Class-A Amplifiers and the Krell KSA-150

Many readers will be aware of how much I love Krell audio gear and their class-A designs. The Krell KSA-150 Class A Power Amplifier is a great example of one of these hot, power-hungry designs. In this article, I discuss the basics of class-A amplifiers, with service details and images of the gorgeous KSA-150!

You may have already watched my restoration video, where I completely refurbish one of these beasts.

The video has generated a lot of interest and I receive many queries about this restoration. Powerful class-A amplifiers like the Krell KSA-150 need regular inspection to ensure they don’t go up in smoke. This is largely because of the prodigious heat they generate, a by-product of the class-A amplifier topology.

What is ‘Class-A’?

Class-A configuration means that the output devices are biased or “on” and working in the linear part of their transfer curve 100% of the time, whether there is a signal present or not. This is achieved by feeding current into the base of the transistors, biasing them so they never switch off. The quiescent current is defined as the current drawn by the device when zero signal voltage is present.

It helps to think of duty-cycle or the number of degrees of phase of the music signal through which the transistors conduct. In a class B amplifier, the transistors conduct through 180 degrees of signal, or half of the waveform. With class-A amplifiers, the transistors conduct through 360 degrees of signal, ie continuously. In a pure class-A amplifier, the maximum peak load current should be conducted by all transistors at all times. Because a single transistor conducts through 360 degrees of music signal, one output device can reproduce the entire signal. This gives the simplest amplifier configuration of one output device per channel – singled ended class-A.

In many modern so-called class-A amplifiers, the output devices are arranged in push-pull pairs and set to conduct around 1/2 the peak load current at all times. This is not really class-A but actually a high-bias class-AB configuration. Class-AB operation brings power savings, but let purity and more parts.

Advantages and Disadvantages of Class-A

All this might sound wasteful, but the benefit of class-A operation is that the signal swings from positive, through zero to negative with no crossover distortion at the crossover point. The transistors are always conducting and operating in their linear zone. This is not energy-efficient but offers reduced distortion. In fact, my KSA-150 constantly draws around 300 watts and dissipates that as heat into my listening room. This has some advantages on cool nights, but resistive electrical heating is not a cost-effective way to heat a room…

Naturally, transistors drawing several amps between them at all times produce lots of heat and heat kills capacitors. So capacitors tend to fail in class-A amplifiers and the KSA-150 is no exception. Krell also modified the protection relay driver circuitry over the years to reduce heat-related failure of parts on the protection PCB. When I service a KSA-150, I assess the state of these parts and upgrade them where necessary.

The Krell KSA-150

This particular customer KSA-150 was in excellent condition, my customer had just purchased it and wisely decided that an inspection was in order. My customer was sure that the amp was a locally supplied unit but the backplate specified 110VAC meaning it was not. The safest option was to open the chassis and inspect the line voltage configuration. In fact, this amp was set for 240V, which is as close as we can get to the 250VAC we see here in Western Australia.

The big Krells do suffer failures and they seem to be fairly predictable. Common failure points occur on the soft-start board. There is a design flaw, with the inrush limiting resistors being under-rated and mounted too close to the PC board. They are also mounted in a limited space right next to some electrolytic capacitors and a protection relay.

These resistors run very hot, scorching the board and cooking everything located nearby. I had to change many parts on that board to restore normal function. Fortunately, these parts are easy to get and have standard values. The KSA-150 produces a lot of heat and on every other board, there are capacitors that need to be replaced if they haven’t been already.


I recommend a comprehensive overhaul of any KSA-150 or KSA-250 every 10 years or so to avoid costly damage from dying parts. I’ve seen first-hand the damage that can result when parts inside a big Krell go bad and it’s not pretty!

The KSA-150 will deliver 300 watts per channel into 4 ohms, 600 watts per channel into 2 ohms and 1,200 watts per channel into 1 ohm. The KSA-150 is fully capable of driving impedances below 1 ohm. Consider this thing able to weld speaker cables, so despite the excellent protection circuitry, BE CAREFUL!!

As always, you can visit the HiFi Engine for an owner’s manual and other details, or get the manual directly from Krell!

class-A amplifiers
Battleship grey, the big Krell really is big, and heavy!
img 0263
The insides are a joy to look at and work on. There is a reason why these were $AUD10,000 – in 1991!!!
img 0267
The orange ERO caps are original, the brown Nichicon parts are new though, so someone has been in here and worked on the beast before.
img 0268
This is the soft-start board, designed to prevent blown fuses in your home when you turn the amp on. This is also where most of the problems start. All capacitors have been replaced on this board. The big white components are the power resistors, as is the brown part on the back of the board. A lot of heat is generated by these parts.
img 0265
This image doesn’t really convey the sheer size and weight of the mains transformer. It must be 2 or 3 kVA, which makes sense when you consider that the amplifier can drive a 1 ohm load and deliver over 1 kW or power continuously, per channel!
img 0272
Here we see a few more new capacitors, and a handful of original ERO parts.

img 0270

img 0266
Note the 20 amp circuit breaker – no fuses in the Krell to potentially reduce sound quality, just a 20 amp circuit breaker and high-speed protection circuitry!
img 0271
Close-up of the protection board and the regional voltage selector switches. Get this wrong and the whole thing will go up in a puff of magic smoke – NOT something you want.

10 thoughts on “Class-A Amplifiers and the Krell KSA-150”

  1. What are the voltage settings on the soft start board for 250/240 volts- i have three switches on my board on my Krell KSA-150…..It came from Spain.

    1. Hi – not sure, I wuld have to check the service manual and I don’t have access to that right now. Will see if I can find that when I get a chance.

  2. Hi! can i ask where possibly the problem when there is a DC voltage at the output about 45VDC and cause the unit to protect.

    1. Hi Ranz, if only it were as simple as that! These are complex amps, I’d need the unit on my test bench and several hours to test and diagnose a fault with an amplifier like the KSA-150. It helps greatly to have the service manual. Speak to Patrick at Krell about getting that if you don’t already. You might want to pull and test all output devices – a Herculean task but something you’ll need to do I think. Regards, Mike.

  3. Hi Mike, very nice pages abut the Krells (almost sound like London gangland hoodlums…). One tiny question, if I was to buy a Krell amp from the US and bring it to Sweden would the “only” change I had to make is to set voltage internally to 240 Volts and fuse to 6 amp slow. What about 60 to 50 Hz, is that of no significance once voltage and fuse is changed/set???

    1. Hi Martin, thanks for your feedback and question. Some Krell products need only line voltage selection, but others need both local voltage and line frequency to be correctly set, and in some cases like KSA100S, 200S etc, there is software which checks these conditions and prevents power on unless tricked. This can create real problems as you might imagine!

Thanks for visiting, feel free to leave a comment. Advice is accessible via our contact page.

%d bloggers like this: