70's Distortion box build diode question. (pics)

[redshift]

Hi all, I've just completed my first stompbox build. It's a copy (or as near as I can get it) of a distortion pedal I bought back in the 70's.

The query I have is the diodes that are linked together, (see picture) does anybody know why this was done?

I have just used one diode on my circuit and it seems to work fine apart from a bit of noise when the drive pot is set to max.

Also the IC I used is a UA741CN as I couldn't find the original CA741CE. Could it be the diodes that are causing the noise?

This is a great distortion box with a wide tone range, I just wish I could get rid of the noise at high settings.

If anyone would like to build one here's the link to the circuit diagram.

THE CASE


THE ORIGINAL


MY EFFORT


THE CIRCUIT
http://myweb.tiscali.co.uk/merlinsigns/CIRCUIT_LAYOUT.pdf

[tiges_ tendres]

first off, LOVE the tortoise shell look.

The diodes are clipping diodes.  I cant tell you much about them, but probably not them that is causing the noise.  If you do a search here for 'clipping diodes' you will be inundated with types of diode, and the kind of distortion sound they help you create.

what's the name of the original pedal?

[Toney]

Hi there, check the FAQ for clipping diode configurations and you'll see there are no rules. All combos can fly. Let the ears decide.
The IC is a bog standard 741 the other letter codes are unimportant for this application.

[twabelljr]

     The 2 diodes in each direction increases output. Check out the first mod explained here: http://www.diystompboxes.com/cnews/mods.html.

Most distortion circuits have noise at high gain settings. (hiss). You may be able to find a better single op-amp than a 741 to help cut down on the noise though. NE5534 and TL071 seem to have good reputations.

[redshift]

Thanks for the info and twabelljr for the link, I did'nt realise diodes were so important.

Tiges_tendres The pedal is called an 'EEZEE Sustainer' and was an own brand made by my local music shop which closed down about 20 years ago. My original was getting a bit clapped out so I decided to try and make a replacement.

If you fancy building one it's worth the effort.

This has been really helpful and I'll try doubling the diodes to see what difference it makes.

[jeroen_verbeeck]

Can you make a little sound sample ?

[Toney]

Or a schem, if you can be bothered.

It wasn't a Mom and Pop store who commissioned the production of just 200 of what is perhaps the best overdrive effect ever...by any chance?
 
ahem.

[jrc4558]

If I read your layout correctly, the diodes are in a feedback loop of the op-amp. This is regarded as a "soft clipping" and is used by such pedals as Ibanez TS808 or Boss SD-1.
Now, the slew rate of the chip used is really low. Higher harmonics will not be as prominents, and the input imedance of the device is ~50K, which rolls off high end. Good for an overdrive. Why two diodes in series? Yes, an increase in output, but also, better chance to hear the op-amp saturation distortion. The gain of the circuit is ~10000, if the Gain pot is at max, and the 10 ohm resistor is really 10 ohm. That alone will overdrive the opamp to the point of severe clipping. The diodes in this case will only "groom" the signal to better stability. There are several designs based on a single op-amp chip which is not used as a pre-clipper amplifier, but rather is overdriven on its own. Some people say (i dont remember who, and i dont know for a fact that the following is true), that the Hotcake overdrive is working like that. But again, those are rumours. This pedal has not and probably will not be pubished as aproject i nthe nearest future for reasons going beyond the scope of this post.
Try it for yourself. Remove the diodes. Change the gain pot to 500kA, turn it to maximum and listen to IMHO best asymmetrical distortion after fuzz-face. Good luck!

[redshift]

Hi all. Sorry 'Toney' but I'm a newbie to all things electronic so I'm afraid schematics mean total zilch to me. The only way I can follow a circuit is if it's drawn out as a layout!!

Thanks 'Constantin Necrasov' for all the information, I intend on trying the diodes in series later.

Here's a quick sound clip of the distortion pedal. I don't have much in the way of recording gear so this is just my Gibo 335 and the pedal played through my old peavey amp on a clean setting with a touch of reverb. The amp is fan cooled, so it's quite noisey.

http://myweb.tiscali.co.uk/merlinsigns/eezee2.mp3

[8mileshigh]

Redshift,

Sounds great !  Nice !

8

[twabelljr]

Very nice! I like the tortise shell too. That's a pick guard cut down right?

Hi all. Sorry 'Toney' but I'm a newbie to all things electronic so I'm afraid schematics mean total zilch to me. The only way I can follow a circuit is if it's drawn out as a layout!!

If you can post a photo of the other side of your PCB, One of us will be able to draw and post a schematic.

[burnt fingers]

I think we can probably draw up a schem based on the circuitboard layout provided.  Looks pretty simple although I wonder about the output. Is there a decent amount of output level or does the volume on the effect have to be all the way up.  It looks like a lot of passive loss on the tone stack.

Scott

[twabelljr]

I think we can probably draw up a schem based on the circuitboard layout provided.  Looks pretty simple although I wonder about the output. Is there a decent amount of output level or does the volume on the effect have to be all the way up.  It looks like a lot of passive loss on the tone stack.

Scott

Duh! I forgot about the PDF file attached of the layout. You are absolutely right Scott, my bad.

[redshift]

Hi all.

'Burnt Fingers' I don't know what you mean by "passive loss on the tone stack" but you are totally right, the pedal does lack a bit of output, so the volume control has to be very high....Do you know how I could rectify this? (in laymans terms).

'twabelljr'  your right  the tortoise shell is a piece of pickguard I had leftover from a guitar mod I did years ago. I just shaped it with a dremel.

Once again many thanks for everyones help.

[PenPen]

Hi all.

'Burnt Fingers' I don't know what you mean by "passive loss on the tone stack" but you are totally right, the pedal does lack a bit of output, so the volume control has to be very high....Do you know how I could rectify this? (in laymans terms).

What he means, is that there is some volume loss with any tone stack. Nearly all pedals have either an output buffer or gain stage immediately following a tone stack. You will probably want to add on the final gain stage from say, the Big Muff to recover from the losses. I use that gain stage as a generic recovery stage in nearly all of my pedals.

In layman's terms, you're going to want to run the output line from your tone stack into a 0.1uF cap, then to the base of a transistor, like a 2N5088. After the cap, but before the transistor base, set up a voltage divider, meaning put a 390K resistor, connect one end to your 9V power line, connect the other end to the base of the 5088. Then, add another resistor, 100k, from that base to ground. This sets up the bias for the transistor. Next, put a 10k resistor from the 9V power line to the collector of the transistor. Put a 2k2 resistor from the emitter of the transistor to ground. Finally, put a 0.1uF cap from the collector of the transistor, and run it to your volume pot. this is your signal out. I hope that is easy to follow, I know I personally can't follow someone putting schematics into words, I have to have a schem to see what they are talking about. Words and circuits are just in two separate portions of my brain and they don't speak the same language to each other. If it doesn't make sense, take a look at the schem for the Big Muff on generalguitargadgets and look at Q4. I described the section starting with C12 and ending at C13. Thats two caps, four resistors, and one 2N5088 transistor.

[burnt fingers]

I like the idea of the 5088 buffer.  I did a modded tube reamer and used the big muff tone stack.  I found my self using a jfet output to up the level on that one.  I will try this one with the 5088 and see how it goes.  this will probably be in a few months since I have a few other things to finish up.  I quickly sketched a schem last night on my sons spongebob notepad cuase it was handy.  I will try to put it into some electronic format soon.

Scott