[ZIG]

Saving up for a LIFEFORCE 55 .......can't wait.....would have been happening now if it weren't for the need for a new fridge, microwave and washing machine!!! .......yep......you guessed it......all crapped themselves at the same time.........unbelievable bad luck

Anyhow, for the moment I am using the above amp, modified with IXYS rectifier bridge and some good filter caps. bypassed by some big ass GE oil caps etc.
My reason for the interest in Aspen amplifiers is that the power output and simplistic circuitry is similar to a point

The sound, after being a valve nut for over a decade, has opened my eyes to the possibilities and transparency, speed and dynamics of solid state amplifiers - not necessarily high powered arc welder types either .It sounds absolutely superb - pacey, musical and toe tappingly involving.

I have the service manual for the kenwood and am a bit worried about a small SMT op amp which is labelled as a ''BUFFER''. I don't know whether this ''buffer'' can be bypassed or if it has any gain on the inputs.
The I.C is a NJM 4565M. The output pin and - pin are connected together without any components what so ever, and this output signal goes directly to the left and right input pins of the electric volume I.C.

I realise this is hard to comment on without a shematic, but I would like to get the most out of this amp and avoid any unecessary circuitry.

Can anyone help?

[AKSA]

Zig,

It does sound like a buffer, it's certainly being used as a follower prior to the volume control.  It's an integrated amp, I presume?

A few of the Japanese integrated amps are very good, no question, I would never dismiss them out of hand, and they are very good value.

An opamp generally has a limit of about +/-18V, so can only be used at line level and into the front ends of power amps. You really do need to get a circuit schematic for this amp if you want to improve it.  Be aware, however, that you are in for a seriously difficult exercise.....

Hugh

[ZIG]

Thanks for your response Hugh. I presume that in this case, as all inputs are switchrd in to it, this ''buffer'' has gain for the front end of the amplifier?......so, it's really just an op amp as such and shouldn't be removed?

Anyhow, as you say, this amp must have been one of the better ones at the time, because it absolutely trounces my Pioneer home theater receiver at $2,500 +.
The amount of detail retreival and speed is truly outstanding(the rectifier mod , I think was mainly responsible for this I suspect).

[konut]

Sorry to intrude with a somewhat off topic reply but, select Kenwood amps reflect some serious engineering chops and can sound surprisingly good in the right set ups. They are often mistakenly labeled as mid-fi dreck and can power a sleeper hi end system righteously.

[ZIG]

Right on konut!........it's these engineering chops that do way with complexities in the signal path - and that's probably the reason why they can sound stupendous in the right system as you say.
There will be some cap mods in the class a and a/b areas + volume i.c areas soon so it will be interesting to see what happens.

Hugh, or anyone, there are some electrolytics with values of 3.3mf, 2.2mf and 1.0mf that are directly in the signal path. Can they be replaced with some 0.22 paper in oils or something similar(may need to be raised off the main board with another larger veroboard or something)?

[rabbitz]

At a guess I'd say those caps are on the pre input, pre output and power amp input. Changing the values to lower values would more than likely upset the frequency response such as the bass rolling off earlier so if changing, use the same values (some cases a bit larger).

Generally on the older amps I've found the best results have come by changing the main power supply caps to the next higher value using a good quality cap and change the signal caps <1uF with MKT or MMK and those >1uF to Nichicon ES or Blackgate N (can also be used for <1uF). Other changes were not worthwhile nor not worth the effort and cost. Using physically larger caps offboard can lead to pickup of garbage plus can end up costing more than the amp. Better off putting that coin towards good diy amp and believe me they can make a huge difference over a commercial amp.

You can check your schematic against buffer circuits to be sure it's a buffer only with no gain. You could change the opamp to a LM4562 or maybe a OPA2134 and perhaps get a better sound. These opamps do require the power supply to be suitable with power supply caps at the power input and by-pass caps etc. You can get that info from the data sheets and diy pre amp projects.

Don't go nuts as you'll start hearing improvements that aren't there once the money gets poured in.

[ZIG]

Thanks for your comments rabbitz.You are quite correct on the position of the caps.

As I have quite a few 1.5mf MKT caps on hand, I'll change a couple at a time to see if there are any noticeable improvements.

 I have the schematic of the entire amp and the IC5 is labelled ''BUFFER'' and looks like it is configured as a buffer - going straight to the volume I.C. All other inputs lead to it via the switching I.C 's.

Problem is....this is a tiny surface mount device and is a pain to remove or replace.
Would it be possible to bypass it entirely or would that affect the dynamics or power/pace of this amp? I would hate to loose that sonic trait.

At present the amp is ver ''quick'' ''dynamic'' and definately ''musical''. It does however seem to have some midrange ''Glare'' which is not that offensive and has already been reduced quite noticeably by using a high quality rectifier and better, bigger caps with multiple bypasses.
Luckily, the experimental power supply bypassing doesn't seem to have caused any nasty unwanted noise.......I think.

I'm hoping that this glare is the result of the electrolytics in the signal path and not the power supply caps and bypassing?

[rabbitz]

I'd leave the buffer intact as would set up the impedance for the next stage. All the amps I've seen have the buffer / active stage after the volume control but there are some where it's on both sides.

You can get adaptors that replace the SMD with a DIL op amp that's replaced in an IC socket. Similar to this but would have to be dual SO-8 to dual DIP-8. It allows easy op amp rolling but still have to remove the exisiting SMD.

http://www.soundlabsgroup.com.au/p/BD-031101/Single+to+Dual+Op-Amp+SMD+or+DIL+to+SMD

Op amp manufacturers do however recommend the op amps be placed on the PCB and not in sockets etc.

[zygadr]

Thanks again rabbitz.

Sorry about the Moniker change - had to do it due to some glitch with my user name and password
 I think I'll replace the smd op amp with another smd device.

Any recommendations on a brand/model number in a dual op amp that would allow a noticeable improvement? I'm looking to smooth out the sound, perhaps warm it up a tad?

[rabbitz]

I've had good results in CD players and stand alone buffers with the LM4562. The OPA2134 is very good but can be a bit foward in it's presentation and more for a laid back amp that needs a lift.

A lot like the OPA627 (not me) due to it's warmer presentation but it's a single and would have to be mounted on an adaptor. I've tried a few AD opamps but none floated my boat except for AD825 in CD players.

This link may give you some ideas. http://www.soundlabsgroup.com.au/c/ICs/ICs.html

The other option is a Burson Buffer op amp module but could mean a bit of surgery to adapt. They are fickle on supply voltage and don't start to sing until they have at least 15V rails.

[rabbitz]

One other thing.

Replacing an op amp can be more than just swapping as they depend on circuit design and application for their success. Some require power supply modifications as well as power supply caps situated next to the power supply pins plus the use of by-pass caps or by-pass across the rails. Have a look at the data sheet on any you consider to check the requirements.

[zygadr]

All noted rabbitz......you obviously know your op amps

Well,.........I went crazy over the weekend and completely bypassed everything : all that works is the main amplifier stage and a passive volume pot that I connected up with one input facility as a quick test.
I connected a tuner up to it at volume set midway - speakers were some old JVC mid- fi 6 inch two ways of medium sensitivity.
The sudden burst of volume nearly gave me a heart attack!

Haven't had an opportunity to hook it up to the main system in my listening room, but I can already hear a clearer, richer and more natural tonal balance. I have used these speakers with the Kenwood on many occasions during the power supply/cap mods, so I know their sound quite well.

Will report back this coming week on the sound quality.

I would expect some change as I have bypassed a lot of extra circuitry and I.C.'s including the buffer.We'll have to wait and see and I'll try to be as realistic as possible on the results of this brave move.

[zygadr]

Just while I'm playing around with this Kenwood, I notice there are numerous ceramic bypass(?) caps near some of the to92 transistors.
Is it worthwhile to replace these with mica, polystyrene, or any other film cap of small size?

[AKSA]

Yes, Zig,

Try polystyrene.

Hugh

[zygadr]

Thanks Hugh....I'll try the polystyrene caps......have heard they can make quite a difference.

Report on total bypass sound quality : as expected,a massive improvement - sound stage, air, smoother, detailed and ''glassy glare'' has gone.
Looking at the datasheets of the electric volume I.C and switching I.C's it's not hard to see why. The bypass loses many components in the signal path that are purely there as ''features'' in a non audiophile way - didn't realise that the volume I.C had a whole chain of bass treble etc circuits within(that I thought I had bypassed externally)....same goes for the switching I.C.'s - signal path goes through mosfet switching!

Hugh, I can now see why your amplifiers sound so good with all those positive reviews and comments. You keep the K.I.S.S. method in your design and use good quality components that are obviously selected with care.
Can't wait to own one of your amp's

[zygadr]

 I know I can't stop my raving on.........but I have found the sound of this amplifier is now extraordinary

Having layed off the FM radio for a couple of nights(my main listening source of late) and played many of my well known CD's, I am hearing things that were not previously evident before. I'm talking about comparisons to the sound obtained from the so called revelatory single ended valve amps and other high end stuff that I stiil own and friends who have compared their main amplifiers to it(some very expensive stuff ).

I can only say that I never expected this to happen and was only fooling around with components I had at my disposal and a service manual that I got with the amplifier.

I say tweak, modify and rip out the unecessary guts out of anything similar(if you know what you're doing ) and try my mods and bypassing.....you won't be sorry IMHO.

[zygadr]

Well, originally The main supply caps in this Kenwood were just a couple of smallish 4,700mf 63v standard Elna's - the usual rubbish found in price compensated amps.

Being a highly knowledgeable audiophile hero, I removed them then ran two sets of wires to a triple bypassed besswax encased, ( ...yes, you read correctly, I enclosed them in to a melted beeswax casing, forming a square block with the caps suspended inside!) pair of Samwha ''for audio'' (HA!!) electros (I found a capacitor test recently on the net that revealed these to sound ''harsh''  ....now they tell me!) salvaged from a Sherwood home theatre receiver, and bypassed with Solen polypropylene caps and and a couple of Sprague Orange drops.
 This Improved the sound or so I thought, from the stock caps.

The problem all along was a harshness or metallic ''glare'' that persisted to overshadow the overall sound despite my power supply and extensive circuit re-routing and bypassing mods.
I attributed this left over ''glassy glare'' to the remaining components associated with and around the amplifier circuitry as discussed before here on this forum.

Boy, was I wrong!..............yesterday, luckily enough, I scored free of charge, two gigantic blue, 5 inch diameter, 8 inchtall, baked bean size cans   :Nichicon 4,500mf 400volt electrolytic capacitors
I thought, why not?.....let's give 'em a go!...........so I disconnected the other caps and replaced them with these.
Result?...............WOW!....................AMAZING!..........no more glare......completely gone - sound is smooth , warm and fast/powerful.I only had a couple of hours to listen to the set up as we had to leave the house to visit friends.

As the monster caps have been dormant for some time, yet measure perfectly, I have left the amplifier running for some 15 hours or so and will resume listening tonight(which would have given them a 24 hour re-form period - maybe that's not enough still?).

So the point is ................watch that power supply!............they're all correct when they say it's all about the supply, then comes the topology(which is also important of course)

[zygadr]

Well, after a 24 hour re-form, the new caps are a far step ahead of the Samwha/besswax/bypass concoction

Sound quality has a transparency and speed that is truly enjoyable to listen to. Initial warmth has now been replaced with clarity........hey.......I'm not complaining

I will still, however, this weekend, replace the remaining electrolytic caps in the amplifier circuit (only - as I haven't yet got the polystyrenes )with Black Gates.There are also a few resistors in the signal path that will be replaced with metal film types.

 The amplifier will stay ON at all times from now on to allow for the B.G.'s to burn in.

I counted a total of only 8 transistors per channel (if you count a Darlington pair as 2 transistors?) that make up the amplifier circuit which puts out around 80 watts/channel at 0.09% total harmonic distortion. Pretty simple eh?........could be the main reason for it's unusually good sound quality

[zygadr]

Ripped out all the power supply guts : transformer, power supply caps, power switch/ switching relay board.........everything to do with mains/grid/A.C. 

For fun, rigged up two 2.0ah SLA's to provide +/- 12 volts( about 1/3 of normal supply voltage).Had to connect 12 volts B+ to another point in the circuitry to make it work(as there was a separate transformer winding and D.C. supply for this previously.

It works!! ..............surprisingly well considering the reduced power output and especially compared to the sound with mains power and all those wonderful caps I've been ranting on about.
What I heard briefly was the unmistakable battery sound : truck loads of deatil and a crystal clear sound with the obligatory inky black, silent background

 I have only tested sound quality so far on some old 10 inch 3 way speakers, but it can go damn loud before distortion and these are not high efficiency speakers.

At a guess the power output would be 12 watts per channel or so. May or may not be enough for my main speakers which are about 90db sensitivity.

It'll be easy to up the power if necessary.....they're only batteries