Circuit Overview

Let’s start with a power supply, which is always “make or brake” place for a class-B amps. In this unit we have a mono power supply. This means that one power transformer, rectifier and supply caps are shared between two channels. That’s a typical configuration for a mid-end product, certainly not what you would expect from a top of the line 8300. There are good technical reasons, why it’s inferior to dual-mono design and hopefully we will reveal them later in measurements section.

Power transformer seems to be adequately rated at 300W and sized for 2x80W at 8Ω outputs, but unfortunately, same can’t be said about the main caps. With just 7500 μF per rail/per channel it’s really underrated.  Rule of the thumb is at least 10000μF per rail/per channel if we are talking of any sort of hi-fi here. So, main caps should be increased to 18000 – 20000μF when doing a recap as a must. That’s as high as I would recommend, without upgrading diode rectifier bridge.

Stabilized power supply for ±30V is a joke too. With just zeners and 1W shunting resistors it looks… well, cheap! That what it is.  Again, very not top-of-the-line’ish. But hey! It does the job, as there are enough CMRR build in to powered circuits.

Nothing worth mentioning in pre-amp and tone control sections, except maybe usage of voltage-amp IC for pre-amp input.

By this time, I was really puzzled on what could make this amp worth it’s top of a range position at all. Don’t get me wrong, there are obvious technical reasons to go with output IC’s – better complementary matching, power dissipation and most importantly – price. It’s cheaper to put all circuit on one wafer substrate and call it a day. That’s what you’ll find in all modern mass consumer amps and it all started right here it seems. There is just one little problem though, if it goes puff – say goodbye to your amp as these are unabtonium now. Yes, you can find them on Ebay but don’t even bother as they all are 100% fakes and will not work. Don’t ask me how I know that.

The rest of the output circuit is text book example, with Qe1 and Qe2 forming LTP phase splitter and Qe3 class-A amp, loaded by Qe4 current source.

But wait ! It’s not all lost. After some digging I found more info about this  IC. It’s seems this is just a darlington output pack with build in over-current protection.

By this time, I was really puzzled on what could make this amp worth it’s top of a range position at all. Don’t get me wrong, there are obvious technical reasons to go with output IC’s – better complementary matching, power dissipation and most importantly – price. It’s cheaper to put all circuit on one wafer substrate and call it a day. That’s what you’ll find in all modern mass consumer amps and it all started right here it seems. There is just one little problem though, if it goes puff – say goodbye to your amp as these are unabtonium now. Yes, you can find them on Ebay but don’t even bother as they all are 100% fakes and will not work. Don’t ask me how I know that.

The rest of the output circuit is textbook example, with Qe1 and Qe2 forming LTP phase splitter and Qe3 class-A amp, loaded by Qe4 current source.

But wait ! It’s not all lost. After some digging I found more info about this  IC. It’s seems this is just a darlington output pack with built in over-current protection.

It uses the same darlington transistor packs for output, just rated at 80W.  This shouldn’t bring any worries, unless you are planning on driving some really heavy speakers. But then, you should probably be  looking to something else all together.

It also has much simpler and fully discrete signal path. Again, more resembling Model 600.

With total 7 coupling caps, it’s ahead of 8300 in being “Capacitor-less” too. All in all, it’s a much cleaner design and I like it better. It should really pay off sonic-wise, so if you’re hesitating between two – grab 7300.  Unless, of course,  you are one of those people listening with their eyes. Then 8300, with it’s mesmerizing big UV meters, might be more up your alley. Again, to each its own.

Measurements

After every restoration I do some basic measurements to make sure equipment is conforming to its factory specs. It sometimes  also reveals any defects that might be still present. This is not meant to be in-depth technical analysis using state-of-the-art measurement equipment. Here goes my usual disclaimer.

Measurements are just that – measurements. They are meaningless without context. I see people to often emphasizing on numbers and graphs and I strongly advise against this over-interpretation. They don’t tell us anything about how good or bad particular piece of gear will sound, unless it’s seriously flawed. Audio reproduction chain should always be analyzed as complex system, composed of many components which may and will interact with each other. Only when there is established certain correlation, between measured data and perceived sonic properties, numbers start to make sense, and can become a powerful tool for intelligent development.

Kenwood KA8300 Specifications

• Power amplifier section   80 watts per channel minimum, RMS at 8 ohms from 20 Hz to 20,000 Hz with no more than 0.1% total harmonic distortion;
• Both Channel Driven   85 + 85 watts 8 ohms at 1,000 Hz; 100 + 100 watts 4 ohms at 1,000 Hz;
• Dynamic Power Output   380 watts (4 ohms);
  
• Total Harmonic Distortion   0.1% at rated power into 8 ohms; 0.04% at 1 watt power into 8 ohms at 20 Hz ~ 20,000 Hz;
• Intermodulation Distortion (60 Hz. : 7 kHz. 4: 1)   0.1% at rated power into 8 ohms; 0.04% at 1 watt power into 8 ohms;
• Power Bandwidth   5 Hz to 40,000 Hz;
• Signal to Noise Ratio (lHF A)  100 dB (short circuited);
• Damping Factor  50 at 8 ohms;
• Input Sensitivity / Impedance  1.0V / 50 kohrns;
• Speaker Impedance  Accept 4 ohms to 16 ohms;

As always let’s begin with a baseline measurements to figure out our noise floor.

That’s really not bad, and close to a declared 100dB. Some room still left for improvements on Left channel for better grounding.

Total Harmonic Distortion

Let’s continue by checking THD numbers for a rated power output. I tend to use 6Ω resistor for a dummy load. It’s a happy average between 4Ω and 8Ω and it saves me time and effort to make measurements for both ranges.

All in specs and still far from declared <0.1% THD. It’s a really a good place to measure dynamic cross-talk too. One channel is driving  6Ω load, while other is measured. This is where 8300 falls on its knees and the price you have to pay for cheaper single power supply is obvious. Only 47dB of stereo separation ! That’s ok for vinyl, as you rarely have more than 30dB from cartridge at ideal conditions anyway, but if you’re planning on listening to hi-res digital material – this amp will be serious bottleneck. It’s not a disaster, as we all know how good vinyl can sound with even worse cross-talk, but something to watch out for.

Here is a more “real life” measurement of 1W output to 6Ω. That’s a typical background listening level, with at least 90 dB sensitivity speakers.

Hmm… maybe they missed one zero when writing specs, as this is ten times better then declared 0.04%. Nice overall spectrum, with all higher harmonics below -110dB. Also notice how well channel amplitudes are matched! All the restoration efforts have paid off it seems.

Inter-modulation Distortion

Here we are looking for sharpness off that 7khz tone. The less wide “skirt”, composed of 60Hz inter-modulation products it has – the better. Again – all in specs, with a lot of headroom.

Up until now I was quite satisfied with somewhat conservative and well written specifications. And we come to the line were it says “Power Bandwidth  – 5 Hz to 40,000 Hz”.  Ouch…  The general definition of  power bandwidth is “the frequency range for which half the rated power of an audio amplifier is available at rated distortion”. That sounds simple, we just check at what frequency extremes, distortion becomes higher than rated. But it begs a question – what is a half of rated power ? Should we use minimum rated 40W or nominal 42,5W at 8Ω ? Or maybe 50W at 4Ω ? What distortion figure we should use ? THD, THD+N or maybe even IMD ? You see, it doesn’t make any sense, because it’s incomplete. It should be written as “Power Bandwidth  – 5 Hz to 40,000 Hz at 0,1% THD with 40W power in to 8Ohm load”. But the way it is now – this spec is pointless and can’t be checked.

Frequency Response

Even though it’s not in specifications, this is one of the most important specs for amplifier. I like to measure it at different power levels and see were gain compression sets in. It gives me clear indication on power supply robustness. This time however, for some reason I saved only 1W graph (oops). So it must suffice.

No problems here and we can specify it as “-0.2, -0.4 dB, 20Hz to 20khz” or “-3dB, 10Hz – 55kHz” at “1W to 6Ω” or “19dB below rated power”. No matter how you put it – it’s the same thing really. You see, how easy it is to manipulate those specs ? But surely someone will notice that 10Hz – 55kHz is much better ! I’m starting to see my bright future in sales department now, so much possibilities, so little time…

The Bottom Line

All in all, this is a really nice amp. I wouldn’t put it at the top of the line, because of previously mentioned reasons, but it does the job. Overall build quality is really decent, and these  mishaps with broken traces and budge wires could be isolated to this one unit. General serviceability is good, but is somewhat limited by the availability of Output Power Modules. It has enough power and slam for all except those seriously heavy speakers and should drive everything that you through at it. I will retract my self from those murky sound descriptions and just say that it sounds easy and laid back. It lacks this sparkle and micro details, even after full recap to Nichicon audio grade caps, but on the positive side – it’s easy on your ears. I had this amp playing at my workshop for whole two days straight as a background music source and haven’t felt any fatigue. So if you find one for a reasonable price – grab it. Otherwise, it has an average bang-per-buck ratio.