Power Acoustic output stage upgrade

[pergo]

Hi everyone!

today arrived a broken Power Acoustic 2APC-2200 from Miami and in a couple of hour i've repaired it. Easy things: new power supply with IRF3205 (+ gate resistor upgrade) and new diode bridge AKA KAK.
In add, it has 2 output BJTs shorted.

I replace them and it works very well.

I was thinking, because i'm crazy to upgrade the output stage with better BJTs.

The original ones are 2sb688 and 2sd718. 3 pair per channel. The rail voltage is +-40V unregulated (@12Vin).

I have at home 5 pairs of Toshiba 2SA1302 + 2SC3281. (from an old Audison HV16)

I substitute one channel, for testing, and the amplifier works without any changes to bias or driver section.

Drivers are TIP42C & complementary. Pre-Driver are 2sd600 and complementary.


I've a little problem, due to lot of work and 0 time for testing
I've a little DC offset in the channel upgraded: 30mV.

The other channel (original) has less then 10mV.

I was thinking about this problem. Could i change the drivers? I have (by HV16) some pairs of 2sa1837 + complementary.

They are all hi-fi components

I'm quasi-sure that i've to boost the bias. Now, the amplifier draws 700mA @ idle from 12Vin... For me, for 3 pair of those BIG output BJTs, is too little!
I don't have time to see o-scope for crossover distortion, but surely i'll boost bias

Any suggest for the DC offset problem?

Tomorrow i can post photos, if you want

thank you!
ciao!

[Perry Babin]

30mv is insignificant. If you want to try to reduce it, replace the pair (or two pair) of differential amplifier transistors in the channel with 30mv of offset.

Most of these amps are class B and should have 0 bias current. If this one has current flowing through the emitter resistors at idle, you should drive it hard enough to get it to go into thermal protection. Every few minutes, remove the signal and confirm that the idle current hasn't increased.

Sometimes, if the replacement transistors have a lower Vbe threshold, they will go into thermal avalanche as the amp heats up, causing the amp to fail. This has happened to a lot of people (myself included) when using TIP35C/36C as subs for the various outputs in the budget amps. The problem can be overcome by changing the biasing slightly but isn't apparent until the amp heats up.

[pergo]

Thank you Perry,

fortunately, the new output BJTs don't go into thermal runaway. They're cold as the originals.

I'm not glad to hear that they are class B. It seems "more Orion": tipical AB with low low low (very low) bias.
Obviously, no crossover distortion.

Tomorrow i'll make some measure.


For your experience, with 3 pair of that BJTs (large SOA), can i handle 1 ohm bridge??
You know, original amplifier said that the minimum impedance is 2ohm bridge (1ohm stereo). But original BJT are very inexpensive and only 80W each.

This Toshiba have 160W each and maximum 2A @80Vce for 100ms

Or, i can buy MJL21193

The original power supply (now with 12 IRF3205) can handle 2kW without problem. There are two 2.4" toroids @80kHz
And the amplifier ""only"" got 170Wrms @4ohm @12V

[Perry Babin]

You'll have to run it up to thermal shutdown to know if there's a problem. The problem rarely shows itself when the amp is cool.

Most of these amps are class B but the drive circuit is fast enough so that there is no crossover distortion. Generally, there is 0 bias current. When you have bias current, it typically indicates that there's a problem.

[pergo]

for your opinion, these upgrades i'll made, are useless?

Remember that i use only recycled component, for unfixable amplifier (explosed, burned, etc.). No buy, No wasted cash.

I love American amplifier. I have 5 Orion at home, some PPIs and Zapco. I never try a Power Acoustik.
I hope that it isn't a Chinese-like amplifier

[Perry Babin]

Not useless but you need to thoroughly check the operation of the substitute parts so they don't fail (possibly taking the power supply FETs with them).

[pergo]

thank you,

today i make some measures and maybe the burn-in test

[pergo]

Amplifier works fine. without only output BJTs changed.
The new channel tested as a little less voltage gain then the other.
For example with 1Vpp input, @maximium gain, the original channel goes on hard clipping, the other not much.
Like 1.5V less voltage than original channel.

I don't think if it's a problem. For me, not.


A question.
The original VAS is made by 2SD600 & 2SB631 from Sanyo. Datasheet seems good performance.
I have new 2SA1358 & 2SC3421 from Toshiba.
For your experience, are better for this application?
Datasheet are very similar.


And for differential BJTs? Original are 2SA1268.
I have some BC556...are they better? I don't think so

[pergo]

any suggestion for 1ohm bridge operation?
i have 3 pair of BJTs each channel. (6+6 BJTs for total 2 channel)

I can mount TO247 or biiig TO-3PBL.

is it possibile with only 3 pairs? for example MJL21193+MJL21194 (high SOA: 2.25A @1second)

Now i have 2SA1302+2SC3281 in TO-3PBL. Sure for 2ohm bridge...but not for 1ohm


MJLs have less Hfe than Toshiba

[pergo]

Hi everyone!

today i finished the amplifier

It works very well, but i wanna continue to tweak it in the spare time

Up to now the tweaks are:

- output stage: from B688 & D718 to original Toshiba's 2SA1302 & 2SC3281

- operational amplifier: from shitty SIL8 JRC, to DIL8 NE5532 (with diy-conversion board) for now..maybe some Burr Brown in future.

- power supply mosfet: from #12 IRFZ44 to IRF3205 (it's dual mono)


The amplifier provide 170Wrms @4ohm @12V and 250Wrms @4ohm @14.4V...not bad.


Original project (with B688 and D718) declare 2ohm bridge operation stable..i don't think with that small SOA.
Now, i think it will be stable

Pre-driver stage is made by 2SD600 & 2SB631 from Sanyo what about 2SA1358 & 2SC3421 from Toshiba??

Driver stage is made by TIP41c & TIP42C, i wanna replace them...any advice?
I have some 2SA1837 2SC4793


Some measures:

- left channel --> bias +-380mV simmetrical, no DC-offset
- right channel --> bias +360mV & -420mV (unsimmetrical), -30mV DC-offset


any comments?