[sdiy] sources of hum and hiss in a solid state guitar amp (LM1875) (slightly OT)

[harry bissell]

Hey Anthony....

(do you have a scope or are you working by ear ?   you keep mentioning 
that you are
hearing rather than seeing things. For serious design or mod work, a 
scope is a must-have)

That said...
the 5532 is a poor sub for a 4558. The 5532 will only be 'low noise' in 
very low impedance
applications. I aviod it unless I can keep the input resistors less than 
1K ohm.

The 4558 is really quite a GOOD low noise opamp for most audio work. I 
made the mistake
(once) of thinking it was "for sh!t and I can do better..."   until I 
actually LOOKED for a better
one. I like it and would not swap it.

Most hiss (imho) comes from noisy resistors.  Carbon comps are the 
worst. They excel in peak
power dissipation so replace them WITH CARE in tube cirucits.

Metal film are usually the quietest, until you get to really high 
resistance values. I'd say they are probably
good up to 1M ohm or so. After that you need to be really selective in 
what you use.

High gain circuits can be trouble is the PCB was poorly laid out to 
start with...  Look for oscillation
with a scope.

The Power Amp chip you are using can be prone to oscillation and you 
will NOT notice it without
a scope, but it CAN make audible (unpleasant) distortion.  Especially 
watch out that you are not
picking up AM radio etc. There are components you should have on the 
output to supress oscillation...
it should (iirc) be in the data sheets. The manufacturer may have 
SKIPPED them "trying to save paper"
(as the Beatles used to say...)

Try swapping all the resistors for metal film first, ESPECIALLY in the 
preamp. You might be totally
surprised.

I assume you have a humbucking pickup in the guitar... if not that is 
the most likely source of hum and the
rest of the circuit just amplifies it.  Proper decoupling caps should 
kill most power supply hum... but it can also be induced by a cheap 
transformer. This will 'leak' magnetic flux and induce a voltage in the 
circuit if it is nearby.

Move the power supply FAR away, or use a high quality toroid 
transformer, or shield the transformer with
steel, iron or (best) Mu-metal  (hard to get and very expensive, works 
wonders though).

Also... you needn't gild the lily...
don't try to make a silk purse from a sow's ear etc...
there is only so far you can go from a cheap design. If you are willing 
to change EVERYTHING
then it is not their design anymore, its yours !!!

Good luck !!!

H^) harry

anthony wrote:
> I've been so impressed with the performance of the tube amp that I 
> just made that I am now completely unsatisfied with the performance of 
> my Epiphone EP800R solid state 15 watt amp. The hiss and hum on it are 
> atrocious. But since it has a cheesy but super-cool spring reverb I've 
> been willing to overlook this. I'm sort of at a loss on where to start.
>
> I do not have any sort of empirical baseline performance experience 
> data because when I got this amp at a Pawn Shop it was a total basket 
> case. The PC board and one of the pots was broken. I did a really good 
> job fixing the board, but I only had a 100kOhm pot that would fit 
> where the 250kOhm pot that broke was  - that may be a small part of 
> the problem. What happens is there's a lot of hiss at high gains and 
> at various points along the the travel of the gain and volume 
> controls. Added to the mix is what sounds like some garbled 
> oscillations at various points. This also is prominent when the reverb 
> control is turned way up (which is really all the time since lower 
> levels are virtually inaudible). What it ends up happening is a sound 
> much like what you get from a BBD device with the lock running really 
> slow. But of course there are no BBD's anywhere inside.
>
> The preamp looks like a Tubescreamer except the clipping diodes are 2 
> red LED's. I changed these to 4 red LED's and 4 Ge diodes - 
> symmetrically. This mod didn't change the hiss and hum (as would 
> probably be expected). The original op-amp was an NJM4558  (the newer 
> ones with with a 5-digit date code). I switched this to an NJM5532DD, 
> but this actually sounded a lot worse. (The 5532 was also a newer 
> 5-digit date code unit - I don't think any of their newer parts are up 
> to snuff.) I replaced this with an older JRC4558DD whcih was a little 
> better and I liked the tone from it a lot so I kept it in. I don't 
> think I like the way LED's clip so I think I'll be making a small 
> board with around 10 Ge diodes on to fine tune asymmetric soft 
> clipping while retaining a relatively high level of clean headroom. 
> That was really the first thing I didn't like about this amp when I 
> got it going: it clipped way to early. Since I already have a rad 
> distortion pedal I don't need to pretend this is a tube amp (OK so 
> maybe giving it asymetrical soft-clipping is pretending it's a tube 
> amp, but oh well...). I only used LED's because it was easy to get a 
> lot of headroom with few diodes. Too bad it sounds like crap... I 
> think I'd get great tone with a bunch of Ge diodes with only a 
> difference in one or two diodes. I've tried experiments using Si in 
> combo with Ge and concluded that all Ge diodes sound better (but this 
> was in a pedal that used diodes to shunt the signal to ground).
>
> The power amp is an LM1875. I read a little about these and was 
> surprised to see people using them in sort-of hifi DIY audio. Actually 
> I guess the preferred chip is the LM3875. Anyway I had thought of 
> either swapping in an LM3875 or increasing the power supply voltage. 
> It looks like this amp is using  a PS voltage a lot smaller than the 
> max rated PS voltage. I'd actually prefer to not have to buy any new 
> specialty chips - especially if I'm not even sure I'll like the result.
>
> I'm really having a problem justifying keeping it as a solid state amp 
> - especially given my early success getting a great-sounding amp. It 
> has this cool tweed covered box and just about the perfect number of 
> controls for what I'd want on a tube amp. (The only thing lacking 
> would be tremolo controls, but reverb has priority over tremolo 
> anyways.) There's a schematic for a perfect amp with tons of gain and 
> reverb and everything on the AX84 site. But I think - since I have 
> enough Mullard EL84's - I want to do a push-pull amp: something like 
> an AC15 or a GA15 with reverb. I'll still have 1 EL84 left over to 
> make a single-ended amp (or mod an Epiphone Valve Junior - which is 
> most likely what I'll do with it...) I'm not sure if the reverb spring 
> in this amp will work with a tube amp. But that's not really a big 
> deal since I want a "surfier" spring anyway.
>
> The big problem with making a push-pull tube amp in this enclosure is 
> that the enclosure is a lot smaller than I'd like to mess with, so if 
> anything I'd probably just end up making a simple single-ended 7W amp 
> and save the pair for a 12" combo.
>
> But in reality that's not really a simple solution to the problem at 
> hand. What I need to do is figure out where the hiss and hum are 
> coming from and do things to fix those. I had thought of putting an 
> LM380 amp right before the reverb spring because the reverb on this 
> amp is wimpy. That made me ponder how maybe at least PART of my 
> problem is impedence matching. I noticed that the clock hiss from my 
> DOD flanger/doubler is horrible with my solid state amp (even way down 
> the chain), but barely perceptible with the tube amp (and sounding way 
> sweeter with the tube amp). I guess it's obvious that the major source 
> of hum to eliminate is the power supply. I have some good caps to put 
> in so that's easy. The jacks are isolated from the chassis so no 
> worries there.
>
> What would be the best way to cut down on hiss and oscillation while 
> maintaining the best possible tone for a guitar amp? It just now 
> occured to me that maybe the LM1875 is a source of hiss and maybe 
> reducing its gain (and making up for it in the preamp since I actually 
> modded it for more headroom anyway) would cut down on the hiss a lot.
>
> I could - even using the existing power supply - make a preamp using a 
> 12AU7. I have some sweet RCA side-getters that would be perfect. 
> Getting a quite tube preamp seems more staightforward to me at this 
> time. Maybe I'm missing something.
>
>
>