# Clean Preamp for DIY practice amp - suggestions please



## Grubb (Oct 15, 2022)

Hi folks, I have decided to build my kids a practice amp for Christmas seeing as they have started playing recently. So far the plan is to use Erik Vincent's 10 Minute Amp circuit (7W class A-B power amp) and a Belton brick reverb like the Boing. I just need a Fendery clean preamp as the front end of the amp, preferably something I can DIY. I was considering waiting for the Doxie but then I read that they perform best in front of a full guitar amp, whereas I want something that can go straight into the power amp section. Any suggestions?


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## Elijah-Baley (Oct 15, 2022)

Maybe a classic ROG Tonemender could be ok for your project.


			Tonemender
		


Fendery, but even something more.


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## Grubb (Oct 16, 2022)

Yep, I think that's perfect, thank you @Elijah-Baley

Here's what I've got so far, the only part of the circuit I'm not confident about is the speaker protection segment.  I found a circuit that uses a 555 timer IC to drive a relay closed for about 5 seconds, which I'm hoping will help me avoid thuds and pops through the speaker cabinet. I might try and trim it to 3 seconds if I can work out how to set the R and C values.





I took some visual cues from the Keeley Aria with the look of the enclosure design, it's currently planned for a 1590XX but I'll see how big it ends up. My daughter has no idea about this project and today asked for an amp for Christmas so there's a fair chance this will be a winner as far as presents go!


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## JamieJ (Oct 16, 2022)

That looks awesome @Grubb 
Can’t wait to see the finished product!


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## Grubb (Oct 23, 2022)

Updates: The good folks at DIY Stompboxes helped me simplify and improve the design. It now has a Mach 1-flavoured clipping section going into the Tonemender tonestack. Switching for the reverb was removed as the circuit has a dry path that is always on, so the knob can control whether or not the reverb is active. And the amplifier chip has built-in soft-start capability, so the speaker mute circuit was removed. Overall I'm pretty happy with the results so far. 

Schematic:




Updated exterior (1590xx, Tayda light gold):




PCB:




Can anyone spot the neat symmetry cheat I learned from Robert?


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## JamieJ (Oct 23, 2022)

I love it @Grubb 
What an awesome Christmas present!!


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## Feral Feline (Oct 23, 2022)

Grubb said:


> Updates: The good folks at DIY Stompboxes helped me simplify and improve the design. It now has a Mach 1-flavoured clipping section going into the Tonemender tonestack. Switching for the reverb was removed as the circuit has a dry path that is always on, so the knob can control whether or not the reverb is active. And the amplifier chip has built-in soft-start capability, so the speaker mute circuit was removed. Overall I'm pretty happy with the results so far.
> 
> Schematic:
> View attachment 34344
> ...


Re "symmetry cheat" ... you mean like where your lightning bolt logo is to replace an electrolytic and the "NRG" text & logo to make up for the cap and resistor ?


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## Grubb (Oct 23, 2022)

Haha that's exactly right. I was looking at PCBs here the other day and I noticed some "caps" with logos rather than values and I was floored by the genius of it 😂


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## benny_profane (Oct 23, 2022)

Looks great! That’ll be a very cool gift! Is that power amp ic obsolete?


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## Grubb (Oct 24, 2022)

benny_profane said:


> Is that power amp ic obsolete?



Not according to Mouser. It's not cheap though, $11AUD. That's nearly as much as an LM308 costs from a bricks and mortar electronics store here ($15) 😅 I may go for a cheaper preamp IC too.


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## Feral Feline (Oct 24, 2022)

Grubb said:


> Haha that's exactly right. I was looking at PCBs here the other day and I noticed some "caps" with logos rather than values and I was floored by the genius of it 😂


Do I win one of your Christmas boards?


Seriously, though, I think your kids are gonna be well chuffed.


I might try to layout a perf of your schematic, if that's okay with you.


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## Grubb (Oct 24, 2022)

Feral Feline said:


> Do I win one of your Christmas boards?
> Seriously, though, I think your kids are gonna be well chuffed.
> I might try to layout a perf of your schematic, if that's okay with you.


There may well be spare boards that I'm unlikely to build. The downside with rolling my own PCBs is that I always end up with spares as there's a 5-board minimum order. But perf away if you prefer, I shared in the hope it would be useful to someone else. None of these circuits are mine in the first place!


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## Grubb (Dec 4, 2022)

Oops, so I made a slight mistake with my board for this project. I accidentally put the wrong DPDT switch DipTrace component in my PCB design and didn't pick up the error. Now I have physical boards that won't fit a mini or sub-mini DPDT toggle. 

Anyone know what kind of DPDT switch might work with 2.54mm pin spacing in both horizontal and vertical directions?


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## grin (Dec 4, 2022)

Hi. I'm interested in a similar project. Can the Neurocyton Preamp be used as a preamp? Opinions?


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## Feral Feline (Dec 5, 2022)

Grubb said:


> Oops, so I made a slight mistake with my board for this project. I accidentally put the wrong DPDT switch DipTrace component in my PCB design and didn't pick up the error. Now I have physical boards that won't fit a mini or sub-mini DPDT toggle.
> 
> Anyone know what kind of DPDT switch might work with 2.54mm pin spacing in both horizontal and vertical directions?
> 
> View attachment 37284



What about a 3PDT toggle with one column of lugs cut off? Problem with that is the switch will be off-centre.

Offboard wire a micro-switch, if possible. Maybe... Offboard wire the mids switch & the Mids knob and manoeuvre their positions a tad, swapped or ?

I've looked at some slide switches, too, but nothing comes up with 2.54 on both X & Y axis.



How'bouit a pushbutton that you have to bend the pins a tad? This one has a 2.54 spacing vertically, and 2X that spacing horizontally, so maybe the pins could/would
be able to reach with a bit of bending...






						LEAP#244 Switches/DPDT
					

all about double-pole, double-thow (DPDT) toggle switches




					leap.tardate.com
				



Ahh never mind that one, it's common pins are 3 and 6. Never seen that before in a DPDT, common's always been 2 and 5.

...still, a pushbutton might be out there that fits...


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## Mike McLane (Dec 6, 2022)

Get on the SushiBox FX thread and check out the Echo Foxtrot for a simple clean TUBE driven preamp.  It's got a EF86 preamp tube which is not a Fender approach. . . . BUT. . . . if you can wait I know that Nathan is working on a DIY blackface Fender preamp that I expect (hope) is not too far off from release.  It should be a simple straightforward AB763 circuit w/ Gain, Bass, Mid, Treb, Vol.  I am salivating just thinking about it.


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## Mike McLane (Dec 9, 2022)

Go here for tubey goodness.  .  .  . CLICK HERE


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## Grubb (Dec 22, 2022)

Update:










For the eagle-eyed among you, I did actually remember to install the ICs before closing the enclosure for once!


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## Bricksnbeatles (Dec 22, 2022)

Awesome! I hope your kids make some wonderful music with it! Merry Christmas!


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## giovanni (Dec 22, 2022)

Very cool! What did you do about the switch? And what speaker are you using?


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## Grubb (Dec 22, 2022)

I ended up ordering the PCB again with the correct switch footprint. Couldn't work out a fudge that I was actually going to be able to pull off because the holes were already drilled in the enclosure. 

I ordered a Pulse PM112 cheapo guitar speaker cabinet. It will do fine for the kids, and if they outgrow it I'll just swap in a decent speaker.


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## Feral Feline (Dec 23, 2022)

Awesome, and just in time, too!

A very Merry Christmas will be had, I'm sure.


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