Over the past few years I have worked on a number of capacitor-couple amplifiers, and having listened to them all, I can appreciate their sonic attraction. This is not meant to be a detailed technical discussion, rather an observation of a period in audio design when the characteristics of these amps was deemed to be highly desirable. In the early (and not so early) days of transistor amps, many of the lower-power amps were capacitor coupled, in that the output of the output transistors was fed to usually large output capacitors and then to the speakers. Generally, there was no speaker protection circuitry and the capacitors were at least 1000uf, however some amplifiers had protection circuits; one example – the Sansui AU555 – has an SCR-based protection circuit.
One example I will use is a Luxman SQ707, which I recently upgraded and refurbished, though I will also describe a number of other amps with similar topologies. The Luxman is a 17 watts per channel amp produced around 1975. Other contemporary amplifiers that I worked on with similar topologies include the Sansui AU 505 (1972-1974), AU 555 (1969-71), AU 555A (1971-1973) and Sugden A21 (1973). Having refurbished and upgraded a number of Sansui’s in particular led me to explore this part of audio history.
In all the cases these amps are not particularly powerful, however with efficient speakers they can deliver a very pleasing and “full” sound.
The topology of all the amps is similar; a phono stage for moving magnet cartridges feeds a line stage for tuner, auxiliary and tape inputs. The tape outputs provide a line level output. This section then feeds the volume and balance controls which in turn feed the tone control sections. Each of the amplifiers adopts a slightly different arrangement of volume, balance and tone controls. The Sansui AU 555 also has separate outputs for the preamp. The preamp section then feeds the main amplifier section, with two NPN main output transistors per channel driving the output coupling capacitors, usually through a speaker selection switch.
All of these circuits, of course, had electrolytic capacitors in signal path, as well as coupling of the output, which given the elapsed time (measured in decades) and the quality of capacitor engineering at the time means these should be replaced .
The Luxman used a 2.2uf electro capacitor on the preamp input, which could be replaced either with the same value in polypropylene film, or increased in value to 10uf with a bipolar electrolytic. The 10uf cap will give a lower bass response, though with minutely higher distortion than the polypropylene cap. It is always wise to replace the input resistor (1.5k in this case) with 1% metal film, such as Vishay Dale CMF or similar, as these are typically carbon and can be noisy.
The inputs on the Sansui AU505 used a 2.2k resistor and 1uf capacitor, while the AU555 used a 1k and 1.5uf, and the Sugden was similar. One factor influencing this choice of values was the source audio material(e.g. vinyl, tape AM/FM) all of which had some low-end frequency rolloff, particularly as many of the turntables of the day had rubber drive wheels, which could give low frequency rumble. With modern inputs, lowering the roll-off point with a larger capacitor can be advantageous.
Generally, phono stages are comprised of a pair of transistors, usually with emitter bypass capacitors – again, electrolytic – and an output capacitor inline with the signal. These capacitors should be replaced and all the resistors if one is inclined; these were carbon composition and these do impact the noise floor. The emitter bypass capacitors can generally be increased in value, and on the AudioKarma forum, Leestereo has done some amazing upgrades using this approach. Generally doubling the value of the emitter bypass capacitor is a good step, as often the value chosen was, in part, determined by cost and physical size. He also describes changing the RIAA curves of the Sansui amplifiers to obtain a more accurate response. The phono topologies for all these amps is similar, and in some of these amps the transistors originally used may be known to be noisy and should be replaced.
The capacitors in the signal path should be either film – ideally polypropylene – or audio types, ideally bipolar. Although none of the capacitor manufacturers state what the measurable benefits of their audio caps are, I assume there is more to it than just marketing and different cover colors. Sadly, Nichicon are discontinuing their range of audio caps. In most cases, if there are power rail decoupling caps present, these can be increased in value using low impedance/low ESR types.
Most of these amplifiers then have a single or dual transistor for the volume control and in some cases the tone controls. Also, most capacitor-coupled amps have the tone controls in circuit at all times, so replacing the electrolytic capacitors is necessary. Depending on what was fitted originally, some of the ceramics or film caps can be replaced with polypropylene or polystyrene depending on value and size. If there are silver mica or Shosin-dipped mica these should be left in place, as they are unlikely to have deteriorated and are excellent for audio. Depending on how detailed and extensive you want to be with your work, you can replace all the resistors with modern 1% metal film, which in aggregate will reduce the noise floor.
The main amplifier output stages generally involve fixed bias using multiple diodes; the Luxman for example uses a triple diode, such as the SV-03 which has three diodes in series in one package. The amplifier stages have a signal coupling capacitor ranging from 1uf to 10uf which can be replaced with either polypropylene or bipolar electrolytic. I generally use a 10uf MUSE UES Bipolar to lower the bass roll-off.
As with the phono stage, additional decoupling and emitter capacitors can be increased in value, and a good approach is to increase the working voltage of the capacitor as well. For example, if the original capacitor is 35v, then install a 50v. Of course, never install a capacitor with a lower working voltage, unless you enjoy magic smoke.
The main output capacitors are usually in the 1000uf-2200uf range. For example, the Luxman uses 1000uf 35v, the AU505, 1500uf 63v the AU555 uses 2200uf 63v. On the Luxman I installed 4700uf 50v, on the AU505 6800uf 80v and on the AU555 6800uf 80v.
One of the main shortcomings of these original capacitor-coupled amps was the power supply, and it is here that significant improvement can be made. Most all of the transformers manufactured at the time were on the larger side, and if anything, over-specified for the task at hand. This enables the use of larger power supply capacitors, as the original amps were to a large degree limited by the physical size of the capacitor. For example, the Luxman had a 2,200uf 50v which was replaced by a 10,000uf 63v, while in the Sansui AU505 the main capacitor was also 2,200uf at 63v, and the AU555 had 2,200uf 63v, which were replaced by 6,800uf 80v units. Rectification is undertaken by 2 diodes in all three units, and these should be replaced with 3 amp/400v or higher soft-recovery types.
Both the Sanusi AU505 and AU555 have different voltages for different sections within the circuit, similar to the approach used on tube amps, with a resistor and capacitor for each voltage. The electrolytic capacitors should be replaced and uprated; for example the AU505 uses a 100uf 50v electrolytic for supplying the B4 power rail, which provides power to phono, volume and tone stages, with an additional decoupling capacitor for the phone stage (C4 = 470uf 35v). I increased these to 1000uf low impedance types. The Luxman has a single voltage for all the circuitry, and with all of the decoupling caps (C119 = 470uf 25v, C117 = 10uf 25v and C109 = 33uf 6.3v) I increased them to 1000uf, 100uf and 100uf respectively. All of these amps use a single sided supply – that is, only positive DC voltage, rather than the more common dual supply of positive and negative DC used in modern amplifiers. The output transistors are all NPN types.
It is worth replacing many of the smaller transistors used in the preamp and VAS stages of these amps as many are known to be noisy. For example, in the Luxman, I replaced all the 2SC693 with KSC1845 and in the Sansui AU555 and AU505, replaced the 2SC871 with KSC1845. These replacements have made a measurable reduction in the noise floor.
The impact of all these upgrades is a measurable reduction in the distortion the amplifier adds to the signal, which for the AU555A was an order of magnitude, that is the original specification was 0.5% distortion at rated output, whereas after the refurbishment this became 0.05%. The AU505 was similar, with 0.5% being the original specifications and 0.09% being the result after refurbishment.
And finally clean all the switches and controls with DeoxIT, as here the noise demons live
Thanks to all on AudioKarma and Diyaudio from whom I have learned so much, and if you are thinking of working on the Sansui’s Leestereo’s Audiokarma forum postings are essential reading.
If you wish to explore the more technical aspects, read Rod Elliot’s excellent explanation at https://sound-au.com/articles/cap-coupling.htm