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The
Attack of the Clones
In
the two weeks preceding our third Analog meeting in Sneek I worked hard to finish
the GeenKloon in time. After all, Sodejuu and I promised to each other that
this would be an excellent occasion to show our GainClone models and have our
peers review both projects. To my surprise it needed more work than expected
to finish the GeenKloon in time and the day before I was still in doubt whether
it would be a good idea after all to show the GeenKloon to the public.
It is also the main reason that initially I did not put pictures of the GeenKloon on this page, as I did not find the time to make nice pictures of the GeenKloon in the various phases of the building process. Therefore, the GeenKloon has been opened more than once after completion and most pictures on this page are taken afterwards..
And after finishing building this fun project, which got slightly
out of hand by the way :-) , I sure hope that everybody agrees with me on the
looks and hopefully also on the sound.
Before
starting this project, I had to take a few decisions myself about components
and basic design. My goal was to make a GainClone with a better than average
sound for an acceptable budget. And as I had no experience with PowerOpamps
I did not feel the need to start off with very exclusive components such as
Black Gate capacitors and very rare resistors. After all, these modifications
may be done at a later time anyway should the GeenKloon sound OK.
But first of all I needed to decide on which Power OpAmp to use and which components to use and which ones to leave-out. At that time I was looking at all kinds of DataSheets on the web just to find out that there is lots of PowerOpAmps available but in my country nobody is selling them for private use. Just at the right time I received a terrific offer from my hear.nl colleque Sodejuu who could offer me some spare LM1875 and LM3875 units for testing. As the housing of the LM3875TF package was completely made of plastic which does not need electrical isolation, I decided to go for the LM3875 design which also delivers more output power than the LM1875.
As shown on the picture above, I mounted the LM3875 on an aluminium tube of 2mm thick. The piece of aluminium is tightened to the bottom of the aluminium housing, and the LM3875 is mounted on top with its pins easy accessible. In order to have the star-earth point nearby I drilled a small hole 1.5 cm below the Opamp on which some lugs are mounted that should provide the central point for ground connnections.
Just
in time I changed my mind on the attenuator. An integrated amp setup would allow
me to make much better use of the amp. And since I promised to bring the amp
to a show it would offer much more flexibility if I did not need a pre-amp in
order to show the GeenKloon to the public. Initially, I calculated the resistor
values with a program of SDS
labs, but as it is very easy to make your own resistor array using a spreadsheet
I ended up writing something myself which I like even better.
Please find more background on building the attenuator as well as an Excel spreadsheet on this page.
This is where I ended up with as I needed to use resistors from the E96 range (sold by Conrad) which were not used by the SDS program per se.
I used silver wire with a PTFE shield for all internal wiring. the exception is the wiring of the input section which is done by Prefer microphone cable. The black wire that runs from the attenuator to the input of the OpAmp is 0.35mm silver with PTFE isolator.
Why did it take so long to finish this project? After all, soldering 9 components together and put it in an enclosure sounds like a simple job. First of all, it often pays when spending as much time as possible in the design phase. So I spent a lot of time thinking how I should put all 9 components together on such a small footprint minimising the length of the signal path and at the same time allowing for later modifications when necessary. Therefore I was in doubt whether I should put the OpAmp upright, on the floor of the box or whether I should make a construction for cooling which would allow me easy access to the OpAmps connections. I choosed the last option. And I think I succeeded in getting this very short signal path that is given so much attention in reviews of the Gaincard.
Secondly, I had some spare enclosures of harddisk drives (old ones) and initially I planned to make this a real low-cost project. However, later I decided that the two housings did not offer me trhe freedom to make something really nice and therefore I used the two Conrad housings (see picture on top of page).
And as the transformer and the two housings took most of the budget, I decided to spend some time to at least get the most out of the components inside the amps.
Anyway, first of all find below the schematics I used for building my amp:
The Schematics above are based on the standard non-inverted GainClone
configuration. I did some calucaltions on the feedback resistor and other components
in order to check the gain etc. Please find some more background on the calculations
relevant for GainClones on the background section
<LINK> to GainCalc background
Not found in the figure, but present in my amp, is a 12-step attenuator between the input busses and the input impedance. The resistor values used on this rotary switch are found in the table below. Please make sure that you assemble the switch in the correct way to avoid having to redo it all again (yes, like me):
| from | to | resistor | attenuation | attn (dB) |
| 1 | 2 | 5600 | 1 | 0 |
| 2 | 3 | 3900 |
0.719 |
-2.87 |
| 3 | 4 | 3000 | 0.523 | -5.63 |
| 4 | 5 |
2000 |
0.372 | -8.59 |
| 5 | 6 | 1500 | 0.272 | -11.32 |
| 6 | 7 | 1100 | 0.196 | -14.14 |
| 7 | 8 | 1300 | 0.141 | -17.01 |
| 8 | 9 | 750 | 0.076 | -22.40 |
| 9 | 10 | 390 | 0.038 | -28.37 |
| 10 | 11 | 270 | 0.019 | -34.62 |
| 11 | 12 | 100 | 0.005 | -45.98 |
Simpler
than this is almost not possible (better is). As the original Gaincard power
supply the Power Humpty makes use of only one transformer, 4 diodes and maybe
some small capacitors I strictly adhered to that same diet.
As shown on the picture, the box I used for the power supply is really too big
and much less elegant than the boxes used for the amplifiers. Well, in any case
there is room left for another transformer and stabilisation circuitry enabling
private power supply for each amp in a later stage. This is in line with the
original; It is possible to bi-power the gaincard by adding a second Power Humpty
also for even better sound quality.
Electrical Safety is a point of concern. The transformer is capable of driving lots of currect and therefore it is very much advisable to mount a IEC chassis connector with fuse in the powersupply. Optional but definately to be considered are fuses for the power lines ging to the two amps. (-30 and +30 volts, 3A), I use one at the moment as seen on the picture above, and I tell you it worked last week.
The power Supply design could be a little bit too simple: I connected two secundary wires of the transformer such that it delivered -22,0,+22 V AC. The connected wires are used for earth-grounding and are the 0V reference point for the amps. The 44 Volts AC is rectified by 4 diodes and two small capacitors in the powersupply do the inital flattening of the DC. After all, the main 1000uF capacitors are found in the amplifiers on the pins of the LM3875 power opamp.
For the power cable I made use of my new "Net-3" power cable which uses twisting and braiding to increase the inductance (and eliminate HF). Especially since the 47labs website and other GainClone sites warn us for using a netfilter, the Net3 power cable may be a better alternative.
The
good thing of building dual-mono designs is that it takes only a half-finished
amplifier to be able to test and listen. OK, still mono and far from ideal but
at least you get a feel for the sound qualities.
Well the first thing I noticed when switching the amp on was that I had built the 12-step attenuator the wrong way. After having been unhappy with myself for some time I desoldered all 11 resistors and mounted them in the correct sequence and after that the music started.... (and I was able to really control the volume)
The second thiong that struck me was the low temperature of the LM3875. in articles about the power opamp a lot of attention is paid to cooling the chip. Not that a high temperature would destroy the chip, it is well protected, but it would prevent us from playing loud. Well, even when playing for a longer time the opamp does not become very hot, it may be that my cooling solution works, but I take it that I do not push the opamp to its limits in the current setup.
Third, the sound is entirely different from my other (5) amplifiers. The dynamics are very good, there is lots of bass and both my wife and I heard things in our setup that we did not notice before. It may be the combination of a cheap 5-year-old Sony CDplayer and the older B&W speakers I use for testing equipment, but it was definately different (and good). You can imagine that I worked hard to finish the amp after hearing one channel for 10 minutes.
It was already late when I wrote these lines (in Dutch) for the first time, and I remember playing Eva Cassidy too loud that day. Every now and then i switch back to my main hifi setup to hear the difference and make my judgements. The GeenKloon is less polite in sound than the big Maranz/Dolce-Vita setup, and basses seem to be more present also. The first may be caused by the cheap Sony CDplayer and that's why I switched back to my very old Philips CD104 with guest performance of the new Sony SCD 940 SACD player. Indeed we lost some of the harshness but still the GeenKloon is less polite and we like this setup for songs and bluess music. For the basses, it may be that the sound is somewhat impacted by the powersupply and its less than perfect regulation. Hmm, read the last paragraph on this pag on Power Supply modification in order to know why I think we're doing OK now.
It will be hard to make the design any simpler, but I think there may be some room left for improvement. There is no room for cost-savings in my design if you want to keep the sound quality on the same level. Let me share with you the areas where I think the GeenKloon may be improved.
And now, lets get on with listening to our favorite music...
Today (somewhere in November 2002) I decided to change the power
supply. The existing 1N4007 diodes used for rectifying were exchanged with 
BYW29E/100,
faster types and more power. Also I did some measurements in order to check
the DC stability of the powersupply for the amp especially when playing loud/difficult
music.
On the scoop it was clearly shown that the power drops from
30Vdc to around 26Volts (and thus from -30V to -26V on the Vmin line, settings
are 10V/div). This may seem a lot, but it was much less than I expected and
the amp behaved nicely and remained reasonably stable.
After changing the rectifier section it seemed that there was
more punch in the basses 
and
that the amp was able to cope with dynamic music on full power without distorting
too much. I did not compare one-to-one with the scope and don't want to give
the impression that changes were so obvious. There are already too many people
noticing earthbreaking differences after minor mods, and I'm not one of them
so leave it to them to judge my modifications and to yell about it.
Anyway, this is how the rectifier section of the power supply
looks inside after the modification.
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