Krell KSA 100mkII Clone

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AndrewT

R.I.P.
Joined 2004
to really get the most out of this amp, it's necessary to regulate LTP/pre-driver.
i'm thinking of all discrete parts
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I agree.
What part/s get the brute force cap input supply and what gets the regulated?
How effective is the dual supply compared to stock?
What current works best? (not what current does the section draw?)
Is there any benefit in rough regulating the driver pairs? to separate from the gross ripple on the main rails?
How much capacitance is beneficial for the main supply?
How should the caps be arranged? single pair per channel or twin pairs or multiple pairs?
Worth adding snubbers? or just some film caps? how big?
Should the rectifiers be RC snubbered or just C or not at all?
or high speed rectifiers? or soft recovery?
Do RF filters at the primary side of the transformer kill the transient audio at the output?
 
P

PWatts

Member
Joined 2003
AndrewT raises a number of important questions. As there are a plethora of options that can be simulated, measured and evaluated it's unlikely that all will be adressed.

However, a good starting point would be to just separate the front-end from the drivers, and letting the drivers run from the main supply. Then to run the drivers and front-end together and the main supply on its own, and lastly all three sections separately.

The boards do not provide for separate power for just the LTP, only for everything upto the drivers. To regulate the LTP separately, the 1k resistor should be removed and power applied there, or remove the 39V zener as well and use a properly regulated supply.

It should be mentioned that active regulation will not necessarily guarantee better performance, especially at higher current levels. The drivers in particular should be happy with a separate but unregulated supply. If regulation is used, it must be designed for high enough bandwidth. Snubbers should only be necessary if the supply is located far from the amp, but they won't do harm either.

However, due to the low power levels involved it may be worth investigating on-IC switching regulators that requires minimal discrete parts.
 
K

K-amps

Member
Joined 2003
Re: current...

Sorry to be so late in jumping into this...

Mark A. Gulbrandsen said:


K-Amps got them all!

.....

Mark
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Yeah!! But they were like 6 or 8 of them, not like 20-30 :cannotbe:

Stuart Easson said:
...is a strange thing in an output stage...

According to most everything I've read, and so far tried, the more transistors there are in the output stage, the less current is needed from the drivers, all else being equal...ie using 3 pairs of the plastic power transistors will stress the drivers less than using 2 pairs of the to3s...

Stuart
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I know you have been a proponent of this concept Stuart and I tend to agree. However at some point in a class-A amplifier where the driver needs to load up (bias) the OP device, the class-A loading will require some DC current to be fed from the driver. More the OP devices you have, more the loading, so from this aspect, we have an inverse relationship of number of OP devices to drivers needed as far as driving the signal goes. If you plot both current requirements for different number of OP devices, I am guessing the curves would intersect at some point (break-even point) below which you are better off using more OP devices for one driver, and above which you need to add another driver.

I guess this variable is less of an issue in class-AB amps...
 
Stuart Easson

Stuart Easson

Member
Joined 2002
Re: Re: current...

K-amps said:
"...However at some point in a class-A amplifier where the driver needs to load up (bias) the OP device, the class-A loading will require some DC current to be fed from the driver. More the OP devices you have, more the loading, so from this aspect, we have an inverse relationship of number of OP devices to drivers needed as far as driving the signal goes. If you plot both current requirements for different number of OP devices, I am guessing the curves would intersect at some point (break-even point) below which you are better off using more OP devices for one driver, and above which you need to add another driver..."

If all else is equal, more output devices will each need less bias to achieve their fraction of the standing current, ie if 2 pairs are used with 2 amps total idle current they get 1A each while 3 pairs will have 660mA each, and will actually use LESS total current from the drivers, this is easily demonstrable. If you plot the curves as you suggest, you'll find you will never need more drivers, assuming everything else is equal.

Don't misunderstand me, I'm not saying there aren't scenarios where more drivers are necessary, just that arbitrarily adding output transistors while keeping everything else the same isn't one of them.

I've done a little research on this now, (as did Doug Self) and it is clear the appropriate 'size' of the driver stage is determined by a fistful of factors, including output stage Hfe and the load current. More transistors (within reason) have a higher HFe, so I am confident that more output transistors driving the same load will NEVER need more drivers...OTOH different load, all bets are off, higher rails another story...but since we are all making the 'same' amp, +/- 20%...and Dan wasn't a penny pinching designer...

While a lot of the time I am a big advocate of overbuilding for it's own sake, sometimes you just have to go with reasoning...

Stuart
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K

K-amps

Member
Joined 2003
Re: Re: Re: current...

Stuart Easson said:
K-amps said:
...

While a lot of the time I am a big advocate of overbuilding for it's own sake, sometimes you just have to go with reasoning...

Stuart
Click to expand...

I have experienced BJT amps with one pair outputs (& drivers) sounding better in the HF and mids, but having flabby bass.

Adding more outputs gave me better bass/midbass... for the same power supply. So yes there might be situations where going optimal may be a good thing, point in case, I love the sonics of the 669/649. not so the older 15030/31 (haven't tried the 34/35 though).

As for the biasing requirements, the example works out well till you get to a minimal bias required to sustain linear operation per OP device i.e. you don't want to go below (pick a number?) 40mA perhaps. Granted these low numbers will happen only if you use an unGodly number of OP devices or a weaker driver.

Glad to have you back in the fray.


:)
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apassgear

apassgear

Member
Joined 2002
pollypocket said:
Hi

I realize the Wiki for the project is closed but thought it would be worth posting...I would like to participate in the project...anyone have a set of boards they are willing to sell?

Thanks for the consideration.

Paul
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If thats ok with Mark I can pass my boards to you. Presently I have my hands full with other things and audio projects, so let me know.

:xmastree: :xmastree: :xmastree:
 
B

bobgroger

Member
Joined 2005
Output heat sinks

Is anyone seriously interested in a GB for heat sinks? I am planning on 4 monoblocks (8 heatsinks). We should be able to come up with a decent order given the board interest. Conrads seem to be best bang for buck, but I am open for suggestions. Saw here somewhere estimate for .09C\W? .18C\W X2 per channel? Does this sound right, or was that for 2 channels?

Bob G.
 
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