Krell KSA 50 PCB

[Mark A. Gulbrandsen]

Yes, Its a good idea to post data here so stupid people like me won't burn their house down . Math is not my forte.....

Mark

 

[NUTTTR]

Thanks for the Calcs K-amps! (ignore my email!)

I did know i'd have to get rid of stupid amounts of heat... I am going to have 800va toroids, so the rails will hold fine...
My current setup stays quite cold... another 100w of heat (as i see it) should be absolutely no problem... However i have a funny feeling (i tested it) my current toroid i'm using isn't suited
It does "28.7v @ 2.8amp" +- obviously, so it has a 160va output
Which is possibly why it hits 55deg (C) sitting there after 3 hours!
If i put the bias up at all the voltage just drops, so i've almost hit the point of no return with this toroid...
Speaking of which Mark is perfectly correct, i've noticed how poorly recorded CD's sound horrible! But good ones do sing...
I've found that the amp definately sounds BETTER (not just me a lot of people) than my Peavey PV8.5... which overall is a pretty good sounding amp.... It is WORLDS ahead of my other amp i modded.... Also sounds a LOT better than driving straight out of my Harmon Kardon surround sound amp (which isn't a base model!)....
So i'm very happy so far!
Aaron

 

[NUTTTR]

For 4 ohm load and 150 watt class-A, Aaron needs to crank up the bias to 0.867 amps per device. At that level the amp will be dissipating over 300 watts at idle plus 150 watts at max power i.e. 450 watts total? As opposed to his current dissipation of 60-100 watts. Also the PSU will need to keep that rails at a minimum of +/-35vdc or above to get 150 watts class-A into 4 ohms.

Lastly assuming his rails will drop, I have used a figure of +/-36vdc for this calc: (adding a volt or so for losses etc).

Class-A output Calculation: Krell KSA-50 Mk. II 150watts into 4 ohms

Inputs
Number of output devices: 5 Pairs
Voltage rails (per rail): 36 volts
Emitter resistance (per device): 0.68 ohms
Bias voltage per Emmiter resistor: 589 mv
Idle bias per device 866.2 mA
Speaker ohms 4 ohms

Results
Idle bias per device: 0.866 Amps
Total Amplifier bias (per rail)** 4.33 Amps
Total Amplifier bias (both rails) 8.66 Amps
Total Dissipation (per channel) 311.8 Watts
Dissipation per device pair at idle 31.2 watts
Class-A output: Peak 300.1 Watts peak
Class-A output: RMS 150.1 Watts RMS
Efficiency 48.12 %

Assuming he can sink that much heat and keep the thing alive, he will have a KSA-75.

Wouldn't that make it a KSA-300? The KSA-50 does 50 w @ 8 ohm - 100w @ 4 ohm (but 25w class A only?)... so i'm going up a LOT from that.... so i'd have something like a KSA-300?

 

[K-amps]

Aaron,

Yes at 4 ohms, its the same as a KSA-300 but only as far as class-A delivered is concerned.

A true KSA-300 would require the following stats:

Class-A output Calculation: Krell KSA-300 simulation @ 4 ohms

Inputs
Number of output devices: 15 Pairs
Voltage rails (per rail): 80 volts
Emitter resistance (per device): 1 ohms
Bias voltage per Emmiter resistor: 289 mv
Idle bias per device 289.0 mA
Speaker ohms 4 ohms

Results
Idle bias per device: 0.289 Amps
Total Amplifier bias (per rail)** 4.34 Amps
Total Amplifier bias (both rails) 8.67 Amps
Total Dissipation (per channel) 693.6 Watts
Dissipation per device pair at idle 23.1 watts
Class-A output: Peak 300.7 Watts peak
Class-A output: RMS 150.3 Watts RMS
Efficiency 21.68 %



The key stat here is the idle dissipation of about 700 watts and 30 OP devices needed. (The KSA-250 has 24 OP devices)

 

[NUTTTR]

You mentioned that it does half the power @ 4 ohm than it does @ 8 with Class A, so running on my previous specs, what am i looking at for Class A into 8 ohms (from my post above, so finalised specs)...... Just so i can get an idea.... It should do more like 300w into class A? As it does 150 @ 4 ohm, you said before 4 ohm rating is half of 8? Correct me if i'm wrong, i just got up!
Thanks

 

[NUTTTR]

Thinking about my power and drain, etc....
I'm currently running my sinks @ 15deg above ambient.... which is not bad for 100w dissapation... I figure then about 45deg above ambient (quite warm....) That i'd be able to dissapate 300w? Is it a constant scale like that? Just interested

Aaron

 

[NUTTTR]

first i must...

Apologise for wasting thread space with so much posting!!
Second for putting up incorrect values
I am running 250mV (yes, millivolts) not milliamps per device as i previously stated *slaps head*.... This makes more sense... i figure i'm not far off 150w dissapation with approx 75w @ 8ohm class A and 36w or so @ 4 ohm? Approx obviously...
I was wondering how this was working So i pretty much have to at least double my bias plus a little.... which means that i shouldn't have to be hotter than 65deg on the heatsink (40 above ambient) approx... That's if my calcs are correct...
Sorry guys
Thanks for the help K-amps!

Aaron

 

[K-amps]

You mentioned that it does half the power @ 4 ohm than it does @ 8 with Class A, so running on my previous specs, what am i looking at for Class A into 8 ohms (from my post above, so finalised specs)...... Just so i can get an idea.... It should do more like 300w into class A? As it does 150 @ 4 ohm, you said before 4 ohm rating is half of 8? Correct me if i'm wrong, i just got up!
Thanks

Aaron,

Not a problem,

Yes you are right but only as long as your rails can provide 300 watts RMS (i.e. 80volt rails) so While it works the other way i.e. 4 ohms half of 8 ohms, it does not work in reverse i.e. 150w going to 300w. (unless the rails are high enough).

As opposed to regular output, the class-A drive depends on a constant current. As the load halves, so does the class-A drive proportionally because it does not increase with lower impedances. The relationship is actually 1/4th of power, i.e. for a class-A 50 watt drive, if you go to 4 ohms, class-A is 25 and class-AB is 100 (assuming your 8 ohm clipping was 50 watts)

Power on the other hand works by doubling the current needed so 50 watts into 8 ohms is 20 volts and 2.5 amps. Lets say your amp is biased to 2.5 amps class A constant. Now you go to 4 ohms, the voltage remains 20, but the amps required to drive 4 ohms is now 5 while your OP stage is biased to only 50% of the current required, therefore the output in class- will also be 50% or 25 watts class-A.

A simple way of calculating class-A power is by the formula IXIXR

Current squared multiplied by load, (In this case 8 ohm class-A is 50 w. And as 50 w is 20 volts and 2.5 amps,

so for 8 ohms 2.5 x 2.5 x 8 = 50watts and for 4 ohms, 2.5 x 2.5 x4 = 25watts. This current is not your bias current but the current required to drive a given load.


I am sure others can explain this better, because I know in the measurements, Peak currents are calculated for class-A.. too sleepy to reverse engineer it for you right now. ;-)


Based on your latest numbers, here's what you have for 8 ohms

Class-A output Calculation: Krell KSA-50 Mk. II

Inputs
Number of output devices: 5 Pairs
Voltage rails (per rail): 40 volts
Emitter resistance (per device): 0.68 ohms
Bias voltage per Emmiter resistor: 250 mv
Idle bias per device 367.6 mA
Speaker ohms 8 ohms

Results
Idle bias per device: 0.368 Amps
Total Amplifier bias (per rail)** 1.84 Amps
Total Amplifier bias (both rails) 3.68 Amps
Total Dissipation (per channel) 147.1 Watts
Dissipation per device pair at idle 14.7 watts
Class-A output: Peak 108.1 Watts peak
Class-A output: RMS 54.1 Watts RMS
Efficiency 36.76 %
Max Class-AB RMS (1 kVA Toroid p/CH 5% reg) 90.99 Watts RMS


and for 4 ohms:

Class-A output Calculation: Krell KSA-50 Mk. II

Inputs
Number of output devices: 5 Pairs
Voltage rails (per rail): 40 volts
Emitter resistance (per device): 0.68 ohms
Bias voltage per Emmiter resistor: 250 mv
Idle bias per device 367.6 mA
Speaker ohms 4 ohms

Results
Idle bias per device: 0.368 Amps
Total Amplifier bias (per rail)** 1.84 Amps
Total Amplifier bias (both rails) 3.68 Amps
Total Dissipation (per channel) 147.1 Watts
Dissipation per device pair at idle 14.7 watts
Class-A output: Peak 54.1 Watts peak
Class-A output: RMS 27.0 Watts RMS
Efficiency 18.38 %
Max Class-AB RMS (1 kVA Toroid p/CH 5% reg) 181.98 Watts RMS

 

[NUTTTR]

Thanks for the calcs!
Makes more sense now...
I'm "finishing" one channel off at the moment... no case or final toroid, etc yet, but hey, at least i have it working! Now for the other 2...
27wrms @ class A does go fairly loud it seems too, it has no problems filling the room with music/noise/whatever!
Aaron

 

[Mark A. Gulbrandsen]

Yes, those calcs do really help to see what one is up to. Mine was up and running just fine all day then I shut er down about 5 PM. Went to turn it back on and blew the bridge, amp channels are fine though. I back a couple hours later and not thinking just threw the switch and there is no current limiting yet. I shoulda run the variac up as I was doing. Will install a new bridge tommrrow and a CL-60 inrush limiter so this doesn't happen again!

Mark

 

[NUTTTR]

Ouch... blown bridge? Hmmm...... What's the best way to avoid it? Other than CL-60?

Here's my single channel completed so far... Next step is to add RCA out's on my amp (it's easy)...

Here's the setup:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

 

[Mark A. Gulbrandsen]

Aaron,

Looks really nice! Are those sinks cast or extruded? Yea, a CL-60 is probably the best route to go for now and uf needed it can be shorted out by a time delay relay. Krell used a 5 ohm 50 watt time delayed relay in the KSA-100... could be implemented in the 50 as well.

Mark

 

[K-amps]

Nice hefty sink there Aaron.

For a temporary manual softstart, a nice 500 watt lamp in series with the toroid primaries will work well too, then as a second step, short it.

CL-60 is 10 ohms cold and will drop a few volts (and rails). At max current draw it drops to 0.18 ohms, but be sure not to exceed the 5 amp rating for it. Probably not a problem for you guys with 240vac mains.

 

[NUTTTR]

I believe the sinks are cast... No way to extrude these... They work very well and are very very well made (with small fin spacing, etc) for cast sinks! They should be up to the task though, definately... Hopefully
K-amps - thats why i wanted to avoid the CL-60.... i guess a time delay circuit could be used.... I still have to have each toroid turn on individually though i think - unless you know of something that can "soft start" 2400va worth of toroids (800va x 3)...? Any ideas?

Thanks
Aaron
P.S. Now to make the other two

 

[K-amps]

Aaron,

I edited my post after your read it, the CL-60 should be fine once warmed up but should not be exceeded for it's Imax. But it would still drop a few volts under normal operation, if you want max power from your toroids, then just short it with a relay.

 

[Upupa Epops]

To Aaron : This size of heatsink is too small for declared dissipation ( without fan ). Longtime temperature rise ( past minimal four hours ) must not be over 60 °C ( safety norm ).

 

[Stuart Easson]

Cl-60's

A cl-60 has a hot resistance that is less than an ohm, about 1/5ohm at 6 amps. Using one in the primary isn't really going to cost you much voltage in the secondary, if it were even one volt I'd be surprised...if total capacity were a problem, I'd use one of the cl-60's for each transformer.

The fact that this is a class A amp makes it a perfect candidate, the power is always being drawn so the thermistor stays hot...

That Nelson Pass, he knows a thing or two...

Stuart

 

[NUTTTR]

To Aaron : This size of heatsink is too small for declared dissipation ( without fan ). Longtime temperature rise ( past minimal four hours ) must not be over 60 °C ( safety norm ).

I will be using fans... potentially 3 per heatsink as they fit nicely and cover the whole area....
It does (by my calcs) 15deg ABOVE ambient for 147w dissapation.... So by guess work (anyone?) i would have a 30deg above ambient rise for almost 300w dissapation... Is that right? Even if it is 40deg rise, i can handle having sinks that are 65deg as they will be semi-enclosed... On a side note, i need to add the bias switching for hot days... being that it hits 40+ (deg C) ambient on the odd days of the year (inside and out, no a/c!) a 40deg rise makes the sinks 80+ deg potentially, which is too hot... IMHO!
Thanks
Aaron
P.S. I guess i could use a CL-60 (3) and just short them out with a relay after a few secs... that's a pretty simple circuit and will mean i only need 1 lower power switch for all 3 toroids (i have a 12v circuit planned with separate toroid, etc, for fans).... so should be ok

 

[K-amps]

Aaron,

45c above ambient on a 40c day is 85c.... thats too much for having a safe reliable amp. Hopefully your fans will keep it below 65c in total.

 

[Upupa Epops]

To Aaron : Probably it will be OK with this three fans, but look at Fischer or PADA ENGINEERING catalogue and look there at examples and graphs. At pic is my soft start delay circuit ( designed for two traphos ).