Made me my first amp
May 31, 2003 at 4:12 AM Thread Starter Post #1 of 17
myself, aka me

myself, aka me

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It is apparently similar to a CMoy circuit - as stated in http://www4.head-fi.org/forums/showt...highlight=MSCA .

I hadn't been able to get it to work, but as usual, something really simple. In this case, wiring left to ground and ground to left. D'oh. The opamp is a TL072. :\. But I can't find any good chips here... The other PC board is a power supply, +/-15v. I don't know anything about it... This was just a kit, and i'm not very skilled with electronics..

It sounds pretty good except for a little bit of hissing. How to say this.. I don't know much about describing sound, but this thing just sounds full, rather than thin.. it's giving somewhere near 18mW into my 40 ohm eggo. It can take my ancient very high impedance Pioneer headphones to bleeding levels. Anyway, I like it.
 
May 31, 2003 at 5:44 AM Post #2 of 17
Hey,

This isn't similar to CMOY amp. This one has complementary pair buffer stage.

ef0k.gif


either this or this,

eaton2.gif


Above should be the identical design used for this amplifier. (Zero Imp mod? Hehe, is that what they call it nowadays?)

Yellow caps are the coupling caps. The electrolytic you see is the bypass cap, but that is the ONLY supply bypass cap on the board.

Four transistors you see form a complementary pair per channel. They are biased by the two diodes and two resistors (per channel) you see on the board.

I think hiss is coming from two things (one or the other or both). First, anyone should notice, TL072, isn't all that good. Try NE5534 or LF356. Second, the supply lines are POORLY bypassed. (which doesn't help one bit, probably making it worse.)

Tomo

P.S. You can use even more expensive opamps like OPA132 or any other we talk about. But I think you should solve this with generic opamps first, because the hiss may not be coming from opamps at all.
 
May 31, 2003 at 1:24 PM Post #3 of 17
Hi, thanks for advice
biggrin.gif
.

Locally it seems only the NE5534 is available, so i'll pick one up sometime and see what happens. I take it the supply lines being bypassed poorly is a design fault, and not much I could do there? Either way, it's just a little worse than my MDS-PC1 MD deck. I think my shoddy wires account for a bit of hissing too. Perhaps i'll replace with something better and better soldering. (well just now the wire snapped off at the joint, so it's little wonder it was a bit hissy).

I recognise the second schematic, 'cept my power supply is different. (also, I have no idea what parts J3 and J4 are). My electronics knowledge isn't very great, lol.

I was listening to it earlier. Still haven't cased it up, but it's not nearly as detailed as either my DR7 or MDS-PC1, but it's a LOT fuller sounding than either. I'll swap the op amp and try again when I have time.. But at this stage it's promising!
 
May 31, 2003 at 3:34 PM Post #4 of 17
HOLD IT,

I made a typo. You need NE5532. It is NE5534's dual opamp version. It should be pin compartible with TL072.

LF356 is a single opamp as well. So you will need to modify in order to fit this one.

Pic is supposed to references. It probably have some differences, but topology is very similar.

Lastly, if I were you, I will check the output DC offset.

T
 
May 31, 2003 at 8:48 PM Post #7 of 17
Hail, Almighty One!

I tried using the delete option, but it denies my request and tells me I should contact the omnipotent administrators.

I am burning a capacitor as a sacrificial offering at your temple.

T
 
Jun 1, 2003 at 10:49 AM Post #11 of 17
Hey,

I knew it. Hehe loose socket. That happened to me long ago as well. Took me forever to track down.

Anyway, if I were you, I will go and try to bypass both negative and positive power supply line on the amp board. Only one of them is bypassed and it is not good to leave mergins where the amps can let you down. (After all, your amp should be pretty good, if tamed.)

LM833 is pin-compartible with most commonly available opamps. It should work. I told you about NE5534 and LF356 because they are commonly available. If LM833 is readily available, you can try those.

Try contacting some of the members here. Some of them may send you few premium opamps with some cost.

T

P.S. All Hail, your Rick-ness. I thank thee. ...
 
Jun 4, 2003 at 8:27 AM Post #13 of 17
Hmm,

It hums. I take it above the carpet maybe 10-15 cm holding by the PCB, and it's hum fades. I repeat this while holding the metal tip of each power regulator, and the hum almost disappears, but a slight hissing (like wind) is now audible.

I'm confused as to how I can stop this happening?

The bypass cap is over the + and - rails of the opamp.. You said to bypass both supplies, but how to do that?

Thanks for the help so far, though. I read up on bypassing but it confused me more..

The power supply is described as dual polarity half wave rectified.. However I could change this to a dual polarity full wave, centre tapped supply with a couple extra diodes and a different transformer...

Also, I get about +9V on one regulator, but -8.5V on the other. Is the 0.5V difference worrying? Cause of my problems? These are LM7815CT and LM7915CT. Further experimentation found that while removing the ground from the left produces a LOUD buzzing (expected), removing the ground from the other channel doesn't seem to affect it at all, which I find bizarre.
 
Jun 4, 2003 at 12:44 PM Post #14 of 17
sounds like your regulators are not regulating

two possibles

wrong pin connections or you do not have at least 17 volts going into the regulators

The voltage drop of a semiconductor regulator is 1.5 volts so you need to have at least that much voltage over the regultor voltage
 
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