Two Amp Projects: 8x100W DIY & H/K 770 Restoration

MikeC's Audio Craft
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This is a show-and-tell about two amplifiers I worked on in the past year. One is a DIY custom assembly of class-D amp modules and an SMPS PSU into an 8-channel power amp in a single chassis. The other is the restoration of a classic 1980 Harman/Kardon HK770 dual-mono power amp with complete replacement of all 44 capacitors.

THE DIY 8-CH AMP

A 40w/ch @ 8-ohms 8-ch Rotel RMB1048 amplifiers has been the power amp for my Linkwitz Orion speakers since I built the speakers in spring 2014. The electronics of the system live in a custom shelving unit I built into the fireplace in my living room. The Rotel amp is a bit too deep to fit fully into this space, and it's always annoyed me that the front portion sticks out beyond the fireplace by a good size inches or so.

I'd read about class-D amp modules based on single chips, some that actually contain 4 channels on a single module. One active DIY speaker enthusiast in Australia, gainphile, described building an 8-channel amp from two such modules on his blog. These were to drive his Orion-inspired active crossover 4-way speaker system. I examined what he did, based on his blog, and his comments about them, and decided that perhaps something similar could replace the over-sized Rotel amp.

After much scouring the web and ebay, two "4x100W" STA508 amp modules by Sure Electronics and two 24V 14.6A 350W switching power supply were ordered. Two different sized aluminum cases were chosen and ordered as well. I chose two different sized ones just so I'd have options. The amp and power supply modules are very similar to those chosen by gainphile. The STA508 is based on two TC2000 and four TP2050 chips from Tripath, a leading class-D chip maker.


STA508 4-ch amp module


24V 14.6A PSU (350W)


The smaller aluminum case with thick front panel.

First examination and power up proved immediately that the small fan on the STA508 amp module is too noisy for me. I'd replace them with high quality 80mm Noctua fans, loaded down to spin at ~500rpm. At that speed, they are inaudible in a quiet room till you get to within 1-2 ft, then it is a soft, barely audible whirr. With music playing, this would be a complete non-issue.

The fan in the PSUs had to go as well, of course, but this was a harder nut to crack because of the tight space in the PSU casing and the tiny size of the fan. I took the PSU apart, examined the parts that needed cooling, and then decided that I could devise a way to clamp those parts to the chassis front and side panels for effective passive cooling.

I read more of the notes on the gainphile blog. He could not hear the difference between running his amp modules on one or two PSUs. I rigged up the amp temporarily into my Orion system, and did the comparison myself. It was not a particularly rigorous comparison, but I was satisfied that the sonic difference between one PSU and two wasn't audible to me. I would go with one PSU.


The end result, before closing it up.

Key heat-generating components clamped to aluminum plates, which in turn bolt into the front and sides of the case. That was a huge amount of work to get right, mechanical alignment of all the parts was no piece of cake.

It all got done, and the final assembled amp worked AOK. All the wiring inside is soldered except for the speaker terminals at the amps, and the DC power output at the PSU, for which I used the existing screw terminals. My soldering skills are good, not the most elegant looking joins, but solid.


Finished class D 8-ch amp. It's quite small, given its "800W" output rating: 12.5w x 3h x 13d" including feet and back panel connectors.


Four extra holes were drilled by accident.


The screw heads on the side show where the transistors or regulators are clamped.


Underside.

The amp took me some 10 days to build. A big part of the work was the metal work on the enclosure, always a pain with thick panels. I'm not an expert metal worker, so I took it slow... but still made some minor errors.

I adjusted the supply voltage to just under 27V, about 1.5~2V less than the maximum recommended for the amp modules. The most realistic power output spec for the amp is 64W @ 4ohm, 1% THD+N; this probably translates to around 40~50W @ 8 ohms. Hence, it makes sense that with music, two of these amps are highly unlikely to draw more than the 15A the PSU can deliver, which is why neither gainphile nor I could hear any difference when the power supply was doubled.

This amp ran my self-built Linkwitz Orion 3 speaker system for about a month. The Rotel RM1048 used before is a good amp rated for 40W/ch @ 8 ohms and [email protected] ohms. The DIY class-D amp matched the Rotel for sound quality, loudness, and punch, but occupied less than half the space. That's what I was looking for: A similar amp in a much smaller package to fit nicely in my living room. The best thing was that the total cost, even including shipping and import tariffs, was only under US$200. OK, add the cost of the Noctua fans and it exceeds $200. Still, it is an incredible bargain!

Despite the amp's good performance, I decided that I wanted higher maximum loudness capability, some 4-6 dB higher, which the speakers can probably produce, but only if driven by amps capable of at least 4X higher power. Perhaps 250W or more for the bass drivers and at least 100W for the midrange drivers.

This amp plays the Orions plenty loud enough in my 14x30' LR. I measured ~97 dB peak SPLs at full tilt from 10' away. But in warm weather, I like to have the music on while lounging on the back deck with the French doors open, and that's another 25' away, through the kitchen, through a double door opening in a dividing wall. I know driving the Orions to near max capability from >50' away is not exactly the most efficient way to get good sound out on the deck, but I'm not prepared yet to install speakers on the deck.

So this 8x100W amp was sold on ebay to a happy buyer who said "Impressive little machine. If he builds another I would buy :) "

Notes: The IEC AC socket was fused with a 6A slow blow, though there is another 6.5A fuse in the PSU. The steel (?) mesh grill is affixed with double-sided carpet tape, which is really strong, but I added a single sided adhesive tape around the edges to be safe. The carpet tape + silicone glue would make it even more permanent. Input sensitivity is quite high; it only takes 520mV at the input (10k ohm impedance) for full output, so this might be best padded down depending on the output of your gear. A 50k pot is recommended for this purpose, but I'd calculate and use the right value resistors for better performance.

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