Apr 26 2011
Introductory Aside: Class-A inefficiency?
There are many who say that Class-A is a gross waste of energy – that over 75% of the energy is “wasted” as heat. What about in winter in a closed home with the central heating running? Is the amp then a 75% efficient heater (with 25% “wasted” on music)? Or is it 100% efficient in some way?
Having lost the earlier “Silicon Chip” Class-A tweeter amp and the ESP AB midrange amp to my brother’s stereo system, it was time to build a pair to keep. The midrange amp sounded so good that I decided to base this new “tweeter amp” on the P3A as well, but to do it a bit differently. The complementary “lateral MOSFET” (P101) midrange amplifier is on another page. It worked flawlessly from Day 1. This one was a bit more effort, but they now work beautifully together with a pair of 800W UcD subwoofer plate amps in an active 3-way speaker system.
The chassis was ordered from China. I just added taller rubber feet to the floor, step-drilled a larger front panel hole to accept a mains voltage power switch, added a separate front panel LED and removed a silk screened “fake” logo from the front.
This one was built using the same type of bisected P3A PCB as the earlier midrange amp, but is biased into Class-A operation following the P3B project article. As it turns out, the bisected PCBs were not well suited to Class-A bias (1.5 Amps quiescent current) and installation at opposite sides of a chassis. See “buzz problems” below.
It would want to run quite hot, but the extruded heat sinks are up to it and adequate heat transfer is assured by adhesive-free Kapton tape, thermal paste and clamping bars at the output transistors.
The project article called for a CLCCc power supply with six 10,000μF caps and a pair of 10mH inductors. An Altronics/Silicon Chip Class-A power supply PCB was a good starting point, but it had to be butchered to add the filter chokes after the first pair of capacitors.
That was tested for over-heating using the DC bench supply:
And 3 Amps (continuous current of two channels) was too much (the inductors got hot and robbed too many Volts from the rails) so I had to duplicate the supplies – one for each channel.
The transformer is a custom made 625VA with 20V secondaries – a significantly more substantial transformer than the unit used in the first tweeter amp and weighing in between 5 and 6 Kg:
The transformer has bifilar secondary windings ending in common PVC tubes:
Almost like having two transformers in one. The isolated windings were originally extracted as separate circuits to left and right bridge rectifiers:
I was subsequently advised by the transformer manufacturer that this was not a good idea due to the single PVC tubes. Although it worked and both channels drew the same current at all times, I later stripped back the heat shrink and combined the bifilar windings with solder (see below in relation to a minor buzz issue).
The four centre taps are tied to that central chassis-isolated stainless steel star grounding bolt:
… which ties to chassis ground (and the mains socket earth pin) only via the safety ground loop breaker. The whole thing ended up somewhat like an over-the-top development of the dual power supply described here.
The dual power supplies left little floor space free, so the muting relays had to go on the back panel:
Here’s a mistake. I stupidly increased the ballast resistance on the soft-starter. The tranny growled slightly after the second relay activated with these in place:
Easy fix was to piggy-back another resistor on top:
The optional grounding terminal of the above module as well as the speaker protection module are connected to the centre bolt and are both powered by an EI auxiliary transformer as before:
The auxiliary transformer has the “engine mount” treatment to minimise mains vibration noise:
It had to be strapped to ground in case its frame contacted a stray active wire during testing!
The P3A PCBs were populated as before, but with 0.75W metal film resistors and heat sinks on the driver transistors:
Those driver heatsinks must be in place during bias adjustment, so had to be sized to clear the adjusting screw.
25μm Kapton tape (about ten times thinner than sil-pads or mica washers):
Thermal paste both sides:
These tended to bend plastically if tightened too much (which wasn’t much), so they were subsequently replaced with stronger 8mm bars made from a door handle shaft:
The heat shrink provides a gap under the middle of the bar to ensure even force over the output transistors without stressing the PCB.
Being a Class-A amp, the chassis maintains a stable temperature. These 60°C thermal switches cut the low voltage AC auxiliary supply if necessary. That shuts down the amplifier. They are not intended to cycle the amplifier between ON and OFF states. With around a 10-20°C drop required to reinstate the ON condition, there is sufficient hysteresis to get up and turn the thing off at the power switch and come back another day. So far this has not happened.
Powering up and Calibration:
This was practically the same as for the previous AB version – using the dual bench supply. It just had to be done more slowly so that the heatsinks had time to warm before fine tuning. Drift was negligible. The amp definitely runs warmer than the standard version – about 25°C above ambient.
The power supply produces the target figure of ±25V DC at the rails. The transformer has a 240V primary winding and 20V secondaries, but the amp is fed with 225V mains, the inductors take around 1.4V and the amps themselves create a sag of a few Volts with the high bias.
Although it was suggested to me that the amp would sound the same as the standard AB version of P3A and that Class-A was over-hyped, I was quietly enthusiastic despite an underlying reservation about the likelihood of power supply hum with all that current drawn at idle.
Anyway, for the initial music test it replaced a borrowed Audio Research D200 in the upstairs system – after the 2-way active crossover to power the midrange and tweeters (which were passively crossed). This is where the original AB P3A was tested, so it’s a direct comparison with identical equipment albeit some months apart.
Thankfully noise was very low and indeed below the noise floor of the valve preamp (which is itself below the noise floor of any recording). Just a slight general noise with ear to midrange driver and a slight buzz in the tweeter. Probably about the same as the AB version.
The first impression with music was that it blows the AR D200 out of the water, but that was to be expected. I’d love to say that it sounds better than the standard P3A, but overall it is about equal. The sound is pretty much the same at ordinary listening levels. It gives the same great 3D impression.
Downside: Lower power means that at higher listening levels, that lock-jaw dynamic grip on the music of the standard AB P3A is missing.
Upside: It is cleaner and even more 3D on vocals and actually sounds less distorted in the high frequencies.
It will make a perfect tweeter amplifier and thrashes the Silicon Chip Class-A amp in the midrange department hands down.
This infrared thermometer is calibrated to a surface having a emissivity coefficient of 0.95. Black anodised aluminium is about 0.88, but that’s close enough.
I measured a few critical places on the amplifier chassis. The following temperatures were read:
- Ambient metal temperature (before powering up): 23°C.
Then after a good long warm-up period:
- Base of heatsink between fins directly behind any output transistor: 54°C (+31°C)
- Between fins directly behind 60°C thermal cut-off switches: 48°C (+25°C).
- Middle of front panel: 42°C (+19°C).
All seems OK – protection circuit might cut in on a 35°C day.
Addendum (removal of the slight buzz):
Each channel of the amp was completely silent when a source was connected individually to just one of them. Now used solely for the tweeters in the new tri-amplified speakers, when both channels were connected to a stereo source, there was a very slight mains-related buzzy noise with ear close to either tweeter. It was below the noise floor of recordings, but annoying nonetheless and presented a worthwhile challenge, so I had a little fuss.
Some investigation suggested that the strong magnetic field radiating from the power supply AC cabling (around the bridge rectifiers) due to the high quiescent current may have been passing the shielding of the thin internal signal coax cables and/or entering the amplifier modules directly and combining with the signal. A major strip-down and overhaul was carried out.
OK, not knowing for sure what the cause was, I made a list of things to do:
- Combine the toroidal’s secondaries for single circuit extraction to the “stereo” rectifiers. This would remove any unwanted inductive coupling across the bifilar windings previously extracted separately to the left and right power supplies.
- Add a steel disc under the transformer for further magnetic diversion (the chassis floor is just aluminium).
- Use large/heavy steel (high magnetic permeability) plates to isolate the amplifier modules from the AC wiring and rectifiers.
- Move the input RCAs away from the PSU to alongside the heatsinks.
- Use Belden/Blue Jeans (brand) double braid low capacitance coax instead of the thin Teflon coax.
- Pass the coax through heavy steel tubes en route to the modules.
- “Crack” the rear corners of the chassis to prevent circulating currents near the coax cables. This required Kapton tape insulation and the substitution of Nylon screws for the steel ones joining the panels.
- Add copper plate to the underside of the chassis lid as an HF barrier (it should be grounded but isn’t – yet).
Some progress pics:
Combining bifilar secondary windings (I think this was crucial as it removed channel-to-channel inductive coupling within the transformer):
A weapon (12mm OD 2mm wall thickness steel tube to encase the input wires):
Not intended as RF shields, these need not be grounded to the chassis. They simply divert any impinging LF magnetic field lines through their ferromagnetic/permeable mass en route to the opposite pole – i.e. around (not through) the coax.
A bit of trouble getting the crazy amount of copper in the double braids down to a size:
Kapton tape stuck down around the floor and lid corners where the heatsinks and back panels connect (else circulating currents around the side walls might simply cross the floor and lid corners):
New CMC (brand) RCA sockets – better than the old Neutriks. See Nylon (non-conductive) screw to ensure a proper electrical break:
RF copper shield for chassis lid (held on with thin thermal transfer adhesive pad):
3mm thick mild steel plates now bolted down by the rectifiers (and tranny bottom disc laser-cut to size):
Spray painted and installed:
Although the steel panels are grounded electrically to the aluminium chassis floor, this is purely for safety. They do not pass LF magnetic flux to the aluminium (magnetically reluctant) chassis floor. They simply provide a ferrous mass to “grab” incident LF magnetic fields (which might otherwise pass straight through to the amplifier modules) en route to the opposite pole.
While waiting for some parts to arrive, I was looking for other possibilities as to why the amp might buzz only with both channels connected and came across the Silicon Chip article for the original Class-A amp that I built. That amp sounds nothing like this one, but has no such problem. I read/remembered that the amp has an optional “ground lift” resistor of 10Ω at the input of each PCB. It is to be replaced by a wire link in a monoblock version and the resistor is said to “reduce circulating currents in a completed stereo amplifier” and to improve channel separation! So I studied the Silicon Chip circuit diagram and decided to modify the ESP PCBs to do likewise (it turns out that the high-power ESP AB amps exploit a similar “lift”). That it is not included in the PCB is beyond me. Perhaps being designed as a single stereo board, circulating currents are not an issue and the bisection to mount on opposite sides of a chassis was not properly considered. Who knows?
Here is my hack on one module. The other got the same (I also think this was crucial):
With that all done, there is absolutely no noise at either tweeter! Sorry I can’t say for sure which modification did it, as they were all done at once to avoid repeat strip-downs.
One caveat is that the tweeters have a DC protection capacitor in series. These provide an F3 somewhere around 530Hz, so if the amp does still produce any noise around say 100 Hz (my guesstimate of the original buzz) it would be suppressed somewhat by that.