Diode substitution question

[Kipper4]

The rothwell love squeeze im building has D2 and D3  listed as 1n5817
ive googled alternatives but cant seem to find any usefull information regarding possible substitutions.
i have in stock some
1n914
1n4148
1n4007
and some 9.1v Zeners

http://revolutiondeux.blogspot.co.uk/2012/06/rothwell-love-squeeze-compressor.html

please can some one clarify for me what role these diodes play in the circuit make up.
and the same goes for the two red leds. (my guess for the leds is some kind of clipping stage and it is just a guess)
Plus why do they have to be red?
Thanks for any help you can give
Rich

[Kipper4]

i asked a friend at work to give me some 3mm red leds but the ones he gave me have a clear lens so i asummed they where clear ones. well im shocked when i tried it with the meter in diode test mode it lights up red.
how does that happen?

I have more questions than answers as ever.
it remains to be seen if i can retain any of the information imparted to me here
Good thing is i can always revisit the forum for a refresher.

[gritz]

Hi Rich, referring to the schematic in your link:



D2 and D3 are schottky diodes. The symbol looks similar to a zener, but at the head of the triangle is a squiggly "s" shape, which is how I remember it. The schemo shows BAR43s. Schottkies have a lower forward (switch on) voltage than silicon diodes (like the 1N4148), so will make the compressor a tad more sensitive. I thought the BAR43s was a surface mount device, but I may be wrong (it's not unknown!). I have a bag of BAT85 small signal schottkies that I use for everything like this, but any small signal schottky that specifies low leakage (reverse) current should be ok ('cos you'll be able to use 'em for other stuff too).

A silicon signal diode (like the 1n4148 again) will work, but the compressor's threshold will be a tiny bit higher, because silicon switches on at .6v instead of the schottky's .3 - .4 volts. Alternatively, germanium signal diodes would work too (albeit slightly differently again), but you don't have any of those either. As well as switching on at a lower voltage, schottkys switch on a bit more progressively than silicon, which are pretty abrupt at .6 volt. Germaniums are a bit more progressive still (and can leak a bit when reverse biassed, which adds to the possibilities).

Regarding your red leds: Yeah, they're there to provide a little clipping. Red leds will start to conduct at ~about~ 1.7 volts, and that's where the clipping will start. Green leds would clip at about 2.2 volts - a tiny bit louder. White / blue leds might need north of 3 volts to clip, so it's possible that the opamp will start to run out of output swing before the clipping starts there. Your diode tester lights your leds because it's pushing a small current (maybe 1 milliamp) through the device in order to measure it's forward voltage. That's enough to light your led.

Hope this helps a bit.

[Kipper4]

I had no idea the BAR43S was that important i just ordered some 1N5817s from teyda
heres the spec sheet will these be ok? sorry to keep asking stupid questions.

http://www.taydaelectronics.com/datasheets/A-159.pdf

[Kipper4]

Oh shoot i just realised my IC numbers dont match up either
i have TL072CN and the schematic says to use TL072P
i'm guessing a dual opamp is a dual op amp so i can use it right?

[R.G.]

The letters at the end of the IC number are used by TI to note different packages. The only bad result would be if the "p" type is an SMD package and that's not what you wanted. I believe the "CN" is a plastic DIP.

Most low-signal Schottky diodes can be used interchangeably. the 1N5817's will probably work fine in this.

[gritz]

Don't worry about the opamp thing - a TL072 is pretty much a TL072 (the "2" denotes that it's a twin opamp). I always thought the "cn" might have stood for "China"  

The 5817 diodes are more for power rectification than small signal purposes. They have a bit of a reverse leakage current to 'em, but I'm not sure how much it (if at all) it would noticeably affect this application. They may well work just fine, but I'd prefer to use signal diodes (mainly 'cos you can order a few extra and use 'em for other things too). The 5817 would for instance be fine to use in place of the 1n4001.

Edit: everyone else says go with the 5817, so I'll have to buckle. 

[Mike Burgundy]

What he said ;P
Just to add: the Schottky diodes are part of the envelope detection circuit around IC1B. This listens to IC1A's output, and from a certain voltage (threshold), the Schottkys start to conduct storing voltage on C10. The negative side of that voltage is fed to Q1 which acts as a variable resistor - it starts to alter IC1A's gain. Q1 is an N-channel Jfet - with nothing on its gate ("middle" connection) it is normally ON - very little resistance between source (bottom connection) and drain (top connection). Some negative voltage on the gate increases the resistance between source and drain - we have a variable resistor! For higher values of that variable resistor, IC1A's gain goes down. Once the signal gets low enough for the diodes not to conduct, R11 bleeds off the voltage on C10 acting like a release control. Hey Presto, an automatically variable gain thingy, aka compressor.
The LEDs work like a crude limiter, I think.

The regular Si diodes (1N4148: 0.6-0.7V/BAR43: 0.45V, 5817: same, so yes, that will work) conduct at a little under twice the Schottky's forward voltage. I think it's possible to put those in and increase IC1B's gain to compensate for the threshold difference, say by increasing R8 to 82K?
Or just get some Shottky's, as you just did ;P

[digi2t]

Oh shoot i just realised my IC numbers dont match up either
i have TL072CN and the schematic says to use TL072P
i'm guessing a dual opamp is a dual op amp so i can use it right?

Just slight power rating differences between the two, but should work just fine. I say use it.

Quick symbol reference guide to diode symbols;

[Kipper4]

Wow thanks for all the help guys.
Mike thats some depth but i think i follow it thanks.

[Kipper4]

http://www.ebay.co.uk/itm/20x-BAT85S-30V-200mA-Small-Signal-Schottky-Diodes-BAT85-/170869973896?_trksid=p5197.m1992&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D14%26meid%3D5011427713567160565%26pid%3D100015%26prg%3D1006%26rk%3D1%26sd%3D170869973896%26

what about these any good?

[ashcat_lt]

Mike, I think if you add gain to IC2 you also change the ratio of the compression.  The diodes will turn on at the same time as the stock circuit, but when they do they'll be pulling the transistor further toward off.  The voltage could be divided back down along the way, though.  Maybe a 1M to ground after R12?

[bluebunny]

http://www.ebay.co.uk/itm/20x-BAT85S-30V-200mA-Small-Signal-Schottky-Diodes-BAT85-/170869973896?_trksid=p5197.m1992&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D14%26meid%3D5011427713567160565%26pid%3D100015%26prg%3D1006%26rk%3D1%26sd%3D170869973896%26

what about these any good?

Yep, but I wouldn't pay a quid for postage!   

[Kipper4]

I found some more after that but its only 10 for £1.53 inc pp UK dealer
maybe not such a good deal it depends how you look at it.
I've decided to wait for the ones i ordered from Tayda to arrive as it will only be a matter of days after the transistors arrive so i'm not going to get much done until then and the pots should be all in the same order.
I got a 10 voucher from tayda and only paid just over $5usd for a load of stuff thats way more expensive elsewhere.
Especially from the uk dealers.
I want to support them and do when i need stuff in a rush, but a std shipping charge from bitsbox is expensive by comparaison.
I'll order some of the BAT85's later. i'm sure they will be a handy stock item to have a dozen of.


Anybody got any good ideas for a next project that will further my knowledge and be a bit more of a challenge,
I'd like to do a TS808 but im not sure, Anyone of you done the DOD 250 grey spec?

[Kipper4]

This is the schematic i've been looking at for the DOD


http://www.diystompboxes.com/smfforum/index.php?topic=97133.0

[bluebunny]

I'd like to do a TS808 but im not sure ...

A tube screamer build is mandatory, isn't it?   

[Kipper4]

What like First rule of pedal makers club.
You must make a tube screamer.

[midwayfair]

Best, and cheapest substitution would have been 1N60P there -- that's the silicon Schottky workalike of the germanium, and they're 4c each from Tayda. (I used most of the rectifier from the LS in my Bearhug comp and did a bunch of testing.) 1N5817 might actually clip too low to be effective. 1n34A is also pretty good, a little more compression.

Everyone's got it backwards in this thread: the lower the Fv of D2/D3, the MORE subtle the compression. Using a 1n4001 would actually make it compress like crazy.

[Mike Burgundy]

Could you elaborate on that, I'm not seeing that at all.... Youth wants to learn ;P

[midwayfair]

EDIT: removed.

Mike B you already explained it above. Sorry! But yes, high Fv in D2/D3, more compression.