ASL Wave-8: More Mods

I hope there are tube fans reading the blog. If not, do not worry. This is the only tube device I have :-). You can read part 1 here: [link]

In reality, I am not really that much of a fan of tubes but this amp has served me well in my secondary system and consistent with the theme here (“lot of value, little money”) the ASL Wave-8 is great value in audio (unfortunately no longer manufactured). But the mods are a lot of fun to do and improves this already good amp up “a notch or two”.

Even compared with the current, higher power version of this amp (the Wave-25), the Wave-8 still has “sweeter mids” according to the designer. The bigger brother of course has more power and extends further in its frequency response, but with mildly efficient speakers (the KEF Q15.2 I use are 91 dB), this is the amp to have.

Two primary mods for the ASL Wave-8 were described in the previous post, along with rearranging the ground connections:

• Better regulation of the power supply
• Better coupling capacitors

The PS mod is the most elaborate. Based on simulation, the ripple is reduced from 2V to apporx 20 mV, an improvement of 100X. A we know,  the quality of the power supply is perhaps the most important factor for good sound.

Based on simulations (Duncan’s PS Designer II -Scott sent me the simulation files) we can compare the output of the two power supplies:

Original PS: ripple ~ 2.4 volts

Modded PS: ripple ~ 27 millivolts

HUMMMMM

Hum has been a problem with this Amp since day one.  Many people have reported hearing hum and mine are not an exception. However it is low level and only audible if you put your ear close to the speaker. After the mods, which included rearranging the ground connection, I can report that the hum is much, much reduced. With a RS sound meter and only after putting the meter inside the port, I can measure 57 dB on the modded amp and 66 dB on the original, un-modded amp. If I put the meter right next to speaker surface (or anywhere near), the meter (which has a sensitivity of  50 dB) does not measure anything on the modded amp.

This is a reduction of 9 dB which translate to about 80% reduction in sound power. Very nice. According to some, it may not be possible to totally remove the hum in this design:

My experience has been that getting all the hum out of a push-pull amp can be difficult. When one tube of the push-pull pair is drawing more current than the other tube there will be a hum. I can hear this hum from about 1 foot from the speaker. I usually set the DC balance to try to minimize it. The Wave8 does not have a DC balance, so all you can do is to swap output tubes around such that each tube of the push-pull pair is producing equal amounts power. [link]

But more can be done…

There is an excellent tutorial on heater wiring. Essentially: heater wiring – the Good the Bad and the Ugly

• Tight twisting of the wires -to cancel the magnetic field (shielding reduces electric field but no magnetic field)
• Move wires away from circuitry

I redid both the AC wiring and the heater wiring. The AC wire goes from the back of the amp to the front where the switch is located. Originally I used stranded zip-wire with only 6 turns along the length of the wire. This time I utilized18 gauge solid core and twisted it very tight (at least 6 turns per inch). Solid core is better because it keeps its shape. [Tip: you can buy thermostat wire which has two 18- gauge wires for 24 cents/foot at the Home Depot]. Same for the heater wire. The first photo is the mains AC switch, the second photo is the filament/heater wire (which is also AC)

I measure the sound level as before and now I read about 56 dB. We have an improvement of about 1 dB. The original wiring was pretty good. In any case, a reduction of 90 % of hum (10 db) is already pretty good.

HEATER WIRING: we can do more

Here is the original heater wiring:

Here is a diagram and instructions from http://www.el34world.com/charts/commonhookups.htm:

If you do not have a heater center tap on your power transformer, you must run two 100 ohm 1/2 watt resistors to ground to create an artificial center tap. If you do not have a center tap, you will get 120 cycle hum. Each 100 ohm resistor is soldered to one of the heater wires. The other ends of the 100 ohm resistors are twisted together and then soldered to ground.

The heater wires are usually run up in the air, above the tube sockets in a twisted pair. Twisting the heater wires cancels hum. This is why phone line wires are run in twisted pairs. The twisted pair wires drop down and get soldered to the tube socket pins. The twisted pair continues down the line to every tube in the chain.

Keeping the wires in phase helps with hum sometimes. In other words, pin 7 on one power tube goes to pin 7 on the next power tube. Pin 9 on a pre amp tube goes to pin 9 on the next pre amp tube. EL84 power tubes heater connections are pins 4 and 5. Most other 8 pin power tubes use pins 2 and 7.

Also, there are some discussions on heater wiring with PCB (as opposed to point to point)

There is NO good way to get low hum with PCB heater wiring.

Live with it, or fix it. First verify that 6VAC flows in PCB “wires”, simple circuit tracing. If so, cut the heater lines AT the socket pins, leaving only the pin solder-blob. Get some hookup wire and run heater power OFF the PCB, in twisted pair cable AWAY from all audio points (basically everything except the heaters). Study some of the excellently-wired amps posted here. [link]

The original heater wires are connected “out of phase” (don’t know why. Perhaps to keep the wires as short as possible). Also, the artificial center tap is done at the end of the wire run with the ground connection at the end of the ground line. This means the current travels the whole length of the ground trace to the star ground. Will try implementing the artificial center tap near the power ground and also connect the wires in-phase.

Notice that the artificial center tap in the PC board has been removed (R18  and R19) and the heater wire  has been shortened, more twists and connected “in-phase”:

The artificial center tap is now implemented towards the back of the amp near the transformer, with direct ground wire to chassis:

Results: I measure 55 dB, an improvement of 1 dB.

ELEVEN dB REDUCTION IN HUM

All the AC and heater wiring modification resulted in about 2 dB reduction of hum. Not too bad. The total reduction combined with the previous mods is in the order of 11 dB. Remember that this measurement is sticking the meter into the port of the speaker. If we measure the sound pressure at the speaker, we measure 57 dB with the un-modded amp and unmeasurable for the modded amp (which should be 46 dB, 11 db lower).

How loud is that? I can start hearing the hum at about arm length (~32 inches) from the speaker with the unmodded amp and at about 7 inches with the speaker with the modded amp. This is with good ears and in the quietness of midnight… In the morning hours, the distances are about 24 inches and 3 inches respectively.