Need help wiring transformer

[J M Fahey]

RG: solid, practical, experience-based reasoning. :tu:
This power supply (and probably many more things) come from a Mr. Carlos Filipe (carlosfm) from Lisbon, Portugal.
I was curious and tried to see some other of his brainchilds, I found his Fet preamp suggested to drive chipamps:
http://myweb.tiscali.co.uk/nuukspot/decdun/gainclonepre2.html
Specifically:

Here we see a BS170, powered with +47 Volts (or +41 regulated) , and with a 75 ohms load resistor, which is described as: 
"this circuit has a very pure signature. Detailed, transparent, with plenty of drive, it will be the answer to your prayers particularly if you are using items like the NOS DACs that have a lowish output."
Well, you'll have to pray a lot , considering that:
a) The BS170 will have to dissipate around 7 Watts, being rated for 400 milli watts (it's a plastic TO92 fet) and
b) It will have to supply 23V/75r (ohms)=306 mA, being specified as a low noise preamp at 1 (one) mA.
Enough said about the technical expertise of the designer and of many who find those products "detailed ... transparent ... etc." without, obviously, having never ever built them.
Of course, if you say "what everybody else is saying" nobody will contest you.
Oh well  :loco
Of course, using 10000uF (or 66000 as Mr Carlos suggests elsewhere) has never hurt anybody, except, maybe, the diodes at turn-on , but Mr Carlos takes care of that by specifying diodes capable of 10x of what's really needed (Now I know why).
Never shy, he specifies 10000 uF (and 2 power transformers in series) to filter a 1 transistor preamp supply.
I wanted to finish this here, but I can't resist posting other two "pearls" (should I call them Bloopers instead?)
1) When a reader worries about the "transparency" of an input filter that already has a cutoff frequency of 48 KHz (1k5/220pF), he suggests :"the input filter (before the volume pot) can be reduced or even removed. If you want to keep a gentle RF filter there that doesn't impact the sound, then I suggest 51R + 100pF"
Gentle is the word, it will slowly (6dB/oct) cut everything above 32 mega Hertz.
2) When another one worries that the output capacitor muddies the lows, he suggests not using it, but warning that "never forget that your preamp has tens of DC voltage on the output. In case you connect it to a power amp without an input coupling cap. (!)"
Really, considering his preamp has 23 DC volts in the output, and gainclone builders avoid like Hell input and feedback capacitors, it will be interesting to see what that does to "low frequency transparency."
Oh well.
I'm doing this for fun, but I guess I'll have to set a PayPal account and start supplying *Guitar* chipamp PCBs to our friends worldwide.
Not as a business, but as a public service.

[R.G.]

I don't doubt that it could be done, but could you describe in more detail (maybe a schematic?) how to wire the rectifier to produce bipolar output from the dual secondaries (as shown in the power supply in the link)? I already have one of these power supplies (with the 8 diodes) built and was considering building a second one for an additional LM3886 chip I have. If I can get away with a cheaper power supply configuration then I'll do that

Do this:

The board is laid out to full wave bridge rectify two independent secondaries, then to connect the positive of one of these to the negative of the other.

That's not necessary. You can connect two identical secondaries into one centertapped secondary.  This is in fact what happens in side the transformer for centertaps. When you do that, you can run the two outer legs of the CT secondary into a full wave bridge connected to two capacitor in series and connect the CT to the center of the two caps. If you then ground the CT and the two caps, you get two equal and opposite DC outputs, both of which are full wave rectified.

You can prove this to yourself by looking at the full wave rectifer in most tube amps. This uses a CT winding, but uses two diodes, each of which conduct when that half of the winding happens to be most positive. The output cap is connected between the CT (usually grounded) and the junction of the two diodes.

If you think about that for a moment, you realize that each half of the secondary winding is not conducting at all when it's going negative. Each side of the secondary is effectively a half wave rectifier, and the diodes add these together.

If you then take two diodes and another cap and hook up one diode to each half of the CT winding so the diode conducts when the winding goes negative, and connect that to an appropriately-polarized-to-ground filter cap, you get a negative supply of the same voltage as the positive one. This is the circuit that is used in 99+% of all bipolar supplies. The double-bridge rectifier circuit works, but it eats four more diodes (and you lose two more diode drops out of your DC) and cannot use a CT secondary. The single bridge rectifier circuit can use either a CT winding or two independent windings connected in series.

I've heard some noise in hifi circles about the double bridge being somehow better, but really folks - they're rectifiers! They pulse current into filter caps!

[joecool85]

Ok, the other thing with this board, it has 8, count em, EIGHT diodes on it -- what the??

I hadn't remembered, but the LM3886 mono kit I did that came from Brian several years ago also had 8 diodes, but I used a toroidal transformer that had two sets of secondaries on it so it was easy to hook up.  After that I did one of his LM1875 kits (not on his website) and that only uses 4 diodes and is real simple to hook up to a CT tranny.

[R.G.]

RG: solid, practical, experience-based reasoning. :tu:
This power supply (and probably many more things) come from a Mr. Carlos Filipe (carlosfm) from Lisbon, Portugal.
I was curious and tried to see some other of his brainchilds, I found his Fet preamp suggested to drive chipamps:

Wow. Words fail me. Stuff I put out may not necessarily sound like angels singing always, but it doesn't burn up.

I was intrigued by your comments on the design and looked it up. He really, really, no fooling did that. The 2SK170 specified and thoughtfully provided with a pinout diagram has an absolute maximum drain-source rating of 40V. Putting 41-42V on it as in the design MIGHT not destroy the device for some devices. Maybe.

I'm doing this for fun, but I guess I'll have to set a PayPal account and start supplying *Guitar* chipamp PCBs to our friends worldwide. Not as a business, but as a public service.

Like this? I had a few minutes and whipped that up yesterday.

[R.G.]

I became curious about why someone could have done a circuit like that preamp. I didn't have any 2SK170s, so I ran it in the circuit simulator; easier to measure that way.

Maybe he did build one. It turns out that Idss limits the drain current and it does run.
Kind of.

With a 41V power supply, the drain voltage is 40.1V. The rms power in the transistor is 550mW, only mildly beyond it's 400mW rating, so it will run good and hot and only die sometimes. That probably improves the tone... 

What's funny is that the output is visibly distorted. With a sine in, it has a nice soft squashed side and a normal side. It's got soft JFET asymmetrical distortion. I'm sure it sounds good - human ears take to soft even-order distortion like flies to honey - but it's not very accurate, nor transparent.

[teemuk]

I don't know if all that "high end" stuff is even supposed to make any sense. In many cases it seems to be more about the image than about the actual specs and performance.

For example, there's a countless number of Gainclone amps with a cathode follower buffered input, a design which honestly makes no sense to me at all if their purpose has been adding some "tube magic" into the design. First of all, no realistic input source can overdrive such buffers and even if it could the clipping, due to 100% degenerative feedback, would be far from the characteristic "musical soft tube clipping". The way I see it, such designs can only add more non-linearities do the design making it less HiFi. Not to mention all the complexity of adding in a heater and high voltage supply. But hey, that tube in the chassis looks nice so it must improve the sound somehow.

Then we have these class-A SE amps with tons of distortion and countless other absurdities even real tube audio designers tried to get rid of already several decades ago.

There's so much snake oil and image marketing in that scene that it's not even funny.

[J M Fahey]

Hi RG.
Yes, I also knew it *might* work, in a horrible, very marginal way; not blowing to the roof in a microsecond just because the "short circuit current" of a FET auto-limits it.
Anyway, the 400mW maximum dissipation is specified at 25ºC , still air, and is one of the "absolute maximum" limits, not to be surpassed , ever. (The others being the maximum voltages , which he does not respect either).
The preamp *may* work if still in the protoboard, in a light breeze.
And with some signal input, *something* will come out.
What worries me most, isn't the fact that there are some nuts roaming free out there, but the *thousands* of devout and fierce followers they get, and the fact that outright lies become true, by constant repetition.  :loco :duh
I liked your PCB, looks very professional.  :tu:
I was thinking about concocting a simpler one, that might be made by anyone at home, single face, using iron-on transfers (or photopositive ones), no PTH, real simple.

[J M Fahey]

Hi Teemu.

There's so much snake oil and image marketing in that scene that it's not even funny.

That's the worst part of it: IT AIN'T EVEN FUNNY
Not only that, many of those "audiophiles" are very angry people and get very mad at whoever tries to "open their eyes"
They also discuss angrily between themselves.
The "impartial judge" which would solve those fights, the *experiment* , including *measuring* results, doesn't even cross their minds.

[joecool85]

I will say one thing, the LM3886 kits Brian sells at chipamp.com right now sound great, are easy to put together, and take the abuse of being a guitar power amp no problem.  When I built mine I put it through it's paces playing it VERY loudly for hours at a time and never had any issues.  That same amp is now being re-done as a bridged LM3886 for my brother's bass rig.

I can't vouch for this new board he sent me as I haven't made it work yet.  Although I suspect it works just fine with a toroidal anyway.

All that said, my favorite chip amp is the LM1875 Brian had me test a couple years ago, as far as I know he hasn't ever sold it on his site though.  Which is sad because it is quite loud, clean, and works well for everything from computer speakers to a guitar amp (I became enthralled with this amp and tested the heck out of it lol).

[BrianGT]

Joe,

Sorry for the delay, was out of town for work.

Here is a schematic for the board:
http://www.briangt.com/gallery/3886mono/3886mono

As for the CT transformer, you will want to remove the center four diodes, exactly as R.G. described.

Let me know if you have any more questions,

BTW, the LM1875 kit that I sent you has been available for quite a while on my site.

-Brian

[J M Fahey]

Hi joecool
Please rest assured that nobody is attacking Brian, who clearly is a clean, straight guy who supplies state of the art kits to us hobbyists.
We are concerned about unscrupulous or even plain crazy people who abuse of everybody's natural wish to advance and progress.

[J M Fahey]

Hi Brian, we seem to have posted at the same time.
Maybe joecool thought we were finding your job less than stellar; it´s quite the contrary, as stated above.
Take care, friend.

[BrianGT]

No worries at all.  I do this in my spare time as a small side business to help people build their own amplifiers, and welcome criticism.  If you think the dual bridge rectifiers are a waste, I can't imagine what you think of people who build these things using $5 each Mil-Spec resistors and $50 each capacitors

-Brian

[joecool85]

Joe,

Sorry for the delay, was out of town for work.

Here is a schematic for the board:
http://www.briangt.com/gallery/3886mono/3886mono

As for the CT transformer, you will want to remove the center four diodes, exactly as R.G. described.

Let me know if you have any more questions,

BTW, the LM1875 kit that I sent you has been available for quite a while on my site.

-Brian

My mistake, I didn't even see it there on the left hand side. 

[R.G.]

I liked your PCB, looks very professional.

Thanks!

I was thinking about concocting a simpler one, that might be made by anyone at home, single face, using iron-on transfers (or photopositive ones), no PTH, real simple.

OK, go that one done. It turns out that doing a good single sided layout was harder than doing a double sided one. I got it down to four wires and gave up. See the attachment. Note that R3, shown off the PCB, is the resistor/inductor component for decoupling the speaker at RF. It's actually on the PCB as the two concentric circles below the "R3". R3 actually stands on its end and is wound with large-gauge magnet wire to make the inductor.

I noticed some differences with the schematic that was posted. I used four times 4700uF filters, instead of six times 1500uF, so there's about twice as much filtering on the power supply. It's really possible to leave two of those off.

I didn't because I used this as a mental exercise in current-loop oriented layout. In any circuit, the DC and AC currents follow complete loops. The trick in getting a quiet layout is to think about what currents flow on a loop and then making those currents not radiate and not cause ohmic voltages for other parts of the circuit to amplify.

So on the AC power input side, I set up two current-pulse loops. Each one goes from the input AC connection to the rectifying diode, then to the first filter cap, through the cap, then back to the other side of the winding to "ground", which is ground in name only, as it's where the transformer winding CT connects. The paths to/from the filter cap carry pulses with quite high currents, maybe 10-20 times the average DC currents coming out of the cap. There are current spikes in this loop which are huge that happen 120 times per second. The rectifiers turning off can cause RF ringing  on the parasitic LC components of the traces and rectifiers. The current loop can also act as an antenna, broadcasting these pulses or their harmonics into space. Use of fast, soft recovery rectifiers can prevent diode-ringing, although with small loop size, this isn't much of a problem. But I*R voltages on the return is a problem. The currents are big enough to make millivolts of voltage across copper traces, even wide ones. It is important that your amplifier input NOT be referenced to places where the pulse voltage shows. So the return line to the CT point is isolated per cap. Even though the two caps are connected for DC purposes in the returns to CT, the current pulses will find the lowest resistance path back to its source, the CT. So isolating the return path from amplifier ground by even a longer bit of circuit traces forces the pulse voltage back to the CT.

At the second set of filter caps, the trace length to the first filter caps helps keep the highest of the pulse currents out of the actual voltage to the amplifier. This is a subtle difference, but measurable. The longer path for charge and pulse currents lessen the pulse noise on the caps that actually supply current to the amplifier. This effect could have been improved by thinner traces from the first filter cap set to the second one as long as the traces could carry the current, or by using a small, low value wirewound resistor in the non-ground path to the second filters. This could be sized to lower the DC voltage only trivially, but to reduce pulse noise quite a bit by further isolating the 120Hz pulses to the first filters.

A similar thing is going on at the second filter caps. The second pair handles its own set of half-wave-rectified pulses. These pulses are the positive and negative halves of the output signal. On the second filters side, it's not the ripple on the + and - power supplies that matter so much as it is the spikes on the return lines. So, as the author notes in the Lm3886 application note, the returns for the speaker load, output compensation filter, feedback return, at-chip filtering, and chip reference are returned by separate paths to the exact center (as much as I could!) of the center trace between the second filter caps. The speaker return gets special attention. I dinked with it for quite a while before doing the right thing: returning it to the exact center of the path between the two second filter caps. Careful measurement can show an error voltage caused by returning this current path to anyplace else. I had to use a wire for this, given the single sided board. It's clumsy, ugly, and offends my sense of elegance, but elegance is not useful if it doesn't work well, so I did it. Plowing is not elegant work, but you get to eat the grain.

I went with National's recommended 470uF of capacitance as close to the chip as I could get it. I put the mute circuit on the PCB, even though the only use of this for a guitar amp is perhaps as a standby switch.

This PCB could be adapted to a better mounting for guitar amps if one used an LM3886 with straight pins and bent the pins up perpendicular to the plane of the mounting tab so they enter the PCB from the back side. This would let you mount the PCB and chip flat on a flat-backed heat sink and have the shortest thermal path and sturdy mounting. Mounting these things on a surface at right angles to the heat sink mounting is problematical. Unless you also build a right-angle bracket for holding the PCB, there is always some flexure between the heat sink and the chassis where the PCB is bolted down. That pressure is on the soldered connections of the chip pins, and will eventually lead to failure from stress and creep of the solder joints.