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 Post subject: 5902 I-pod Amp Progress
PostPosted: Nov Sun 01, 2009 9:56 am 
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A few weeks back I started on this project with the thread below .. and now it seems like time for a new thread for this update:

viewtop ... highlight=

So here's the latest.
The project was found at RVRLABS' Site and this is what I'm attempting to duplicate:
Image
http://www.rvrlabs.net/page57/page57.html

Here's the schematic that I'm following.
(EDIT update the latest version of the schematic can be found on page 6 near the end of this thread ) or here: http://www.balazsy.com/radio/5902/Peter ... nal-2B.png

Image

The only change that I made was to replace the individual cathode bias resistors and by-pass caps with a single common cathode bias resistor of 420 ohms.

I don't know if I should be using a bypass cap in the cathode resistor here or not.
I thought Norm Leal indicated that with a single common cathode resistor that by-pass caps aren't needed.
Any ideas?


Here's my finished assembly of BOTH preamp channels on a single tiny 3"x2" perf board from Radio-Shack.

I'm not used to working so small since we usually are digging around these 1940s radios where EVERYthing is ginormous comparatively speaking.

So I simply put on an extra pair of eye-glasses and cranked my solder iron temp back a bit and came up with this little crowded circuit.

I know it seems like there's a lot more stuff on there than the schematic indicates... but I now believe that the schematic was just meant to "trap" you into thinking it's a simple little project!...lol

Image

The project, as shown at RVR Labs, did not include a power supply.

So after researching some similar project supply circuits I came up with the following schematic:
Image


Now...I don't know if what I've ended up with is the most optimal configuration possible.... so I'm anxious for feedback from you guys for any ideas that are perhaps more inventive and/or more efficient. ( cooler running and small)

So PLEASE give me your thoughts.

I have not considered this PS final and would love to have a better idea if possible.

Here's the assembled power supply. It seems to run a bit hot (especially the step-up transformer) and so I added a fan to keep it cool.

The total B+ current is only about 80ma and then there's another 15ma feeding the 40v Zener and op-amp loads.

I would rather not have a design requiring a fan but I want to house it in the same wooden box under the tube circuit-board if possible, so I think a fan will be required to circulate air around and out the bottom vent hole anyway.
But here is where I'm open to ideas.

Image


I chose the 2amp Triad 115v/25v CT because I bought a 1/2 dozen of these on sale last year for about $2 each. It seemed like a good beginning.

I needed a source of filament voltage for the four 6.3 volt 450ma tubes ... as well as two other voltages.
1. B+ = 150v
2. Vcc for the pre amp = 40Vdc

Then I bought the Tamura dual-wound 1.6 amp 8v/115v transformer (3fd-516)
http://www.tamuracorp.com/clientuploads ... FD-5XX.pdf
because I had to step up again to get to my 155 B+

The 8volts is actually 9volts unloaded... so that calculates to roughly about a 13:1 ratio (115/9=12.7)

The Triad will actually put out about 28v CT with a typical 125v input.
So I figured that from CT to one side = 14v x 12.7 ratio = 177v .. or there about.
And that's what I get.

However after thinking about balancing transformer loads.. I decided to leave the 115v windings in parallel... but I put the two 8v windings in series to 16v... because I felt that I'd be drawing a more balanced load from the Triad by taking the whole 28v output.
So that still works out the same :
Since the 8 volt winding unloaded = 9v that means with both 8v windings in series (16v) ... then I'd actually have 18v unloaded.
( 115v/18v= 6.3:1 ratio.... and using the full 28v out from the Triad secondary = 28 x 6.3 = 176v )

I ordered up some copper-clad PC board and cut it to the size I needed for the box of my choice that I'd be using to house the amp.

Image

I was able to cut the board with a hacksaw with relative ease.
I had a devil of a time finding some sub-miniature tube sockets and finally found 6 or 8 of them at some remote place on-line and had to pay over$4 per socket plus shipping!

The 5902 tubes themselves, ( also quite scarce) were supplied to me by a very kind fellow ARFer for shipping costs.

Drilling the holes for the sockets was a bit tough for me. I bought the largest drill bit that would fit in my drill chuck. However my chuck doesn't hold very well so it kept coming loose.

Then I still had to ream each hole a bit afterward for the correct fit. I needed #2/56 screws to hold 'em in place so that meant 8 more tiny holes to drill.

Boy do I wish I had a nice drill press...lol

I decided to run all of the output transformer wires through one common center hole. That meant mounting them slightly above the board with a nut as a spacer and then I routed the primary wires under the transformers.

Quite luckily, I found a pretty good box at the craft store for only $1.

It's a nice little Chinese-made wooden tissue box cover with a sliding removable bottom panel.

It seemed great to simply turn it over and remove the bottom panel and instead I could slide in MY copper board. That puts the oval tissue-box hole on the bottom making for a great exit-way for wires and airflow.
Image

Image

I will be mounting the power switch and audio in/out jacks on another small copper panel on the front-side of the box.

The hardware will not mount properly through the thicker wood so I decided to cut away part of the wood front wall and I will mount another smaller copper panel from a hinge on the main board that will hang flat down against and be attached to the front wall with access to it from the cut away wood section behind it.

I also stained the wood box with some Minwax Special Walnut. It will later receive a nice top coat of polyurethane.

Image
Image

After mounting the tube sockets and building the preamp board it was time to wire and assemble the tube amp components to the socket pins. This was another carefully thought out process as there was little room for error.... especially since I don't have but a few more of these little sockets and I wanted to make everything layout as efficiently as possible and keep lead lengths nice and short as well.

Image

The Hammond transformers are "ok" but for $30 each I was hoping to get a better performer.

In my bread-board test circuit I used a different output transformer.

Typo. Correction follows in Blue:
It was another (smaller) 1 amp dual-wound Tamura 6.3v/115 transformer ( 3FD-412).


I used it as a PP output transformer which performed (to my ear) much better that the Hammond 125A.

I used the two 115v windings in series (230 ct ) and the junction as the CT for B+ input.

Then I paralleled the two 6.3-volt windings for the speaker side.

Remember... to calculate the proper winding ratio.... the 6.3-volt winding actually produces 7.2 volts un-loaded.. right?

( I actually tested the turns ratio of this transformer by putting a 400hz signal in on the 230v side @ 1v rms and measured .0319volts out on the 6.3 volt side. 1/.0319 = 32. Conversely I put .072v rms in the 6.3v side and measured 2.3v rms on the 230v winding side. And calculating that we get 2.3/.072=32 )

[You can measure the ratio with 60hz as well. Just feed in 10 volts in on the primary and then measure the secondary... then divide to get the ratio.]


Anyway... I'm wiring the the two 115 volt windings in series to get 230.. right?
So 230/7.2 = 32 :1 turns ratio.

Since the primary impedance is equal to the square of the turns ratio times the speaker impedance.

So we have a 32:1 turns ratio.
32 squared = 1024
.... and if we use an 8 ohm speaker then 8 x 1024 = 8,192 ohms


This is just about right for these tubes.

When using the Hammond 125A I used taps 2 and 5 giving what seems like the best performance for it with about 8,200 ohms according to the Hammond tap table for an 8 ohm speaker.

Okay..
So, I installed the output transformers and inserted the tubes and fired it up. I didn't like the writing on the top of the Hammond transformers so I cut out some thin shiny brass covers for them to dress it up a little.

Image

It seems to sound very nice at this point. I still have the little pre-amp board mounted external on the bench right now with long leads ....so everything isn't exactly optimized yet.

As far as heat goes... I would like to operate this, if I could, from wall-wart power sources to keep away all heat from the amp box.

I tried running the filaments from an external 13vDC 800ma wall-wart and it works perfectly... however, as odd as it seems, by removing that big 900ma load from the Triad transformer didn't really seem to cool things down very much at all.

Running with external filament power I find that both the Triad and the Tamura transformers still run hot without a fan.

With a fan connected however the extra filament load on the Triad doesn't seem to matter at all.
It runs very cool.... with a fan that is!

If I could come up with a nice simple way to get B+ from something the size of that 13vDC 800ma wall-wart I'd be happier.

Any ideas?

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Last edited by Pbpix on Jul Fri 29, 2016 8:57 pm, edited 40 times in total.

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 Post subject:
PostPosted: Nov Sun 01, 2009 10:49 am 
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Looking good ,Peter, glad to see that you are underway again, David

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 Post subject:
PostPosted: Nov Sun 01, 2009 3:06 pm 
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Looks like good work as usual Peter.

My only question is if the tubes and bases were challenging/expensive to procure, wouldn't it have been better to substitute a different tube?


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 Post subject:
PostPosted: Nov Sun 01, 2009 3:24 pm 
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Peter,

It looks great. You've really done a lot of work in the past couple weeks.

Did you find why you weren't getting a full 36 Vpp on the output of the op amps?

I don't have any ideas right now about changes to your power supply. I'll think about it a bit.

About the cathode bypass cap, if the amp is operated purely class A, a bypass cap is not needed. In class B or AB operation it's best to use a bypass cap. In your case probably 100 uf would be fine. Try it and see if you can hear any difference. You probably will only notice a difference with the volume up near maximum.

Very cool project and sure beats those cheap powered PC speakers all to heck.

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 Post subject:
PostPosted: Nov Sun 01, 2009 3:43 pm 
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Nice work Peter! Only thing missing are the 6L6's :D


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 Post subject:
PostPosted: Nov Sun 01, 2009 6:06 pm 
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Hello Peter,

A couple of suggestions.

1) I can't remember if you have a scope or not. With these home brew projects, a scope is extremely valuable tool which helps quickly troubleshoot problems. Being able to see the waveforms to observe distortion and clipping is very helpful.

2) Your stepup transformer (Tamura) in the power supply is overloaded. The heat issue alone tells me this. The transformer is rated for 12VA. With a 16v/115v ratio (~7.2:1), you should be getting about 205VAC on the output with a 28.6vac input. Since you are getting only 142vac on the output, this is another indication that the transformer is overloaded. At 205vac * ~0.1A load = 20.5VA, so I recommend switching the stepup transformer to one rated at least 25VA or more. This will keep the transformer cool. With the higher rated transformer, your secondary voltage will increase to about 205vac.

3) You are getting excessive ripple on the B+ due to C1 being too small. Another indication of this is that the B+ DC voltage is not high enough. This will result in a 60Hz hum in your amplifier. The capacitor should be large enough such that the B+ is not drooping between every half a cycle. A scope will show this clearly, although measuring the ripple with a AC voltmeter also works. I try to get the ripple below 0.1VAC (just my general rule of thumb) unless the supply is drawing over 2A. The DC level should be very close to the theoretical 1.414*(VAC from transformer - bridge rectifier voltage drop). Since the IR drop across the diodes is small, I'll ignore that. Thus you should be getting about a 142VAC * 1.414 = ~200VDC. Increasing C1 should get you close to this value.

4) The zener will work, but they don't give good power supply noise rejection. If you have hum problems with 60/120Hz still getting into your amplifier, switching to a voltage regulator will give you better rejection.

5) Just one suggestion on the amplifier section. For opamps, its always good to decouple the power/ground connections of the opamps very close to the opamp leads themselves using a capacitor, whose value is not critical and can be in the 0.01uF to 0.1uF range. I usually put these either right next to the opamp or solder them directly to the bottom of the board on the power/ground pins.


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 Post subject:
PostPosted: Nov Sun 01, 2009 6:20 pm 
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John Kusching wrote:
Hello Peter,

A couple of suggestions.

1) I can't remember if you have a scope or not. With these home brew projects, a scope is extremely valuable tool which helps quickly troubleshoot problems. Being able to see the waveforms to observe distortion and clipping is very helpful.

2) Your stepup transformer (Tamura) in the power supply is overloaded. The heat issue alone tells me this. The transformer is rated for 12VA. With a 16v/115v ratio (~7.2:1), you should be getting about 205VAC on the output with a 28.6vac input. Since you are getting only 142vac on the output, this is another indication that the transformer is overloaded. At 205vac * ~0.1A load = 20.5VA, so I recommend switching the stepup transformer to one rated at least 25VA or more. This will keep the transformer cool. With the higher rated transformer, your secondary voltage will increase to about 205vac.

3) You are getting excessive ripple on the B+ due to C1 being too small. Another indication of this is that the B+ DC voltage is not high enough. This will result in a 60Hz hum in your amplifier. The capacitor should be large enough such that the B+ is not drooping between every half a cycle. A scope will show this clearly, although measuring the ripple with a AC voltmeter also works. I try to get the ripple below 0.1VAC (just my general rule of thumb) unless the supply is drawing over 2A. The DC level should be very close to the theoretical 1.414*(VAC from transformer - bridge rectifier voltage drop). Since the IR drop across the diodes is small, I'll ignore that. Thus you should be getting about a 142VAC * 1.414 = ~200VDC. Increasing C1 should get you close to this value.
4) The zener will work, but they don't give good power supply noise rejection. If you have hum problems with 60/120Hz still getting into your amplifier, switching to a voltage regulator will give you better rejection.

5) Just one suggestion on the amplifier section. For opamps, its always good to decouple the power/ground connections of the opamps very close to the opamp leads themselves using a capacitor, whose value is not critical and can be in the 0.01uF to 0.1uF range. I usually put these either right next to the opamp or solder them directly to the bottom of the board on the power/ground pins.


John,

I agree with all your points except #3. Since this is a push pull output stage theoretically all even harmonics and PS hum are canceled. In practice there is never perfect balance so a bit of hum may exist, but you can get away with a fair amount of 120 Hz ripple on the +B to the output stage without objectionable hum. Obviously the +40 V for the op amps should be decoupled very well.

Also I would guess the 16V/115V transformer is saturated due to being overdriven by the 28V and is the cause of reduced output. This core loss is causing the excess heating.

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PostPosted: Nov Sun 01, 2009 10:02 pm 
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Hello Peter,
Well i might have to build a one channel version of your amp.
I have one of those movie drive-in speakers that you hang from the window that needs a amp to power it.

Far as your amp it turned out nicely later if you want you may want to make your own printed circuitboard .

Also you should Buyt your self a small tabletop drill press it would be a lot easier then hand drill for you .

With my limited use of my right hand A drill press comes in handy for me.
Rich


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PostPosted: Nov Mon 02, 2009 4:25 am 
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Cdoose wrote:
Peter,

It looks great. You've really done a lot of work in the past couple weeks.

Did you find why you weren't getting a full 36 Vpp on the output of the op amps?


Hi Chuck:
Well last test showed that I could, now, definitely crank the gain pot to get just about 38 volts PP ... but there is serious flat topping.
The 400hz wave form NOW remains undistorted up to about 32vPP.

But then one 1/2 ( one peak) begins to show distortion on the tops while the other peak remains clean. When I look at the other 1/2 of the opamp's signal it is reversed but identical.

If you remember, previously I started seeing distortion at about 28vpp.
It may have to do with long leads at this point.

I'll be checking it again and give an update.
Perhaps decoupling the ground close to each opamp will help with .01 or so.

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PostPosted: Nov Mon 02, 2009 4:48 am 
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John Kusching wrote:
Hello Peter,

A couple of suggestions.

1) I can't remember if you have a scope or not. With these home brew projects, a scope is extremely valuable tool which helps quickly troubleshoot problems. Being able to see the waveforms to observe distortion and clipping is very helpful.

2) Your stepup transformer (Tamura) in the power supply is overloaded. The heat issue alone tells me this. The transformer is rated for 12VA. With a 16v/115v ratio (~7.2:1), you should be getting about 205VAC on the output with a 28.6vac input. Since you are getting only 142vac on the output, this is another indication that the transformer is overloaded. At 205vac * ~0.1A load = 20.5VA, so I recommend switching the stepup transformer to one rated at least 25VA or more. This will keep the transformer cool. With the higher rated transformer, your secondary voltage will increase to about 205vac.

3) You are getting excessive ripple on the B+ due to C1 being too small. Another indication of this is that the B+ DC voltage is not high enough. This will result in a 60Hz hum in your amplifier. The capacitor should be large enough such that the B+ is not drooping between every half a cycle. A scope will show this clearly, although measuring the ripple with a AC voltmeter also works. I try to get the ripple below 0.1VAC (just my general rule of thumb) unless the supply is drawing over 2A. The DC level should be very close to the theoretical 1.414*(VAC from transformer - bridge rectifier voltage drop). Since the IR drop across the diodes is small, I'll ignore that. Thus you should be getting about a 142VAC * 1.414 = ~200VDC. Increasing C1 should get you close to this value.

4) The zener will work, but they don't give good power supply noise rejection. If you have hum problems with 60/120Hz still getting into your amplifier, switching to a voltage regulator will give you better rejection.

5) Just one suggestion on the amplifier section. For opamps, its always good to decouple the power/ground connections of the opamps very close to the opamp leads themselves using a capacitor, whose value is not critical and can be in the 0.01uF to 0.1uF range. I usually put these either right next to the opamp or solder them directly to the bottom of the board on the power/ground pins.

Hi John:
Thanks for your detailed response:
YES I do have a scope.
On my power supply schematic you'll notice that I indicated that I measured 60mv PP ripple there at the 155v B+ point.

That is my ERROR though because it is ACTUALLY 600mv not 60mv. I was reading the wrong scale for the x10 probe..lol
My big goof.


I am sure you are right about the transformer being overloaded somewhat.. That excess heat is what surely tells us that.... right?

But I came about MY figures differently.
I was calculating the turns ratio based on the actual un-loaded voltage. So the 7.2:1 ratio (115/16 ) that you arrived at is based on 16v... but I learned (however accurate or not) from another PS design where the author stresses that the ratio should be calculated on the un-loaded voltage.
So in this case the unloaded voltage is roughly 18volts.
In my text I was using that figure. So 115/18 = 6.4 ratio
So 6.4 x 28volts = 179v
So do you feel I should still increase the first filter cap John?

You indicate that the ripple is too high on the B+ John.

I was thinking that 60mv pp ripple was extremely low.
But since it was a BIG ERROR and actually 600mv ... I guess you are right!
However I am not hearing ANY real noticable hum.
You said" I try to get the ripple below 0.1VAC (just my general rule of thumb)"

Updated:
So I have just now Added another 47uf at C1 for a total there of about 100uf for C1.... and I added another 200uf at C2 for a total of 268uf.
Now I get well below 100mv ripple there on the 155 B+ line


Thanks a lot about the de-coupling cap idea on the op-amps I'll try adding those tonight.

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Last edited by Pbpix on Nov Mon 02, 2009 8:39 am, edited 4 times in total.

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PostPosted: Nov Mon 02, 2009 5:00 am 
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Peter,

That make total sense that the grid drive starts to flatten on the positive peak of the signal. The grid bias is about 16 V, so as soon as the positive peak value of the grid signal reaches 16 V, grid current will start to be drawn and this will prevent the signal from going much more positive. At this point you are producing the maximum undistorted power available before clipping, so sounds like this is just fine.

I am a bit concerned about the 16V/115V transformer you're using. When you drive a transformer with more voltage than it's rated for, the magnetic field in the core will most likely saturate the steel and this will cause excessive hysterisis and eddy current losses. The tip off is that the higher voltage side is much lower than would be expected from the turns ratio. You can always use less voltage on a transformer, but unless it was over designed in the first place, it's not a good idea to exceed the voltage rating. This is the same reason old PTs which were designed for 110 Vrms heat excessively when energized with modern 125 Vrms line voltage.

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PostPosted: Nov Mon 02, 2009 8:41 am 
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I added corrections in BLUE to my last post response to John and also to my original post in the area of the output transformer info.

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PostPosted: Nov Mon 02, 2009 11:59 am 
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Power Supply update:
Well I spent several hours after my last post evaluating readings, changing components and checking heat.

First of all I wanted to optimize and/or reduce ripple as much as was reasonably possible. I just happened to have a couple 330uf/200vdc caps so I used them both for C1 and C2.
This does get ripple way down to 30mv at the B+ Point

I considered John's conservative advice for increasing the VA of the Tamura transformer. The current 3fd-516 model is rated at 12VA. I had another here (why waste it?) so I put them both in parallel now that should double the VA rating from 12VA to 24VA now.. I believe.. right?

I put it all together mounting them side by side on top of the Triad transformer and fired it up.

The first problem was that after 15 minutes of running all transformers were much too hot to touch.

So I decided to determine how much heat is being caused by what.

So I took the two Tamarua transformers off the Triad to avoid the heat rising up to them from the Triad. In addition I removed the filament loads from the Triad to see how hot it gets just feed power to the DC supply.

So I am running the filaments from a separate 13v/800ma DC wall-wart for the time being.

Image

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Last edited by Pbpix on Nov Wed 21, 2012 6:42 am, edited 6 times in total.

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PostPosted: Nov Mon 02, 2009 12:17 pm 
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hmmm... *** are those triangle things doing in the schematic? :wink: :lol:


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PostPosted: Nov Mon 02, 2009 2:26 pm 
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Just letting my mind drift here now...
It almost seems to me that from a heat stand point I could dump all these transformers and design this using a direct AC fed dropper capacitor for the filaments and a voltage doubler feeding the DC supply.

Is that a reasonable alternative?

For safety.. maybe just a small 120v isolation transformer feeding a voltage doubler? Would that be cooler running and draw a lot less over-all current?

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PostPosted: Nov Mon 02, 2009 2:29 pm 
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Peter,

Go back and look at my previous post.

The reason the transformers are getting hot is that they are saturated! Even with no load you're going to get too much heat!

You can't drive a 16 V winding with 28V it will saturate the core and cause major heating.

Try using just half the secondary(12.5V) from the first transformer to drive the second transformer. You should get about 80 or 90 Vrms on the 115V winding with this hookup.

Then make up a voltage doubler instead of the bridge. This should get you about 160VDC +B

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Last edited by Cdoose on Nov Mon 02, 2009 3:10 pm, edited 1 time in total.

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PostPosted: Nov Mon 02, 2009 2:41 pm 
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Peter--
You should make 2 changes: correct the bridge rectifier drawing--you have AC going in where DC should come out. Also, don't use a zener as a shunt regulator--have it feed to the base of a transistor.
The transistor should have a rating of at least 120 volts, and be heat sinked.
Do I understand that you want to parallel 2 transformers at T2? That's not going to work correctly.


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PostPosted: Nov Mon 02, 2009 2:50 pm 
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Johnnysan wrote:
Peter--
You should make 2 changes: correct the bridge rectifier drawing--you have AC going in where DC should come out. Also, don't use a zener as a shunt regulator--have it feed to the base of a transistor.
The transistor should have a rating of at least 120 volts, and be heat sinked.
Do I understand that you want to parallel 2 transformers at T2? That's not going to work correctly.


That op amp only draws a few milliamps, a series pass regulator is way overkill for this.

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PostPosted: Nov Mon 02, 2009 3:11 pm 
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Cdoose wrote:
Peter,

Go back and look at my previous post.

The reason the transformers are getting hot is that they are saturated! Even with no load you're going to get too much heat!

You can't drive a 16 V winding with 28V it will saturate the core and cause major heating.


Hi Chuck;
I did read it... you are right..
But the big heat generator is the Triad... the other Tamura's don't seem burning hot now that they are in parallel.

.. So you are.. saying I should get a 28v to 115v ... right?
or a dual wound 14v/115 or such.. and rated for 25va ... maybe 30va.. right?

Ok.. sure for that part.. but do I need a heftier replacement for the Triad too?

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PostPosted: Nov Mon 02, 2009 3:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1974
Location: Hinsdale, IL, USA
Yes but the first transformer has to supply the extra power to the other two that are saturated. If they weren't saturated the first transformer won't have to supply so much power.

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Chuck D. KB9UMF


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