F5 power amplifier

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here is the affected area. ignore the red wire, as its the V- from V- confusing i know:(

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here is my setup if it helps any. Thanks

Hi,

In your photo the solder of R4 & R14 seem to be very close...

Oh, actually that won't make a difference if they were bridged
 
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Dunno, but 3 watts seems marginal for this part.
Double the value and parallel two.
Two 1 ohm are close enough. You can add a "trim" resistor if you need to pull the value down some... don't forget that unless you have 1% values, that the actual value of two 1 ohm in parallel might be the same as a 0.47 depending on tolerances...

Permitting air to circulate freely around the resistor(s) is required for them to meet their full power specs... you'll get a very hot spot between the board and the resistor (any resistor, any board) if they are mounted flush. I space them up using the end of a tweezer as the spacing guide. A little more is ok too.

_-_-bear

_-_-bear
 
Yeah, according to schematic they are supposed to be connected

R12 tho appears to have been overheated, and may be damaged

I don't know if you saw my saga, but R12 is where smoke was pouring out of. I have since placed an order for extra 3W resistors, as well as a new 5k trimpot to replace the damaged one (mounted it backwards in the beginning and had to flip it and resolder it.)

Ichiban & bear, thanks for the advice, I wasn't aware of the hot spot issues. If I popped them up 1/16 would this be enough? I already clipped the leads and don't want to sacrifice the joint. I guess I was going by the philosophy that flushed components sound better than raised components (I've heard it multiple places, especially with caps (from vibration?) don't know if it's true though)
 
jackinnj, seems like a whole lot of folks have smoked these resistors due to operator error... so IF the mosfet survived, either they are good fuses or else they are maybe on the edge...

If I wasn't lazy and unable to calculate accurately in my head (what is in my head?) I'd figure the peak current through them and so the peak watts... if it was within 50% of the resistor's rating, I'd double it up myself... Ymmv.

_-_-
 
Many power resistors have a rating that is based on destruction over time, in addition to max voltage etc. Don't really know the details but, I don't like the idea of something inside my amp hot enough to start a fire or burn something else :mad: The resistor might survive 150C but that is to hot for me. On Source R type things this typically happens. I tend to use a derating more like 20%. That is, 5X the necessary power capability or more. Acttually I like the heat radiating area that this will provide. Parallel Rs ussually is better than 1 big one in this regard. Or, Rs designed with heat sinking properties like the Vishay Dale RH/NH aluminum cased, gold thingies with 2 mounting ears.
Also note, most film resistors go over the edge easily. If you heat them to much they burn the resistive film and change their properties forever.
just my 2 cents :Pawprint:
 
power supply question

Using the standard F5 power supply as an example --

1. Does anyone know how to calculate (or at least approximate) how long it will take for the filter caps reach full charge upon start-up?

2. Now, if you used a resistor w/ relay (instead of a thermistor) for soft start duties on the primary of the tramsformer (say a 20 ohm resistor for example) -- does anyone know how to approximate or calculate the time it would take for the filter caps to reach full charge after start-up?

Thanks,
Steve
 
1F charges up 1V if we apply 1A for 1second
dv=di/dt / C
but dv and di change with time and state of charge.
If V is at the start (no charge), then after RC seconds the charge will have risen by ~70% of the starting V. R is what limits the starting current.

All done and charged in a few tenths of a second.

BTW,
the resistor in the soft start (primary) is there to limit the start up current while the transformer is pulling up it's socks. It is not a transformer until it is dressed.
This resistor only needs to be in line for a few cycles of mains frequency. It should not be used to slow charge the capacitors.

Use a current limit in the secondary circuit to slow charge the capacitors.
 
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Hey Andrew,
Plus, it's a bit of a pain slow starting from the secondary side. With two power supplies (for dual-mono), you need 4 resistors (one on each of the secondaries), which all have to be bypassed with either a 4-way relay or multiple relays.
If it's doable from the primary side, you can do the whole thing with one resistor (or combination of resistors) and one relay as long as the ratings are high enough.

Now for that matter, if you plan on slow starting from the secondary side, wouldn't it be even better to slow start from just after the rectifiers. this way the rectifiers never see a shhort at start-up?

Thanks,
Steve
 
jackinnj, seems like a whole lot of folks have smoked these resistors due to operator error... so IF the mosfet survived, either they are good fuses or else they are maybe on the edge...

If I wasn't lazy and unable to calculate accurately in my head (what is in my head?) I'd figure the peak current through them and so the peak watts... if it was within 50% of the resistor's rating, I'd double it up myself... Ymmv.

_-_-
I've had folks with issues regarding the 10 ohm source resistors -- but not the 0.47R Panasonics, Vishay or Dales. At peak the JFET source resistors dissipate 0.57W, definitely not a place for a 1/4 or 1/2 W resistor.

The machine inserted Panasonics are specified to be mounted 16mm (about 5/8inch) above the PCB.

The Ohmite TWW5's are very efficient at getting the heat off the board. They aren't quite as flat as the Panasonics or Dales, but are rated at 5W. There was a brief period of time when Ohmite switched over to ROHS and they were out of stock at all the usual suspects. Janneman appears to have used them in one of his designs for Elektor.

I have gotten my 576 curve tracer to smoke on occasion. sometimes you can make the current limiting resistors unhappy.
 
I was going to bring up the 10 ohm resistors... hadn't gotten there yet.

So, maybe 2 x 20ohm 1/2w then?


sekess, if you put a resistor in series with the primary (the usual way to go) for a soft start the caps will never fully charge because of the vdrop of the resistor...fwiw.

How long before you throw the relay shorting out the series resistance depends on the resistance and the amount of capacitance you have to charge up in the real world. A little bit of experimentation will be sufficient. Watch an ammeter in line or else look to see if the house lights blink? :D

Of the many ways of doing the switching are the RC + transistor, 555 timer, manual switch, and comparator that looks at the rail voltage...

_-_-bear
 
Hey Bear,
Yea, I know while the resistor is in place the rail voltage won't reach 24 volts. But, the caps will stop charging at some lower voltage. then, when the resistor is switched out, the rails will reach about 24 volts and the caps will finish their charging.

Basically, I am trying to figure out the following (for soft-starting and slow-starting):

-- What size resistor(s) to use - both voltage and wattage
-- Where to place them - primary, secondary, or at the rails
-- How much time before I bypass them with the relay

If practical, I would like to soft start as well as slow charge the caps (as Andrew T has alluded to).

I got a decent deal on some very large caps. So, I wanted to try those guys. I am going to end up with 280 000uF per power supply (dual-mono -- so 560 000uF total). With that amount of capacitance, I thought it would be best if I could slow-charge them at start-up.

Thanks,
Steve
 
Right, absolutely necessary.
I'd suggest 50watt wirewound ceramic resistors at minimum.
You can get away with less, but in the event something goes wrong or delays your relay you will smoke smaller resistors.
A value around 8-10 ohms is likely to be good.
With the lower voltages maybe a higher value will suffice...
I would only put this before the transformer at the primary.
Also, do NOT use a single relay contact. Use multiple high current contacts, imo. If you just have to use a medium size "cube" relay, at least get one with 4 contacts...

My Symphony No.1 amp has 500,000ufd @65vdc rail voltage. That and >2KVa of iron... so I use very large P&B relays. You can see a pic on my website if you hunt down in the amplifier section a bit...

As I said you can figure out the time thing after you put the supply together and have it running... just look at the pulse when you bypass it... that and the time it takes for the rails to come up. On my amp I used 10 seconds... more than sufficient to bring up the rails and let the amp stabilize (no speaker relays, so don't want thumps)

I'm thinking that a soft recovery type rectifier will be best in consideration of the current waveform that will be charging the cap banks...

How many caps to achieve this total capacitance??

_-_-bear
 
I got a decent deal on some very large caps. So, I wanted to try those guys. I am going to end up with 280 000uF per power supply (dual-mono -- so 560 000uF total). With that amount of capacitance, I thought it would be best if I could slow-charge them at start-up.

I'm very curious how your transformers/mains will behave with this amount of capacitance. I often experienced lots of humming and buzzing in the toroid due to the very, very bad power factor (cos phi). One of my F5's has 130.000uf per channel on a Amplimo 2x18V, 625VA and I can't go further without having the buzz.....
 
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Right, absolutely necessary.
I'd suggest 50watt wirewound ceramic resistors at minimum.
You can get away with less, but in the event something goes wrong or delays your relay you will smoke smaller resistors.
A value around 8-10 ohms is likely to be good.
With the lower voltages maybe a higher value will suffice...
I would only put this before the transformer at the primary.
Also, do NOT use a single relay contact. Use multiple high current contacts, imo. If you just have to use a medium size "cube" relay, at least get one with 4 contacts...

My Symphony No.1 amp has 500,000ufd @65vdc rail voltage. That and >2KVa of iron... so I use very large P&B relays. You can see a pic on my website if you hunt down in the amplifier section a bit...

As I said you can figure out the time thing after you put the supply together and have it running... just look at the pulse when you bypass it... that and the time it takes for the rails to come up. On my amp I used 10 seconds... more than sufficient to bring up the rails and let the amp stabilize (no speaker relays, so don't want thumps)

I'm thinking that a soft recovery type rectifier will be best in consideration of the current waveform that will be charging the cap banks...

How many caps to achieve this total capacitance??

_-_-bear

Bear I'm prepared ... tell me more ! how low can we go .......