The "Forum Amp" design thread

[Ben N]

I'm working on a gyrator-based mid section, with Baxandall bass and treble.

An Ampeg man, eh? Nice choice.

[therecordingart]

Any updates on this?

[Taylor]

Just waiting on the proto boards. Should have them Wednesday. If all goes well with them, the final boards would be here about 2 weeks later.

[MetalUpYerEye]

What are we lookin' at cost-wise for the power-amp with PCB, ICs, etc.?

Was thinking it could be cool to have an option for a tonestack on the clean channel without built-in compression and overdrive.

I'd probably re-house my Dr. Boogie with this for a hot channel. Maybe a switchable 4x12 cab sim on the end? 

Could be cool to build a Mesa/Boogie recto clean channel preamp to use for l33t Mesa clean tones. 

[Taylor]

Don't know an exact cost but it'll be fairly cheap. The only special parts are the power amp (TDA7240a) the regulator (LM338T) and the laptop power supply, all of which I will sell on my website, or else Aron will sell them in his store, for reasonable prices. If you want to source that stuff elsewhere, I encourage that, but I thought it would be nice to save on shipping if you could get those parts at the same time as you order the PCB.

Since there's no transformer it's about as cheap to build as any small pedal.

I wanted to point out if it's not clear that it's just the straight power amp PCB that's on the way. There's still some work to do on the full amp with EQ and everything. But what's nice about the power amp is that it's so small you can pair it with whatever you want to use as a preamp and fit it in a small space.

[Taylor]

I'm working on a gyrator-based mid section, with Baxandall bass and treble.

An Ampeg man, eh? Nice choice.

Actually, I hadn't really looked at any Ampeg amps for inspiration - I mostly just played with the various tone stack calculators and concluded that not having a mid boost is really limiting. What Ampeg amps use this kind of setup? I'd definitely be interested to see how some other designs have implemented this combination.

[bassmannate]

I'm working on a gyrator-based mid section, with Baxandall bass and treble.

An Ampeg man, eh? Nice choice.

Actually, I hadn't really looked at any Ampeg amps for inspiration - I mostly just played with the various tone stack calculators and concluded that not having a mid boost is really limiting. What Ampeg amps use this kind of setup? I'd definitely be interested to see how some other designs have implemented this combination.

Yeah, I'd love to see some too! Looking for a good bass preamp to put in front of this thing! As quickly as this seems to developed, I'm really curious to see a schematic and BOM.

[Taylor]

Well, fellas. Here we have a complete 20 watt clean instrument amplifier (clean preamp and power amp) in a 1590A box.

The good news is that it works and sounds massive (even with a bass). The bad news is that I need to change a couple of things in the PCB design which means another rond of prototype PCBs.

To clarify what this PCB is/does: it has a clean preamp with high input impedance, and a decent amount of clean gain so you can plug your passive guitar straight in without losing high end. This then goes to a 20 watt power amplifier. There is an onboard power regulator to take a variety of power supplies and get them to where the power amp is working as it should.

There is no EQ or clipping on this PCB. It's intended to be the clean power, which you can pair with whatever sound shapers you want.

[JKowalski]

How did the heat dissipation turn out,

And where do you get your PCB's made? I was looking to do a prototype run myself for something I've been cooking up. It's a 6.5x2".

[Taylor]

Gets slightly warm to the touch, not hot at all. But I haven't been able to really stress test it as its way too loud for my house and I'm not a gigging musician. I'll see if I can get somebody to really punish it with loud volume for an extended time to see how it goes.

For prototype PCBs, pcbfabexpress.com.

[waltk]

Go Taylor!  I'm drooling now...

I take it that the IC at the top-left is an LM338?  And the thin brownish thing is an insulator to keep the tab from shorting against the enclosure?

-Walt

[azrael]

Neat! I've been looking into little power amps, this one looks great!

Now I just need a more powerful one to replace my clunky Crate Powerblock for gigging situations.

[Taylor]

Go Taylor!  I'm drooling now...

I take it that the IC at the top-left is an LM338?  And the thin brownish thing is an insulator to keep the tab from shorting against the enclosure?

-Walt

As you know, I stole most of what's going on here from you!  Yes, it's an LM338 and the brownish thing is a very thin insulator meant just for this purpose. The screw that holds the chip to the enclosure has heat shrink around it to avoid shorting to the enclosure. When I get the real boards made, I'll offer some plastic screws to make this simpler. I really wish there was a regulator that could handle this amount of current but didn't need to be isolated. Haven't found one yet.

[zambo]

Taylor that is some sweetness goin on there and you are kind of my hero now. Well at least you are up there the Doug H and Rick H. G

[bassmannate]

Go Taylor!  I'm drooling now...

I take it that the IC at the top-left is an LM338?  And the thin brownish thing is an insulator to keep the tab from shorting against the enclosure?

-Walt

As you know, I stole most of what's going on here from you!  Yes, it's an LM338 and the brownish thing is a very thin insulator meant just for this purpose. The screw that holds the chip to the enclosure has heat shrink around it to avoid shorting to the enclosure. When I get the real boards made, I'll offer some plastic screws to make this simpler. I really wish there was a regulator that could handle this amount of current but didn't need to be isolated. Haven't found one yet.

Wait...I'm new to this so it may be a stupid question, but the heat sink tab is connected to something other than the ground pin?   

[Taylor]

Taylor that is some sweetness goin on there and you are kind of my hero now. Well at least you are up there the Doug H and Rick H. G

I don't know about that - those guys are kings to a peasant like me. But thanks. 

Wait...I'm new to this so it may be a stupid question, but the heat sink tab is connected to something other than the ground pin?   

Correct. The heat sink tab of the power amp is ground, but the power regulator (the other tabbed IC in a TO-220 package) is "floating" (no ground) and the tab is connected to the voltage output. So if you connect this to the case, it would short the positive voltage to ground.

[bassmannate]

Correct. The heat sink tab of the power amp is ground, but the power regulator (the other tabbed IC in a TO-220 package) is "floating" (no ground) and the tab is connected to the voltage output. So if you connect this to the case, it would short the positive voltage to ground.

I guess that makes sense...Most manufacturers I guess would put a dedicated heat sink on something like this so you wouldn't worry about the tab grounding out. I still think it would make more sense to have the tab on the regulator be ground to give you the option to sink it to the case. Oh well. engineers with a heck of a lot more education than me design these things so who am I to question it!   

[JKowalski]

Go Taylor!  I'm drooling now...

I take it that the IC at the top-left is an LM338?  And the thin brownish thing is an insulator to keep the tab from shorting against the enclosure?

-Walt

The screw that holds the chip to the enclosure has heat shrink around it to avoid shorting to the enclosure. When I get the real boards made, I'll offer some plastic screws to make this simpler.

I assume you haven't seen the plastic washers (shown below as "bush") made specifically for this purpose - they have a lip that goes inside the TO-220 metal tab and prevents the screw from touching the transistor.

[Taylor]

You are correct in your assumption! Thank you for the tip, and I've now located a whole assembly with mica spacer, bushing, etc.

BTW, googling "bush screw" does not turn up electronic info. 

[bassmannate]

You are correct in your assumption! Thank you for the tip, and I've now located a whole assembly with mica spacer, bushing, etc.

BTW, googling "bush screw" does not turn up electronic info. 

I can only guess that whatever you get when you google "bush screw" is not "family friendly"