HELP: wiring a simple amplifier

[mac]

I want to build a simple amplifier like the one in the picture, I need help wiring, grounding, etc., I never build an amp.
It is a two fet preamp stages and a power silicon driving a speaker. The final design could be a little different but this is not important.

I'd like you to correct things, and I'll redraw the circuit ASAP. This could be helpful for others also.



I do not know if the speaker gnd should be connected to point B, if the preamp filter cap should go to point A, and if the chasis connection should go to C or D.
How bad is NOT isolating input jack from chassis?
I'd like to hear your opinions.

Sorry for the schem, I'll add labels to next graph.

mac

[PRR]

What you have drawn could be correct.

It probably will NOT do what you want.

You seem to have a single transistor resistance-coupled into a loudspeaker. This is a very inefficient "power amplifier".

You seem to expect to run on 50V DC. However nearly all loudspeakers are very low impedance. Which means their power amplifiers draw a large current for their voltage.

Assuming 50V supply, 8 ohm load, and a maximum-efficiency resistance-coupled power amplifer, you will flow 3.9 Amperes of current, have 152 Watts of heat in the resistor, 43 Watts heat in the transistor, 195 Watts total heat pouring out of this thing. Output power is 7.7 Watts RMS.

The almost 200 Watts will burn your perfboard.

And a power transistor flowing 3.9 Amps and working to put all of that to a load will need about 100mA of Base signal, which is more than a small FET can deliver.

For perfboard power, we could choke back to 0.2 Amps of output stage DC draw. That's 10 Watts of total heat, but most of this could be in an off-board power resistor, only about 1 Watt in the transistor, so it can be on the perfboard. Output power is about 0.16 Watts, ample for bedroom playing. Base drive signal could be several mA, which you might barely be able to drag out of a selected FET.

Oh, in either case the distortion is around 26% THD at full power.

Practical power stages are more complicated. (It is very practical to use a chip.)

It takes more than three semiconductors to rise from guitar level to speaker level well.

[mac]

Thanks for the Power info.

You are right, it is going to sound bad. I forgot to say this is a small almost clean pratice amp for bedroom use, about a watt. Higher wattage makes no sense since I own a Valve Jr. I have to polish the output stage, look for power stages on the web, etc.
Output transistor could be a TIPxxx darlington, mounted on a large heat sink close to the perfboard.

What about changing the coupling cap for an OT?



I recorded a simple demo which can be found in my gallery.

http://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/crunch.mp3.html

Power darlington transistor runs at no more than 300ma and there are 9w across it.
I wired the OT to meet specs.

mac

[Brymus]

I thought it had a nice medium distortion,perfect for the sample you played.
I didnt hear any hiss either,again good.
How does this compare to your FET Jr. ?

[PRR]

Yes, a Darlington helps.

> What about changing the coupling cap for an OT?

OT is always good for guitar.

As a rough load, 35V at 0.3A suggests 110 ohms. Is there a readily available 100 ohm 300mADC audio output transformer on the market?

> [/i]there are 9w across it.[/i]

A good SE amp can output 40% of input. That's 3.6 Watts, which is not a lot less than a Junior.

> about a watt

Then say 2.5W dissipation. At say 30V that is 80mA, and about 360 ohm load.

Edcor XSE10-8-3K wired pin 3 to 5 would present 480 ohms, and with the 40% turns might pass 80mA easily. Under $20.

You could also go with a 120V:12V 24VA power transformer. The impedance is good, and the oversized core might not be so bothered by a little DC. It is "too big", but VERy widely available, and possibly cheaper than any for-audio transformer. Frequency response may be as good as a Champ/Junior, or much worse, depending on specs they don't tell you for power transformer application.

[mac]

I thought it had a nice medium distortion,perfect for the sample you played.
I didnt hear any hiss either,again good.
How does this compare to your FET Jr. ?

I recorded from the breadboard. I went crazy trying to eliminate oscillations and the like. Since what was on the breadboard is similar to the schem I asked if this is the way to wire.
The FET jr. is just the preamp. It's just a way to get 30W of VJ fat distortion out of my Laney LC30 which is more like a heavy overdrive. I'd like to go higher voltage but I can't find high voltage fets in my town.

As a rough load, 35V at 0.3A suggests 110 ohms. Is there a readily available 100 ohm 300mADC audio output transformer on the market?

At least not where I live. I had to build it:

http://www.diystompboxes.com/smfforum/index.php?topic=80047.0

mac

[petemoore]

  I tried a few of that kind of thing.
  Power Mosfet Sources seemed to be able to drive the speaker currents 'ok'.
  Bipolars used to beef the signal voltage for the mosfet gate.
  IIRC I was over at Elliot Sound and lookin' over various little SS type amps.
  I had a stereo amp deal, Wall Warts and all [for a while] the WW's kinda gave up IIRC.