Hamlet Delay and Preamp: Schematic, build doc, layouts, and demo

[midwayfair]

Project: The Hamlet Delay and Preamp

The Hamlet Delay is my take on a PT2399 delay with tails. The dry path is an always-on preamp with available boost, very high input impedance and low output impedance. The delay line is switched on separately in parallel with the dry signal. A tone trimmer allows the user to fine-tune the repeats, going from bright digital to tape-like to darker narrow-bandwidth analog-style repeats. Unlike many PT2399 delays, there is very little treble filtering otherwise, and combined with some progressive bass loss, the repeats take on a shimmery, singing quality that is not readily found in other delays. The circuit also contains what is, to my knowledge at the time of this design, a unique method of limiting the signal in the PT2399 via a single LED, making it impossible to distort the delay chip.

My goals with this design:
1. Bright repeats without too much noise
2. Tails and a transparent dry path that people won't mind having on all the time
3. High headroom.
4. Low parts count with common parts and values where possible (for the most part ...)
5. 1590B-sized or smaller

Background:
Not too long back, I built a friend of mine the Joshua Tree Echo, a multipedal with the Multiplex, Fatpants, and Rub-A-Dub. I had also promised another friend a delay for Christmas, but I didn't want to build him just anything.

While I was working on the JTE, I had breadboarded a delay circuit so I could work on some of the mods. I also had a couple transistor stages already sitting on the board, so I decided to use them instead of breadboarding an op amp, and that led to working out a way to actually get the delay to work with transistors ...  and I realized immediately that this meant I could come up with something, well, a little different. So after the JTE was done, I started tweaking the delay, working on the filtering and coming up with ways to make the repeats as bright as possible without the PT2399 distortion, then adding a charge pump to increase the headroom of the preamp, and eventually I had something that solves a lot of the issues I've had with PT2399 delays.

Today I verified a prototype PCB. It fired up the first time with no issues, and it only took me an hour and a half to populate, including the pots. Yay! The final version of the PCB will have a very slightly revised layout but won't require another round of prototyping.

This is NOT the PCB sale thread. Please do not ask for a board to be reserved for you here! This is only for questions and discussions about the circuit, layouts, etc.

Here's a link to the build doc:
https://docs.google.com/document/d/1P7uOYFRH6ZW5eGoquhAiMQMyUU76bod_gNKkiSlPNJM/edit

The build doc contains the perboard layout, a primitive etch image, the bill of materials, notes on mods and substitutions, and an extensive "How It Works" section.

The schematic:


Here's the PCB:


Here's the demo again so that everything's in the same place:

[pinkjimiphoton]

wow, jon,
that's freakin' sweet~

finally a pt2399 that don't suck!!

well done!!!!!

[psychedelicfish]

Cool!
Something very loosely based on this will probably make it into the amp I'm building ATM (by ATM i mean the amp build that has been going on for about a year now )
Still not entirely sure what the tone control does... does it change the amount of treble cut on each successive repeat?

[meffcio]

As far as I know, you shouldn't use junctions in the kind of points I coloured red. Junctions are for connecting more than 2 wires meeting. Components are connected to wires just like that - no need for junctions there. Btw. If you don't, you should use Net function rather, not Line/Wire.
You should use junction in the point I coloured blue, if the wires are meant to be connected.
Anyway, the idea is great, and from what I see in the video, works just as well. Shame that only half of the pots are outside. ;p At least the tone pot should also be on the outside, I think.

[samhay]

Very nice job Jon. Certainly a few ideas that will probably end up in my next PT2399 project.
The tone control is a nice addition. Did you consider bringing it out of the box?
I assume it will work OK if we run VA at +9V, but I guess you are using the charge pump to give the preamp more headroom? Seeing as the PT2399 can only cope with 5V signal, I can see why the clipping diode became real important.

Edit: Shouldn't pin 5 of the charge pump be grounded, or is that only the MAX1044?

[midwayfair]

As far as I know, you shouldn't use junctions in the kind of points I coloured red. Junctions are for connecting more than 2 wires meeting. Components are connected to wires just like that - no need for junctions there. Btw. If you don't, you should use Net function rather, not Line/Wire.
You should use junction in the point I coloured blue, if the wires are meant to be connected.
Anyway, the idea is great, and from what I see in the video, works just as well. Shame that only half of the pots are outside. ;p At least the tone pot should also be on the outside, I think.

Eagle can be funny about connections when using the line tool sometimes. I threw in junctions to be safe because I got tired of going back to the schematic to add them. :/ I had a few components that just wouldn't stick together, though weirdly the one you circled in blue worked without a junction (both of those pins are in fact grounded in the PCB).

Anyone's free to mount the tone pot outside. I just prefer an uncluttered panel and I wouldn't change it once I had it set. And it's easier to turn a trimpot into an external pot than vice versa.

samhay: pin 5 apparently does not need to be grounded in either chip for a basic +18v operation. And according to a recent post it might cause the charge pump to whine. I just did it the way it's done in tons and tons of madbean projects and the like.

As far as running it at 9v, the +18v also helps ensure that the single transistor has enough output for the split signal. (You can't use a high-gain transistor like a Darlington because it will freak out the PT2399 -- oscillation, not distortion. I'm not sure why.) But yes it's also a headroom thing. I was able to overdrive a 9v transistor setup biased to 4.5v with a booster. And since using the charge pump gave both headroom and enough gain, whereas using a second clean gain stage would have only given more output, it seemed the right call here. I thought pretty hard about using a charge pump in the design, since they're a fairly expensive part and I usually avoid those in designs I release.

The limiter LED is just for protecting against boosted signals -- the output on the emitter is below unity with the guitar, so even a high-output humbucker by itself wouldn't overdrive it even without the diode.

[Thomeeque]

Eagle can be funny about connections when using the line tool sometimes. I threw in junctions to be safe because I got tired of going back to the schematic to add them. :/

There is an error check function (Erc) in schematic editor, which will point out suspicious junctions (besides the other errors)..

[midwayfair]

Eagle can be funny about connections when using the line tool sometimes. I threw in junctions to be safe because I got tired of going back to the schematic to add them. :/

There is an error check function (Erc) in schematic editor, which will point out suspicious junctions (besides the other errors)..

Yes, I've used it, and sometimes the net isn't created when using the line too. Is there a compelling reason to change it if it's functioning, or is this just telling me for next time I create a schematic? Or is it actually confusing for other people reading the schematic? p.s. not trying to be hostile, I guess I just don't understand why it's even come up.

[Thomeeque]

Yes, I've used it, and sometimes the net isn't created when using the line too.

These situations are pointed out by Erc too (unfortunately I don't have example now). It does not happen to me when drawing lines but usually when I connect segments by moving parts (or segments) around - sometimes it attaches them correctly, sometimes it does not. I use Erc after these actions (or just drag part and move it around to see if all connections follow) and clean it immediately, no big deal IMO.

Is there a compelling reason to change it if it's functioning, or is this just telling me for next time I create a schematic? Or is it actually confusing for other people reading the schematic?

It makes schematic just little bit harder to read to me but it's not that bad (circuitry is still readable correctly), I would go with "next time"

[midwayfair]

Yes, I've used it, and sometimes the net isn't created when using the line too.

These situations are pointed out by Erc too (unfortunately I don't have example now). It does not happen to me when drawing lines but usually when I connect segments by moving parts (or segments) around - sometimes it attaches them correctly, sometimes it does not. I use Erc after these actions (or just drag part and move it around to see if all connections follow) and clean it immediately, no big deal IMO.

Is there a compelling reason to change it if it's functioning, or is this just telling me for next time I create a schematic? Or is it actually confusing for other people reading the schematic?

It makes schematic just little bit harder to read to me but it's not that bad (circuitry is still readable correctly), I would go with "next time"

okay, thanks! When I get home I will see if I can just delete the junctions without changing anything on the board.

[SISKO]

Great work!!

[slacker]

Sounds great Jon, nice work.
Not to derail the thread but I always enjoy your demos if only for your guitar playing, do you play in a band or anything be interesting to hear it.

[midwayfair]

do you play in a band or anything be interesting to hear it.

This is the sort of derail I'm perfectly fine with!

The band linked in my sig (Midway Fair) is on a more or less permanent hiatus until some stuff gets sorted out with a couple band members, but our records are free downloads. I'm hoping we'll be able to practice some more in the Spring and put out an EP early next year, because the last four songs we worked on are really, really good and definitely need the full band treatment since our keyboard player did a lot of the arranging. I also play mandolin with a bunch of people in the Baltimore area, most recently an Americana group Whale Show. I've hada solo EP being in the works for what seems like an eternity. I recorded it over the summer; it's just not done mixing. And I'm doing FAWM this year, so ironically I'll have a solo "record" done before the EP ...

[samhay]

samhay: pin 5 apparently does not need to be grounded in either chip for a basic +18v operation. And according to a recent post it might cause the charge pump to whine. I just did it the way it's done in tons and tons of madbean projects and the like.

Cool. I have only used charge pumps for bipolar supplies, so I have learned something today

As far as running it at 9v, the +18v also helps ensure that the single transistor has enough output for the split signal. (You can't use a high-gain transistor like a Darlington because it will freak out the PT2399 -- oscillation, not distortion. I'm not sure why.) But yes it's also a headroom thing. I was able to overdrive a 9v transistor setup biased to 4.5v with a booster. And since using the charge pump gave both headroom and enough gain, whereas using a second clean gain stage would have only given more output, it seemed the right call here. I thought pretty hard about using a charge pump in the design, since they're a fairly expensive part and I usually avoid those in designs I release.

The limiter LED is just for protecting against boosted signals -- the output on the emitter is below unity with the guitar, so even a high-output humbucker by itself wouldn't overdrive it even without the diode.

Fair enough. Ultimately, it sounds good, so I guess it is/was worth it.

Oh, and like Slacker, I always enjoy your playing. I too play sans plectrum - just not as well as you unfortunately - and enjoy trying to rip off your riffs.

[slacker]

The band linked in my sig

Doh, right there in front of my eyes, or it would be if I didn't have sigs turned off.

[midwayfair]

The band linked in my sig

Doh, right there in front of my eyes, or it would be if I didn't have sigs turned off.

Huh! I didn't even know people could do that. It's www.midwayfair.org.

errr.. Back to the delay discussion now. 

[soundclone]

Can I use 7660 SCPA for charge pump?

[midwayfair]

Can I use 7660 SCPA for charge pump?

As long as you connect pin 1 to pin 8. As noted in the build doc, if you do this substitution, you'll also want to use a 10v Zener.

I tend to use the LT1054 because it's less likely to blow up. It's just worth the extra $1 to know I won't have to replace it if my power supply spikes.

[soundclone]

OK TANKS. But i can't find LT1054 in local store.

[midwayfair]

OK TANKS. But i can't find LT1054 in local store.

I usually pick a couple up if I have to order stuff from Mouser. I don't even try to buy most stuff locally ... too difficult to find most parts.