First attempt at discrete amplifier design

[mnturner]

Hey guys, this is my first post although I've been a member of the forum for a while.
I've recently decided to design and build a discrete SS amp. So far I've built the power supply circuit and designed the power amp stage but have not yet fully tested it; it looks ok in simulation though. Initially, I intended only to build a small (~5W) practice amp, but once I started, I realized how easily I could scale the design up to be much more powerful.   
The design seems able to put out about 140W RMS into a 4 Ohm load, limited to about 100W by the rating of the power transformer I'm using.

The problem is, at the moment, I have no cab/speakers able to handle that much power. All I have available for testing is a junked 20W hi-fi speaker.
Also, I haven't fully decided on the plan for the preamp. I think I'm going to go OpAmp based with a pretty stock standard Fender or Marshall tonestack, but I want to build in a reasonable distortion circuit and possibly an octave effect as well. I'd also like to be able to switch preamps/effects without having to pull apart the whole box once I've built it. What I'm thinking of doing to get around this is initially built the poweramp in one box with a master pot to limit the output to something that won't destroy whichever speakers I happen to plug it into, and build the preamp in a separate box which I'll just plug into the poweramp, so if i want a new sound I can build another and plug that in instead.

All of this leads me to ask a few questions:

With regards to giving the poweramp a variable power output to enable me to switch speakers as i go without blowing up the smaller ones, I'm thinking of putting a pot in the feedback loop to enable me to vary the gain. Does that sound good, or would it be better to set the gain in the feedback loop to a reasonable value with fixed resistors and maybe just put an attenuator pot on the input of the poweramp?

Does anyone know a good place to get guitar/instrument speakers in Australia? I don't want to buy a cab, I want to get the drivers and build the box myself. I don't necessarily need the highest quality, at this stage I'm just looking for pretty much any guitar amp speaker that will handle about 50W.

Does anyone have any suggestions as to a decent distortion/overdrive circuit for blues rock? (I play a lot of White Stripes and Black keys type stuff)

I'm planning on running my preamp circuitry off +/- 9V or 12V for the OpAmps (so far I've been using TL072's); is it better to get this supply voltage from a separate power transformer and rectifier circuit or will voltage dividers (possibly with regulators) from the main power rails suffice? I do happen to have a small centre tapped +/-12v transformer.

Does the whole plan sound reasonable?

Anyway, sorry about the epic post, thanks for reading and any comments, suggestions or random thoughts anyone has are totally welcome!

[mnturner]

Got a speaker, Celestion Seventy 80 bought off ebay.

[J M Fahey]

Hi mnturner.
You may try your 100/140W amp with your 20W speaker, as long as you don't use it *that* loud.
You can place a volume control between the preamp and the power amp, which will effectively become a Master volume.
I know in Australia you have this guy who used to work for Rola-Celestion , who still is producing what amounts to be a clone of them.
As of pedals, try any which has "Blues" on its name, you won't be far off; besides the obvious Tube Screamer.
Good luck.
PS: if you wish, you can post your schematic here.

[mnturner]

Thanks Fahey.
I just got some LM3886's in the mail so I might put the discrete project on hold for a while, the schematic's attached anyway, let me know what you think. I'll attach the LTspice file if anyone's interested.
What do you think is the best way to power the preamp circuitry?

[phatt]

Hi mnturner,
                 Another Aussie ay?
might make life much easier if you just purchase the *Jaycar SC480* kit.
$50 for the PCB and all the bits but you supply Heatsink and PSU.
You should get change out $100 all up.

Or go to Rod Elliot's site *ESP* to give you some ideas to ponder.

Also have a look at ; http://www.ssguitar.com/index.php?topic=1609.msg10267;topicseen#new

Bassbuddy's got a nice preamp setup he built.
Cheers Phil.

[J M Fahey]

Hi mnturner.
On first sight it should work.
You'll need beefier transistors on T8/10/11/12 , usually TO220 cased, something like TIP31/32C or similar.
It *may*work with TO92 units for a while, but I would not risk it.

[mnturner]

Hi phatt,
yeah I saw those kits at Jaycar; I didn't want to buy one to begin with but I've pretty much ended up building an LM3886 gainclone type amp which is basically the same thing, just with a little more power. Thanks for the tips on the preamp!
Hi Fahey,
when I get around to building that thing I'll have to do that. I had it running at really low power on a breadboard and even then T11 and 12 were getting way too hot.

[phatt]

Hi phatt,
yeah I saw those kits at Jaycar; I didn't want to buy one to begin with but I've pretty much ended up building an LM3886 gainclone type amp which is basically the same thing, just with a little more power. Thanks for the tips on the preamp!
Hi Fahey,
when I get around to building that thing I'll have to do that. I had it running at really low power on a breadboard and even then T11 and 12 were getting way too hot.

Um?   I think you will find the LM3886 (although reasonable) is no match for pure grunt of SC480.
LM3886 only has 2 driver tr's ,,whereas the SC480 has 4 giving it far greater current ability.

LM3886 is only 40Watts @ 8 Ohms and I believe that is the outer limit.
68 Watts on paper sounds nice until you see the 4 Ohm tagged on the end.
but I'm sure as you have spent the money you will have fun with the chip amp, 40Watts is still loud.
Phil.

[mnturner]

sorry phatt, you're exactly right. i have two 3886's that i'll parallel. that should put me up to the 80 watt region right?

[mnturner]

I've put the discrete idea on hold for a while and decided to go with the gainclone.
I've built a single LM3886 gainclone in an MDF box which is sounding fine but I have a few questions;
I'm using a 100VA transformer with a 4Ohm speaker and its getting fairly hot. With only one LM3886, it should be pulling maximum about 68W right? Its not painful to touch, but definetly very warm. How hot should a transformer usually run?

Also, the chip is attatched to a fairly large heat sink (black anodized aluminium, 125x75mm with about 45mm fins) but that also is getting fairly hot when playing at moderate volumes.

There's very little DC offset at the output (~10mV) and nothing seems wrong, and I know the LM3886 has inbuilt thermal shutdown which has never kicked in so I guess it can't be running *too* hot, I'm just wondering if this sounds normal?

[J M Fahey]

How hot should a transformer usually run?

Very short answer:
If you feel it hot but you can leave your hand there grabbing it for a couple minutes, you are within spec.
If you can't stand it, it's too hot (over 65/70ºC, you "Fahrenheit" that)
If your hand bubbles, hisses, and a nice barbeque smell fills the air .... grab some bread and Ketchup and serve a "hand-burger" to your friends.