Adding bass to an F5?

[BigE]

Now we're talkin'! What is the power rating of this amp?

That looks like a simple change to any existing PCB. Do the 546/446 BJT need heatsinking?

 

[Zen Mod]

Yes, my request seemed a little open ended as well, maybe if I narrow a little....The schematic I see shown around here for the cascoded front end indicates up to 45 volts, most of the threads I've looked at have been around 30 or just over. I'm just wondering if 45 volts is given as a max, and there is no advantage to going that high. Just wanted to know if there was a particular sweet spot in terms of rail voltage in this setup. Also wonder if durability of the jfets falls off at higher voltages. Similar for the bias on the dual output F-5. Threads show this as well, just wondering if there is a place where no more advantage is gained, or life span of the Mosfets is compromised?

Russellc

I wrote up to 45V as some practical limit , at least from this quick and dirt rewriting of origin . going further , voltage vise , is certainly possible , but I wasn't in the mood to advise it without real life confirmation .
anyway - if one is keen to increase voltage envelope , in that case is needed also to increase number of output pairs ; that means you also need to double input jfets or resort to some other way of increasing current capability of input stage ( followers or some other sort of intermediate VAS) .

life span of mosfets is primarily defined with their dissipation , at least in this case , where working voltage is much lesser than their breaking voltage

 

[Bksabath]

Tanks a big bunch as Big E said now we are talking.

Lets see bin the trafos and the caps as vrong voltages and job done

QuestionJust keeping thing simple) what about current supose 50 W top for mosfets dissipation = about 1 A bias

supose a 40 V swing as listening level on a 4 homs speaker = 5 Amps on each rail so 5 pairs need to dissipate about 500W of heath.

500 W heath ok can do that and BigE may just do even beter leaving the amps outside the door in winter.

BUT now thinking about the model 10 have 2 10 in WOOPHING Woofers

No Bass ? Sometink does not sound right. Big room? dogy caps on 18 dB cross over

And F6 caming up.

Me think before troving 400mU of caps and traffo out of window I am going to vaith a bit + by amp and balanced are still an option if al else fail.


Still muchas gracias for the pointers to Russellc and mighty Zen IMO worth considering.

What about sound Papa said that they (mosfet) sound better at hier current would taking bias down to 1 A change things much?

 

[BigE]

It's not that there is NO bass at all, just not acceptable. The bass is fine with a modified phase linear 400 -- thought it could be tighter. The bass was EXCELLENT using a BAT VK-250 w/bat pak. So, no, not a crossover fault.

I think AndrewT nails it: putting a 2 ohm load on the end of a single pr of transistors is daft.

You need more transistors for that, and at class A, a liquid cooling system to wick away the heat.

 

[Russellc]

that means you also need to double input jfets or resort to some other way of increasing current capability of input stage ( followers or some other sort of intermediate VAS) .

By "doubling the input jfets", you mean cascoding with the other jfets, not doubling the 170s and J74s right? or do you mean cascoding PLUS doubling up on the exisiting J fets?

Thanks,
Russellc

 

[BigE]

Perhaps just bigger ones?

 

[AndrewT]

Question:...what about current suppose 50 W top for mosfets dissipation = about 1 A bias

suppose a 40 V swing as listening level on a 4 ohms speaker = 5 Amps on each rail so 5 pairs need to dissipate about 500W of heath.

500 W heat ok,..........

The F5 is Push-Pull ClassA.
It remains in ClassA if the output current does not exceed twice the quiescent bias of the output stage.
1A bias of a 2pr output allows ~4Apk of ClassA output.
Similarly, a 3pr output allows ~6Apk of ClassA output.

40Vpk into 4r0 requires 5Apk. A 2pr output stage would transfer over to ClassAB on the highest current peaks.
A 3pr output stage would stay in ClassA for all non reactive loading upto maximum power.
The total output dissipation for a 3pr biased to 1A each will be [50V+50V] * [3*1A] = 300W.

You may find that due to very stiff PSU that you will be able to get 100W into 8r0 from much less than +-50Vdc, probably more like +-43Vdc when fully biased and rising to +-45 to 46Vdc when the bias is turned down to a ClassAB 100mA/device.
Expect approaching 180W of ClassAB into 4r0 from those +-43Vdc supply rails. With the bias set to 850mA/device and supply rails @ +-44Vdc would result in a dissipation of ~225W and still capable of 100W ClassA into 8r0.

 

[Zen Mod]

By "doubling the input jfets", you mean cascoding with the other jfets, not doubling the 170s and J74s right? or do you mean cascoding PLUS doubling up on the exisiting J fets?

Thanks,
Russellc

doubling jfets - I meant on paralleling them : 2 x 2SJ74 + 2x 2SK170

cascoding remains , as measure of voltage umbrella for Jfets , and also useful umbrella for nasty Mr. Miller's liquids



btw - for Mr. Sabath - you don't need bloody class A all up to heaven ..... at least not in this topology

take Krell KSA50 as example

 

[Bksabath]

Krell sorry did not like the recent (i think I400 or someting like that) 400 W integrated
sound wise like nails sratching black board and volume reading 250 sorry I am to Old for that and I am quite hapy with the volume at 11 (Just thinking about the load of components soldering joint and bends in the track make me cringe)

I did like a gold fronted one but she did not like the gold fronted bit.

What almost made the grade was litle Sugden but Much more hump and beter sound from F5
My F5 sound much beter as it was on take one I should say.
I just wants it beterer still

And well I know as tried that 12 Volts peak mesured at the speaker terminals where enoug to fill bass in the sabath tummy (Smalish room and woofer firing to the wall).

But still I would like to go to Heaven (altrought I don't think any of my mates be there +IMO partyes be quite boring) or to know how to get there so 45 V looks real good and tanks for that.

I dig the paralel J fets, again at risk of leting you exposed to the surmentioned do not use switch, what are the advantages as I tried with single ones and it was driwing 3 TOSH in parralel with no problems ( then decided that 6 amp bias where to much and wanted to keep traffo quiet as I was taking about 12 A for the stereo pair and any way 12 V / 4 Homs = 3 A so all it was doing was keeping my sinks a tad warm)
Would those be :
Less noise
More gain


Sems like bit of confusion about stereo pair and lets call it quadrants and me putting same numbers in a post got the do not use button well used.

Told you so litle voice in my head and why do I bother to post here second voice in my head.

So just to get childish and booring and with no better thing to do at the momemnt sorry for the folowing total waste of space.

my sinks can dissipate 250 W for each channel to stay at 50C whit ambient themperature of 20C 312 W as mentioned abowe is easy peasy with litle fan.
Papa sugest top 50W dissipation for TO247 (if I read corectley when it mention bigis ones) so 25V 2A so 500W of previous post meant for stereo pair
With 2 A Bias and 4 Homs speakers amplifier is class a up to 16 V (simple multiplication of 2 times the bias by the nominal impedence of the speaker (which me think all can do if you can not buy a calculator or wait for some one to state the obwious) singles mosfets 32 V for twin mosfets but 25 V rails realy let you get up to a very conservative 20 V before clipping so quite a waste of currants.
(Number made up of the top off my head and may be a total figment of my immagination.)

If you got a calculator and newer build anithing and or do not want to waste electricity and contribute to global worming thats fine.

If you actualy build the thing... Papa mention that mosfets sound beter at hig bias voltagge + the 4 homs nominal on the speaker may be Just a made up number and IMO in UK a bit more global warming be velcome so we could grow our own grapes.

So as Papa said I stick with bias as hi as your heat sink lets you

 

[Russellc]

something like this ;

it's possible also to double input Jfets , but that demands rearranging some values of drain resistors , same as feedback net

Yes, this is the schematic I spoke of. The mods required to double the 170s and j74s are a little over my head. The schematic you have is what I want to do. So rails 32-45 volts or so, biasing confuses me here, still .59? Seems like that would alter because of the dual pairs...I'll go reread Cvillers blog again on the dual output boards.

Russellc

 

[AndrewT]

If one requires more current from the input stage, there seem, to me, two methods of getting there.
1.) double up the bl grade jFETs to get ~ double the current rating but without any increase in voltage rating.
2.) use a v grade with cascode. This too could give double the current rating and the cascode limits the device dissipation.

If the voltage is not to be increased, do either of these options operate well? Or does one have an advantage?

If voltage is to be increased it seems the cascode route is the only choice and then the v grade seems to logically follow that.

 

[Zen Mod]

I wouldn't decrease their operating voltage under , say , 12V

off course - there is always issue of which grade of Jfets is available

there is already enough trouble finding proper BL , let alone V grade .

and ..... there is always xconductance .... better to have more , if you're going to feed more Miller capacitance with them

edit :

there is always second/third route - as using current mirror intermediate stage between input and output stages ...... but that will not be F5-like anymore

 

[Zen Mod]

"there is" ...... no less than 4 times

 

[Bksabath]

Would dubling the J fet increase gain of the F5?

 

[AndrewT]

No, the gain is set by the GNFB ratio.

 

[flg]

It would increase OLG and that would allow for more overall gain with the same amount of feedback.

 

[Bksabath]

Tanks FLG

Much apreciated.