Kagliostro 12AU7 push pull schematic

[Ritchie200]

On another thread Kagliostro posted this schematic with two 12AU7s on the output.  I've always wondered for several years: first, if you could even do this, and second, how the heck would you do it?  Looking at the schematic it looks like 4 triodes working push pull - too cool!  However....  What is the 4 and 5 tap for on the input side of the OP tranny?  What is the metering switch?  Also, how the heck would you figure out the input impedance for this setup?  How many watts is this capable of?  How would a PP design differ sonically from an SE design?  AHHHH!!!   Can someone go through this and explain in first grade tubeology what the heck is going on?  It would sure be nice to see some voltages at various points...

Thank you!
Jim

[kagliostro]

I don't see anything of magic on a PP that uses 2 double triodes

Unfortunately I don't know much about that circuit, I recovered it because of the use of the 5879 tube, not more

if you want a guitar amp schematic that uses double triodes in PP ....







To double the number of triodes isn't a problem, only use an OT with an adequate primary impedance and cathode resistor + bypass cap

--

All the schematics are Kit you can find (with complete documentation) at https://www.tube-town.net/ttstore/

there you can find other similar schematics (low power version of famous amplifiers)

a similar design is the FireFlay, but there you see a self split power section, not a real PP section



If instead you want only tecnical info .... I'm not the right person for that

Here you can find very useful info about SE

http://www.valvewizard.co.uk/se.html

and here about PP (first read SE info)

http://www.valvewizard.co.uk/pp.html

Franco

[jjasilli]

it looks like 4 triodes working push pull: Yes, 2 double triodes per side.


What is the 4 and 5 tap for on the input side of the OP tranny? Tap 4 goes to ground; tap 5 seems to supply feedback to the cathode of V2a.

What is the metering switch?  Probably to connect a meter to monitor performance of various test points


Also, how the heck would you figure out the input impedance for this setup?  No figuring required.  The input impedance is users choice of 150 or 600 Ohms per the chart in the schematic.  Same for the OT.


How many watts is this capable of?  Probably 1 or less.  See Firefly.


How would a PP design differ sonically from an SE design.  Probably same as bigger amp.  Though I must confess I don't hear a difference.  I.e, my VibroChamp sounds much like my Princeton to me.

[jeff]

I don't like how that volume pot on the first schematic is wired.

[DummyLoad]

On another thread Kagliostro posted this schematic with two 12AU7s on the output.  I've always wondered for several years: first, if you could even do this, and second, how the heck would you do it?  Looking at the schematic it looks like 4 triodes working push pull - too cool!  However....  What is the 4 and 5 tap for on the input side of the OP tranny?  What is the metering switch?  Also, how the heck would you figure out the input impedance for this setup?  How many watts is this capable of?  How would a PP design differ sonically from an SE design?  AHHHH!!!   Can someone go through this and explain in first grade tubeology what the heck is going on?  It would sure be nice to see some voltages at various points...

Thank you!
Jim

that is for a mic preamp: the secondaries are line level outputs. has some cool tidbits, such as the tap off the cathodyne and gain stages for meter driver, etc.. can't answer the sonics question, that is subjective after all and it is a line level device. i imagine that it does sound decent if one were to use the correct and quality transformers.


--pete

[DummyLoad]

similar to this.


--pete

[terminalgs]

On another thread Kagliostro posted this schematic with two 12AU7s on the output.  I've always wondered for several years: first, if you could even do this, and second, how the heck would you do it?  Looking at the schematic it looks like 4 triodes working push pull - too cool!  However....  What is the 4 and 5 tap for on the input side of the OP tranny?  What is the metering switch?  Also, how the heck would you figure out the input impedance for this setup?  How many watts is this capable of?  How would a PP design differ sonically from an SE design?  AHHHH!!!   Can someone go through this and explain in first grade tubeology what the heck is going on?  It would sure be nice to see some voltages at various points...

Broadcast, television and radio had big installations of mic preamps, cueing amps, talk-back amps, monitoring amps (both speaker and headphone), power amps for studio audiences, power amps to send signals thousands of feet from the studio on the 3rd floor to the transmission control room,etc... plus limiters, compressors etc.. and  clear voice/speech was overriding objective.  distortion was the enemy. So everything had vu meters and gain controls so the next device wasn't over driven, .etc.. When you look at schematics like this, know that the engineer's objectives were different than we usually talk about here.  At CBS, Gates, or Raytheon,  you'd never hear "overdrives nicely".   

• taps 4 & 5: what jjasilli said. those high end transformers with tertiary taps dedicated for NFB. $$.  you can't steal it off 600ohm output with affecting that output like you can an 8ohm output.
• the dual triodes:  in a competitor's design you might have 6V6s there.  two 12AU7s would run a lot cooler than 6V6s, and in a console of 40-100 tubes, heat was a big issue. especially in the days before wide spread air-conditioning! Imagine a radio station owner from El Paso  at a trade show asking about heat.
• impedances: see chart lower left.

[DummyLoad]

this should make about 4W. for the output stage set the the bias for 10mA per triode. voltages indicated are estimated. low power plexi - call it the JTM4ish. 


ref-des in schema are probably effed up: ignore or fix.


--pete

[shooter]

should make about 4W

that's about what I got with my Crayola's!

thanks for the re-draw!

[DummyLoad]

thanks for the re-draw!

 ??

--pete

[kagliostro]

There are more things in heaven and earth, Pete, Than are dreamt of in your philosophy.


 


Franco

[Ritchie200]

Gentlemen, thank you!

Terminalgs, that is a great history lesson!

Pete, so.... when are you going to build it?

Thanks again!
Jim

[shooter]

 ??

Ok, completely engineered amp schematic.
I put it in my top 5 list, but will probably leave it self biased if I ever steal your work

[DummyLoad]

??

Ok, completely engineered amp schematic.
I put it in my top 5 list, but will probably leave it self biased if I ever steal your work

still confused.


--pete

[shooter]

makes 2 of us, I thought the .sch in reply #7 was your work, just saying thanks for the conversion to guitar ready AU7 amp.

[DummyLoad]

makes 2 of us, I thought the .sch in reply #7 was your work, just saying thanks for the conversion to guitar ready AU7 amp.

YW, however, those are BH7 and running a bit hotter than AU7.

have you done something similar? was thinking about using 6S4 as well for a bit more power and perhaps 1/2 power switch switching 4 toi 2 tubes.

--pete

[shooter]

something similar?

Nope, all my PA's have used multi grid'rs
I have a 4SE low watter at the breadboard stage, then I have more au7's than any other tube

[PRR]

> taps 4 & 5: what jjasilli said. .... can't steal it off 600ohm output with affecting that output like you can an 8ohm output.

You "can". But a guitar amp, the load is on the same stage, totally floating. A broadcast line amp, the load may be across town and connected to a different power utility. You don't want your 600r output connected to "common" at both ends. So the 600r winding is full floating, and you add a tertiary winding just for NFB. Cost is not high (couple bucks at most) if you buy a lot (or if such transformers are in wide production).

Likewise the current taps under all tubes are Broadcast culture. The VU meter (or a meter under the console) has a switch to read the current in every tube. The sense resistors are scaled so they all show "nominal 100%" (note 560r on first stage but 36r on the power stage). Every day the junior engineer logs all the readings. If looking back, V1 was 98% last month, 90% yesterday, and 85% today, it may be failing, he puts "replace V1" on that night's to-do list. (Or if the box just quits, a quick scan of the switch tells if one tube just crapped-out dead; might be a 60-second fix.) This goes with broadcast economics: a minute off-air may be $100 of lost advertising revenue, so it is worth it to proactively know how your tubes are doing.

In guitar-land, you don't have a load miles away, and you don't have a junior engineer or a log-book. If you can fix it fairly quick (or borrow from another band), you still get paid. It is generally not worth it to add tertiaries or tube monitoring. Ignore these other-world frills.

[drgonzonm]

I vote for about 6 watts output.  and possibly higher if amp is operating in class AB.  Without voltage numbers it's hard to figure. 

Each triode is capable of 2.75 watts dis.; 2.75x 4 = 11 watts.  x .5= 5.5 watts  for class A.

Double triodes on each power tube, not unusual for the audiophiles.  Some of their dual triodes tubes have 100w output

I haven't seen any data for class AB operation for a 12au7

My take, the amp takes a either a balanced or unbalanced signal and outputs an unbalanced signal.  Output to a set of horn speakers? 

[kagliostro]

As far as I can know, the max plate dissipation is for one triode at a time, having both triodes to the max will hurt the tube

so, with both triodes engaged you must stop at a more conservative dissipation

Franco

[jjasilli]

I vote for about 6 watts output.  and possibly higher if amp is operating in class AB.  Without voltage numbers it's hard to figure. 

Each triode is capable of 2.75 watts dis.; 2.75x 4 = 11 watts.  x .5= 5.5 watts  for class A.

Double triodes on each power tube, not unusual for the audiophiles.  Some of their dual triodes tubes have 100w output

I haven't seen any data for class AB operation for a 12au7

My take, the amp takes a either a balanced or unbalanced signal and outputs an unbalanced signal.  Output to a set of horn speakers?

OK let's revisit that.  No doubt this circuit is designed for maximum linearity, not max plate diss.  From the 12au7 tube chart that looks like maybe 150 plate volts @ 7ma = 1W per section X 4 sections = 4W total plate diss.  Actual output maybe around 2 - 3W???  However it would be possible to re-design the circuit for for more W output, but with less linearity (higher distortion).

[DummyLoad]

this one should make 10w fairly clean.


--pete

[DummyLoad]

OK let's revisit that.  No doubt this circuit is designed for maximum linearity, not max plate diss.  From the 12au7 tube chart that looks like maybe 150 plate volts @ 7ma = 1W per section X 4 sections = 4W total plate diss.  Actual output maybe around 2 - 3W???  However it would be possible to re-design the circuit for for more W output, but with less linearity (higher distortion).

sure. if you push them into heavy class b operation you can double the power but you also need a b+ of over 400V to get that kind of power. use them as presented with respectable reliability. if you want more power add another couple of bottles or pick another tube.

if you want 10W or thereabouts use 6AQ5 or a 10watt-ish triode.

--pete

[PRR]

> 4W total plate diss.  Actual output maybe around 2 - 3W???

Triodes at reasonable voltage rarely get over 25% efficiency.

Small triodes have high plate resistance, which needs higher load resistance, which takes us to the edge of what is practical in a wideband audio transformer. 6BX7 is practical. 6080 is a beast to drive but very low-Z which eases OT selection.

Never forget that triodes have NEVER been front-line g-amps except in the earliest years.

I would expect four sections 12AU7 to give over a Watt, but not a lot over 2W.

Why do we care?? There are billions (well, dozens) of well-tested recipes for higher power. 2A3 will do 3.5W for one, 15W for two (and was used in early g-amps).

[jjasilli]

Yes, the circuit presented is clearly meant to be a voltage amp.  Don't know why the thread digressed into power issues.

[shooter]

Don't know why the thread digressed into power issues

from initial post;

Kagliostro posted this schematic with two 12AU7s on the output.  I've always wondered for several years: first, if you could even do this, and second, how the heck would you do it?

[DummyLoad]

> 4W total plate diss.  Actual output maybe around 2 - 3W???

Triodes at reasonable voltage rarely get over 25% efficiency.

Small triodes have high plate resistance, which needs higher load resistance, which takes us to the edge of what is practical in a wideband audio transformer. 6BX7 is practical. 6080 is a beast to drive but very low-Z which eases OT selection.

Never forget that triodes have NEVER been front-line g-amps except in the earliest years.

I would expect four sections 12AU7 to give over a Watt, but not a lot over 2W.

Why do we care?? There are billions (well, dozens) of well-tested recipes for higher power. 2A3 will do 3.5W for one, 15W for two (and was used in early g-amps).

what i present are two 12BH7 running in class b with a higher than usually seen b+ in fixed bias . so yeah, they'll make 4 watts.

so as you say, use a small-ish pentode from other well proven plans - unless you have to have triode output stage, then a low-er b+ 6AQ5 plan would be better suited for anything over 3 to 4 watts. i have some 6F6 i want to fiddle with for something similar. i have a plan for triode output stage that should make about 20W - just to pass the time and nothing worth posting, it's just for my own edification. 

--pete

[PRR]

> the circuit presented is clearly meant to be a voltage amp.  Don't know why the thread digressed into power issues.

Jim: "Kagliostro posted this schematic with two 12AU7s on the output"
http://el34world.com/Forum/index.php?action=dlattach;topic=21867.0;attach=64780;image

There was talk about the pentode voltage-amp stages.

Jim saw the heap-of-triodes stage and asked about it here. This is clearly a miniature Power stage. Background is remote broadcast amplifier to boost a microphone to drive miles of random telephone wire back to the studio. Line level must be high-ish to overcome static, crackles, and adjacent telephone signals. There may be losses. There may be deliberate padding and passive EQ. Distortion is not wanted, and should be <1% at operating levels.

Reference level is +8dBm or 6mW, indicated on an averaging meter. Peaks will run 10dB to 20dB higher, up-to 600mW. While one 12AU7 can get into that zone, THD may run a bit high; also it has to run HOT which may lead to early failure and unexpected "technical difficulty". A second 12AU7 is not much more cost, better THD, more reliable at this power level.

So Power dominates the stage that Jim is asking about. You can go any which way. One 12AU7 is a part-watt, or can be "unloaded" for pure voltage gain. Put a dozen 12AU7 together and you can run with a Champ (at far higher heater consumption).

[shooter]

this from reply #18;

and outputs an unbalanced signal.

are you talking asymmetric outta the plates or outta the line out tranny?

If I ever get to a quad PP AU7 I was thinking of using a UL 40% tap SE tranny just to make things even more complex

[Ritchie200]

Actually like PRR and others said, I am glad this got into a power discussion.  I am not a big fan of the single ended 12AU7 Firefly setup, although I have built a bunch of them.  As in my original post, I always wondered how a set (or 2) would work PP.  Would it still have that SE fizzy characteristic? How hard would they be to drive?  If you were to run them conservative, could that be a path to low wattage sonic nirvana? I mean a triode is a triode - right?  PRR, that's why I care! (plus the fact at last count close to 100 AU7's in inventory....)

I remember doing some searches a while back for this very type of setup and came up empty.  That is why I was so excited to see the one Kagliostro posted.

Thanks again guys!
Jim

[PRR]

> this from reply #18;

I have no idea why that says unbalanced or talks about speakers.

[sluckey]

consider the source.

[DummyLoad]

consider the source.

exactly.


--pete

[drgonzonm]

consider the source.

exactly.


--pete

Its blind squirrel finding an acorn.  The output from OT T2 is unbalanced.  Look closely at T2 (says the blind squirrel).  If it was a balanced output, there would be three leads the + the - and the ground.  There is no ground shown from terminals 7 &8, just a jumper.  This would be the location to provide the third lead.   Look at T1, and the switch, input is selectable balanced or unbalanced.

Since the output is unbalanced, I believe you are looking a short distance between this preamp and the power amp. 

Regarding maximum dissipation, from a 9 pin tube, a data sheet for a E182CC, gives a max of 8 watts for two plates.  (The E182CC is a computer rated tube)

[PRR]

It is strictly a "floating" output. Since it has no ground (as you say), it can be connected balanced or unbalanced.

There are easier ways to give just an unbalanced output. The general design points to "broadcast" market where balanced is common.

Yes, strictly balanced-only would ground the CT. However a hard-balanced line is rarely better and sometimes worse. Floating will self-balance to the line's asymmetry (if any).

[drgonzonm]

It is strictly a "floating" output. Since it has no ground (as you say), it can be connected balanced or unbalanced.

There are easier ways to give just an unbalanced output. The general design points to "broadcast" market where balanced is common.

Yes, strictly balanced-only would ground the CT. However a hard-balanced line is rarely better and sometimes worse. Floating will self-balance to the line's asymmetry (if any).

Where is the neutral reference point?  An artificial center tap at the end in manner similar to hum reduction using 100 ohm resistors on heaters?  Please provide an example, and distances if possible a floating output can be used. 

[terminalgs]

You "can". But a guitar amp, the load is on the same stage, totally floating. A broadcast line amp, the load may be across town and connected to a different power utility. You don't want your 600r output connected to "common" at both ends. So the 600r winding is full floating, and you add a tertiary winding just for NFB. Cost is not high (couple bucks at most) if you buy a lot (or if such transformers are in wide production).

Ah! very cool.  I didn't realize that aspect of it.

Likewise the current taps under all tubes are Broadcast culture. The VU meter (or a meter under the console) has a switch to read the current in every tube. The sense resistors are scaled so they all show "nominal 100%" (note 560r on first stage but 36r on the power stage). Every day the junior engineer logs all the readings. If looking back, V1 was 98% last month, 90% yesterday, and 85% today, it may be failing, he puts "replace V1" on that night's to-do list. (Or if the box just quits, a quick scan of the switch tells if one tube just crapped-out dead; might be a 60-second fix.) This goes with broadcast economics: a minute off-air may be $100 of lost advertising revenue, so it is worth it to proactively know how your tubes are doing.

very cool indeed.   


Also interesting on that schematic is the pin connector to the chassis R29,R30,R31 look to be a balanced L-pad with 8,9 as input and 5,6 as output. 


Kagliostro,  we need the schematic for the chassis too!! :-D

[Ritchie200]

Terminalgs

As my daughters would say, "I know, right?!!!"    This is cool info!

Jim

[drgonzonm]

It is strictly a "floating" output. Since it has no ground (as you say), it can be connected balanced or unbalanced.

There are easier ways to give just an unbalanced output. The general design points to "broadcast" market where balanced is common.

Yes, strictly balanced-only would ground the CT. However a hard-balanced line is rarely better and sometimes worse. Floating will self-balance to the line's asymmetry (if any).

I accept the correction of it being a floating balanced output. A poor example, but an example anyway, is the creation of a lighting circuit from one phase of a 3 phase delta power system.  By creating a ground in the middle of the transformer windings, you take power from two hots (120° phase shift, not 180°), and create two lower voltage sources. 

Another balanced system, are those twisted pairs used in many landline telephones. 

I hope Franco, can find additional information on the supplied circuit.  As I would like to see voltage number also.  I believe he posted the circuit to show an application of the 5879 tube.

[PRR]

> Where is the neutral reference point?

"Floating will self-balance to the line's asymmetry (if any)."

> Please provide an example, and distances

Telephone wires were, for 100 years, run floating. (As you seem to know??)

Around 1908 AT&T had run a passive (no amplifiers) line 1,000 miles west. The limit was that the signal got weak (despite fat wires), not interference. Amplified floating lines ran transcontinental and broadcast-quality.

> floating balanced output. A poor example, but an example anyway, is the creation of a lighting circuit from one phase of a 3 phase delta power system.

That example is not "floating" except when a wire has burned-off. Power utilization circuits are almost invariably tied to earth. You'd never deliver Delta to a customer unless there was also a Y winding deriving a mid-point. Power systems "must" ground due to lightning and other power services faulting. Audio system, if full floating, do not "have" to be grounded because in a lightning strike the thin wires are gonna melt, nothing you can do about that. Surge and line-drop protection is done with a carbon-block (now crystals) which does nothing until the line is hundreds of Volts away from ground.

[PRR]

> You'd never deliver Delta to a customer

OK, there is jack-leg or high-leg 3-phase where a corner or a mid-side is grounded. This is going out of style for good reason, but you sure may run into it in farm-country.

Still not floating.

[drgonzonm]

> You'd never deliver Delta to a customer

OK, there is jack-leg or high-leg 3-phase where a corner or a mid-side is grounded. This is going out of style for good reason, but you sure may run into it in farm-country.

Still not floating.

Delta is still the cheapest way to provide power to small three phase customers.  Star(wye) requires three hots, and three transformers.   Open delta can provide three power using two 120°out of phase hots.  (Actually reviewed the technology yesterday 4/24, when a friend asked me to look at his three phase power in his shop. )   A remote pump takes three wires, two hots and a neutral, two transformers, and what looks like odd-ball wiring at the transformers.   

The mine I worked in 70's was three phase delta, and converted the 440-480 volt to  star in the early 2000's. The 4160 remained delta.  MSHA, (Mine Safety Health Administration) required the change for safety reasons related to some very unhealthy shocks from equipment. 

 Tell me about not delivering Delta to a (retail) customer.  I was working as a PE consulting with GC/EC remodeling a building for a dental clinic.  Electrical Engineer spec'ed star three phase so the 10 ton AC could be powered three phase.  Local ulitity provided 3 phase delta, so new transformers would not be required, and shops in the area were provided 3 phase.
Long story short, 1/3 of three phase panel was not usable.  Mechanical Contractor fried an expensive board on A/C start up (We did warn him about delta and not wye.)  Owner had to pay for a sub-panel.  Mechanical contractor replaced expensive board at his cost. 

The high tension wires that pass overhead are hot on the three legs, each 120° out of phase. You do not need a ground to get power from two hots.  The power source is generators at the power plant.  At the location or very near, the power is used, a reference ground and neutral is provided. You can do the same thing at home on a 240 volt plug. (In this case the the hots are 180° out of phase) grounding is suggested to avoid unpleasant shocks and possible death, but not necessary. 

Your high leg in 3 phase delta is the potential between ground found in the center tap of one transformer and the hot leg not attached to the centertapped transformer, its phases. Lets say transformer 1 is centertapped, N line , transformer 2 and 3 are not. and there are hot Lines A B C.  Wiring is A-N-B, B-C. and A-C.  The voltages ratios expected are A-N & B-N = 1; A-B, B-C, & A-C = 2; N-C = 1.73. 

I answered the examples after some research. After I asked for examples, The twisted pair telephone wire bugged me, so I also researched telephone technology. So, when I asked for examples, I did not know the answer about phone lines.       

Regarding the early telephone lines, amplification was done by coils, it wasn't until a few years later, tubes were used, legal problems. 

Not sure about phone lines being broadcast quality.  The acceptable frequency range was a little over one magnitude, and cross talk a common problem.

[drgonzonm]

I saw some lightning protection in mid 80's by the Brandname of E-clips. (Company out of Clearwater)   It looked like some MOV's were part of the protection.  The company I worked for protected IBM 286 technology on a radio pump control.  Lightning struck the power pole closest to the pump station.  We expected to see about $5,000 in boards and computers damaged.  The e-clip literally blew apart pieces 40-50 ft from the installation point, but the computer and its boards were fine. 

Look at the grounding schemes on powerlines.  The neutral is typically grounded at every pole, and at the main panel. 

Take the power grid, Power is generated in Farmington NM and high tension wires take three phase power all the way to LA about 600 miles.  The ground wires are not tied into the grid except at the end points. 

[jjasilli]

. . .Don't know why the thread digressed into power issues.

[sluckey]

. . .Don't know why the thread digressed into power issues.

I do.

[shooter]

I do.

I saw the after-math of a fluke 87 and the fool that used it ,(alive n wellish), when used to read 13KV, does that count?

[DummyLoad]

. . .Don't know why the thread digressed into power issues.

troublemaker!   

[drgonzonm]

the question at hand was a review the push pull schematic, and then a discussion, as to whether the output from this circuit was balanced or unbalanced.  This question has been answered.  The output can be balanced or unbalanced without a reference point.
Some of the moderators became trolls, posting smart-alec remarks. 

When an example was provided, delta three phase as an example of a system where power, (yes the circuit above is about power0  one of moderators challenged the assumption that a neutral is not necessary.  Look at "www.guora.com/what is the significance of the neutral line, in 3 phase electricity distribution? What happens if the neutral line does not exist?"  bottom line it is not necessary, but is required for safety reasons.  Same reason, most moderators recommend three prong power cords. not necessary but should be required for safety reasons. 

I accept that I make mistakes but the trolling moderators need to admit they make mistakes too.

[jjasilli]

Ok, but reasoning by analogy has its own issues.


Meanwhile, I'm still not getting the Power importance of the schematic as presented and without alteration. Why? Because it's feeding a line-out tranny.  Presumably, whatever device it's feeding is expecting a voltage input, not a current input.  If this is true, then the schematic depicts a voltage amp by definition.


The fact that it puts out current is beside the point.  All real life (non-ideal) voltage amps put out current.  E.g., a voltage gain stage puts out more current than the guitar signal which feeds it.  But it's still considered a voltage amp, and not a current amp.


Applying this to the schematic at hand, clearly 4X 12au7's put out more current than 2 tubes.  But to my mind this does not make it a current amp for the reasons already stated.


We can replace the line out tranny with a "speaker friendly" tranny; and thereby take advantage of the current. With that alteration, then I agree that the PP 12au7 section becomes a power amp, by definition.


Critiques are welcome.  It's OK to play rough.

[shooter]

If this is true, then the schematic depicts a voltage amp by definition.

isn't the OT the "shift" from V to I?  in my veggie brain if I provide big enough stable V outta the PA tubes then the step-down feature of the OT provides the needed I at the speaker.
 I get that P is P on either side of the = sign, (minus the engineers precision math:)