ODR-1

[dlazzarini]

This makes an interesting read: https://nordland-electronics.de/en/blog/odr-1.html

Thanks. I’ll check it out when I get home later

 

[Flying]

I'm just looking at the ODR1 Plus schematic, in particular the boost bit, but not understanding how the boost works, can anyone enlighten me?

 

[Chuck D. Bones]

I'll keep this as simple as possible. The gain of the 1st stage is determined by the impedance of everything between pins 2 & 3, which we'll call Zf, and the impedance of everything between pin 3 & ground, which we'll call Zg.

The gain, G is given by:

G = 1+ Zf / Zg

Notice this if Zg gets smaller, the gain gets larger. The Boost switch puts R15 & C15 in parallel with R12, R13, C12 & C13. Engaging the Boost switch makes Zg roughly 3x smaller, which will make the gain approx 3x larger.

Because of the capacitors and diodes, the gain varies with frequency & amplitude, but the general principle is still valid. Note that this is a gain boost and not a volume boost. The apparent volume will go up a little when Boost is engaged, but the main effect of Boost is more distortion and sustain.

 

[Flying]

Thanks Chuck, and for keeping it simple, this boost can so easily be put on a switch and wired into the PCB I think give it a go.

When I saw the 2.2u cap in the boost section I thought it was going to make an already bass 'rich' pedal have even more bass! I appreciate that this because I don't have the knowledge to understand what's going on, so thanks again for clarifying.

 

[Chuck D. Bones]

Engaging Boost will boost frequencies above 250Hz, based on C15 & R15.

 

[Flying]

I've just been looking at the schematic for the ODR-S, this is the Bass Mid and Treble eq version, the bit that caught my eye was that the hard clipping diodes are AA112s rather than the 1N4148s I've seen in the other schematics.

These AA112s are germanium, rather expensive and seem to have a higher forward voltage compared to 1N4148, I know that in Aion's version he's suggested 2 1N4148s in series to increase the forward voltage, making me think there is some milage in playing with different diodes in the hard clipping part of the circuit.

Any suggestions for diodes that might be a good substitute that are not silly money?

 

[Chuck D. Bones]

A couple of thoughts...

Can you point me to, or share the ODR-S schematic?

AA112 are $0.90 ea at PedalHacker. Not expensive. Their advertised Vf is 0.28V-0.29V, which is much less than 1N4148.

Lower Vf in either diode pair increases compression. There is interaction between the soft and hard clippers which means that the effect of changing Vf in one pair depends on the Vf of the other pair.

 

[fig]

I'm in!

 

[Chuck D. Bones]

Hmmmm, interesting. Thanks. I stand by my previous statements.

 

[Barry]

Took me a minute to get my head around the way they do their pots

 

[Flying]

Interesting, this data sheet has a forward voltage of 0.95v max (no minimum)

https://datasheetspdf.com/pdf/155557/ETC/AA112/1

 

[Chuck D. Bones]

Vf = 0.95V max is at If = 10mA. Small signal diodes like these have fairly high resistance and that's what causes the high Vf at 10mA. The ODR-S circuit will put less than 0.4mA into the hard clippers, so the Vf given on PedalHacker is more applicable.

This points out the pitfall of comparing Vf without know If.
I measured an OA74A. Vf = 0.36V @ 1mA; Vf = 0.84V @ 5mA.
I measured a D18. Vf = 0.37V @ 1mA; Vf = 0.57V @ 5mA.
Both are Ge diodes, have about the same Vf @ 1mA, but their V-I curves are very different because the resistance of the D18 is substantially lower.

 

[music6000]

I used NOS Sylvania 1N34A's in my ODR-S Build!

 

[Barry]

I used NOS Sylvania 1N34A's in my ODR-S Build!

That's exactly what I was thinking about using

 

[fig]

I thought the ODR-1 had like 7 transistors?

Edit: It does, Nobleman has 2 transistors. OP "exact replica" threw me`, but it's all good.

 

[Flying]

The ODR-1 is not true bypass and so has a number of transistors doing the switching. Pedal PCB's is true bypass, when the pedal is on it should sound exactly like the original.

 

[fig]

Right on.

The ODR-1 is not true bypass and so has a number of transistors doing the switching. Pedal PCB's is true bypass, when the pedal is on it should sound exactly like the original.