NAD C272

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The NAD C272 is a 150w per channel power amp from the early 2000’s. This is one of the later NAD amplifiers I have worked on. As with most all NAD equipment, the basic circuit is well thought through and was intended to offer very good audio quality at a reasonable price point.

In common with other NAD amps, revising some of the inevitable compromises that led to those price points, creates a tangible benefit. Additionally many of today’s components were not available when these amps were designed and built. My philosophy is to use the components that the designer would have used had they been available and had cost not been such a concern.

The C272 uses a large toroidal transformer which provides an adequate source, though I did increase the total available capacitance in the power supply. There were also some places in the circuit where it was worth increasing the available capacitance for power rail decoupling.

The 272 is common with other NAD equipment of a similar vintage employs an input buffer module. This, to me equates to a discrete component IC. Given the way it is connected I opted to leave it as is, though there a couple of caps that would probably benefit from replacement.There are signs of the NAD heritage in the design and replacing the electrolytic caps with low impedance, audio and bipolar types made a significant improvement.

The original main power supply caps all measured very well, and were the later versions, as in earlier versions of the C272 the power supply caps had proved problematic. This particular amp also had the original protection IC rather than the daughter board.

Each main channel amp board had new caps fitted as shown below.

The amp design is modular which made it relatively easy to work on, the picture below shows the completed unit, with additional main power supply caps fitted.

The customer has a Bryston set up as well, and the upgraded C272 is giving his Bryston 4B-ST a run for its money, as he states “In a nutshell the C272 compares equally well to the 4B-ST in many respects and may exceed the Bryston in some”

NAD 2155

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I recently upgraded a pair of NAD 2155 power amps. These are stereo 50w per channel units, and as such are an excellent small power amp for efficient speakers. However when switched to mono, they produce 100w per channel, and this is where they represent extraordinary performance.

The circuit differs from their bigger brothers, the 2200/2600/2700 series in that the input circuit uses discrete components rather than the IC used by 22xx series. When compared to a 2700, which had also been upgraded there was a perceptible increase in the detail at the same volume level. The 2700 delivers 150w per channel in stereo and over 400 in mono, but with efficient speakers, the 2155 performs as well if not a little better.

The upgraded 2155 internals

As with most NAD upgrades the power supply was improved, with new main reservoir capacitors and faster rectifier diodes. There was considerable improvement in the low end distortion through this. All the electrolytic caps were replaced with low impedance, audio or bipolar caps, and some values were increased where appropriate. All of the small ceramics were replaced, generally with WIMA FKP, polypropylene, capacitors as were the bias trimmers. A new relay was also fitted, which was attached to the internal frame.

The 2155’s in the owners home set up, the 2700 and 1130 have also been upgraded.-as the customer stated” I am very impressed, even considering the 2155 is about half the rated power as the 2700, I think it sounds as good if not better. Detail and clarity is very good.thanks for a great job.”

Sansui Au505 Restoration

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I recently upgraded a Sansui Au505, which is a small low power amp that is capacitor coupled. Much credit for the results must go to Leestereo at, absolutely brilliant breakdown made this project a delight.

In common with other capacitor coupled amps, in this case Nichicon 4700uf 63v FW types, there is warmth to the overall sound, that is often described as tube like.

main amp board showing new output caps

The power supply caps were all upgraded, using low impedance types, as were the power rail decoupling following the suggestions made by Leestereo. A number of Nichicon MUSE Bipolar caps were used in the audio path as where Panasonic and WIMA polypropylene film caps.Most of the ceramics were also replaced. A number of transistors were changed, as was the dual diode in the power supply. Everything was deoxed. The RIAA equalizer stage was rebuilt as per Leestereo’s suggestions. The Blaxandall bypass and Vbe multipler mods were also done and the trimmers. Overall the majority of the components were changed, which took longer than I thought, but the end result was very satisfying as the customer then stated in a text- “It sounds wonderful.So much more open and clean and crisp. The detail is really present. I ma very pleased and happy”- so now I am working on his Au 555.

Partially finished showing the upgrades to the pre- amp section


NAD 2200 upgraded review

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One of our customers had a NAD 2200 that had been upgraded submitted to the well respected Audioscience review web site. ( The review is very complimentary, and I am pleased that an independent review confirms my own experience with these units.

A couple of further details are worth explaining. When NAD built the 2600 and 2700, both of which use much of the same circuitry as the 2200, they made certain improvements in both quality (e.g. Film and Silver Mica for ceramic caps) and power rail decoupling (additional capacitance), and these have been incorporated in the NAD 2200 upgrades.

Additionally there are some modern components that were simply not available, such as the Schottky diodes and audio IC’s, which have also been incorporated into the NAD 2200.

Please read the full review at:

Superphon revelation preamps

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Superphon is the name Stan Warren used for his range of power amplifiers and pre-amplifiers. Stan was the “S” of PS Audio, which exists toady and makes a fine range of equipment. I was introduced to these units some time ago, and have developed a strong affiliation, especially withe the preamps.

There are three predominant models, though each has some variations.

Revelation Basic: This comes in three versions each of which uses a separate power supply, with a common transformer, for each channel, effectively making the unit a dual mono. I am  not fully certain that this information is totally inclusive, so please reach out if there is further detail or information.

Original revelation basic dual mono: This has two separate boards, one stacked on top of the other, for the power supply for each channel. The components are the same, and these feed the single PCB for the phono and line stages. The main PCB is symmetrical, with the phone stages at the rear of the unit and the line stages at the front. Each channel has a separate volume control.

Revised revelation basic dual mono. This uses the same components at the original, but with a single board for the power supply, though each channel has a separate power supply, which is fed to the main PCB via separate wiring. This unit has the dual volume controls. Early versions has a white line across the bottom of the front panel, later versions did not.

Early version Dual Mono Basic

Later version Dual Mono Basic


Rear panel of Dual Mono basic

Updated revised basic dual mono. This version replaced the dual volume controls with a single stereo control for volume and a balance control. Otherwise it is the same as the previous version.

Superphon revelation II. This preamp uses a 19″ 1u rack format and has the power transformer inside the case. The circuit is quite different from the original basic, specifically the use of FET front end (dual FET devices) rather than the BJT’s used in the basic. There is a single main board with the PS components mounted on it, but the separate power supplied for both channels and symmetrical board layout are maintained.


The superhon II had dual input selectors making it easy to switch between inputs.

Rear of the revelation II.


Superphon SP100– this is a completely different design based on a single IC that acts as a line stage buffer preamp.



The internals of the Dual Mono basic comprise the power supply board, or two of them in the original dual mono, and the main signal PCB.

Original main signal PCB from early (white stripe on front panel) Basic

Power supply board from early Basic


Original internals for Superphon revelation II- not the board is symmetrical, and has a protection relay fitted, which was an additional feature over the Basic.

The SP100 internals, this one has been updated with the original electrolytic caps replaced and the original tantalum caps replaced with WIMA film. Originally there was a TL082 IC fitted, and this was replaced with an OPA2134.



The component selection was very assiduous, and Stan made mention of caution in changing the components. However is various posts in DIYaudio ( he mentioned that given the increases in quality of some components, there were improvements to be had.

Upgrades to the Basic dual mono.

The power supply caps are increased in size and the tantalum replaced with either Nichicon KL or Wima film. The two bridge rectifiers are also upgraded, all in line with Stan’s comments.

On the main board, the tantalum’s are all replaced by Wima film caps, and the electrolytic’s by either low impedance Nichicon PW or similar, or ELNA SIMLIC II or similar.

The result was a lower noise floor and slightly better THD+N figures. Sonically there was an improvement in dynamics and transient response.

Upgraded Basic-note that the Nichicon VX series capacitors are now longer available

Upgrades to the Superphon revelation II

The power supply was upgraded in a similar manner to the basic, larger power supply caps and bridge rectifiers.

The main board also had the tantualms and electrolytic’s replaced. On one unit the relay was replaced (as it was faulty), but there was no measurable difference between a unit with the original relay and one with a new relay.


Upgrades to the SP100

As mentioned new electrolytic caps and replacement of the tantalums, as well as a new IC to replace the TL082.



Mitsubishi DA-30 an underrated gem

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In the 1980’s Mitsubishi made a series of amplifiers that are somewhat unknown, however they are really standouts in the their class. The DA 30 is a plain looking 105w RMS per channel power amp. It is not till you open the case does the design reveal itself.

The amp is basically a dual mono design, with two transformers and separate power supply for each channel, and unlike others of this vintage (NAD, APT etc) this unit can only be operated in Stereo. The power supplies are completely independent, with 2 10kuf caps per channel. The input/voltage amp and bias sections have their own independent regulated power supply which is tapped off each transformer separately form the output stage power supply.

The front end of the amp uses a dual FET for the input stage (2SK109) which then feeds the voltage amp stage. There are two inputs marked AC and DC. The AC input has a cap (non polar) across the input whilst the DC input does not. There is a volume control on the back panel- though generally this is set and full open.

There are no capacitors in the audio path through out the whole amp. The main outputs are the well respected Sanken 2SC2837/2SA1186. There is an over power detection circuit and a protection circuit with independent relays for reach channel. The two front panel leds indicate power and protection status.

The other unique feature is the heatsinks which use a copper “heat pipe” coupled to a set of fins, which appears to do a very fine job of handling the thermal energy from the amp.

So having examined the amp, of course this looked like a great amp to upgrade! So all the electrolytic caps have been replaced with low impedance types, Nichicon PW and HE, as the are all associated with some form of power delivery. Where appropriate some values were increased, though the original design provided adequate capacity.

The outcome is a very good sounding amplifier, neutral and quiet and transparent- in fact so much so that I use one as my “reference” in the lab- connected to a modded APT pre amp and B&W speakers. The sound is simply whatever the music is delivering, the bass is tight the mids and highs neutral, with plenty of power for transients and excellent dynamics.

This amplifier is certainly underrated and deserves its place with the other stand outs form that period , the NAD 2200 and APT 1.

After the upgrade ready to have the cover put back on- note the dual power supplies and symmetrical nature of the PCB layout

on the bench fully restored
There is support for 2 sets of speakers, which can be switched on the rear or remotely via a separate meter bridge which is connected via the 8 pin socket seen here. note the “bumps” which are where the heat pipes are situated.

APT 1 Power amplifier

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Finished pair operating as mono blocks with APT1 pre amp

I have worked on a number of APT Pre amps and have been impressed with their performance, so I decided to see how the APT Power amps performed.

These are truly amazing amplifiers, the circuit is DC coupled- so no capacitors in the audio circuit path- which matched modern design principals and was very innovative for the time.

They include a DC servo which tracks input and output to reduce distortion to a minimum- again innovative for the time. A single IC on each power amp board uses the two channels of the original TLE072 as an overload detector and a DC servo. In the updates I installed TLE2072 as a lower noise replacement.

On the power supply there are two 10000uf 75v main supply caps, to which I have added a 4.7k 2w resistor to drain them, as they have a nasty habit of retaining their charge(which is clearly explained in the manual and on the amp itself. I have tried to find replacements that fit, but to no avail and all the ones I have worked on (5 to date) have tested well.- Also they are -10% +50% types.

Power supply board with new capacitors – the relay and mono/stereo switch yet to be replaced

The Power supply board electrolytics were all replaced with either low impedance or low leakage types for the small values the larger values were increased somewhat. The IC acts as a comparator, again a TLE072 which was left as is. The mono/stereo switch and the relay were replaced as both were noisy.

The power amp boards have had all the electrolytic caps replaced- with low impedance, low leakage and where appropriate film caps. Most of the of the ceramic caps have also been replaced with film types, or NG0 MLCC ceramics as appropriate. Also the input resistors have been replaced and the input decoupling cap removed.

One channel amp board, with two ceramic caps yet to be replaced-the main PS caps are partially visible as well

The wiring can present challenges when working on the boards, so each connection is checked, and as they are relatively compact there is some fiddling involved in removing the amp boards from their heatsinks.

Amp board mounted on the heatsink

The result though, especially when run as mono blocks is astounding- they have great dynamic range, transparency and detail. The combination with the upgraded APT pre amp punches well above its weight in terms of cost/performance- and of course they have that great retro cool look.

A set finished with rack mounted panels

Revox B780

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Revox was the consumer brand of Studer, who made some of , if not the, best analog tape machines used in the worlds great recording studios. Back in the day I worked on the A80 (24 track and A800 (24 track) machines, sometimes slaved together to create a 48 track machine. These all used 2inch tape (my favorite was Ampex 456) and were run at either 15ips or 30ips. My all time favorite was using the Studer 1/2″ tape machine running at 30ips (inches per second) to master onto as this captured all the information that the mixing console (SSL/Neve/Harrison etc) was capable of providing. Over the years I had owned a number of Revox tape machines, the A77 and B77 which were also excellent examples of electro-mechanical  construction.

So when my friend asked me to “recap” his B780 FM receiver, of which he had two- one purchased as a spare-I leapt at the opportunity. Revox HiFi gear has always been appreciated and was at the more expensive end of the spectrum when new, though as you were paying a premium, you got a top quality product!

The Revox, in line with many Swiss and German engineered products has a very specific method of construction, which when understood makes working on these units very straight forward- or appearing to be so. There are a number of circuit boards connected by a wiring harness, each board can be removed from the chassis and worked on separately.However there area couple of complexities……

Power supply board: This board supplies the low voltage rails to all parts of the receiver, except the main power amps. At the time Revox, and a number of other European brands, for example Tandberg, used “Frako” electrolytic capacitors, which have not stood the test of time well at all. If you have a unit with these installed, then it is a very good idea to get rid of them. It was also very popular at the time to use Tantalum capacitors both for their small size and low leakage, and without wishing to add to the long running debate on their applicability for audio, I come down on the side of always removing them from the signal path or any audio related functions (including power supplies), however I do tend to leave them in place in RF circuits where they are not involved with the audio chain.

So the power supply board is now fitted with new Vishay Electrolytic capacitors, as the mounting is axial rather than radial, and where applicable I tend toward the Vishay units over the Nichicon VX series. Additionally Nichicon KL low leakage types replaced some tantalums as did WIMA film for the lower value types. Finally Nichicon low impedance types were also employed. The three bridge rectifiers were also upgraded.

Power amp boards: These have separate power supplies for each channel to provide sufficient independent power capability so that a heavy load on one channel will not drain these resources for the other channel. Although not quite as isolated as two independent transformers, this deals with most of the issues created by using a single supply for both channels, or worse a single supply for all the amplifier/receiver functions. I have read that there is a cogent argument for sharing a single transformer, though my personal preference is to have two completely separate power supplies. The photo shows new and old power amp boards side by side, with a polystyrene cap in place of one of the ceramics.

Now the fun began. The power supply boards mount the reservoir capacitors and bridge rectifier on the track side of the board- which makes their removal and replacement somewhat difficult. The capacitors have small stand offs and four connectors, of which only two are active, though fortunately a company ( provides a replacement kit of 4 10,000uf 63v capacitors that replace the original evo 4700uf 63v units. Without these it would have been extremely difficult to undertake this work. Having mastered the challenge of removing these, involving the use of a jewelers saw to get underneath the caps to remove them, the rest of the upgrades went relatively smoothly.

There is another limitation  to these units, which is that some of the PCB’s have tracks that lift with even the smallest amount of heat (applied with a Hakko vacuum desolder system), which resulted in having to track down minute and invisible breaks in the PCB or use the leads of the new components to replace lifted track. On one of the units there had been previous repairs undertaken, and I was relieved to see from this unit and read from others experience that I was not the only one with this challenge.

On both units the output transistors had been replaced, in one case by a good choice and in another by an less good choice, so I replaced the now unavailable 2N6031/2N5631 with MJ15004G/MJ15003G respectively on the unit with the less good choice. The original bridge rectifier was also upgraded in support of the larger value power supply capacitors.

All the electrolytic capacitors were replaced with either Audio or low impedance types, and the low value tantalum by a WIMA film type. Two ceramic capacitors were also replaced by WIMA film types (or on one unit by polystyrene).

Finally the input stage of the power amp employs an LF357 op amp, which has a high slew rate and relatively low noise, however op amps, like most all electronic components have progressed over the last decades and I decided to try some alternatives, however the only drop in replacement is the very expensive OPA627, which at $30 each  certainly impacted the budget. On reflection it would have been a good idea to add some further decoupling to the IC power lines (there is none originally fitted), however with the experience with the PCB tracks and lack of space, I opted to forgo this. Given the cost of the IC’s and one unit being a spare, we left the original IC’s in one unit and installed the OPA627 in the other.

There was a difference in sound, the OPA unit having a more transparent and dynamic overall sound compared to the “warmer” LF357 unit, though I was really impressed by the overall balance of sound delivered by both of these units and would be happy with either configuration.

The rest of the unit received the same treatment, use of WIMA film types where appropriate and possible, especially in the replacement of the tantalum units, Nichicon Audio (FG/KZ/KA) types and low impedance types (PW/HE), ELNA SIMLIC II and some MLCC CGO ceramics as appropriate. In all cases the original capacitance values were replaced like for like, though usually with a higher voltage rating. The RF sections only had their electrolytics changed and the CPU board was left alone.

The result is two fine sounding receivers, which deserve their premium reputation, though working on them certainly provided some challenges.



Photo show the second unit prior to upgrades, with the rear heatsink removed and feet with appropriate footwear

UPDATE for OPA 627 as LF 327 replacement.

There was an intermittent problem with the OPA627, a low frequency oscillation that would occur randomly. The OPA 627 shows pin 8 as NC, as does the original LF 357, however on the revox pin 8 is tied to earth. So after some assistance form the folks at the TI user message board, the OPA 627 was removed and replaced with an OPA 134, with Pin 8 lifted so that there is no connection between that pin and the IC socket. The amp now sounds fantastic again, and there was little or no difference (measurable or listenable) from the OPA 627.

Harmon Kardon 730

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I have worked on a couple of HK330 and wanted to know what the bigger brother would sound like. The Marantz 2230/2235 and Pioneer SX737 are the equivalent contemporary peers of this receiver and share a number of common design philosophies, however the HK*30 series has a dual power supply as the most significant difference. It has always been my belief that two independent power supplies are superior to a single power supply in power amplifiers, so when the opportunity to upgrade a HK730 came along I could not resist.

There are a number of areas which can be effectively upgraded in this receiver and each of these is outlined below.

Power supply: The original power supply used 4,700uf 50V reservoir capacitors which have been upgraded to 10,000uf 80v (the voltage only because of the physical dimensions) and the bridge rectifiers  have also been upgraded in capacity (amperage) and voltage. The other electrolytic capacitors have been replaced with low impedance types (Nichicon) of the same values. The lower voltage rail that supplies the lamps and indicators has a separate power supply board, keeping this supply away from the audio chain. Again all the electrolytics have been replaced with low impedance types.

Power amp boards: The electrolytics have been replaced with either Audio quality (SIMLIC II) or WIMA film types for the audio path or low impedance Nichicon types for power supply decoupling. Some of the original ceramics have either been replaced with WIMA film or CGO MLCC ceramic capacitors.

Preamp: All the capacitors have been replaced by either low impedance or audio electrolytics for the higher values or WIMA film types for the lower values with some original ceramics having also being replaced by MLCC CGO Ceramics.

Equalizer(Phono): There are a number of modifications that improve the RIAA equalization curve that have been published and tested and these have been implemented,with larger capacity electrolytics, both Audio and low impedance types with film WIMA used in lower values and replacement of some ceramics as described above. The overall result is an improvement in the accuracy and bass response of the phono section, with a number of people preferring this to the Pioneer and Marantz equivalents.

RF section: Here there was a minimal intervention approach, replacement of the electrolytics with low impedance and audio types and replacement of of those tantulums in the audio chain with WIMA film types.



Finally the lamps were replaced with LEDs giving that great green glow.

Overall the receiver sounds excellent. Subjectively there is more dynamics and transparency  with a lower noise floor and certainly less distortion than the original. There is a “warmth” to the overall sound that is quite appealing and with suitable speakers, say JBL or other relatively high efficiency types, the overall sound will be certain to impress.  In comparison to the  Marantz or Pioneer- and this is all from memory, so highly subjective, the HK has more “oomph” and a relaxed sense of “power”.

So yes my hypothesis that two power supplies is better than one is borne out in the power amp section, and the phono stage is really very good indeed, which makes these HK series very appealing….and then there is that great green glow…



The picture to the power supply shows the power supply under rebuild with the new capacitors installed on the left.The picture below shows the rebuilt equalizer board, which in this example also includes new low noise transistors, which have been matched for Hfe. There are also a couple of resistors that have been replaced with modern 1% tolerance types.

The picture below shows the rebuilt power amplifier board


McIntosh MC75 overhaul

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I have had two of these since around 1978, and they have traveled the world with me. In the late 1970’s I did a recap of most of the capacitors and replaced all the resistors and the main diodes. Then in the mid nineties I replaced the main power supply capacitors again (the 250uf units), but left in place the lower values (100uf and 30uf) from the earlier refit.

So I had just finished a pair of NAD 2200’s, so I thought now is the time to take these out of my listening rig and give these mac’s some love and attention.

So I have replaced all the capacitors, electrolytic and film, which you will see in the photo’s- The main PSU caps are Cornell Dublier 250uf 350v, the 100uf and 30uf are double 100uf and double 30uf 500v cans.

The 12uf are all Sprague 250v and the films audiocap.

The original diodes are now UF types. Most all the resistors have been changed as well. I also reinstalled the input caps which I had removed previously, though I may remove them again.

Aside from the usual drilling out of rivets, the whole exercise went very well. They received new tubes “golden dragons” and Sovtek 6550’s I had bought in the 90’s as spares.

They do sound great once again , though I am tempted to try some different  tubes, especially that initial 12ax7, and perhaps some other KT88/6550 types.