The "Forum Amp" design thread

[Taylor]

I voted on the dual channels with separate EQs. I guess because I was thinking that it would be more useful to have two different full channels of amp emulation, like a fender clean and a marshall dirty. If you aren't going to have two full emulators, all you need do is have a footswitch that switches between two gain pots. That's easy enough to do as a mod and doesn't require any extra PCB space. Although, if you were to include it on the PCB, it would be easy to leave out.

Now that I type that out, What do you think Taylor of having that as an 'option' on the PCB. A second gain knob (channel) with switching as an Option, just like the Effects Loop or Reverb could be optional?

Jacob

Well, to have a footswitch would require a relay, or maybe electronic switching with a 4066 IC, otherwise you'd have your audio zooping through the footswitch cable, to the footswitch and back up the cable. So it does require the extra space for the relay and driver circuit. It's not a huge deal, but something to think about, as a lot of people who build their own pedals are suspicious of relays for some reason. Otherwise, I'd design all my DIY PCBs with relay true bypass, as that's how I do it with all my stuff. It adds an unfamiliar part which people don't seem to like. But again, not a huge deal.

Oh, unless you were going to build the footswitch into the amp's enclosure... somebody on the circuitworkshop thread linked a page or two back suggested that.

An optional second channel is a good idea, the key is just in making it simple to build either way. Often, when you add lots of "forked roads" where you can do things one of two ways, it really confuses people. So it's a tricky balance of making the project versatile but not really confusing to people with a little less understanding.

[culturejam]

I think one channel is plenty. Switching from dirty to clean (the whole point of channels) should be as easy as bypassing whichever pedal is providing the dirt.

I also vote for a super-squeaky clean preamp so that it won't be overdriven by hot signals. If I want a nice JFET preamp to overdrive, I'll stick one in front of the amp in the form of a pedal.

Go with the laptop power supply. 

[jkokura]

Dang, you and I gotta have a talk and you explain Relays to me then Taylor. I've been trying to read up on them and see how to make them work for me for weeks!

Jacob

[phector2004]

I'm really interested in seeing how you'll build this! I've got a similar but smaller amp powered by a 24V 1A SMPS. Still figuring out a suitable preamp...

Laptop power supply is a great idea, though. The one I'm using now (on my laptop) puts out 19V @ 4.7A... my friend's laptop "Mobile Animation Workstation" adaptor puts out a whopping 24V @ 6A 

144W?!?

Guessing most of it will go to waste... How are you gonna deal with heat? Mine's got a massive heatsink and its only 5W....

[Taylor]

I think one channel is plenty. Switching from dirty to clean (the whole point of channels) should be as easy as bypassing whichever pedal is providing the dirt.

I also vote for a super-squeaky clean preamp so that it won't be overdriven by hot signals. If I want a nice JFET preamp to overdrive, I'll stick one in front of the amp in the form of a pedal.

This is what I figured most people would want, but the majority in the poll seem to want channel switching.

For those who want channel switching, tell me a little about why you'd prefer this over using pedals to switch from clean to dirty.

[culturejam]

For those who want channel switching, tell me a little about why you'd prefer this over using pedals to switch from clean to dirty.

Yeah, I want to hear the reasoning behind this as well. I can't imagine why anyone would want a clean and dirty channel if they build their own dirt pedals.

I think this might be a case of "if you give people an option, they'll choose it". 

[joethugs]

hi..

here's my take on chip amps



http://www.diystompboxes.com/smfforum/index.php?topic=86840.msg728338#msg728338

[JKowalski]

You could use a bridged amplifier configuration if you ultimately decide on a laptop PSU. Basically, two single supply amplifiers amplifying the same signal but inverted from each other. Connecting a speaker across the outputs negates the DC at Vcc/2 since the same potential relative to ground is at either end of the speaker, and results in only the AC output.

Plus , you get a larger output headroom. A typical single supply amplifier can only output a voltage that is around 1/2 Vcc because it uses the midway point as one connection to the speaker and the power amp output as the other - it can only go maximum peak 1/2Vcc negative and 1/2Vcc positive.

In a bridged amplifier, at the peak, one amplifier is going negative while the other goes positive. Each amp can go to 1/2 Vcc peak +/-, so when one goes up and one goes down that's actually the full Vcc across the speaker (peak).

Just be careful about the design, keep in mind that without decoupling (transformer/capacitor) you have Vcc/2 on the output of the amplifier that if shorted to ground may shut down the amp or cause disastrous damage... Ideally I would put decoupling on the output... but if you have the speaker in the same box as the amplifier, you can just hook up the speaker direct to the bridged outputs without much worry.

[hday]

Channel switching would just be a convenience. This little guy should be pretty portable, sort of an all-in-one package, and having a few different choices in tone would make things simple. But a bomb three-band EQ design might make it unnecessary. With the title of "Forum Amp" it should easily impress, and I'm not sure a basic, clean amp is that impressive. Not that I'm not stoked to build this thing! 

[jkokura]

Taylor, I thought this would be more of an on the pedalboard kind of amp, where it would be easy to have a 3PDT to switch channels at your feet. No need for the signal to travel down a cord. If built into a head or combo cabinet, switching could simply be a 3PDT toggle switch on the amp.

I do want EQ though, so if the choice is channels or EQ, I vote EQ.

Jacob

[derevaun]

I'm a total noob lurker here, but here's my take on this:

I like the idea of a "main configuration" with options to mod, replace, repurpose, etc. So, having a laptop supply as the nominal power source, with affordances for other options, is brilliant.

Likewise, having a preamp stage that could also feed a balanced line to a mixing board (and maybe a dedicated camcorder audio feed for youtube demos  ) would be great; having separate buffered instrument-level and line-level effects loops in the preamp would likewise be neat, if they could be optionally unused too. Being able to bypass the preamp with one's own DI-type pedal would be nice too.

I don't use a second amp channel because it tends to de-motivate me from moving around onstage. I prefer to use a Gigrig Loopy-2 in the pedal chain.  But if the dual channels were nominally identical and had separate tone controls, that would make it worth it, I guess. As a bass player I don't often go to a fuzz sound but I do sometimes need to fill a different tonal niche, and it's a pain to kneel down and dial in a second tone at the pedal board. In any case, if I can leave it out and/or populate it in later, all the better.

Which, having a bass-optimized parts list option would be awesome as well.

In any case, thanks Taylor!

[markeebee]

Oh, ok then, nobody likes my idea about modularisation. Taylor, please don't let me be misunderstood.......let me try again:

How about semi-modular? 

A couple of people have (IIRC) mentioned how a TDA power stage plays nicely with a ROG pre amp. I've made a few with English Channel and Prof Tweed as preamp sections, and they've always been good enough.

So how about a power stage and physical package that is specifically optimised to work with ROG designs?  After all, the ROG designs are proven to be good, and it would take many many hours to design a pre for the Forum Amp that is any better.

If the masses decree so, it could also incorporate a switching arrangement to select between two (or more) of the preamps - you could switch between, say, Vox/Supro channels rather than just clean/mucky.

And if it could be made so that you can slide out/slot in the preamps to change flavours, that would be cool. Means we might need to change the for factor of the ROG circuits so that the inputs and outputs are in the same place (or maybe come up with a "pcb caddy" affair) but are we not men?

[Taylor]

Oh, ok then, nobody likes my idea about modularisation. Taylor, please don't let me be misunderstood.......let me try again:

How about semi-modular? 

A couple of people have (IIRC) mentioned how a TDA power stage plays nicely with a ROG pre amp. I've made a few with English Channel and Prof Tweed as preamp sections, and they've always been good enough.

So how about a power stage and physical package that is specifically optimised to work with ROG designs?  After all, the ROG designs are proven to be good, and it would take many many hours to design a pre for the Forum Amp that is any better.

If the masses decree so, it could also incorporate a switching arrangement to select between two (or more) of the preamps - you could switch between, say, Vox/Supro channels rather than just clean/mucky.

And if it could be made so that you can slide out/slot in the preamps to change flavours, that would be cool. Means we might need to change the for factor of the ROG circuits so that the inputs and outputs are in the same place (or maybe come up with a "pcb caddy" affair) but are we not men?

It's not that I don't like the idea Mark - I like it a lot actually. It's just that, to me, a pedalboard kind of is a modular guitar processing system. So it seems like that kind of setup would largely be duplicating what people are already doing with their pedalboards. From the perspective of designing neat stuff, it's cool. When I was a kid, so like 2 or 3 years ago, I always built all of my LEGO, Capsela, Knex, etc. vehicles with little detachable mini-vehicles, docking stations, escape pods, etc. That's a lot of fun, and if you guys will buy it, I will have lots of fun designing it. 

But I'm thinking that that might not be adopted by enough people for it to work out.

[markeebee]

Um, yeah, fair enough.  Can't argue with lego.

Having read back through the thread I see now that you've already addressed my idiot raving in previous posts, and I just misunderstood your initial brief. 

Seems to me that what we're moving towards is something like a EHX Caliber, but at a close-to-ebay-chipamp cost?

[petemoore]

  What does it do ?
 Where should we take it from there ?
 Is there something outside of the amplifier design we should be looking at ?
 *Does the final amplification stage do sufficiently well with bass and treble reproduction ?
 What happens with 'this' speaker and cabinet, room and surfaces ?  
   *this part of design can pretty well be nailed, any number of ways, has become extremely easier with use of a chip, but of course still requires 'a' power supply.
 Stereo ? Why not, if the power supply can handle the current draw [without a huge increase in cost], the second amp doesn't add that much cost and then doubles output @x voltage, allowing dual mono or stereo.
  I use a stereo chipamp unit it for MP3/Disc/recorder/playback of stereo recordings makes it pretty useful. I almost built a mixer for 2 into 1 playback.
  "doubles output"...ie requires a second speaker, so a doubling of power output is a bit closer to doubling of percieved loudness. A lot louder and louder with the sweetness is the best description I can offer, 2 speaker cones separated by X distance adds a slight touch of off-phasing effect.

[JKowalski]

Um, yeah, fair enough.  Can't argue with lego.

Having read back through the thread I see now that you've already addressed my idiot raving in previous posts, and I just misunderstood your initial brief.  

Seems to me that what we're moving towards is something like a EHX Caliber, but at a close-to-ebay-chipamp cost?

Well, that's a class-D amp (the 44 magnum is no doubt a class-D)... quite complicated, quite a beast to design. This (non class D) would probably be much bigger... Unless you design around the loads of heat you are going to be dissipating into the enclosure. The EHX amps only manage to be small because of the ridiculous efficiency benefits that class D amps give you...

Knowing EHX's talent with dsp effects I would bet that those amps are designed around a custom programmed DSP chip to control the PWM conversion and switches.

I got the impression that this would be a medium sized to small amplifier, not "miniature".




A stereo chip amp would be nice for the bridged configuration I mentioned earlier.

[culturejam]

So you guys who want channel switching...I assume you want a clean and dirty channel? (I can't see the use of having two clean channels.)

If so, that's definitely not something I'd want. I would much prefer to dial in my own dirt with pedals.

[Galego]

I really don't care about the preamp part. That should be easy enough for people to figure out on their own. So many JFet based preamp schematics around. But the power amp i'm really interested in. I've got a couple of TDA2030's and a 18V transformer, both from some sort of amp, i've never seen it complete, just a broken pcb and cut wires.

[deadastronaut]

+! culturejam..

be nice to have a really clean 20w amp with a nice eq...bass/ mid (very scoopy)/ treb/ pres/ not overly bright though!....

then add our own flavour of dirt as one mans idea distortion is very different to anothers...

[Taylor]

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. Maybe you want a little stompbox amp in one package for taking to rehearsal or traveling. That would give you some versatility without going overboard with adding specific distortion circuits.

For those who want to build their own monster amp with switchable preamps, each with its own EQ, etc., this amp PCB would be a perfect building block, which you could marry to the preamps/overdrives/reverb of your choice. You could build a perfectly tailored amp to your tastes, and it would be much simpler to build than any other guitar amp project.