The "Forum Amp" design thread

[jkokura]

Bang on Taylor.

Jacob

[Brymus]

The channel switching question on the poll is almost tied.

What I think I might do as a compromise is to do a nice, clean preamp, with some switchable diode clipping in the feedback loop. This doesn't add much to the build in terms of parts, but gives you the ability to have a clean channel and simple overdrive channel if you want.

This is exactly what I did with my TDA 1519b practice amp http://www.aronnelson.com/gallery/main.php/v/Brymus/TDA1519b_386AMP/
The problem I had doing that was,that the EQ and volume wasn't right when switching between OD and clean.
What I mean is that with the EQ set for good clean tones adding in the clipping diodes for the OD tones the EQ and volume had to be re-set for optimum sound.
Which is another reason I had the bass boost > more bass for clean less for OD,so then there was two switches...
It did however keep the part count lower and used less space.

This made me realize why so many prefer two channels each with its own EQ.
Same holds true for using pedals,if your TMB is set perfect for clean tone,when you engage whatever dirt pedal you use ,you are likely to need to re-EQ for optimum sound.
Then again this amp is just for practice not ,for mic'ing at a gig.
My problem is I want to practice what I am gonna play ,that means switching between clean and dirty in the middle of a song sometimes.
I guess it really comes down to what the intended use of the amp is to be.
Is it for just demo-ing pedals,or actually practicing for a show,ect,ect.

[Taylor]

I think if you want a multi-channel amp with separate EQ for each channel, it would make a lot of sense to use this amp as your power amp and pair it with your 2 (or 3) favorite preamp pedals with their own EQ. Let's say we made this amp with a Fender-ish preamp channel and a Marshall-ish channel. Then everybody who doesn't like those preamps would be annoyed that these choices were forced on them.

If we keep it simple, make the preamp and 3-band EQ bypassable, then you can build it specifically to suit your needs by adding preamps/overdrives that you prefer, without forcing those choices on everyone building the PCB.

[fatfoohy]

Taylor, i can't wait to see a finished product for this amp, i love your idea for just using preamp pedals for preamps, plus that way it'll be easy to put a tube preamp in front of this thing, I'm definitely interested in this amp, i'm hoping this thing will have some mods to make it more bass friendly

[Taylor]

I'm a bass player, so I will definitely make sure this does well in that application. Although, a lot depends on your speaker(s) of course - you can't expect deep bass coming out of a single 8" speaker for example.

[fatfoohy]

i do have some unused 15" PA speakers laying around....I think those ought to be handle a little bass

[Taylor]

Yep, although don't get too caught up in stereotypes about speaker size, because there a lot of other factors that matter about as much. If you want to build really, really good bass cabinets, check out billfitzmaurice.com. His designs are mostly horn-loaded, which gives amazing efficiency (so you'll get way more volume out of this amp than with a standard speaker cab), great deep bass, way better dispersion than standard cabs, etc. They are a little more complex to build than a plain box, but vastly superior.


So, based on my current plan for this amp, it will be ideal for the following applications:

-Build it as a simple, standalone amp stompbox. Just build it by itself into a stompbox, with overdrive and EQ on their own stomp switches. Plug from this box into a speaker. Great for practice, travel, simple applications, but honestly 20 watts for guitar should be enough for small gigs as well.

-Use it as a power amp building block. Build your dream custom amp by combining switchable preamps/overdrive circuits with their own EQ, and running it all into this power amp. The amp PCB itself will be small enough that you can cram a lot of other stuff into an amp-size enclosure, even tube preamps.

-Build it into a guitar combo. I will include lots of information on building simple guitar combos in my PDF document, so if you want to build a nice combo amp for testing pedals, practice, etc., there will be enough info to get you going.

[phector2004]

-Build it as a simple, standalone amp stompbox. Just build it by itself into a stompbox, with overdrive and EQ on their own stomp switches. Plug from this box into a speaker. Great for practice, travel, simple applications, but honestly 20 watts for guitar should be enough for small gigs as well.

-Use it as a power amp building block. Build your dream custom amp by combining switchable preamps/overdrive circuits with their own EQ, and running it all into this power amp. The amp PCB itself will be small enough that you can cram a lot of other stuff into an amp-size enclosure, even tube preamps.

Taylor, one thing I didn't think of when building a small amp was powering up the separate preamp. If you're gonna go for a modular design, you can also consider adding a power jack to power up other components. Saves a whole jack/adaptor/plug socket/battery/whatever people use to power their fx

Just my $0.02  (And headache to finish that amp!)

[Taylor]

Yep, that's a good idea, I will probably add that.

[earthtonesaudio]

Taylor, one thing I didn't think of when building a small amp was powering up the separate preamp. If you're gonna go for a modular design, you can also consider adding a power jack to power up other components. Saves a whole jack/adaptor/plug socket/battery/whatever people use to power their fx

Just my $0.02  (And headache to finish that amp!)

That could be as simple as leaving extra pads on the PCB for all the various (filtered? regulated?) voltages.

[waltk]

About the power supply... If the idea is to use an external laptop PS, then you probably want an onboard voltage regulator - both to smooth out the input voltage (as some would be switching supplies), and also to accomodate the range of voltages (12 - 20+ depending on the laptop it was intended for).  Because chipamps can draw quite a bit of current, the usual VR suspects are a little light-weight.  LM78XX are rated for 1A, and LM317 are rated for 1.5 amp.  I would suggest a beefier VR - like the LM338 (rated for 5A), or at least the lm350 (rated for 3A IIRC).  These are available in TO-220 packages, so they don't take up too much space, and will save builders a lot of grief when it comes to chipamps that are a little snippy about their power supplies.  Important tip: plan to add an appropriate heatsink, and don't plan to use the enclosure for it, as the heatsink tab on these is connected to the output (not ground).

About the signal input - consider using a pot/trimmer to scale the input to the chipamp - to allow flexibility in the offboard preamp.  Overdriving chipamps usually causes nasty-sounding distortion (or even lets the smoke out of them).

[Taylor]

Excellent advice, thanks.

[earthtonesaudio]

I would suggest the opposite, actually.  You would be very hard pressed to find an unregulated laptop PSU, so the only thing you might have to worry about is residual high frequency ripple.  That can be taken care of with a relatively small choke and filter cap.  Adding a linear regulator is going to increase the thermal load that has to be dissipated by the enclosure, which is still an unknown quantity at this point.

[JKowalski]

I would suggest the opposite, actually.  You would be very hard pressed to find an unregulated laptop PSU, so the only thing you might have to worry about is residual high frequency ripple.  That can be taken care of with a relatively small choke and filter cap.  Adding a linear regulator is going to increase the thermal load that has to be dissipated by the enclosure, which is still an unknown quantity at this point.

Agreed, linear regulators are not built to handle high frequency ripple, rather 60Hz... I doubt they will be very useful at ripple speeds from 400kHz to 2mHz. Laptop PSUs are all "regulated" to different degrees of success. The basic operation of any SMPS requires feedback from the output to the input to keep the output stable and thus has regulation.  

A reactive filter is an excellent suggestion.


Thermal  dissipation is a problem you need to focus on, I think... Since you want it to be able to fit in a stompbox enclosure and be around 20 watts. Like I said, the EHX amps are possible because they are Class-D and very efficient, minimizing the temperature of the box.

[Taylor]

Thermal  dissipation is a problem you need to focus on, I think...

Yes, for sure. I'm not aiming to fit it in a b-size box like the EHX amps, so I will be doing lots of experimenting with heatsinking to find an ideal solution.

[jkokura]

I'll build this in nothing smaller than a BB. The new Smallbear 125BB's sound like a good idea for this. I likely will build it into a 1590DD though, mostly because i want additional options.

Jacob

[Taylor]

Yeah, a 1590D or DD is about what I was thinking. The board will probably fit in something smaller, and if you wanted to get creative with machining the enclosure to accommodate the heatsink, it could maybe work in a 1590bb or the 125bb as you say.

[Cap]

Since has passed 2 years from trying to build a combo...and i'm still unsuccesful..I appreciate your work for the forum Taylor!!!
It's exactly what most of us need..as player i suggest the possibility to plug headphones... sometimes the neighbours..you know
but also can be used before playing while other bands are rocking

Ric

[jkokura]

I was planning on building mine with an optional headphone amp after the preamp also. This is the kind of thing that is an add on I think, because the main focus of the project is getting an easy to build solid state power amp going for us. Lots of preamps out there, not so many easy to use guitar focused power amps. A headphone amp defeats the use of a power amp.

So if you want to add one on with a kill switch for the power amp, I'm sure I will do it as well, so somebody if not me will add details to the 'build your own forum amp' thread that will exist after the PCB has been made.

Also, If you want some people to be guinea pigs, count me in Taylor.

Jacob

[Brymus]

No offense meant to anyone, especially Taylor,just my humble opinion but...
Personally ,I was hoping for more than just another chip amp PCB.  

After all there are entire websites devoted to chip amps as a power amp. As an example these people have been doing it for years.
http://www.chipamp.com/

The web has literally hundreds if not thousands of chip amps as a generic power amp designs and PCB transfers.
Even the data sheets to alot of the TDA chip amps include recommended PCB transfers.
As an example the TDA 2030 was mentioned : http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1458.pdf
Figure 14 from the data sheet IS an already designed and tested to be stable PCB transfer.
Ready for the masses to add their own preamp,and whatever else.

That was a very good idea having the poll/ballot but I am very suprised at the end result.
So if the majority want another chip amp/power amp with the rest left for them to design/add.
Then by all means give them what they want.
To me it sounds like your just going to end up with a 20 watt "gainclone" running off a laptop supply instead of the recommended transformer.

Still I am all for any project that will help Aron/the forum and the store.
So carry on ,and cheers to the success of the "Forum Amp" .

I do see what your offering: A  compact 20watt power amp without the need for mains wiring, a PCB ,and detailed instructions in PDF form.
EDIT-spelling