Hi-Fi Projects

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.  These two work together with a pair of 800W Hypex subwoofer plate amps in an active three-way speaker system currently on the drawing board (Edit: now here). 

The black anodised aluminium chassis came from China.  It has efficient extruded heatsinks.  I just add taller rubber feet to the floor, step-drill a larger front panel hole to accept a mains voltage power switch, add a separate front panel LED and remove a silk screened “fake” logo from the front.

Note the “Aurian” badge!   It’s a telescope of “aural” with my name “Ian” and a contraction of “Australian” and I claim the trade mark rights!

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.  Each channel draws 1.5 Amps quiescent current from a lower voltage power supply to produce 25W (8Ω) in pure Class-A (unlike the Silicon Chip 20W amp which I am advised goes AB relatively early despite what they might say).  It would want to run quite hot, but the 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.

Power Supplies:

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.

Jaycar 9mH laminated core passive speaker crossover inductors were close enough to spec. and were tested for over-heating using the DC bench supply:

3 Amps (continuous current of two channels) was too much for these chokes to bear (around 8.4W dissipation) and they would rob around 2.8V from the rails, so I decided to duplicate the supplies so that each would pass just the 1.5A current of one channel.  The inductors then drop only 1.4V from the rails and dissipate only around 2.1W which is easily managed by natural convection via the chassis floor and lid slots.  As the filter capacitor banks are mounted directly over the inductors, 105°C/15,000μF Nippon Chemi-con caps were used instead of the standard ones supplied with the Altronics PSU boards.  A total of 180,000μF is a ludicrous amount of capacitance for a low powered stereo amplifier, but extremely low ripple voltage and noise were the aim and I was not interested in building a “capacitance multiplier” circuit on VeroBoard:

The resin laminated cores are islolated from the stand-offs with double heatshrink tubing.  There are two reasons for this:  One is that the heatshrink provides good grip for the cable ties; but much more importantly, the cores have an “infinite gap” by virtue of their free ends.  Shorting both ends to the chassis via the stand-offs could cause the cores to saturate with consequences that I don’t want to know about.  Apparently, there is no sonic benefit in providing dual power supplies in a stereo Class-A amplifier because the current demands on the PSU are constant compared to the dynamic demands of stereo music signals in an AB amp in which reduced channel interactions can be achieved.  Here the dual supplies simply serve to increase overall capacitance and to share the current loading.

The transformer is a Harbuch brand made-to-order 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 easily split off to left and right bridge rectifiers:

I was subsequently advised by the transformer manufacturer that it was not designed to extract two separate circuits from single PVC tubes.  Although it works fine and both channels draw the same current at all times, I might strip back the heat shrink and internally combine the bifilar windings with solder later (if I can be bothered ).

The four centre taps are tied to that central chassis-isolated 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 and the speaker protection module had to tuck under the right PSU board with its screw terminals protruding sufficiently for screwdriver access :

The soft start has higher ballast resistance than on the previous amps due to the larger toroidal and higher filter capacitance, and I suspect that the inductors might dampen turn-on current drawn by the connected amplifier modules as well.

This and the speaker protection module are each connected to the centre bolt and powered by an EI auxiliary transformer as before:

The auxiliary transformer has the “engine mount” suspension to minimise mains vibration noise:

It had to be strapped to ground in case its frame contacted a stray active wire during testing!

Amplifier Modules:

The P3A PCBs were populated as before, but with 0.75W metal film resistors and heat sinks on the driver transistors:

Those driver heatsinks took some data sheet searching.  Like all ESP PCBs, high-performance necessitates a compact layout, so there is a premium on real estate.  Screwdriver access to the bias trimmer was required with the amp stabilised and warm, so the heatsinks must be in place during adjustment.

25μm Kapton tape (about ten times thinner than sil-pads or mica washers):

Thermal paste both sides:

Clamping bar:

The heat shrink provides grip/protection just in case the grounded bar loosens and twists into either of the output transistor collector resistors. 

Thermal protection:

Being Class-A, excessive chassis heat could develop on a hot day, so I installed a 70°C  thermal switch on each heat sink.  These are in series with the 9V auxiliary AC supply to the soft-start circuit.  If either heatsink exceeds 70°C everything in the amp except the auxiliary transformer swiches off including the front panel power indicator LED and the speakers mute/protection relays activate.

Powering up and Calibration:

This was practically the same as for the previous AB version – using the dual bench supply on each out-of-chassis module - heatsinks attached.  At ±25V, one channel’s DC offset was a mere 10mV and the other a miserly 0.9mV – both well under the 100mV target.  After attaching the heatsinks to the chassis, adjusting the bias current was straight forward, but I gave the heatsinks a bit more time to warm before fine tuning.  Drift was negligible.  The amp definitely runs warmer than the standard version, but the temperature rise is only about 10°C.

Measurements:

Well Rod Elliott was interested in the chassis, so I lent him the amp and he was very kind to measure it’s performance.  It has less than 0.02% distortion (limit of his equipment) from 10Hz to 100kHz with a slew rate of 6V/μs.  For 25W, you only need 2.2V/μs at 20kHz so it’s the perfect tweeter amp! 

Listening impressions:

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 miniscule and below the noise floor of the valve preamp (which is itself below the noise floor of any CD recording).  Just a slight general noise with ear to midrange driver and a slight hiss 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 on orchestral music etc. at ordinary listening levels.  It gives the same great 3D impression.

Downside:  Lower power means that at higher listening levels it does not have that characteristic lock-jaw dynamic grip on the music as P3A in standard form.

Upside:  It is cleaner and even more 3D on voices and the treble (tweeter) is cleaner.  Nat King Cole is right there in a room bigger than my listening room even more so and Joan Sutherland sounds like she is singing from the heavens.  Please excuse my tastes in test music.  I may be strange.

It will make a perfect tweeter amplifier and thrashes the Silicon Chip Class-A amp in the midrange department hands down.