70V commercial amp for home use?

[2004ex]

I have recently obtained several solid state 70V commercial amp boards (Altec 2280B main frame, Altec 2271 amp cards). I do not see any step-up transformer in the output stage of the 2271 amp, but it is driving by +/- 120V rail. I checked the schematics for this amp and it looks very similar to a standard complementary design. What is the main difference between a 70V system and a home consumer amp when there is no step-up transformer? What modifications should I made to the 2271 (schematics in pg.9 of
http://alteclansingunofficial.nlenet...80B-Manual.pdf
) in order to use it as a home audio mono channel amp?
TIA.

[imix500]

Hi, 70v amps as you may know are designed to feed a distributed amount of speakers all paralelled together over a long distance. At each speaker is a transformer that steps down the 70v line to match the impedance of the speaker.
It says on the first page of the manual, the amp wants to see a nominal 64 ohms. Or the amp modules can be paralleld to accommodate lower impedance loads.
So, you can parallel several modules together, or buy a very high quality step down transformer to drive your speakers.

[jrdmedford]

You need a matching transformer to adapt your 4, 8 or 16 ohm
speakers to the 100V line impedance. They are cheap. Go here:

http://www.edcorusa.com/transformers/speaker.htm

Dont expect much in the bass department.... Once you involve
a transformer, the bass is sure to suffer compared to a direct
coupled amp.

-- Jim

[2004ex]

Thank you for the responses.
I want to modify the amp so that it can drive 8 ohm speakers directly, not to use matching transfomers at all. Where in the schematics (pg.19 of the manual) are the 70V and high impedence determined? The topology of the output stage (complementary MJ15024 and MJ15025 pair) looks similar to me as other home audio amps. The output is high (at 70V) because of the high voltage rail? I also read that the 2271 is the first direct drive (I assume that meant no internal step up transfomer used) 70V amp altec made. So the question is: what has to be changed in the circuit so that the amp will become a regular amp?
Thanks.

[claudiomas]

Hi you d'ont need outputs transformer , each amp module need 64 homs charge so 4 modules in parallel = 16 ohm for stereo or 8 modules in parallel = 8ohm 624watts .

[2004ex]

Thank you again. Sorry I have not made myself clear. I am trying to use the 2280B system (including 8 amp cards (2271, 78W each)) as a multi-channel amp for a home theater application, driven by the sound card in my HTPC. So I shall be running each card amp in "singles" (need at least 5 channels) rather than in "bridged" mode (as noted above, one needs to bridge 8 cards to get down to 8 ohm). I am not limited by the output power, 78W/channel is ample for my application. Basically I am trying to get some help to change (on the board level) the output impedence of the amp (I have seen the same pair of output transistors used in regular home audio amps) and lowering the peak output voltage (for speaker protection?). Would it be OK if I just lower the rail voltage from +/-120V to something like +/-60V and connect to the speakers? Or other bias conditions need to be changed as well? [I should say that I was hoping for a simple mod, if it is too much involved probably not worth the effort. But still curious as to what is the main difference circuitwise between a "direct drive 70V amp (no internal step-up transformer)" versus a regular amp.]
Thanks.

[claudiomas]

so , 1rst thing its to change the R36-37 from 1,5 to 0,22 ohms and R34-35 from 47 to 120or150 ohms , turn bias trim R26 to full 1Kohm . forget Q13/14 overload protect or calculate new polar resistors , i like just two 1N4004 series like 1.2volts zener in place or added to cr8/q13CE and cr9/q14CE. then pray for 0,55 Volts on BE for Q16Q18. if OK start whit 16 ohm and check drivers and outpouts bias . few mV on 0.22 resistors whitout signal means its ok. if toomuch (8 or more after 5') R25 too short.

[claudiomas]

sorry i means 12/15 ohms for R34R35

[claudiomas]

http://www.australianmonitor.com.au/schematics2.html here many schematic diagram for professional 70/100 Volts AMPS free