors in the past, but I can't think
who might have made them. Names like Vishay and Rohm come to mind.

d

Yamaha DSP A2070
 

"Dave Plowman (News)" wrote in message
...
In article ,
Keith G wrote:

"Don Pearce" wrote in message
...
On Sat, 21 Nov 2009 13:20:40 +0000 (GMT), "Dave Plowman (News)"
wrote:

Just to say thanks to all for the tips - a couple of 10µF
electrolytics mounted on the PCB sorted it.


Dear oh dear, when is it ever anything else other than caps or dry
joints?

Expert now, are you? Try reading the rest of the thread before jumping in
with your pearls of wisdom.


Don't be silly. I gave you your best response when you first posted -
someone's using you as a skip.



You think so, do you? There's one on eBay in the West Midlands (collect
only) still at *99p* for the next two and a bit days:

120494159176

Which part of 'well used' 'getting on a bit' 'sold for spares or repair'
didn't you understand?


Grasping at straws again? Wake up, dummy - anyone 'selling' a cheesy old
boat anchor like that is someone looking to avoid a trip to the tip.



Or, if you can't be asked to go yourself (or put a courier in) to
collect and want it delivered, there's one in Claremont NH for a whole
27 quid and *only 280 quid* shipping, but be quick, there's only an
hour left on it:

280422224601

No manual or remote and sold by a dealer in junk.


If it had still got the manual I guess it would have made a few hundred
then?



There are two others (in the UK) described as being in good condition
which have several days to run. I'll see what they make.


So will I - no bids on either one atm. (Wonder if no HDMI has got anything
to do with it? ;-)

How does it go with these things - 'Would sir like his movie 'Recital' or
'Pavilion' tonight?'

Pray tell....


(Pity I didn't have it for last night's Star Trek or tonight's Up ain't it??
:-)))




I got a better idea, but he'll need a length of rope with a noose in it
and he'll have to haul it up to the Thames....

If it was attached to you it would be worth every penny. ;-)


Droll.

Tell you what, I've got a nice little Pioneer hifi amp you can have it for
nowt if you also take the old bathroom suite and ****ter away that I've got
out in the garden? Howzat - sound like a good deal? (You will recognise the
****ter, of course...;-) There's also about a dozen bags of broken tiles to
go as well....

Actually all it needs is a little switch cleaner whooshing in here and
there, but you'll be all set for another nice little *blog* - oh, and don't
forget to replace the caps....




--
*There are two kinds of pedestrians... the quick and the dead.

Dave Plowman London SW
To e-mail, change noise into sound.

Yamaha DSP A2070
 

In article ,
Keith G wrote:
Tell you what, I've got a nice little Pioneer hifi amp you can have it
for nowt if you also take the old bathroom suite and ****ter away that
I've got out in the garden? Howzat - sound like a good deal? (You will
recognise the ****ter, of course...;-) There's also about a dozen bags
of broken tiles to go as well....

I probably shouldn't ask, but why are you collecting old bathroom suites?
Is this how you make all the money you boasted about - totting?

--
*I took an IQ test and the results were negative.

Dave Plowman London SW
To e-mail, change noise into sound.

Yamaha DSP A2070
 

"Dave Plowman (News)" wrote in message
...
In article ,
Keith G wrote:
Tell you what, I've got a nice little Pioneer hifi amp you can have it
for nowt if you also take the old bathroom suite and ****ter away that
I've got out in the garden? Howzat - sound like a good deal? (You will
recognise the ****ter, of course...;-) There's also about a dozen bags
of broken tiles to go as well....

I probably shouldn't ask, but why are you collecting old bathroom suites?
Is this how you make all the money you boasted about - totting?



So much for all your whining about 'OT' and *nonsense* posts...???



--
*I took an IQ test and the results were negative.


Yep, I can believe that....



Dave Plowman London SW
To e-mail, change noise into sound.

Yamaha DSP A2070
 

In article ,
Keith G wrote:
Tell you what, I've got a nice little Pioneer hifi amp you can have it
for nowt if you also take the old bathroom suite and ****ter away that
I've got out in the garden? Howzat - sound like a good deal? (You will
recognise the ****ter, of course...;-) There's also about a dozen bags
of broken tiles to go as well....

I probably shouldn't ask, but why are you collecting old bathroom
suites? Is this how you make all the money you boasted about - totting?

So much for all your whining about 'OT' and *nonsense* posts...???

Which took two lines compared to your 5...

--
*There are two sides to every divorce: Yours and **** head's*

Dave Plowman London SW
To e-mail, change noise into sound.

Yamaha DSP A2070
 

"Dave Plowman (News)" wrote in message
...
In article ,
Keith G wrote:
Tell you what, I've got a nice little Pioneer hifi amp you can have it
for nowt if you also take the old bathroom suite and ****ter away that
I've got out in the garden? Howzat - sound like a good deal? (You will
recognise the ****ter, of course...;-) There's also about a dozen bags
of broken tiles to go as well....

I probably shouldn't ask, but why are you collecting old bathroom
suites? Is this how you make all the money you boasted about - totting?

So much for all your whining about 'OT' and *nonsense* posts...???

Which took two lines compared to your 5...


Well, it don't look too good for your early retirement plan does it, Tee Hee
Boy - 3 out of the 4 of those Yamaha FX boxes I'm watching have gone as
follows: one sold in the states for a whopping 27 quid - which was a
surprise, as they don't usually buy secondhand junk, one sold in the UK for
£12.50 after a last minute *feeding frenzy* and one with a start price of
£40 fell off the table without so much as a sniff.

That leaves one more to go with a start price of *only* £130 in an auction
with wording and spelling that looks like it comes from a moron and which
ends *stoopid late* on a Sunday night, but eBay is eBay and nothing is
*written* until the snipers are done - perhaps I'm judging too early? ;-)



--
*There are two sides to every divorce: Yours and **** head's*

Dave Plowman London SW
To e-mail, change noise into sound.