Bruce Heran writes:
As an avid diyer and fan of the S-5 Electronics K-12 tube amplifiers kits (I have 2) I thought it would be a good idea to get a K-8LS. This tube amp kit is a logical replacement for the S-5 Electronics K-12 and K-502 tube amplifier kits. It is about the same power and is aimed at the same budget tube amp market. The basic amplifier circuit topology of the K-8LS is the same as its predecessors. With the exception of an added preamplifier section, it is not a major redesign, and now uses somewhat more readily available vacuum tubes. The one big plus is that it now comes with a much larger power transformer. The original K-12 ones ran rather warm.
I really want to like this product, but as you will see in the following comments, mine is awful. Sine wave and square wave tests were conducted at several frequencies. The measurements were made at 1 volt output into an 8 ohm non-inductive resistor. This is an easy load for any amplifier. Frequencies checked were from 20 Hz to 40 kHz.
25 Hz Sine Wave
There was a significant drop in output (sine wave) below 150 Hz and by 20 Hz there was not much left (about 9 dB down relative to 1000 Hz). The high frequencies were down around 3 dB at 18 kHz. Sine waves at 50 Hz showed clearly visible distortion. Square wave response at 1000 Hz was fair, but with some tilt. By 500 Hz the tilt was significant and below 150 Hz it was excessive. At frequencies below 50 Hz it no longer resembled a square wave at all. On the high side, rounding occurred from about 4 kHz up. By 10 kHz, square waves looked more like a sine wave! To be fair, there was no ringing or overshoot at any checked frequencies.
1 kHz Square Wave
Now you can see the paradox. I want to like this product, but it is awful! So rather than jump all over the manufacturer, let’s start a dialogue and see if others have the same or different results. I’d like to give the K-8LS a chance, so let’s hear from you about your experience with this kit.
Good listening,
Bruce Heran
Other DIY Tube Amplifiers by Bruce:
4 comments:
Hi,
I bought a K-8LS amp because I live in Europe and it can run on 230V. I was a bit disapointed (I read lot of good things about K-502) when I listened to it : in my opinion, there was no bass and the power was very low (maybe that's normal, I got 92dB/W/m speakers, 6ohms).
Another thing, the internal resistance is about 16ohm (I don't really know much about tube amps, but I think that's high..!), that makes a damping factor of 0.5 on 8ohms speakers.
Bruce, thank you for the notes and measurements about the K-8 tube amp kit. I am a novice and was looking at the K-8 as my first kit. I have read many good things about the K-12 tube amplifier kit, so I will try that one instead.
Hi Benjamin,
Bruce measured the amp to be -9dB at 20Hz relative to 1kHz. That explains the poor bass response.
With 92dB speakers, you only need 2 watts of power to produce 95dB which is plenty loud. What kind of speakers are they?
With tube amplifiers, a high output impedance (and low damping factor) is common.
Hi Gio,
My speakers are JBL XTi 100. Maybe it's because they are 6ohm that the sound level was low ? or maybe the 92dB/W are without the filter ? Or maybe the filter does just attenuate low frequencies to the medium and tweeter and the two bass speakers are taking all the power ? (but I don't think it's that...)
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