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September 15, 2012 / consort3

Cost effective darlington amp Part 2

Created a tidied up layout with a stereo pair and including the rectifier storage capacitors and regulators on the same board. Perhaps a bit ambitious as it is on veroboard! All seemed well until I tried a load giving squegging problems. Should have read this before I started:

A couple of tweaks to the compensation and its now good enough to connect up the speakers. But its not for beginners. It will thermally run away if you are not careful. Not many in the old days bothered to put the standard Vbe multiplier transistor on the heatsink. This exercise has shown me that it is essential. You might get away with domestic use but as soon as you run serious power you might get thermal runaway. Amazingly you can get thermal runaway with valves, see this thread (post by pure sound):

Connected up the speakers and it sounds good. There is a bit of hum but otherwise OK. I think some of the rectifier switching current is getting in to the signal path so will examine the layout to see if it can be improved.


The new layout worked so here is the circuit (note the TIPs are darlingtons). The amp has the desirable characteristic that there are no switch-on or switch-off thumps:

And here is the veroboard layout

This was composed on Veecad a good free tool for veroboard layouts.The leftmost 2 columns of holes on the veroboard do not exist in practice they are only to show the positions of the TIP devices whose legs are bent at right angles and soldered to the copper side. The TIP devices are 3 pin, I chose the nearest standard Veecad symbol with the correct spacing which was 5 pin. Do not build this without the regulators, heatsinks or thermistors as it will thermally runaway. The thermistors and power devices need to be in good thermal contact with the heatsink. Unless a low profile is needed it is probably better to open the appropriate power transistor mounting holes to 1.5mm and use conventional mounting techniques, rather than the right angle bends. The zener voltage of D1 and D2 depends on the transformer, for 15v secondaries use 22V zeners. The SOT resistor (select on test) is about 50k ohms. Bridge and fuseholder are Rapid electronics 47-2958 and 26-7620. The transformer secondaries and the bridge rect. are depicted by L1 L2 and D3-D6. The thermistors are Rapid 61-0400. The thermistors are a tight fit between the transistors so that the thermistor edge touches the heatsink. I used mica washers and thermal compound to electrically isolate the transistors from the heatsink. The bias is adjusted for 6mV at the emitter resistors when cold, using the SOT resistor. After warming up it rises to 9mV.

The Veecad tool wll print out a correct size template on paper which you can lay over the veroboard. The first component you insert pins the template to the board. Also a reverse template to mark the track isolation cuts, shown as crosses on the above layout. Note column 0 and row 46 are not used. The files at the foot are PDF’s of the right size to print out, I had to use 104% on the printer to get the scaling right. Just noted a mistook on the diagram the lower amp output ground should be 1 hole lower.

It is intended as a plate amp for a 2 way speaker and is low profile to fit a slot in the cabinet wall. To avoid drilling mounting holes for each of the transistors I used a strap, but all the transistors need to be the same make (hence same thickness) for this trick. I used ST darlingtons of the TO247 type. Do not use To220 case style unless you want to re-layout the veroboard. If you use another make you may need to redo the compensation. When I tried the strap idea on a TO220 version, the regulator heat upset the thermal sensing by putting heat into the transistors and not the thermistors so do not use the idea with TO22o devices, each transistor needs individually fixing down. Curiously I had to reduce the values of R15&32 for the BDX33/4 and the SAP version to 12 ohm .

The thermal tracking is better with the thermistors bonded to the collector lead and with this mod R7,11,13,14 26,29,30 & 31 should all be 120 ohm

Unfortunately the simple idea ended up as more complex but still a viable alternative to those new fangled digital amps. I do not like all those high speed, high power transients flying about.

The project name is Leitmotif by the way. Light on the wallet and with the motive power for a subwoofer. That was going to conclude my journey into minimalism but somewhere in the east wing of Consort Towers lurks some 2SB1560 and 2SD2390 transistors which I want to try. The topology being true push-pull and with an amplifying output stage is closer to a valve amp than your average transistor amp. Further below is a SAP16 version of the amp. Due to the unusual symetrical pin-out of the devices a new layout was required.

Judging by the site traffic, the op-amp+darlington amp has generated a lot of interest. Bored with a Blameless or Gaga with a Gainclone? This amp has 3 things going for it
Value in the sense of price /performance ratio.
Transient performance. The little secret is that on transients the op-amp attempts to drive the load through the feed-forward capacitor and creates a current demand which kick-starts the darlingtons.
Thermal performance or virtue through necessity. Some amp designers get bothered by the slow thermal response of their creations. Because it was necessary to give each output device its own sensor the transient thermal response can be good.
With fastish op-amps, common base splitter/level shifters and common emitter as opposed to common source outputs, this amp has topological similarities with Musical Fidelity Mosfet amps.
If you try it be aware that the Base resistor R15 and R32 suit the ST TIP142, for most other devices R15 and R32 should be 12 ohms. See Part 3 for more tips

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