808 hiss reducers

[Earthscum]

One thing I don't like about the TP layout is where the input is located. One advantage, though, is that it is right next to a ground (plane) that you could solder cable shielding to. I'm becoming more and more of a stickler about my wires NOT coming from the center of the board. I had one layout I did that the wire laying one way would cause oscillation, and laying the other way across the board would work fine. Since that one, I watch my own layouts more carefully. Probably could've worked with shielded cable, but I took the point that it wasn't good enough for me, lol.

Hope you get this tracked down. BTW, if you are using a socket for the chip, you can sometimes fit the .1 ceramic underneath it. Electronic Goldmine used to have some machine-pin DIP sockets with a .1 monocap installed already, but easy to just do yourself.

[Govmnt_Lacky]

Thanks David! 

The more I think about it, the more RG's comment resonates with me. I am thinking that the problem will be solved with the 100uF electro from DC to GND.

That cap is the only difference in power supply decoupling/filtering between the GGG and TP layouts and I have built several GGG types before with NO HISS.

[roseblood11]

But a battery doesn't need filtering?!?

[Govmnt_Lacky]

But a battery doesn't need filtering?!?

That is what I thought initially. I am running out of options though 

[R.G.]

I have learned after many builds that it is not wise to post until AFTER you have verified parts placement, parts values, and wiring/soldering at least 3 times at different intervals. 

I thought that was probably the case, which is why I hated to mention it.  But I still get tripped up by it too.

Looking at the layout, there really is no "reasonable" place to drill and insert the 100uF electro on the board. I may just temporarily insert it directly on the DC jack for test purposes

That's worth doing for checking, but do try to find a place either on the component side or copper side of the PCB. A 0.1uF ceramic *might* do it all on its own. Those are easy to lay flat on the copper side.  This is by no means certain, but lack of power decoupling is such a problem in all electronic circuits that I put it in all my layouts, whether or not the original circuit had it. It's easy to leave something off a PCB if there is a place for it, but the reverse can be really tough. I started putting them on all my PCBs a couple of decades ago, and now it's like tying my shoes - happens without thinking.

But a battery doesn't need filtering?!?

Depends. Batteries are fundamentally different from other power supplies. The electrochemical nature of the insides of the battery look like a BIG capacitor with resistor in series with it. The resistance is nonlinear, and varies with charge state, but generally gets larger as the battery gets exhausted. Still, it tends to be better than a DC power supply with its output filter cap several feet of wire away from where the power is being used.

Wires are not short circuits. They are low value resistors and inductors. That difference gets more important with higher currents and higher frequencies.

[earthtonesaudio]

Is the enclosure metal? 

[Govmnt_Lacky]

Is the enclosure metal? 

Hammond 1590N1 (125B)

[Mugshot]

ahm, hissy cables maybe? 808 amplifies the hiss, and sends it out to your amp.

[Govmnt_Lacky]

ahm, hissy cables maybe? 808 amplifies the hiss, and sends it out to your amp.

When the pedal is bypassed, the signal is dead silent. It is not the cables 

[yeeshkul]

Did you try to check it by an audio probe? That might tell you a lot.

[amptramp]

One thing that I consider a bad practice in the TS is the tone pot connected to the inverting input in the second op amp stage.  The pot has considerable capacitance to ground which may cause some instability.  A cap across the 1 K feedback resistor would be useful.  A 1000 pF cap would swamp the capacitance to ground of the pot yet not affect the sound (the rolloff would be 159 KHz), and you could afford to go higher on capacitance.  Another scheme would be to modify the tone control to Tone 4 here:

http://www.muzique.com/lab/tstone.htm

which isolates the pot from the op amp input.  If you have any oscillation at the output of this stage, it will add to the hiss, even if it is at a higher-than-audible frequency.

This is in addition to the other changes suggested here.

[Govmnt_Lacky]

Well, It was a NO GO 

I installed new clipping diodes, swapped the 0.027uF input cap with a 0.022uF. I also soldered in a 100uF electro + 0.1uF film cap from the DC jack power in to the DC jack ground. STILL GETTING THE HISSING.

Upon reflection, it may even be a bit of a buzzing sound. Kind of the sound you get with an ultra high gain effect (dr. boogie at medium to maximum)

I guess my next order of business is to try the cap in parallel with the 1K feedback resistor.

This sound would most certainly be squashed with some kind of noise gate circuit BUT, this is for a gigging musician and he wants it as close to dead silent as possible when the strings are muted 

[R.G.]

Well, It was a NO GO 

I installed new clipping diodes, swapped the 0.027uF input cap with a 0.022uF. I also soldered in a 100uF electro + 0.1uF film cap from the DC jack power in to the DC jack ground. STILL GETTING THE HISSING.

Bummer. About this point I'd get out the oscilloscope and start tracing hiss in the circuit. The 0.1uF cap is far more effective on the PCB than out at the power jack, but that's a finer point than there being one there.

[Govmnt_Lacky]

Whilst getting ready to try and install the cap in parallel with the 1K feedback resistor, I noticed something odd.

On the GGG scheme, there is NO cap in parallel with it. Like I mentioned earlier, I have built 3-4 off the GGG layout with no problems.

On the TP layout, there are accomidations for the parallel cap (Ca) however, both of the build versions (808 and TS-9) say to leave it empty 

Can ANYONE confirm a "silent" TP build and/or comment on the Ca cap? Values?

[ubersam]

IIRC, you can build either the TS or the SD-1 on the TP layout. The cap in the feedback loop of the 2nd opamp section is for the SD-1 build and uses a value of 10nF. You can try a small cap in there to see if it will cure your hissing problem. I remove that cap when modding SD-1s as it dulls the sound for me with it in there.

[Paul Marossy]

ahm, hissy cables maybe? 808 amplifies the hiss, and sends it out to your amp.

When the pedal is bypassed, the signal is dead silent. It is not the cables 

Yes, except that the pedal when engaged will greatly amplify any noise picked up by cables. I don't think that is the problem, just sayin'....