Yet another mu-amp (SMD/SRPP)

[waltk]

...been fooling around with SMD lately.  Here's a version of a mu-amp with an op-amp buffer before and after (much like gregwbush suggested in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=97192.0).  It includes "drive" and "volume" pots, and incorporates "noiseless biasing" and SRPP as suggested by R.G. on GeoFEX.

It sounds great; I think in part because the batch of SMD J201s that I got are better (Vp ~-.7) than some of the through-hole parts I've had lately.  Goes from very clean to nice crunchy break-up.

Some of the caps between stages might be unnecessary, but they are not hurting anything.



Layout





Populated board



with wiring



PCB (the 1200 DPI version is here: http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=49828&g2_serialNumber=1)
This PCB image is already reversed for printing.

[kingswayguitar]

cool, thanks for sharing
love the j201 mu amp
i never felt i needed a buffer up front
i can see a bit of benefit on the back end before your volume pot, but i never did and i experienced no loss
i've used buffers before the volume pot on other cicuits though with great results, mostly after a bjt common-collector
cheers

[Mbas974]

what exacly you mean with  "noiseless biasing"  are the 3 caps in parallel ?

I suggest to add a small cap 50p - 100p from leg 3 to leg2 of Drive pot.

[J0K3RX]

It sounds great; I think in part because the batch of SMD J201s that I got are better (Vp ~-.7) than some of the through-hole parts I've had lately.  Goes from very clean to nice crunchy break-up.

Some of the caps between stages might be unnecessary, but they are not hurting anything.

Thanks waltk, great post! I have been using the SMD J201s also and they sound so much better to me than the through hole... and they seem to be much more consistent.  Anyway, cool stuff man! I am investigating high gain designs much like this. If you have any sound clips that would be great!? I think I will give yours a try either way... Got any other stuff like this?

[waltk]

i never felt i needed a buffer up front

I really wanted the "Drive" pot before the mu-amp, and without the buffer, the pot would lower the input impedance too much. (I think - not claiming to be an expert here  )

Besides, it's a dual opamp, so why not use both sides?

what exacly you mean with  "noiseless biasing"  are the 3 caps in parallel ?

"noiseless biasing" refers to using lower-value resistors (10K) for the voltage divider to make the bias voltage - then using a single higher-value resistor to the gate of the JFET.  This reduces the thermal noise introduced by a high-resistance voltage divider.  The capacitor to ground from the voltage divider junction also shunts any remaining noise.  There's an article by R.G. on GeoFex (www.geofex.com/Article_Folders/modmuamp/modmuamp.htm) that describes it better than I can. (and you'll have to ask him for a deeper explanation)

The 3 caps are in parallel - just because it's hard to find small SMD caps with large capacitance.  I wanted to get about 22uF for that voltage divider junction, and you can see on the actual board that I used 2-10uF (B-sized) caps.  I also have some 6.8 uF 0805 caps, and I wanted to leave some room/flexibility on the board to use other combinations.

Anyway, cool stuff man!

Thanks.  I think I'm hooked on SMD now.

Got any other stuff like this?

Yep.  I also have a really tiny verified layout for a mu-amp layout without the buffers, a layout for the Alemic Stratoblaster or Stage3 boost, and a layout for the Electra distortion or LPB booster - plus a bunch of stuff using SMD LEDs.  You might also check out Chromosphere's posts about SMD projects - and his really cool SMD practice board in the "for sale" section (didn't actually try it, but it looks great).

One difference in my approach is that I don't even attempt using a soldering iron for the SMD parts.  I just put a little dab of solder paste on each pad, place all the parts, then blast it with a low-velocity hot air gun (that I got from Sears many years ago - no need for an expensive SMD hot air workstation).  I estimate that that it takes about the same amount of time as a through-hole board overall - maybe a little less.

The biggest obstacle to getting into SMD construction is that you have to acquire a minimum amount of SMD parts to get started with general effect building.  You can find some reasonably cheap assortments of resistors and caps on Ebay, but you will still need to get some nice semiconductors.  I got the SMD equivalent of BS170 mosfets, 1n5817 diodes, MPF102 JFETs, J201 JFETs, and BJT PNP and NPN transistors.  It's amazing what you can build with this handful of parts.

I have been using the SMD J201s also and they sound so much better to me than the through hole... and they seem to be much more consistent.

I agree!  Consistently good.  Maybe the manufacturing process is better for SMD - every J201 on the reel has had Vp and transconductance in a narrow range.  I wasn't worried about matching in this application, but pretty much any 2 JFETs in the batch would match.

[waltk]

P.S.

In my search for SMD parts, I acquired a stock of several large-value caps.  I put together an assortment for sale here: http://www.diystompboxes.com/smfforum/index.php?topic=104389.0

There are still some available.

[kingswayguitar]

woops
didn't see the drive pot (one toke over the line)
i concur
long live the mu-amp!

[GoranP]

The 3 caps are in parallel - just because it's hard to find small SMD caps with large capacitance.  I wanted to get about 22uF for that voltage divider junction, and you can see on the actual board that I used 2-10uF (B-sized) caps.  I also have some 6.8 uF 0805 caps, and I wanted to leave some room/flexibility on the board to use other combinations.

Alright I'm just starting in this smd business so I gotta ask... we are shooting for 22uF there. I have a few Y5V 10uF that are probably not the best choice (?), if the minituarization wasn't a concern could I just use one 22uF electrolyte smd in there?

[waltk]

I have a few Y5V 10uF that are probably not the best choice (?), if the minituarization wasn't a concern could I just use one 22uF electrolyte smd in there?

Yep, you could use anything that fits.  The cap type is not critical either.  It's function there is just to shunt noise away from the bias voltage.  In fact, you could probably just leave it out.

[GoranP]

Thanks!

So I'm just gonna replace all three XuF's with one say 22uF electrolyte (+ to right, on schem?), that should work fine?

[waltk]

Yes, that's fine - except the + side on the cap is the left side on the schematic (bottom side on the layout).  The plus side is always connected to the higher voltage.

It's one of the confusing things about SMD cap markings - EL caps have a marking on their "-" side, and tantalum caps have a marking on their "+" side.

[GoranP]

Awww man I can't believe I wrote that...

That's actually a typo, I was looking where the lower potential was, found that the right side was grounded and typed "- to right" but I apparently hit "+" on the numpad which is all too close to "-"...
Good thing you caught that, I actually re-read it once and completely missed it (just goes to show one shouldn't be typing away on pedal forums while at work)... 

[waltk]

Cool.  Good luck with the build - let us know how it goes.

[deadastronaut]

Thanks waltk, great post! I have been using the SMD J201s also and they sound so much better to me than the through hole... and they seem to be much more consistent.  Anyway, cool stuff man! I am investigating high gain designs much like this. If you have any sound clips that would be great!?

i'll have to check those smd's out then..

nice post walt. i love a mu-amp circuit.

clips?