Hotcake , Flap jack help

Kipper4 · September 13, 2013, 02:00:48 PM

The Flap jack (Crowther hotcake) has been on my build list for a while.
I don't have the lm471 single opamp in stock so I was thinking I'd have a go with a dual op amp and just use one side of it and ground out the other side.
Any reason why I couldn't do this.
Thanks for any help
Rich

induction · September 13, 2013, 02:22:05 PM

You can leave one op-amp unused, but I wouldn't ground it out. I would wire it as a non-inverting buffer with no signal input or output (ie, inverting input tied to output, non-inverting input tied to Vr).

Read this for more info.

Kipper4 · September 13, 2013, 02:26:05 PM

I remember that page now that's what I meant really + input to Vb and -input feedback looped to output right.
Cool
Thanks for the help

Kipper4 · September 13, 2013, 02:29:54 PM

Since they used TL071 in the later models. I'll try it with a TL072 and if it works ok I'll sub out the op amp with some others I have in stock 4558, 1458, and see what I like the best.

Kipper4 · September 13, 2013, 04:33:49 PM

I've run up a perf layout and would appreciate a keen eye to check it for me please

https://www.dropbox.com/sh/7ma9n53c5jqdgsr/dmzK-Jd7wt/TL072%20Flapjack%20Perf%20Layout.pdf

I would help you guys out and post a link to the schemo but I fear i will cause trouble as i got it on the other forum and i dont want to cause offence

google Flapjack Overdrive schematic
thanks
Rich

duck_arse · September 14, 2013, 01:11:23 PM

you could use the spare half tl072 as a buffer for the Vref, which I'm assuming is there, I can't be bothered signing in at the other place to see circuit.

Kipper4 · September 14, 2013, 02:06:51 PM

Thanks mate

jubal81 · September 14, 2013, 09:55:50 PM

Quote from: duck_arse on September 14, 2013, 01:11:23 PM
you could use the spare half tl072 as a buffer for the Vref, which I'm assuming is there, I can't be bothered signing in at the other place to see circuit.

That's an excellent idea.

I recently put one together using Joe Davisson's diode-compression opamp circuit from here and it sounds really great.

aron · September 14, 2013, 09:58:09 PM

WOW! Yeah I love the diode compression op amp. hmmmm

Kipper4 · September 15, 2013, 08:01:13 AM

you built the flapjack (hotcake) using Joe davissons diode compression op amp?

Kipper4 · September 15, 2013, 10:19:28 AM

I breadboarded the TL072 Flap jack today with a few little changes and it works a treat.
I'm now considering using the extra op amp stage to drive a tone control. I'm thinking James or Stupidly wonderful tone control.
Which do you think will work best please.

And just to clarify did you mean use the extra op amp as an output buffer or input buffer or am i mixed up and you meant something completely differant by buffer for the v ref. ?

bluebunny · September 16, 2013, 03:53:59 AM

Quote from: Kipper4 on September 15, 2013, 10:19:28 AM
you meant something completely differant by buffer for the v ref. ?

You know how you build a typical V_REF using two (similar) resistors to make a voltage divider? If you hook this into a bog-standard unity-gain op amp buffer (using the other side of your TL072), you'll get a "rock solid" version of V_REF. Got no handy pictures to hand (at work - yawn!), but I know you know what I mean.

bluebunny · September 16, 2013, 04:01:43 AM

Good ol' Google...

(<-- click on the picture and scroll down to "Making a New Start")

Edit: updated with a better example.

duck_arse · September 16, 2013, 11:27:38 AM

^^^ exactly wot he said, kipper. think of it as "Vref, with bling".

samhay · September 16, 2013, 11:55:16 AM

It works by clipping the op-amp, so why pretty up Vbias? An output buffer probably wouldn't hurt though.

Kipper4 · September 16, 2013, 01:21:08 PM

Thanks guys
I knew as soon as you said it i'd seen this before in a recent build
The Suhr Riot does exactly this
Thanks I will know now for future referance
I built the TL072 flapjack today on perf and to be honest i'm not that impressed.
I think the op amp is misbiased. I'm sure it can take a bigger voltage than its getting but i wished now i'd used two identical resistor values in creating the V ref ( and used the buffer for V ref , It was too late by the time i'd read the replys)
I could have easily used less gain too. I like the topology of the gain control. I havent seen it used like this before.
I could have used a 50k resistor in the feedback loop of the op amp and still had a tone of gain to play with i reckon.
I subbed the 50k gain pot with a 10k.
I have a schemo if anyones intrested and a new perf layout too.
I removed the one in the post above since it was all wrong.
It might be time to go back to the breadboard with this one
Thanks guys
Rich

Kipper4 · September 16, 2013, 01:30:36 PM

I forgot to say i had to try several op amps too
first up TL072
jrc4558
LM1458
and the winner
JR1458

samhay · September 16, 2013, 02:44:34 PM

The gain/feedback topology is really cool in this circuit. Did you build the version with a presence control?

Kipper4 · September 16, 2013, 03:50:52 PM

Certainly did and the more i play it the better it (or I) gets.
Blinding little circuit.
I just bought 10x LM471 so I might even have a crack at the single op amp version Flapjack too.

Theres quite a few differant schemos for the hot cake floating about so I'd be intrested in whom likes what please.
To my ears it sounds better with a Tele and a compressor after it. (I'm using the engineers thumb, I should also try the bearhug too)

Erratta. I wired the drive pot up backto front. I've fixed that now but the gain seems to have bunched up alot on the end of the pot. I was expecting that with a log pot. I much prefer it with mild overdrive the fuzzy stuffs a bit to broken to me and messy too. Over saturated.
I iamagine i has a use i just havent found it yet.

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Hotcake , Flap jack help