Category

Uncategorized

NAD C272

By | Uncategorized | No Comments

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

By | Uncategorized | No Comments

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

By | Uncategorized | No Comments

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 Audiokarma.org, https://audiokarma.org/forums/index.php?threads/leestereos-restoration-upgrade-of-a-sansui-au-555a.816754/whose 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

By | Uncategorized | No Comments

One of our customers had a NAD 2200 that had been upgraded submitted to the well respected Audioscience review web site. (www.audiosciencereview.com). 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:

https://www.audiosciencereview.com/forum/index.php?threads/nad-2200-vintage-amplifier-review.13960/

Superphon revelation preamps

By | Uncategorized | No Comments

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.

 

Internals

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.

 

Upgrades

The component selection was very assiduous, and Stan made mention of caution in changing the components. However is various posts in DIYaudio (www.diyaudio.com) 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

By | Uncategorized | No Comments

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

By | Uncategorized | No Comments
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

McIntosh MC75 overhaul

By | Uncategorized | No Comments

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.

 

 

 

work in progress

By | Uncategorized | No Comments

So I have been very busy with some interesting upgrades and refurbishments. I will post more details latre but as a summary (and aide memoir):

Revox B740

Adcom 545 II

Onkyo A-7

APT Preamp

my own MacIntosh MC75 tube amps

and three NAD 2200’s

descriptions and photo’s to follow