The APT preamp, designed by Tom Holman was a very advanced design for its time and is one of the best performing vintage preamps available. I use one as my reference en my lab set up, and very few other preamps I have worked on can surpass it.
The premap uses a number of IC’s , originally TL072’s, for a number of functions creating a “block” approach to the overall design. This is extremely well explained in the detailed manual, copies of which are widely available on the net.
The Phono section is the only part that does not use an IC, rather closely matched FET’s are employed with discrete BJT transistors.
If there is a compromise, it is the power supply, where due to space considerations a quite small mains transformer was used. This can be considered a limitation, though even though there about 11 IC’s the current drain is not high (less than 100ma for the whole unit). That said when I worked on a Kenwood L08C preamp recently I was struck how much an over engineered power supply can impact. There have been a couple of people on AudioKarma and elsewhere who have upgraded the transformer and claim better results, which I am inclined to agree with. I have planned to have an external power supply built for the APT to test this, but as they say “great plans of mice and men” or more accurately a cobbler nver fixed his own shoes- which in my case is severely limited by my metal working capabilities.
So all that said, The upgrade involves increasing the power supply, with a new uprated bridge rectifier, replacing the original regulator IC’s 7915/7815 and increasing the power supply capacitors using low impedance types. The other crucial area is the relay which comes in a range of versions, and which is replaced with a sealed high speed miniature type. The point has been made that having a relay in the audio circuit (controlled by the mute circuit with a switch on delay)w, is not ideal, and other preamps use different configurations to avoid this. However the APT preamp was designed to be used with the APT power amp, which have no AC switch, so the design would mote the audio for about 5 secs till the power amp was operational. All the electrolytic caps are replaced with the same value using low impedance types, except one low value electrolytic which is replaced with a polyester film cap.
There are different versions of the main PCB, but overall there is no major differences.
The design uses a number of electrolytic caps to separate each functional block of the circuit, and they were originally all electrolytic types. The upgrade replaces all of these with either Nichicon Muse bipolar caps of higher value or polypropylene film caps. Additional power rail decoupling is added at seem points in the circuit as well. The IC’s are all replaced with modern audio specific types and given that the TL07 is a FET input IC, the OPA2134v was chosen, except for the volume IC,which is replaced with a TL207c, given the unique operation of the volume circuit.
The preamp has three PCB’s the main one has the power supply, phono, relay, volume, balance and some switch functions and the upper board has the tone controls, some buffer circuitry and the headphone circuit and the third board is mounted at the rear with input/output connectors.These are connected by stiff ribbon type cables, which can break if flexed too much or too hard. All of the electrolytic caps are replaced, and some are uprated as well. Again a mix of Biploar MUSE and low impedance types are used, with the low value ones replaced by film caps.
There are a number of ceramic caps and many of these are replaced by WIMA FKP polypropylene types as are a number of the original Mylar caps. In the phono section the electrolytic, film and ceramic caps are all replaced and some uprated depending on circuit function.
The switches can also be problematic and are all deoxed, and in some cases replaced. There is a considerable amount of work involved in refurbishing and upgrading these units, and over the years I have evolved my “recipe” to the point where the outcome is optimized. Though one should never say never….
The result is truly excellent, great sound stage, extremely low noise, great detail, transparency and dynamics- a great design enhanced with modern components.
Audio Science Review recently favorably reviewed one of my upgraded units-
I will certainly be keeping my APT.