Solid State OCL power Amp

brianhujung · Post by **brianhujung** » 05 Jun 2012, 09:21

Hello everyone,
I found this OCL schematic yesterday and I'm thinking to use it as power amp for my guitar amp because the parts are easy to obtain here.
Now before I make the boards, here's a couple questions :
1. Does this schematic suitable for this purpose? or is there anything I can do to improve it for guitar amp?
2. If i run this amp with +/- 25V 3A transformer, how much power will I get?

Thank You.

deltafred · Post by **deltafred** » 05 Jun 2012, 10:41

brianhujung wrote:Hello everyone,
I found this OCL schematic yesterday and I'm thinking to use it as power amp for my guitar amp because the parts are easy to obtain here.
Now before I make the boards, here's a couple questions :
1. Does this schematic suitable for this purpose? or is there anything I can do to improve it for guitar amp?
2. If i run this amp with +/- 25V 3A transformer, how much power will I get?

Thank You.

1. It will be ok as a guitar amp but I would use one with variable quiescent current setting, the circuit you have shown uses the 3 diodes D3,4,5 to preset it.

Just looking at the circuit I would like to see 4 transistors in the output stage for that power just for reliability reasons. It has been a while since I built a 2N3055/2955 amplifier but IIRC they will be running a bit close to their maximum collector / emitter voltage.

2. To calculate the output power you have to run 2 sets of calculations, one for available voltage, the other for current and take the lower figures.

(rms)Power = (rms)Volts squared / speaker ohms
so in theory a 25 + 25 volt transformer will give you about 70W into 8 ohms or 120W into 4 ohms.

(rms)Power = (rms) current squared x ohms so a 3A + 3A transformer will have a max theoretical output of 144W into 4 ohms and 288W into 8 ohms.

Taking the lower values that gives 70W into 8 ohms or 120W into 4 ohms.

You are unlikely to get these figures in practice so reduce these by a few watts.

Groovenut · Post by **Groovenut** » 05 Jun 2012, 15:31

Here is another example. This is the PA from the Randall RG series amps from the 80's. Point A is +40 or +20 depending on the output desired, Point B is -40 or -20. This one also incorporates a negative feedback loop and a direct recording output.

Perhaps deltafred could educate us on the inner workings of this particular PA?

meffcio · Post by **meffcio** » 05 Jun 2012, 16:51

Why not try some D-class stuff, eh?

mictester · Post by **mictester** » 05 Jun 2012, 20:44

Really simple, low cost, no adjustments, robust - built with the specified components it's entirely clean and stable:

: Simple, single rail power stage

deltafred · Post by **deltafred** » 05 Jun 2012, 22:26

Groovenut wrote: Perhaps deltafred could educate us on the inner workings of this particular PA?

I am not going into great detail but here goes

Long tailed pair input
Quiescent current set by 250 ohm trimmer
Negative feedback provided by R45 R46 & C34
Additional feedback to account for 4 or 8 ohms by C36, R52, R63
25W into 8 ohms, 50W into 4 ohms @ +/- 20v
*100w into 8 ohms, 200W into 4 ohms @ +/-40v (dependant upon power supply current capability and output transistor rating).

*80v peak to peak = 28v rms (80 / 2 x square root of 2) = 80 / 2.828)
P = VxV / R = 28 x 28 / 8 = 100w

Output and driver transistors are not specified so you are just guessing at them

My favourite schematic is a 150w mosfet amp from the 80s from Maplin UK (when they were still worth dealing with). I built loads of them and most are still working in powered monitors and guitar amps. Unfortunately the mosfets are out of production so are rather pricey. I can post the full build details if anyone is interested.
The output mosfets are TR6 - 2SK135 and TR7 - 2SJ50 (although I seem to remember that they were upgraded at some point)

A friend of mine had a 10" Marshall practice amp, he took out the speaker and cut a way the bracing so a 12" would fit in. Then he bypassed the 12w (I think) IC output stage and fitted a 150W mosfet amp and PSU in the bottom of the cab witch he drove with the Marshall preamp. It was the loudest practice amp I ever heard.

juanro · Post by **juanro** » 06 Jun 2012, 01:03

Take a look here:
http://www.forosdeelectronica.com/f31/a ... s-n-24854/
from a fellow argentine, it's in spanish of course, but you can use google translator and make out the most of it, or if you need help with some part of it let me know. Anyway the schematics are pretty much self explanatory.

Regards,
Juanro

brianhujung · Post by **brianhujung** » 06 Jun 2012, 01:53

meffcio wrote:Why not try some D-class stuff, eh?

I have built a most of IC based amplifier (LM386, TDA2005, LM1875, TDA2050, LM3886 and LM7294) but never had a chance to learn or even build discrete amplifier

I found a lot of schematic on the web but I didn't even know which one I should build..

just trying to build something simpler than http://sound.westhost.com/project27.htm

Groovenut · Post by **Groovenut** » 06 Jun 2012, 04:07

deltafred wrote: Output and driver transistors are not specified so you are just guessing at them

Q13-Q16 are 2N6254 fwiw. sorry I left that out.

teemuk · Post by **teemuk** » 09 Jun 2012, 08:23

brianhujung wrote:...just trying to build something simpler than http://sound.westhost.com/project27.htm

The issue is, for that kind of amp a design like P27 is pretty much the minimum requirement.

Lower output power ratings and such might be more forgiving but increase in output power pretty much introduces certain requirements for the design. I for one like such amps to be at least

- stabile in overall manner
- thermally stabile
- at least moderately short circuit "protected"
- tolerant of overdrive (e.g. no hysteresis, rail sticking, ringing, etc. when overdriven)
- properly rated for the power they are going to dissipate (e.g. not using only a single pair of output transistors when reliable operation at the rated output power really calls for a quad or preferably more)

Most of that simply can't be achieved with a simple design.

tschrama · Post by **tschrama** » 09 Jun 2012, 11:29

If you want to build somehting simple, as a power amp for guitar:

http://sound.westhost.com/project12.htm

Really, no kidding, bullet proof due to the capacitor coupled output, and simple. Kep V+ at 40V or so, for a super simple easy 30W discrete power amp.

teemuk · Post by **teemuk** » 10 Jun 2012, 12:16

Sorry but aside the capacitor coupling protecting speaker from DC if the amp fails catastrophically there's frankly nothing "bullet proof" in that design.

tschrama · Post by **tschrama** » 12 Jun 2012, 12:21

teemuk wrote:Sorry but aside the capacitor coupling protecting speaker from DC if the amp fails catastrophically there's frankly nothing "bullet proof" in that design.

Lol, really? Let's see: Enourmous emitor resistors, absent Vbe mutiplier that doesn't needs to sense any temperature (and can fail at that), nor needs to be set by a potmeter (which can fail), the low overal gain before feedback, the low bandwidth (good stability), the quasi complen output stage that is not sensitive to output trannie overheating (good Iq stability), the single rail, zobel network, and indded, the output Cap prevents any DC current. Everything about that design is about as simple as it can get, keeping it robust and low-fi. Looks like a russian tank to me...keep it as 40V or so, and there not much that can go wrong. You don't have to adjust or set any potmeter to get this amp running.

teemuk · Post by **teemuk** » 13 Jun 2012, 19:00

Enourmous emitor resistors

0.47 ohms is not "enormous", it's more like a typical value. In addition, in the NPN part the resistor is not even tracking emitter current.

absent Vbe mutiplier that doesn't needs to sense any temperature (and can fail at that)

One important function function of the bias circuit IS to track temperature and a VBE multiplier can do it much more accurately than a diode string. It's a sole reason why such more complex circuit is used pretty much as a standard. It has been proven to be a better and more reliable method than a simple diode string.

nor needs to be set by a potmeter (which can fail)

I see proper bias adjustment as a virtue, not flaw. Safe potentiometer failure mode can be noted in the design process.

the low overal gain before feedback, the low bandwidth (good stability)

The current feedback topology actually has an enormous bandwidth. Granted it is more stabile than the (nowadays) more usual long-tailed pair. Ignoring HF local feedback might work with this scheme but overall it might also be wishful thinking that only works on paper. I have hardly ever seen this topology used in practice without requiring some local HF NFB - just like any other generic SS scheme.

the quasi complen output stage that is not sensitive to output trannie overheating (good Iq stability)

Where did you get the idea it's non-sensitive to thermal effects? It's actually a combination of two kinds of thermal coefficiens, those of darlingtons and those of sziklais, both requiring their unique thermal tracking scheme.

Overall the quasicomplementary stage also has much weaker stability than a fully complementary output. The crossover region is very asymmetric and coupled to weak biasing arrangement this can create terrible stability issues where the amp begins to oscillate near crossover regions.

the single rail

...Makes it way more prone to power supply introduced flaws, such as hum, DC offset drifts... And you need a large output cap, which should be able to handle all the load current and huge AC fluctuations....reliably.

zobel network

Has been a standard since late 1960's in just about any decent design to begin with. A good design should really have an inductor zobel too.

and indded, the output Cap prevents any DC current.

Perhaps it wil, but in this particular design the wacky bootstrapping arrangement might make even that doubtful. Since the speak was about "bullet proof" things I'd rather trust generic DC coupling with relay switching coupled to a DC detector, at least it doesn't rely on blind faith that cap protects the thing from everything and it also doesn't need a high capacitance, high voltage capacitor with stupidly high ripple current ratings.

Everything about that design is about as simple as it can get, keeping it robust and low-fi.

I can agree about simple and lo-fi but not about robust.

Do enlighten me....

- What tracks temperature accurately when coefficients of the output pair vs. diode do not match?
- What protects it from overheating?
- What protects it from short circuited output, and how well?
- What protects the voltage amplifier from failing if the output stage starts to hog current above "safe" levels?
- What protects the speaker from DC at output, assuming the amp fails in a manner that there will be DC in the output (e.g. the bootstrap or the output cap fails)?
- What prevents hard saturation?
- What protects the output transistors from reverse biasing during flybacks?
- What protects the speaker from the loud transient "thump" when amp is powered or shut down?

Oh... in closer inspection "bullet proofness" actually seems to be kinda lacking. If we really want to use that adjective then may I kindly guide towards high end PA amplifiers and alike, not ludicrously simple designs dating back to 1960's something.

tschrama · Post by **tschrama** » 13 Jun 2012, 20:13

You're right. I was wrong. Enjoy.