My own Solid state Guitar amp, NEED HELP

[didier]

Hello,

maybe this isnt really the place to ask this but i don't really know where to ask else...

I would like to design my own solid state Guitar amp...
about 60W into one 8ohm Celestion Greenback speaker, 2 channels: a clean channel, and a tubescreamer-a-like overdrive channel, 3Band Tone Control

for now i designed this and i am planning to use it as the preamp:

will it work???
My thoughts:
- i'm guessing the 6k8 resistors to ground might need adjusting, or can i delete them? i dont understand the use.
- i have no idea what signal i'm getting out of the preamp section, V? I? thats why i added the Volume trimpot at the end, so i can adjust the voltage there, but if anyone could tell me what i would get at max volume that would be great...
- the RC network at the end can be deleted? since it is a tone control network and i added a 3band EQ after it...
- i'm also wondering about the clean channel, will it actually be clean? or should i just bypass for the clean channel? now its a gain stage with gain = 10x. should i make it 1x? any other suggestions?


as for the poweramp i would like to use this as i've heard only good things about it:

unless anyone else has some good ideas?
my thoughts again:
- how is the LM3876 chip poweramp? i might use that one in stead because of its simplicity.
- how should i make the powersupply, because like this i need +40V, -40V, +9V and GND. or can i just get the 9V by making a voltage divider or so?

Many Thanks if you can help me figure it all out.

[R.G.]

will it work???

With some work, yes. From your comments, it's going to be a great learning experience, I think.

First some comments related to "will it work?".
- you'll get fairly noticeable treble loss from the loading of the inverting "clean" channel and bass loss from the capacitors at the input if built as shown. Change that 1nF to ground to 100pF. Also change the signal at the clean channel input to the + side of the opamp, not the inverting side, and move the 10K inverting resistor to the bias supply. This will avoid loading down your guitar input. The + input needs a bias resistor to Vbias, just like the overdrive channel has, so duplicate that 1M to bias.

- i'm guessing the 6k8 resistors to ground might need adjusting, or can i delete them? i dont understand the use.

They are there for the same reason we put pulldown resistor on effects - to prevent popping when you flip that channel switch. They could be any value up to about 1M. I'd suggest 100K  instead of 6.8K.

- i have no idea what signal i'm getting out of the preamp section, V? I? thats why i added the Volume trimpot at the end, so i can adjust the voltage there, but if anyone could tell me what i would get at max volume that would be great...

There are several key bits about opamp circuits and guitar signals that you haven't picked up yet. A single coil guitar puts out perhaps a 100mV peak signal, a humbucker up to about 1V max. You have a gain of ten in the clean channel, so you're going to get peaks of 1V on that side and on the overdrive side you'll get slightly more, perhaps 1.7V peak. I don't know what the gain of the tone control section is without digging through the math, but it's probably +/-12db or so; that's common for active Baxendalls, which is what that is. Thia brings up another problem - power supply headroom. If you put 12db gain on the 1.7V peak you get out of the overdrive channel, the tone control opamp will run into the power supplies because the supply only allows +/-4.5V on a 9V supply. This would work out much better if you made your power supply +/-15V for the preamp. That would make the Vbias be signal ground.

- the RC network at the end can be deleted? since it is a tone control network and i added a 3band EQ after it...

It's a high cut for the fizz out of the clipping stage. It's up to your ears. Yes, it can be left out and have the circuit still work.

- i'm also wondering about the clean channel, will it actually be clean? or should i just bypass for the clean channel? now its a gain stage with gain = 10x. should i make it 1x? any other suggestions?

Leave it at ten to help equalize the signal level from the clean to the overdrive channel, but change it to non inverting as I've already mentioned.

as for the poweramp i would like to use this as i've heard only good things about it:

I believe that's Rod Elliot's old reliable power amp. It will probably perform well. It will also die instantly the first time you short the output. And you will. There is no protection circuitry there to prevent instant power device death. Buy several sets of extra transistors.

- how is the LM3876 chip poweramp? i might use that one in stead because of its simplicity.

The LM3886 is very, very good. It eliminates a huge region of ways to get a power amp wrong. If I were you, I would go that way unless you're just dead set on building your own. It's not only simpler, it's probably cheaper (USA$5.00 from Digikey). Get the fully-insulated version and save yourself some heatsink isolation problems. Get two - they're cheap, and easy. I highly recommend you do it this way. Read all of my comments to RDV on his 3886 amp and his replies.

- how should i make the powersupply, because like this i need +40V, -40V, +9V and GND. or can i just get the 9V by making a voltage divider or so?

A power amplifier is NOT primarily the amplifier circuit. It is primarily a robust power supply and heatsink assembly that, almost as an afterthought has this almost-negligible bit of circuitry that lets some of the power out under special circumstances. Design and find the components for your power supply and heatsinking FIRST before you do any of the rest. Preamps and to a certain extent the power amp circuitry are tiny, and amenable to being put any where in a cabinet. The power supply and heatsink components, along with the speakers DEFINE the cabinet.

First, go read "power supplies basics" at GEO. You want a +/- 30V power supply to match with the power foibles of the LM3886. You'll only get about 30-40W out of this. Do not let that stop you. A 10W difference at that level is almost unnoticeable difference in loudness. As RDV put it, he can't stand to play it full loudness at home.

Second, go find a suitable 50W to 100W transformer that does 120Vac to something like 44vct to 48Vct and a heatsink that lets you get rid of 30W of waste heat. I recommend ebay, there are lots of suitable things there. 44vct at 1.5 to 2A is a great power transformer for this. You'll also need a couple of 4700uF to 10,000uF/50V filter capacitors for the raw power and a rectifier bridge. Find this stuff FIRST.

Third, ditch the 9V and make your preamp run on +/-15V made by a 7815 and a 7915 regulator pair from the +/-30Vdc for the power amp.

That's just my opinions, and they're worth what you paid for them.

[gez]

High Power Audio Amplifier Construction by R Penfold, though not the last word on the subject, has plenty of good advice and costs peanuts.  Great intro to the subject and you should be able to find a copy on Amazon.

[didier]

OK GREAT!!

thanks! i'll start adjusting now...

so i could just run the preamp section at +/- 15V? and make a bias circuit with bias to Gnd... hmm interesting

also, i was wondering... i'm going to change the clean section so it is in phase with the overdrive section as you said. but i also read somewhere that for the channel switching i should also switch the gnd? true? or rubbish?

never heard of the 7809/7909 power regulator circuit! it's cool! (and easy:) )

since i like protection, i will go for the LM3886...  since i have a 8ohm load it will give me 38W power? that's good, maybe i will change later to a 2x12 combo resulting in a 4ohm load... unless you really say that this wont have a huge volume change?

thank you!

ps. do you think this amp will sound good? :s

[Joe]

You can add short-circuit protection pretty easily, by adding another NPN transistor (guessing TIP31C) as follows:

-collector to base of TIP142
-base to emitter of TIP142
-emitter to output

When the output is shorted, there is enough voltage across the top .33 resistor to turn on the protection transistor, which in turn shuts down the TIP142.

[octafish]

That's just my opinions, and they're worth what you paid for them.

Oh I'd say they're worth a bit more than that R.G.

since i have a 8ohm load it will give me 38W power? that's good, maybe i will change later to a 2x12 combo resulting in a 4ohm load... unless you really say that this wont have a huge volume change?

Decreasing the load for an increase in power will require you too use a larger heatsink too. Use a large enough heatsink in the first place and you should be fine. Bigger and more numerous speakers also equals louder.

Here is the RDV post that got me motivated to finish my chip amp http://www.diystompboxes.com/smfforum/index.php?topic=37049.0
Cheers, and watch your fingers around those mains.

[didier]

ok...

thanks for the advice on the short circuit protection, but as i'm just getting started on building amps i'm going to use the LM3886

i do have some additional questions...

1. R.G. if you say they are worth as much as i paid for...  i'd better check my financial status and stop asking questions then? 

2. I'm converting the preamp section to +/-15V, but what should i do with the opamp for the 3Band tone control? should i also feed it +/-15V? i guess so right?

3. since i'm running the preamp section on +/-15V i guess now i can't use the polar capacitors any longer? (the ones at the output of the clean channel and at the input of the tone control)

4. Can i connect the gnd from the input with the gnd from the power supply? i have been told its better not to but it depends... can i do it here?

Here's the new schematic:


Again, My thoughts:

- i can get a power transformer for $15, its a 75VA 2x25V one, so that would make +/-35V, could this be a problem? the datasheet says "Supply Voltage lV+l to lV-l 20V to 84V" they say that distortion can be introduced when using large supply voltages, i'd like to avoid this  , it would be usefull to know at what supply voltage the distortion gets noticable.

- reading the datasheet info later regarding the heatsink. i'm planning to get one large enough to be able to switch between 8 and 4ohm loads.

- RDV's thread was very interesting! how much should i pay attention to this: (at this moment)
"Every solid state power amp needs a Zobel network.
Every solid state power amp needs inductive catch diodes.
Every solid state power amp needs a damped series inductor.
Every solid state power amp needs power supply decoupling right at the circuit."

what is all this?
i understand the Zobel network.
maybe the damped series inductor is the one in the schematic on GGG for the LM3886 amp?
i guess the decoupling meand the capacitors need to be on the PCB?
i've never heard of inductive catch diodes.

Damn you are right R.G. i'm only one day further and it's already a great learning experience!

many thanks!

(i understand if you dont feel like taking the time to answer all my questions... but surely anyone must know what a pain it is to want to make something you really want without having some help here and there...)

Greetings,

Didier

[didier]

anyone? answers on these little questions?

[R.G.]

2. I'm converting the preamp section to +/-15V, but what should i do with the opamp for the 3Band tone control? should i also feed it +/-15V? i guess so right?

Yes.

3. since i'm running the preamp section on +/-15V i guess now i can't use the polar capacitors any longer? (the ones at the output of the clean channel and at the input of the tone control)

Correct. Use either (a) TWO polarized 10uF capacitors hooked up + to + or(b) one 10uF bipolar or nonpolar electrolytic. These are available from Mouser at very reasonable prices.

4. Can i connect the gnd from the input with the gnd from the power supply? i have been told its better not to but it depends... can i do it here?

Yes. It won't be a problem inside the box. Do take your speaker ground return back to the power supply ground on a separate wire, not the chassis and not the same wiring the preamp uses.

- i can get a power transformer for $15, its a 75VA 2x25V one, so that would make +/-35V, could this be a problem? the datasheet says "Supply Voltage lV+l to lV-l 20V to 84V" they say that distortion can be introduced when using large supply voltages, i'd like to avoid this  Wink, it would be usefull to know at what supply voltage the distortion gets noticable.

That's a good transformer for this chip amp with an 8 ohm load. It's less good with a 4 ohm load because the chip will only do a limited amount of power, and it reaches that at lower voltage with a 4 ohm load. The extra voltage induces it to go into power limiting. I think +/- 28 is recommended for 4 ohm loads for the 3886. But don't get too hung up in that. That's a good deal for the 3886, and you can always use a second 3886 power amp to drive a second 8 ohm speaker from the sam power supply.

- reading the datasheet info later regarding the heatsink. i'm planning to get one large enough to be able to switch between 8 and 4ohm loads.

Good.

- RDV's thread was very interesting! how much should i pay attention to this: (at this moment)
"Every solid state power amp needs a Zobel network.
Every solid state power amp needs inductive catch diodes.
Every solid state power amp needs a damped series inductor.
Every solid state power amp needs power supply decoupling right at the circuit."

Pay a lot of attention to it. It'll likely save you some debugging misery.

maybe the damped series inductor is the one in the schematic on GGG for the LM3886 amp?

I'll have to look at the GGG layout. It's basically a 10 ohm 2W - 3W resistor with a full layer of magnet wire wound around its body and soldered to the ends.

i guess the decoupling meand the capacitors need to be on the PCB?

It means that the LM3886 needs something like 47uF/50V paralleled with 0.1uF ceramic from +V to ground and from -V to ground as close to the chip as you can reasonably get them. Likewise, the opamps on the preamp need 0.1uF caps from +15 to ground and from -15 to ground quite near the chips. This setup lets the local caps provide high frequency current pulses to the chips, and the pulses are not "telegraphed" to the other chips, causing oscillation.

i've never heard of inductive catch diodes.

They're not common - just a really good idea. An inductive catch diode is a diode from the output to the + supply and another to the - supply, set up so they are normally reversed biased. So if you drive an inductive load with a square wave, the load cannot kill the power amp chip by feeding current back in - the diodes shunt it into the power supply first.

[didier]

Hey R.G.,

wanna switch brains? 

thank you SO much man, who knows how long i would have been struggling through this stuff without you!

i'll start prototyping this one tomorrow (maybe if i have the time  )
i hope it sounds good! jezus i cant wait...

THX again!

[didier]

By the way...

if any other newbs are following this... i came across a nice site with many basics (also advanced) articles.

http://sound.westhost.com/

as i found the geofex site a bit hard to navigate, this site has as many good articles and are easy to access and find again if you are looking for them...

[RDV]

Here's what I'm playing through right now as it is whisper quiet yet sounds really good. Tim E. is yet another creative genius around here.


RDV

[didier]

Here's what I'm playing through right now as it is whisper quiet yet sounds really good.

maybe it will be one of my next ones... as i am also planning to make a smaller, more quiet amp. but the bigger one is more urgent.
also, how can you define a transformer in ohms? or is it more like indicating the turn ratio?

[R.G.]

Transformers don't have ohms - they have ohms *ratios*, like turns ratios, as you suspected.

In fact, the impedance ratio is the square of the turns ratio. So a 1K:8 transformer is a 31.6:2.82 or 11.2:1 voltage ratio.

There are only a few key specifications for a transformer, but they can be massaged around by the transformer-i-ness of the thing. The minimum set (I think...) is
- the turns ratio (which also gives you the impedance ratio)
- the maximum volt-seconds of the core (which defines both the lowest frequency that the core can pass without saturating)
- the inductance of the primary coil which determines the low frequency response point for signal applications
- the leakage inductance (which is related to the mutual coupling factor k)
- the self resonance frequency (which is determined by the self capacitance)
- the maximum temperature of the insulation class in the transformer.

There are alternative ways of expressing all of those and that is one reason they're confusing. Many ways of looking at an elephant.