Hiss

[nickfox]

I have a les paul classic with a marshall DSL40CR. When I play without pedals, it is dead silent. I have a pedal power 2 supply. I have two distortion pedals that are dead silent, my jam rattler pedal and my fulltone plimsoul. I use all these pedals on the clean channel of my marshall.

I have another pedal that introduces hiss into the circuit, my lovepedal purple plexi.

I contacted lovepedal and his response was that this was just the nature of high gain pedals. If that is true then why don't my rattler and plimsoul hiss?

My question is why is one pedal hissing and the other 2 not hissing? Is it poor design? Poor quality electrolytics?

Is there something I can change internally to improve it? (I have no interest at all in noise gates...)

thanks
n

[antonis]

this was just the nature of high gain pedals. Is it poor design?

Mother nature dislikes poor designs..

https://www.diystompboxes.com/smfforum/index.php?topic=118432.0

[nickfox]

Mother nature dislikes poor designs..

That is really interesting. A couple of months ago, I bought a 386 mini amp kit off of ebay for $1.34. That included shipping. I didn't realize the chip was also used in distortion pedals...

I took apart the purple plexi and I found something very interesting. There is a place for a cap (C3) just above the 386. It's attached to pin 7 of the 386 and to ground on the other side. I read the datasheet and some other resources and it said that pin 7 was for a "bypass" cap to reduce noise and hiss. One schematic in the datasheet showed a 10uF cap being used. You can see in the image that I put one in but now I'm getting a lot of static noise from the pedal case. Adding that cap didn't do the pedal any favors...

I saw another post somewhere that mentioned putting a 47uF cap across the power lines to reduce hiss. I haven't tried that yet...

n

[antonis]

Try to raise R3 value, starting from 33R, say..

[MrStab]

hi Nick,

I don't know how different the designs are, but i have the DSL100 and compared to other amps, it is extremely quiet on its own. Perhaps that makes the inevitable system noise of some pedals a lot more obvious.

I gather your problems are with pedals going into the front of the amp? For what it's worth, the DSL100, TSL60, TSL100 and other amps that use the same front & rear PCBs have a really poorly laid-out FX loop Send, which picks up noise like nobody's business. The pre-amp output is hugely attenuated, then sent 10 miles across the amp at high impedance, picking up all sorts of noise along the way. No design issues with the pre-amp input, though.

[nickfox]

I gather your problems are with pedals going into the front of the amp?

Hi and thanks for responding. The problem is with *certain* pedals going into the front of my amp. Other pedals are dead silent. For instance, my rattler, plimsoul and timmy pedals are dead silent.

Try to raise R3 value, starting from 33R, say..

Thanks, I'll try it.

n

[MrStab]

i just searched for the Purple Plexi schematic and one of the first results was a thread from this forum. it's kinda inconclusive, but suggests that you're not alone with this problem:

https://www.diystompboxes.com/smfforum/index.php?topic=118432.0

apologies if you've already found that, ofc

[nickfox]

apologies if you've already found that, ofc

Thanks, I did read it. It seemed like there were 2 main thoughts there. The first was that the gain of the 386 was to high and the second was swapping out for a 386-1.

I'm not entirely sure how to identify if my 386 is a 386-1? Is that clear to you?

n

[GibsonGM]

Most chips have their 'name' printed on them...the '386-1 is no exception!

[duck_arse]

Most chips have their 'name' printed on them...the '386-1 is no exception!

if that's meant to be/claimed to be a NatSemi part, I'd avoid it.

[nickfox]

if that's meant to be/claimed to be a NatSemi part, I'd avoid it.

why?

n

[nickfox]

Do these look alright? It was the cheapest I could find...

https://www.ebay.com/itm/50-PCS-LM386N-1-DIP-8-LM386-Low-Voltage-Audio-Power-Amplifier-IC/291181676148

n

[anotherjim]

That is one of later Nat-semi logos'.
...but why risk it, the things only a quid?
https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=140_159&products_id=1187

I don't think power caps are the answer since it isn't used as a power amp. A larger power filter resistor will be worth having though. I wouldn't bother with the pair of Schottky protection diodes - their advantage is trivial, a single 1N400x would do.
It's non-inverting, so stray positive feedback is the bigger danger with high gain preamps.
It would be interesting to swap the + and - input pins around so that it's non inverting. Probably not easy on PCB.
It might need something from a real amp, negative feedback could be added.

[willienillie]

Do these look alright?

No, that is a horrible copy of the NS logo.  When you see "Shenzhen, China" on an Ebay listing, move on.

It was the cheapest I could find...

There's a reason.

[duck_arse]

if that's meant to be/claimed to be a NatSemi part, I'd avoid it.

why?

n

perhaps i'd better retract that statement, at least in part. I got some parts from the TI samples program back when that were natsemi produced, so I KNOW they are genuine parts. and I went and had a squint at their markings - and yes, they do look quite like the one pictured. but - it also looks very shoddy, much like the markings sighted on chinese knockoffs and counterfeits.

so, parts marked like that from a reputable source, OK. parts marked like that from a chinese backyard, or ebay, no thanks.

[GibsonGM]

{{Picture posted so that OP would be able to see that chips are ID'd, not for a suggestion on which particular chip to purchase}}


I buy from Small Bear, Mouser, Newark and the like, not Ebay or other 'quick market internet places', which are too full of copies to trust.  Not likely that Mouser is running counterfeits, but I guess it could happen.

[duck_arse]

also, Sir Mike GibsonGM is above reproach, a known genuine part. I meant to cast no aspersions upon his good name.

and, I only need one post for an even 8000.

[edit :] well, sir mike, I just won't answer.

[GibsonGM]

Well, Stephen - since the magna carta, I don't think most of us blue bloods are above reproach at all!     Kidding...no New England Yankee would ever have a monarch above him or herself, that is a certainty!

If you reply, you'll have your 8,000!

[antonis]

If you reply, you'll have your 8,000!

And, Stephen, be ready to change your status from Poster2 to Blubbermouth1.. 

[willienillie]

I went and had a squint at their markings - and yes, they do look quite like the one pictured.

Not more like this?