sima amplifier questions

[lmartine3]

I have an amplifier made in the early 80's by Sima Audio of Montreal, Canada. The model number is AB200 and appears to be 100W/ch into 8 ohms.

I rescued it from a local rock'n'roll band (who thought it was a tube amp!) just for the enclosure and power supply, but after looking at the amplifier sections it seems worthy of refurbishment.

It has been around the block a few times but as far as I can tell it has never needed repair (until I started tinkering with it, sigh).

I have reversed engineered the schematic from the pcb (hopefully attached) and I have some questions which I will follow up with.

[Rafael L]

What are questions?

[lmartine3]

1. While playing with the bias adjustment, I went too far and 2N6553-Q6 died (shorted). Is this an inherent limitation of this bias design circuit, or was it just an old (underspec'ed perhaps?) component operating on the edge?

2. I think 29113/114 would be a suitable replacement TO3 for the 15003/004, but what about replacing the 2N6553/6556 drivers with MJE15030/31? The bias circuit also uses the 6553/6556, what is a suitable replacement for these? I anticpate mods to handle the different pinouts on newer devices of course.

3. Why does the high side driver Q6 have a base resistor R9 but the low side Q7 does not? (I am just starting to read Self and Slone books about the theory of operation...)

[Miles Prower]

Originally Posted by lmartine3

1. While playing with the bias adjustment, I went too far and 2N6553-Q6 died (shorted). Is this an inherent limitation of this bias design circuit, or was it just an old (underspec'ed perhaps?) component operating on the edge?

Since it's operating into just 47R of emitter resistance, there isn't much in the way of current limiting. Should have had either more emitter resistance or a transistor with a higher Pc rating.

3. Why does the high side driver Q6 have a base resistor R9 but the low side Q7 does not? (I am just starting to read Self and Slone books about the theory of operation...)

Looks like a quick 'n' dirty fix for oscillation. Putting a base stopper there fixed the problem, and so they didn't bother to add one to the other side.

[lmartine3]

Originally Posted by Miles Prower

Since it's operating into just 47R of emitter resistance, there isn't much in the way of current limiting. Should have had either more emitter resistance or a transistor with a higher Pc rating.



Looks like a quick 'n' dirty fix for oscillation. Putting a base stopper there fixed the problem, and so they didn't bother to add one to the other side.

Hmm, thanks for the info.

Just adding a pic of the internals, check out the heat sinks.

[Laszlo Ferencz]

Nice find!

I'm interested what are the spec's of the power transformer and PSU caps (these huge blue ones)?

[lmartine3]

Originally Posted by Laszlo Ferencz

Nice find!

I'm interested what are the spec's of the power transformer and PSU caps (these huge blue ones)?

The transformer has a nameplate rating of 371VA (exactly!) at 75Vct. The caps are I think 10000 uF at 63V (85V surge). No ripple current rating though.

With no load, the rails are at +-56V, clipping at 30.5Vrms.

With only one channel driven with a sine wave, it clips at 30.1Vrms into 8 ohms (52V rails) and 29.5Vrms into 4 ohms (49V rails).

The 8 ohm load showed 220W input on the primary and 405W with the 4 ohm load, basically only 50% efficient end to end.