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Millett "Starving Student" hybrid amp - Page 441

post #6601 of 6786
Quote:
Originally Posted by tomb View Post

Here's a table I found from Tyco Electronics that details typical contact materials, the melt voltage, the arc voltage, and arc currents:

ContactMaterials.jpg

 

As you can see, we may be in the optimum range with all of these at 24V - some of them even at 12V (I don't know what the current is, either, but it doesn't look like it takes very much.).

 

Tyco describes the use of the protection diode (1N4148 in previous designs) and its result in holding up the relay coil:

"Some relay users connect a diode across the inductive load to prevent

counter voltage from reaching the contacts. When the relay contacts

open, the stored energy of the inductance recirculates through the diode,

not through the arc. While this is an acceptable method of protecting the

contacts, it does result in lengthened hold-up time of the inductive load."

 

So maybe the use of the suppressor diode is what speeds things up?

 

Thanks for the info, Tom!  You might find this interesting.

 

http://sound.westhost.com/project33.htm

 

 

Quote:

I carried out some tests to see just how quickly the relays could be operated. The results were something of an eye-opener (and I knew about the added delay caused by a diode!). The relay I used was a small 24V coil unit, having a 730 Ohm coil and with substantial contacts (at least 10 Amps). With no back-emf protection, the relay opened the contacts in 1.2ms - this is much faster than I expected, but the back-emf went straight off the scale on my oscilloscope, and I would guess that the voltage was in excess of 500V. When a diode was added, the drop-out time dragged out to 7.2ms, which is a considerable increase, and of course there was no back-emf (Ok, there was 0.65V, but we can ignore that). Using the diode / resistor method described above, release time was 3.5ms, and the maximum back-emf was -30V, so this seems to be a suitable compromise.

I was not able to test the zener method prior to publication, since I did not have the 24V zeners needed on hand. I would expect this scheme to be as good or better than the diode / resistor combination. The graphs below show the behaviour of the circuit with and without the resistor and diode. The estimated 500V or more is quite typical of all relays, which is why the diode is always included. This sort of voltage will destroy most transistors instantly. It is exactly the same process used in the standard "Kettering" ignition system used in cars, but without the secondary winding, or the "flyback" transformer used in the horizontal output section of a TV set.

Figure 4
Figure 4 - Relay Voltages

The trace labelled 'Contacts' is representative only, and is not to scale. The peak relay voltage (above left) exceeded my oscilloscope's input range (and I was too lazy to set up an external attenuator), and as shown is cut off at my measurement limit. I estimate that the voltage is greater than 500V.

Note that the kink in the relay voltage curve is caused by the armature (the bit that moves) coming away from the relay pole piece, and reducing the inductance. This causes the stored magnetic charge to try to increase the voltage again, but it is absorbed by the resistance and dissipated quickly. The contacts open at the point where the previously closed magnetic field is opened as the armature moves away from the pole piece. As can be seen, this is 3.5ms after the relay supply is disconnected.

These graphs are representative only, as different relays will have different characteristics. As noted above, I cannot predict what sort of relay you will be able to obtain, but the behaviour can be expected to be similar to that shown. All tests were conducted using a 24V relay, having 10A contacts. Upon contact closure, I also measured 2.5ms of contact bounce. Provided your amplifier is stable by the time the contacts close, this will be completely inaudible.

post #6602 of 6786

I found some more information in some app notes.  Diode + Zener is equivalent to the zener suppression diode.

 

 

 

 

post #6603 of 6786

Wow - very interesting!  That certainly explains the transients we were seeing back in the day.  I never really guessed it was back-emf, but it makes a lot of sense.  I was trying to find out in the documentation for suppressor diodes whether they were a "combo" component, but I couldn't find it anywhere.  Anyway, unless I'm misinterpreting, the suppressor diodes are a good choice to control these transients. wink.gif

post #6604 of 6786
Quote:
Originally Posted by tomb View Post

Wow - very interesting!  That certainly explains the transients we were seeing back in the day.  I never really guessed it was back-emf, but it makes a lot of sense.  I was trying to find out in the documentation for suppressor diodes whether they were a "combo" component, but I couldn't find it anywhere.  Anyway, unless I'm misinterpreting, the suppressor diodes are a good choice to control these transients. wink.gif

 

Yeah, back EMF.  Wow, -750V from a 12V relay, but man it switches fast.  It makes sense for sure, blocking the back EMF keeps the coils charged.  Yep, suppressor is the best choice.  All I found for suppressors is that it's modeled as 2 zener diodes, anode to anode.  But, since I have a ton of diodes and some a hundred or so zeners, I think I might go and retrofit. :)

 

One thing I'm not sure about is the voltage ratings used for the suppressors and subsequent zeners.  For the mosfet max, it's 48V.  In the data above, they also use 2x the relay voltage.  I take it that's "standard"?

 

Anyhow, to make this SSMH related, forgoing the regulator will probably work if one is still using the 48V SMPS.  That leaves, a resistor, a cap, a diode, a transistor, a TVS diode (96V?), and a 48V relay.  It seems simple enough, I might retrofit mine and actually rebuild the whole thing with a mix of the Torpedo and the SSMH in a non-starving build.

post #6605 of 6786

Hi every one. I need some help on debug the SSMH amp DIY (12AU7 version). I decide to make SSMH as my very first DIY project. I plan to seperate two chanels to two board. So I did one chanel first.

 

But when testing:

 

- When no input, output | input V = 0, output V = 0.75V (not ok?). When connect headphone (not input), I hear eh eh eh eh eh eh eh ru ru ru ... in series.

- When connect input, output. I hear the music but have some ru ru ru in delay.

 

Check

- Input V = 47V.

- Tube heater vol is about ~ 12-13 V.

- Tube Cathode voltage = 0.5V (seem not ok?)

 

So what could cause this problem? Any ideals? BTW, I place C3 (470uF) and C6 (680uF) too close to each other. Could this be a problem?

 

EDIT: Turn out to bad solder. I redid some and it work perfect now.


Edited by ilkafrv - 7/12/13 at 12:15pm
post #6606 of 6786

Is this power supply good for the SSMH?

 

http://i2.minus.com/ikKN0HMLDN9Ic.jpg

post #6607 of 6786

It's probably fine but everything depends on it's output noise and you can't tell that without putting a scope on it or just trying it.

post #6608 of 6786
I finally got everything together in my SS Amp, and much to my chagrin... it did not work. I can plug everything in and I hear a *click click click* at about 2 times / second. If I turn the volume all the way up I can hear the music faintly in the background immediately preceding the click. I think this is all related to my C1 popping. For what ever reason my C1 popped, I'm not sure why that happened, I figured I may have damaged the capacitor at some point, so I replaced it. However the second one just popped as well. Any one have any idea what would cause this? Let me get a quick idiot check, the Long wire on the capacitor is the + terminal correct? (It says so on its data sheet...) And that should be attached to the node after the switch with the - terminal on the capacitor going to ground correct? ... Time to order some more caps :(. Thanks for the feedback.
post #6609 of 6786
Well it sounds like reverse polarity, did you get the wires from the DC input socket the right way round?
post #6610 of 6786

Do C1 and C6 have to be the same?  I have one 500uf cap left.  C6 is 220uf.  I could replace the C1 with the 500uf.  And yes, the power supply was opposite of what I thought it was.  Good call & Thanks!

post #6611 of 6786
Quote:
Originally Posted by Whil View Post

Do C1 and C6 have to be the same?  I have one 500uf cap left.  C6 is 220uf.  I could replace the C1 with the 500uf.  And yes, the power supply was opposite of what I thought it was.  Good call & Thanks!

Generally speaking, you want parts of one channel to be identical to the parts of the other channel with stereo electronics/devices.  C1 is the "power" cap for the Right channel and C6 is the same thing for the Left channel.  You might get different responses between channels if the parts are not the same.

 

Just an FYI, but we upsized C1 and C6 to 680uf minimum for the Starving Student PCB amplifier.

post #6612 of 6786
Quote:
Originally Posted by tomb View Post

Generally speaking, you want parts of one channel to be identical to the parts of the other channel with stereo electronics/devices.  C1 is the "power" cap for the Right channel and C6 is the same thing for the Left channel.  You might get different responses between channels if the parts are not the same.

 

Just an FYI, but we upsized C1 and C6 to 680uf minimum for the Starving Student PCB amplifier.

 

Sorry to have to say that but... nop, you misread the schematic on that one.

 

C1 is the first power deoupling capacitor, used directly by both output MOSFETs.

C6 is the second power decoupling cap, used by both tubes.

 

C6 is isolated from C1 by R13, creating a CRC filter for the tubes.

 

So no, they don't have to be identical. 

 

edit: And I'll add that if you have two different sizes for those positions, make C1 the larger one.


Edited by KimLaroux - 7/21/13 at 7:52pm
post #6613 of 6786

I suppose you're right - I was going more by the PCB that Dsavitsk designed and perhaps not by schematic.  You know what they say when you assume ...

post #6614 of 6786

Something about introducing a tertiary condition...

 

Anyways, good call on the power supply having an opposite-of-normal polarity!  And I put a 580uf capacitor in the c1 position (220uf in the other positions) and it works like a charm (Save one of my mosfets not working :P haha.  So close to operational :P.)

 

Next up, I'm going to introduce the 48V DP relay and get the delay circuit working with some LED's in there as well...

 

Thanks!


Pics to come!

post #6615 of 6786
Last Thursday at about 10pm CST, another 12AU7 variant Millett Starving Student was born! I'm thrilled to say that the first power-on went flawlessly, all systems go right out of the gate with nothing to troubleshoot! Dead quiet too, omg. After a weekend of off and on burn in, this thing absolutely sings! I love it! Mates up well with my grubDAC and my slightly modded Grado SR60i's. I did somewhat of a perfboard design to keep the guts from turning into too big of a mess and used a copper board ground plane. It's using some 50's RCA long black plate 12AU7 tubes.


Here's a couple of glamour shots and the obligatory pic of the guts.







I broke up the resistors and capacitors into two perfboard sections, essentially one for the tubes and one for the output. They fit in nicely I think!

Cheers!
Edited by mawoca - 7/23/13 at 10:23pm
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