Audio Proz Service and Sales

APT Upgrades and Repairs Technical Information:


Aug 28th, 2008

The APT 1 amplifier is a truly advanced product and even by today’s standard, a very modern audiophile amplifier design. Some of it’s design technology is only now being implemented in fancy name equipment. Seldom are so many of it’s clever design tricks for correctly stabilized feed back circuits, optimized impedance matching both at input stage design, power supply and output amplifier circuit have been utilized in total (even compared to fancier products.)

First a little primer on sound character and perceived value of upgrades done to APT.

We might reproduce the service manual which would elaborate on these clever circuits and certainly the BSEE mind will be impressed. However I would always want to emphasize to the audiophile, whose claims of “my ear will tell me if it’s good” to seriously consider the now well known phenomenon of “reactive load” speaker designs with troublesome crossover designs and incorrectly tuned cabinets which create more sonic aberrations than a well designed amplifier will ever create. Speakers are more often the real culprit for sonic flaws and claims of poor imaging with different amplifiers. A quick sonic assessment of the APT amplifier must take into account it’s capability to play into virtually any crazy speaker load, from electrostatics to multi way poorly designed behemoths. Keeping in mind my sonic assessments of the amp are really subtle effects I will comment that the amp will exhibit a “tight” bass sound character and slightly more detailed midrange quality, due to it’s high damping factor (the ability to appear as a very very low output source impedance when speakers are loaded to it.) Or with the flick of a switch on the back panel which changes the power transformer to “lower impedance higher current capable output” for driving low impedance troublesome speakers better, now giving a slightly thicker bass sound with possibly a slightly softer warmer sound to high frequencies. Again I must emphasize strongly these sonic effects are mostly related to the speaker loading, the reactivity of their crossovers and again all too often incorrectly tuned bass drivers for the cabinet size. For example, a reflex port design speaker can benefit from the APT amplifier being in the “high” impedance position to help damp bass box resonances. The truth is most manufacturers seldom correctly “hand tune” damp out their enclosures resonances to obtain optimum ‘Q’ of a tuned cabinet because of production line speed. IE most reflex port enclosures sound tubey or boomy or plump in the low end without any fault of the amplifier, more often it’s just short sighted speaker design or an intention to fool the consumer into “hearing” more bass effect by allowing the cabinet/woofer combination to have a few self resonating modes, typically in the midbass frequencies. Many audiophiles claim the APT’s midrange clarity is either extremely detailed or somewhat veiled. Obviously now again we are probably relaying the speakers quality or weakness in reproduction.

I now turn to the matter of upgrading in value and quality. In the era the APT was made, most American manufacturers (regardless of their marketing hype), used similar mediocre quality and now aging capacitors in the power supply and audio circuitry. These famous names have now the same troubles the APT has. Aging capacitors can take more time to “warm” up to work well, cause DC leakage between circuit stages, and also have resistive effects which ultimately can blur the sonic image due to dynamic non linearities. This is even more noticeable with some of today’s super well done speakers. Indeed super clean, well done crossover designs sound better. I will give credit to newer Monitor Audio, Vanderstein, Magnepans, the real German made Dyne Audio designs, the true European made B&W, and of course my favorite modest price speakers such as Paradigm, Definitive Technology, some PSB, (there are others of course) which use high quality parts for good audio clarity.

APT Amp Upgrade Level 2 or 3. Sorry, It’s hard for me to keep to the point in the audio realm since many technologies are combined (preamp, amp, speaker, etc.) to create performance effects. So to clean up the amp, virtually all capacitors are removed from the APT and fresh high quality Panasonic types of corrected and uprated values are installed. In this era higher quality capacitors of a better rating are now smaller and can easily fit onto the Audio circuit boards. All the low level power supply circuit capacitors are doubled up for improved reliability and indeed an improved regulation and cleaning up of supply voltage is accomplished. (Again, in it’s era we could only fit a certain size part on the board. Now twice as good can be installed.) This modification appears to be the crucial one for some sonic improvement. The appearance of a little more “dynamic” clarity and no susceptibility to variable 120 line VAC. We also change out some crucial feedback loop capacitors with mylars, or layered high quality film types. Some low level transistors which are known to age badly are changed out (certain production runs of diodes are transistor with suspect reliability get into virtually any designs. Again, I’ve seen fancy stuff with 5 cent diodes guaranteed to give problems). Also changed out (or cleaned) is the speaker relay. APT used the best quality speaker relays but 20 years of use and the double 5 amp contacts still can arc through the gold plating, especially when peak audio current is passed onto these wacky speaker loads. In fact part of the problem again is the reactivity of the speaker load, returning “back EMF (Electro Motive Forces)” voltage surges to the amplifier. Speaker relays in virtually all 20 year old products almost always require to be replaced. No doubt, ironically it’s this part which causes most of the sonic degradation, as the contacts develop weird intermittent distortion effects. One of the most notable is the contact diode effect, whereby the relay contact which have copper base with gold plating have arced through to create a carbonized (oxided) junction. The two unlike metals and “burned” spot now create a horrible tiny corroded contact causing variable current (the audio) to become “rectified” ie, slightly prohibiting positive or negative voltage signals from passing through cleanly. This same problem in lesser form occurs with most low level switches in preamps and all other audio processors. The moral is good quality switches are worth the investment. (but here I will still scream again because even the most fancy stuff often uses good looking but really cheap parts and fancy knobs – Sorry, I’m not impressed). In all fairness some fancy units do use as few switches as possible (which is good), but high quality switches are easily obtained and there is no good reason the fancy equipment can’t have a little tone control of audio equalization and switches to help correct badly equalized recordings. If Audiophile equipment was truly audiophile it would give the chance for an audiophile to make his audiophile tastes more apparent. How often we find a muddy vintage recording or a modern excessive bright CD recording could be tamed with a little equalization. 10 years from now, maybe less, we will look back and wonder why the manufacturers deprived the true audiophile control over his music. Look into the past and you will see a plethora of equalizers available to the consumer.

Finally on the APT amp we ensure correct amount of thermal paste on the output transistor for correct heat sinking and thermal stability. Also we QC crucial parts such as the DC correction FET feedback and related circuits are upgraded. This is a crucial circuit which is somewhat novel but essential for an audiophile amp. In short, it’s a circuit which helps to correct for the strange “back EMF” from speakers, helping to stabilize the feedback, distortion correction circuits which reduce speaker generated distortion. Again I must emphasize it’s the speaker creating distortion which this amplifier helps to clean up. The APT amplifier is particularly good at this job, with or without my upgrades.

We also change, improve the crucial voltage regulation at the constant current source for the differential input pair transistor. These are a few of the modifications which are important. Although as time went on APT had already incorporated some of these modifications into basic production models. The truth is mostly what we do is rid the unit of suspect aging parts. The sonic improvements will be subtle. Level 2 fix partial upgrade does all the basics as described above but fancy retrofit of spec capacitors or circuit changes.

Level 3 does a few circuit upgrades but more importantly we are changing out and upgrading all capacitors throughout the unit. In most cases, doubling or 3 times to original value. Especially so in the multi-tiered power supply. Indeed today we have capacitors of small size and better quality (if correctly selected) which can now fit comfortably on the circuit board. The Level 3 mod will lessen the noise floor a few DB (amp is already very quiet) and will regulate the power supply a little better, possibly noticeable on impacts, transients of music but no doubt it's theoretical that reviewed longevity is improved. The APT amp circuit itself is very modern and still the equal of the fanciest Audiophile designs, but APT design has a test equipment look to it. That's the philosophy of the designer Holman Tomlinson.

I would also like to address the “audiophile modifications” which many hobbyists believe are effective but I will clearly note as not truly important concerning real sonic improvements. I’ve tried most of these modifications when at APT corp. and later and found that aside of actually fixing a failed circuit these modifications are of limited value for the time and audio improvement.

Many audiophiles love to add “bypass” capacitors to power supply capacitors or power supply rails (which ironically has already been done in the amp) or to replace out diode bridges with specialized types, or use gold plated input or output connectors. I’m sorry to say none of this gains any advantage. Especially considering the real mechanism for fidelity improvement lay much deeper in the circuits actual topology (circuit design). The truth is APT amp already is way past or equal to many of the fancy audiophile amps. It just does not have the fancy name, or colorful looking parts in it. You will have to trust me on this or take the schematic to a BSEE or seriously knowledgeable electronic technician and they will be very impressed but may find it difficult to explain the truly clever design to the layperson. Too much nonsense about bypass capacitors which feed the myth of pseudo scientific modification shops. I’m sorry, don’t pay anyone $600 to add a dozen mylar capacitors to a circuit when in fact for instance the amplifiers bias adjustment has been set incorrectly which obviously creates serious audible distortion. I see many units with fancy upgrades, preamps, amps, etc. but there are failed op amp or low gain off spec transistors producing trouble which the “audiophool” shop could not recognize or repair. Sorry, Sorry, Sorry.

Worse yet is the super thick AC power cords some people have been convinced to add on. This is probably the most egregiously ridiculous and offensive modification. Shops charge serious money for these ‘special’ AC cords and cost to install. Worse yet, I often see the cords installed so as to violate UL approval!! These new AC cords which are so thick often don’t fit well into the unit. I see them patched in often in a dangerous way. We are dealing with 120 VAC here. Finally (just to keep the list short) a supposed audiophile AC cord just isn’t really that. No more power is delivered to the amp because, let’s think about this carefully, the 120vac power coming from the house probably travels 50 to 200 feet before it gets to your wall socket (which can be a little loose, so here is a potential problem). Considering most romex house wire is #14 gauge or worse, then logic has it that 6 feet more of AC cable into the APT amp (even if it is 18 gauge lamp cord) is meaningless. A light bulb draws 4 times more standing current than an APT amp. So I challenge the audiophile to change out his lamp cord to see if the bulb will be a little brighter. It won’t. The APT amplifier power transformer is very efficient, drawing only 250 to 300 MA acquiescent current. Remarkable even by today’s standard.

The moral of the whole story is your tech should be able to explain things clearly about such matters, in a technical but logical way. If the tech starts going on about how this or that capacitor or special resistor makes a huge difference in sound then run as fast as you can with your wallet held tightly. Think about this. The Beatles albums were done with fairly primitive, albeit decently designed electronics with lots of lousy capacitors and resistors but ultimately excellent sound character in the final analysis. (please it’s just a general observation), don’t be too easily impressed with $30 connectors and $10 capacitors… It’s a fools errand. I myself am not claiming to make incredible engineering updates in this amp, just to clean up old parts and do updates which APT engineers were aware of and approved.

Vince used to work for Apt and has developed certain upgrades to increase the reliability, longevity, and also the fidelity of these already remarkable preamps and amplifiers. The preamp upgrade consists of three levels to suit your modification needs.

APT Preamp Level 1: We repair the obvious faults, as well as clean all controls and switches. We replace any obvious bad capacitors and only minor upgrades are done. This level costs $100 - $130 plus the cost to ship it back to you ($15 eastern states, $25 western U.S.).

Level 2: For the Level 2 upgrade, we do everything in the Level 1 upgrade as well as replace certain components that are critical to the fidelity of the machine. There are certain low level transistors which we upgrade for spec value and reliability (remember, these units are over twenty years old at this point). These transistors are often the current source or "sub-regulator" transistors. The critical components are upgraded for the correct type of part for the circuit design (i.e. non-polarized capacitors in certain op amp circuits as well as Mylar caps for other circuits which require higher specs in the equalizer or power supply). Not all capacitors are replaced; some capacitors and power supplies will be upgraded if needed. The speaker relay is virtually always replaced with a 15amp, 4-contact type relay. Silicone thermal transfer paste is double checked to make sure it is not dried out and frequently replaced, especially in units that have seen heavy use. The cost for this level is $225 plus return shipping fees ($20 eastern states, $25 western U.S.) to get it to you when it is done. The one caveat is that if a unit comes in for a Level 2 upgrade but has a major failure such as shorted outputs or damaged parts on board, it will need Level 3 upgrades (we will call to confirm first).

Level 3: This is THE audiophile upgrade! Everything in Levels 1 and 2 are done. We then expand on the Level 2 upgrade and replace all capacitors and other crucial fidelity components. This costs $335 (some units cost less) plus return shipping ($15 eastern states, $25 western U.S.). This price does not include the replacement of the transformer, volume control (which is seldom ever bad), or the main mode switch. If these are needed, the upgrade may cost more. (We have seen cases where machines come in in absolutely terrible condition. Worst case scenario, we replace the entire board assembly and the full cost is $370 or less. We will always contact you if the cost of the repair is more than our original estimate.) It does include the matching of channel parts (FET’s, op amps, etc.) for noise and performance, as well as some “esoteric” modifications. The modifications are done to satisfy those “golden ears” who may possibly hear the subtle difference of some very critical capacitors in the audio path (i.e. there are a few stages that can benefit from capacitor bypassing with best quality Mylar capacitors, improvement of subsonic filter for advanced wider frequency response, less phase shift, etc.). Admittedly, I don’t think the sonic improvements are immediately obvious and dramatic (unless the unit had major off-spec or failed parts), but it is true that listeners who do own high quality, reasonably phase correct speakers playing quality acoustic oriented program material will hear an improvement. Let me stress my opinion (which is based on factual A/B blind testing and reinforced by AES Journal testing competitions) that to hear subtle differences of improved spatial/depth or instrument timbre clarity is mostly perceived due to the quality of the speakers and their appropriate listening position.

Read more about speakers and their appropriate listening position here.

Update August 2014: Minor price increase from $325 to $335 (Level 3) due to a few adjustments in service with slightly improved components, and price of parts. We continually strive to find the best solutions for the highest quality service, performance, and reliability. We wouldn't do it if it wasn't worth it.


PLEASE, when shipping you must wrap the face in 3 layers of bubble wrap and double box with second box which has a little space to fill with paper etc. The APT preamp is very heavy and can punch thru a box which travels across the country. Luckily very few have suffered but double box - don't let UPS box it for you.