6W, Class A, hopefully something I can build. :D

[keantoken]

Hi all.

I have shown a few design ideas on this forum. One of the reasons I have been testing and simulating many different ideas is that I was trying to find an amplifier that I could build to give low distortion into an 8-ohm load loud enough to fill my room (which is quite small).

This amplifier sims good enough for me to try as a project, and I haven't run into any stability problems.

Simulation results:

THD @ 6W, 8ohms, 1KHz is about .0002%

THD @ 6W, 8ohms, 20KHz is about .0012%

I have enough parts (minus bias resistors and heatsinks) to build two of these for a stereo setup.

However, it still might take me some time to buy the few needed components.

What do you think?

- keantoken

[Nelson Pass]

A little emitter resistance on the output devices might be helpful.

[fotios]

Except those mentioned from the good friend Nelson, from my recent experiments on my project, i have the following remarks:
1. C3-R11 network it is placed usually between the collectors of LTP.
2. The C2 of 100pF needed in the case of using an input attenuation pot (before C5) to suppres the oscillations caused from the interaction between pot and C5, and it reduces high frequencies response. C2 and C6 are the components that affects the high frequency responce. You can play with the values of these two caps to obtain the better result which is small rise time and no Miller oscillations.
You can also decrease the value of C5 down to 2,2ìF without loss in low frequencies if you intend to use some good quality foil polypropylene cap, for having the smallest posible inductance.

Regs
Fotios

[keantoken]

Oh, thanks Mr. Pass.

I just tried it but didn't see any affect on distortion. I'll leave them in though.

Things I need to know before building this:

1: How large a heatsink I need.

2: What kind of transformer I need. Rail voltage is 18V each, max current draw is 2A.

Also, I solved a really weird problem with two diodes. The output would sometimes short to one rail because of current through the C-B junctions of Q11 and Q12.

See attachment.

Also, fotios, thanks for the advice, especially for C5.

R20 and C3 work well to keep the circuit from oscillating, as I have discovered recently. A great many of my old designs should be able to become more stable as a result of my recent learnings.

Thanks all,
- keantoken

[fotios]

Originally posted by keantoken
Oh, thanks Mr. Pass.

I just tried it but didn't see any affect on distortion. I'll leave them in though.

Things I need to know before building this:

1: How large a heatsink I need.

2: What kind of transformer I need. Rail voltage is 18V each, max current draw is 2A.

Also, I solved a really weird problem with two diodes. The output would sometimes short to one rail because of current through the C-E junctions of Q11 and Q12.

See attachment.

Also, fotios, thanks for the advice, especially for C5.

R20 and C3 work well to keep the circuit from oscillating, as I have discovered recently. A great many of my old designs should be able to become more stable as a result of my recent learnings.

Thanks all,
- keantoken

For Vcc=18Vdc obtained from full wave bridge rectifier, you can add 1,5 Volt drop from bridge diodes, as well yet 1,5 Volt from ripple (if you are not using a regulated supply), thus you need Vcc = 18V + 1,5V + 1,5V = 21Vp which translated in a transformer with secondary of 21V/1,41 = 14,89Vac thus 15Vac. For a split power supply you need a xformer with 2 X 15Vac secondaries @2A.
Now for the power, you said 2A... i don't know if are peak or rms. Let us to say rms. For Vsec = 2 X ( 2 X 15 ) = 60 Vac and current Isec = 2 X 2 Arms = 4 Arms, the core of transformer must be rated at: P = V X I X cosö (where ö is the phase angle between V and I due to inductive load of xformer and for 45 deg mean phase shift cosö = 0,707) = 60V X 4A X 0,707 = 169,68VA. To be safe, you must get a: 200VA xformer with 2 X ( 2 X 15 Vac ) secondaries made from wire rated for 2A. If you have understand, i talk about a xformer with individual supplies per each channel to avoid the gnd loop caused from the common gnd node. The price is the same. Or you can use two seperate xformers one per each channel. In this case, you need two 100VA rated xformers. All the rest are same.
Keane, i forgot to refer that. Have you seen for OnSemi D44H11-D45H11 for output? They are far better from MJE340-350 in this place. Take a look please on datasheet of OnSemi.

Regs
Fotios

[keantoken]

Thanks for the info, fotios. I was meaning 2A peak, not RMS. My preference would be two toroids, one for each channel...

Originally posted by fotios

Have you seen for OnSemi D44H11-D45H11 for output? They are far better from MJE340-350 in this place. Take a look please on datasheet of OnSemi.

Regs
Fotios

Are you sure? The D4x devices are rated for 10A while the MJE350s are .5A. Overkill! I only need 6W, and I'm afraid if I go higher my breadboard will explode.

- keantoken

[fotios]

Originally posted by keantoken
Thanks for the info, fotios. I was meaning 2A peak, not RMS. My preference would be two toroids, one for each channel...



Are you sure? The D4x devices are rated for 10A while the MJE350s are .5A. Overkill! I only need 6W, and I'm afraid if I go higher my breadboard will explode.

- keantoken

My calculations for 2 Arms consumption of course results in bigger values, but the thing is that the ready offered toroids are rated usually in standard cores of 50VA - 100VA - 150VA - 200VA etc. You can't use 50VA xformer (per channel) it is small, thus you are going again in two 100VA toroids. Moreover the extra cost it is small. Here in Greece, it is only 5 euros extra.
As for D44, because are rated for 10A does not means that they consume 10A! Instead they sustain 10A. I use the dpak versions MJD44H8-45H8 as output devices, in my small operational amplifier implemented with smd discrete devices. It operates with +/-24Vdc and it consumes only few mA. No problem friend, don't be afraid. Simply those are modern and much better devices from the old good MJE340-50.

Regs
Fotios

[MJL21193]

Hi kean,
Greasing and mounting 8 ouputs can get old quick. You could get away with one pair TO-264 for this low power.

[keantoken]

Originally posted by fotios


My calculations for 2 Arms consumption of course results in bigger values, but the thing is that the ready offered toroids are rated usually in standard cores of 50VA - 100VA - 150VA - 200VA etc. You can't use 50VA xformer (per channel) it is small, thus you are going again in two 100VA toroids. Moreover the extra cost it is small. Here in Greece, it is only 5 euros extra.
As for D44, because are rated for 10A does not means that they consume 10A! Instead they sustain 10A. I use the dpak versions MJD44H8-45H8 as output devices, in my small operational amplifier implemented with smd discrete devices. It operates with +/-24Vdc and it consumes only few mA. No problem friend, don't be afraid. Simply those are modern and much better devices from the old good MJE340-50.

Regs
Fotios

The D4x devices aren't suited for this amplifier because of the high Cob. It is about 100 times higher than the MJE340/350. The gain is also lower. I don't believe the MJE340/350 are inferior.

Originally posted by MJL21193
Hi kean,
Greasing and mounting 8 outputs can get old quick. You could get away with one pair TO-264 for this low power.

...Which will have lower Ft, higher Cob and will provide higher HF distortion. The current setup has high gain and high Ft. Plus, I don't want to be wasting 10A transistors on a 6W amplifier...

I will bredboard this first, to check that it works and then I will put it in a case. This is not something that I expect to throw together in 15 minutes.

- keantoken

[ostripper]

By keentoken -Are you sure? The D4x devices are rated for 10A while the MJE350s are .5A. Overkill! I only need 6W, and I'm afraid if I go higher my breadboard will explode.

In some of the old CRT's you said were in the "hood" ,they
use real transistors for the vertical output section , usually
rated 2-3A , and usually faster 2SB/2SD-XXX devices, too.
Just a couple pair would do.
Older compact stereo systems also use 2-5A PNP/NPN
to-220 pairs and have a decent heatsink attached as well.
Find some scrap.

I've built the "big boys" on a breadboard, just up to the
VAS.. put the outputs ,drivers and emitter resistors offboard,
as that much current will fry the contacts.

OS