Help needed modifying a audio amplifier circuit, 2sa1962 / 2sc5242 in parallel - Page 1
Forum section: electronics / beginners

author: paulflan79
author_rank: Newbie
timestamp: November 16, 2018, 01:53:28 pm
content: Would anyone with experience paralleling output transistors just check this out for me please,here's a link to the pic of the driver board circuit and i'm interested in knowing if the 6 pin connector to the right of the board that connect's to the circuit with the main output transistors and is shown on the link below it would be safe to connect to another channel's output transistor's using the same 6 pin connector and joining both speaker output's together and running them in parallel for the higher current capability for driving a 4 ohm's and below load please ??As you will see the Driver board's and output stage that form this amplifier are salvaged from the Onkyo TX-875 along with it's high current power supply however everything else has been removed so there's no protection circuit to trigger or any other part's of the original receiver over complicating it's use and i've already got it working under testing conditions, I'm left with a bunch of 12v relay's to control power and speaker output'sDriver board...Dropbox - Onkyo TX-875 Amp Driver Board.pngThe output transistor circuit layout...Dropbox - Onkyo TX-875 Power Amp transistors 2SC5242 & 2SA1962 layout board.pngYes there's questions if the drivers have enough current to drive 4 pair's of 2sa1962 / 2sc5242 unless someones got any ideas of modifying the second driver board to keep both in perfect sync in order to drive the output transistors in parallel,My idea was to cut the traces on the second driver board at the bases of Q6036 & Q6046 and (inject/modify) drive the transistors with exactly the same signal taken from the base of the same driver transistors on the driver board i'm actually using to provide the extra current to drive the extra 2 pair's of 2sa1962 / 2sc5242 and joining the speaker output's together to run them in parallel checking that there's still the same +0.6v and -0.6v respectively going to drive the 4 pair's of 2sa1962 / 2sc5242 ?I've repaired PA amplifier's that use similar output topology using output transistor's in parallel for better current handling at low loads and reused salvaged modular amplifier's before and modified them to my need's in past project's however need a little guidance as i really do want to get this right and use it as a power amp connected to a AVR using the preamp outputs with my main AV system to drive a (now passive after smps failure) monitor audio subwoofer that's 500W RMS at 4Ohm'sAlso use 4 standard channel's to drive my main stereo pair in a Bi-amped configurationcan someone please give their thoughts an advice please ?cheer's

author: paulflan79
author_rank: Newbie
timestamp: November 16, 2018, 03:10:08 pm
content: I would also appreciate anyone that could also let me know what the sense lines IPRO & VPRO do that formed part of the protection circuit going to the microprocessor along with VOLH ?https://www.dropbox.com/s/l758lcp3dedfnl9/Onkyo%20TX-875%20Power%20Amp%20transistors%202SC5242%20%26%202SA1962%20layout%20board.png?dl=0However i'm going to rely on fuses in the power supply stage and standard DC speaker protection boards from china i think and just looking for a safe way to drive a 4 ohm's load hence me looking into modifying 2 channel's into a paralleled single channel capable of delivering the higher current into a lower impedance load

author: paulflan79
author_rank: Newbie
timestamp: November 16, 2018, 05:03:58 pm
content: Someone on another forum has said it should be OK using the single driver board connected to the 4 pair's of 2sa1962 / 2sc5242 and joining the speaker output's together to run them in parallel checking that there's still the same +0.6v and -0.6v across both channel's respectivelyhttps://www.dropbox.com/s/l758lcp3dedfnl9/Onkyo%20TX-875%20Power%20Amp%20transistors%202SC5242%20%26%202SA1962%20layout%20board.png?dl=0where the 6 pin connector on the left of the pic is from the driver board and the SPR is the speaker outI have an old analog scope but never really used it and i don't have a signal generator but could i use a computer sound card to generate a basic sign wave and use the scope to check the amplifier's output transistors is switching properly reproducing a full sine wave and functioning properly please ?any advice or suggestions gratefully received

author: paulflan79
author_rank: Newbie
timestamp: November 17, 2018, 02:01:19 pm
content: the modification is working under test (still to dig my old scope out and test with a sign wave for possible crossover distortion) and showing no constant DC values on the speaker output's apart from 37mVMy aim with this project is to create a monster power amp for my home theatre modifying 2 (already done) or possibly 3 (yet to try) of the 7 channel's to create a single channel capable of driving a single subwoofer @4 ohm's powered from a separate toroidal transformer providing independent 65v-0-65vdc and separate rectifier with it's own 6 6800uf cap's on board i already salvaged from a Cloud VTX 750w RMS PA amplifier leaving the remaining 4 channel's and with the Onkyo Transformer having two separate B+/B- secondary winding's i'm thinking of splitting them and interdependently rectifying them with a couple of nichicon 10000uf 80v capacitors on each rail and splitting it for driving 2 for the left and the other 2 for the right and running the speaker's Bi-ampedAnyone else into salvaging and reusing part's?

author: paulflan79
author_rank: Newbie
timestamp: November 17, 2018, 03:18:38 pm
content: Now i've just got to finish the power supply modification's but unfortunately i was wrong about the Onkyo transformer it appears to have 3 secondary winding's providing 3 different voltages to the amplifier stage all split rail out, +/-24Vac Gray Wires, +/-33Vac Yellow wires and finally +/-48Vac and a common black wire centre tapped rail coming from the transformer and the +/-24Vac is hard wired straight to it's own rectifier then to the B+/B- of the amplifier,s reservoir capacitors, then the +/-33Vac is controlled by 2 relay's again going through it's own rectifier before connecting to the same B+/B- of the amplifier,s reservoir capacitors and the same is repeated for the +/-48Vac with it's own pair of relay's and rectifier before connected to the B+/B- of the amplifier,s reservoir capacitors,would this be used when you set the speaker's ohm's rating's or required power power output and how do i stand having all 3 supply's connected at once as that's what's thrown me, the lower +/-24Vac supply is constantly connected and the other two are relay controlled ?Scrap's my idea of using the different winding's to power different channel's for left and right as i wrongly presumed they was all the same however added up to more current capability however what's peoples ideas on using this setup with 3 controllable voltages to power 4 channel's aimed at Bi-amping a stereo pair of speaker's and i'll just add to the already 18'000uf reservoir capacitors and drive the remaining modified channel's run in parallel from the separate power supply as stated in a previous postwould this be used when you set the speaker's ohm's rating's on the receiver or for required/demanded power output to the speakersonce i've sorted this final hurdle I'll go ahead and order the speaker protection boards and enclosure from amazon UK to finish the projectany advice on decent speaker protection board's i can buy from amazon UK please ?