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Old 2nd November 2005, 01:25 PM   #1
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Post My upcoming LM4780 design

It's been half a year since I heard about Gainclones, when I got my FX-120 speakers.

I collected a huge amount of informations on this forum, and surfing the net.

I basically wanted to build the basic LM3875 non-inverted GC, but finally I went for a more ambitious project :

- two Bridged or paralleled LM4780 to build a stereo amplifier
- all P2P connections for ultra-short signal path
- many trials concerning the feedback network and amplifier configurations
- separated high cap. snubberized PSU with 300VA trafo and double discrete rectifier bridge
- fully shielded case with 0.8mm iron plates
- good-looking box

I have right now everything, except the most important part, the LM4780 themselves (waiting for the NS samples) and some snubbers, and some RCA sockets.

I begun the assembly with the PSU, as it is the only part where I have all the components.
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Old 2nd November 2005, 01:41 PM   #2
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The capacitors are some Panasonic FC 1500mfd/50V. I have ten of them, and will see if I put some on the chip's pins directly or put them on the PSU board. It would give me the equivalent of 7500mfd per rail, I hope it will be enough.

The rectifiers are some MSR1560 from OnSemi : they are ultrasoft, ultrafast (45ns) and quite oversized : they're rated at 15A 600V. I hope they will be good, since I never saw people using them. On paper, they look far better than the popular MUR860.

The assembly is done P2P on a bakelite board with 1mm˛ OFC cable and electronic solder. I took care to shape the wires in order to achieve the widest contact surface with the pins of the components. I doubt any standard PCB can have this equivalent wire gauge ! I also took care to melt the wire's insulation in the copper strands to prevent them from oxydation (before putting it on the pins of course )


Now, a little question : is it better to have all the 1500mfd caps on the PSU board and put some 100mfd local bypass on the chip, or to have some 1500mfd caps left on the chip's pins and some other on the PSU board ?
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Old 2nd November 2005, 01:42 PM   #3
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And what do you can tell us with this words ?
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Old 2nd November 2005, 01:49 PM   #4
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Little zoom on the caps pins : the + and - rails are not soldered because I'd like to hear what the LM4780 is worth with the basic 1500mfd per rail configuration.

The bad-looking black scrap on the left is just the burnt insulation of the wire, melt in the copper strands
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Old 2nd November 2005, 01:53 PM   #5
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Quote:
Originally posted by Upupa Epops
And what do you can tell us with this words ?

Right now : not much !

This is an ongoing project, I'm just sharing my building experiences, trials... and questions when I have some
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Old 12th November 2005, 08:23 PM   #6
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Well, I received my samples from National a few days ago, and I made the first chip run this evening, in a very "prototype" manner (cf. photo)

I used thin soldering wire for the connexions, if there was a problem, it would blow very fast and save what could be saved.

I first took care to put myself away from the "prototype zone" in the case it would explode or something else (it's the first amp I build !) and switched it on...

What a relief, nothing happened

I touched everything, to be sure nothing was warming up, and measured the DC offset. Good surprise : only 3.3mv ! And I don't use any capacitor !

I plugged my old speakers, and sent the music : immediately, tighter bass than the old pioneer, and much refined highs. Mids appear clearer, though the speakers are not good.

I finally plugged one of my FX-120 TQWTs. First, the difference was not so obvious, but I heard quite fast some differences : more air, more details, better sound separation. It feels very clean.

I now need to make the other chip run, and hear this in stereo !
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Old 12th November 2005, 08:37 PM   #7
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The design is based on the National bridged amplifier but I changed some values :

Ri is 4.3k
Rf1 is 46.7k
Rf2 is 51k
Rb2 is 4.3k (had nothing left for it)
Rin is 47k

I use no cap as DC bloking cap on the input, and no Ci caps.

I bought 1% metal film resistors, and matched them at 0.1% with a calibrated DMM. (hence the 46.7k resistor, not a commercial value). Maybe it is a reason for the low measured DC offset.

The mute is always deactivated, and it doesn't need a switch as there is absolutely no transcient when swithcing on/off.

The PSU is based on 300VA trafo, MSR1560 rectifiers, and I only use 1500mfd on the chip's pin. I'll try later some other PSU config, to see if it is worth.

The pot is a 10k ALPS log pot, one of the most expensive part in this circuit. Gotta find cheaper pots if I build gainclones every month
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Old 12th November 2005, 08:46 PM   #8
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The heatsink is rated 1.4K/W and is isolated from the chip with a silicon pad.

Everything is soldered P2P (quite a long job with the small pins of the LM4780).

The supply rails are soldered on a piece of desoldering braid, it works very well.

On my prototype bench, the chip is fixed on the heatsink with a spring grip.

Everything is star grounded properly on a fast-on connector.
- the signal grounds are first soldered on the ground pins of the chip
- the ground pins are connected on the fast-on with the signal cable ground, the signal cable shield, and the 1500mfd caps ground.

- the fast-on is then connected to the PSU ground with the male connector.

I don't have any hum, and the hissssssssss is hardly audible, even at 2 cm of the speakers. Good surprise compared to the old pioneer's hiss, audible at 1m !
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Old 12th November 2005, 10:23 PM   #9
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well done youyoung, epsecially for a first project I like the use of the desolder braid, I'd thought about using it myself before for beefing up the current carrying capacity of verro board tracks

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Old 12th November 2005, 10:37 PM   #10
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Thanks !

I mainly use the desoldering braid because it is flat and has a very wide surface to solder on the pins. Moreover, once "filled" with solder, it is rigid and helps avoiding to break the pins with heavy PSU wire.


I have to make the other one run : it doesn't use the same circuit and is based on "low impedance" feedback network :

Ri = 1k
Rf1 = 11k
Rf2 = 12k

Maybe the source will not like, or maybe it won't be an issue and the amp will sound better. Just got to try

In fact the big big problem right now is that I don't have a case at all, not even an old shoe box (which would be dangerous lol ). I drew it on paper but have to build it, or more honestly, make it build by my carpenter friend , at least for the wood part.

The heatsink is working lying on the desk, but it only gets warm at body temperature. Hope I won't need the fans I bought !
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