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 Post subject: TwoFer -- Pseudo Reverb
PostPosted: Sat Jan 10, 2015 1:36 pm 
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First project I've posted in a while. Feels good to be back. :D

This is a rehash of an older project:
viewtopic.php?f=41&t=35147

It's essentially a stripped-down PT2399 delay that is optimized for very short delay times, with the idea being to use it as a sort-of reverb. Actually, the idea is to two use two of these in series to make for a more wacky, post-rock type of reverby wash type of effect.

There is no input buffering or amplifying. You're plugging straight into the PT, which actually works pretty well. Humbuckers can cause it to overdrive a little with aggressive strumming, but some people pay big bucks for that kind of thing on tape delays.

I added an output buffer because the previous version had a slight volume drop when engaged. I believe this will also facilitate using two of them in series. I've spec'd a darlington for the transistor since they have much higher input impedance than a typical BJT, but any old NPN tranny will work.

The delay time control is pretty nifty. As the time gets longer, the delay line output gets more and more low-pass filtering (thanks to C4). This keeps any noise to a minimum, although there isn't much to begin with given the shorter delay times available.

Pins 13/14 are not used. I just realized that I forgot to jumper them, which would be the best practice (they are the input and output of an internal op amp. I will fix that after I verify the boards. I went with multilayer ceramic footprint for some of the filter caps to save space.

Proto boards are on order already. Assuming they work, I'll probably order a small batch from China.


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PostPosted: Sat Jan 10, 2015 5:24 pm 
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Sounds cool!

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PostPosted: Sat Jan 10, 2015 9:12 pm 
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Love the filtering on the delay control!

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PostPosted: Sat Jan 24, 2015 7:07 pm 
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Got the boards and they work! Lots of good sounds in here, too. I can get it to cop very similar sounds to some of the echo effects I get out of my Univox Echo Chamber units. I'm not claiming, "OMG this thing totally sounds like a tape delay!!!!!!1!!!". I'm just saying there is some tonal overlap between this circuit and the shitty low-fi tape delays I own. :lol:

I need to make a slight change to the layout to move a part pad out of the way of one of the 9mm pot tabs, but otherwise, nothing needs to change on the layout. Couple value changes, but nothing major.

Next step is to populate a second board and chain them together to see what happens! :mrgreen:

I did notice some things worth mentioning:

Because the Delay control also changes the filtering on the output of the 2399, this also changes the amount of feedback there is. Shorter times will have more feedback than longer times, so that has to be considered when setting the Oscillate pot (what I'm calling feedback). If you've ever played around with the filtering on a delay, you've probably noticed that changing the cutoff frequency also changes the amount of feedback.

Because this thing has no real active preamp or active blending, turning down Mix also darkens up the tone overall (dry and wet). It will color your dry signal, especially with Mix not at max (which is basically a 50/50 wet/dry). So it's kinda like how some people really like the preamp in the EP3. That "feature" is just part of the deal, so don't expect pristine dry signal integrity.

Anyway, I really dig this circuit, and I'll be boxing up a double. But it sounds really good all by itself, in my opinion.


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PostPosted: Sat Jan 24, 2015 7:57 pm 
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Loving this, a very nice and simple add-on for bunch of other circuits!

Well done! 8)

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After reflowing and several attempts at signal tracing and switch testing I plugged in the power.


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