Help needed modifying a audio amplifier circuit & using 2sa1962 / 2sc5242 in parallel

[paulflan]

Help needed modifying a audio amplifier circuit & using 2sa1962 / 2sc5242 in parallel

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 ??


Driver board...
Dropbox - Onkyo TX-875 Amp Driver Board.png

The output transistor circuit layout...
Dropbox - Onkyo TX-875 Power Amp transistors 2SC5242 & 2SA1962 layout board.png

As you can 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 7 for the speaker output's along with 7 line level input's i fed from another preamp
anyway,
Yes 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 with low impedance loads and reused salvaged 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's and a pair of monitor audio main speaker's

can someone please give their thoughts an advice please ?

cheer's

 

[richie00boy]

It should be ok but you are likely to see higher distortion when you use 4 ohm speakers.

 

[paulflan]

Thank you for your reply,
and i will try it and update the thread,

and yes i'm aware of the possible increased distortion and only wanting to modify 2 or possibly 3 if it work's 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 rectifier with 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-amped


anyone got any advice or suggestion's please ?

 

[paulflan]

So this is ok ??
Please check the pic...
Dropbox - Onkyo TX-875 Power Amp transistors 2SC5242 & 2SA1962 layout board.png

where the 6 pin connector on the left of the pic is from the driver board and the SPR is the speaker out i'm ok with connecting two channel's to a single driver board and the speaker output's together to run them in parallel ?

I 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 with a 4, 8 or 16 ohm dummy load to check the amplifier's output transistors are switching properly reproducing a full sine wave and functioning properly please ?

any advice or suggestions gratefully received

 

[paulflan]

It should be ok but you are likely to see higher distortion when you use 4 ohm speakers.

Thank you very much richie00boy,

Also JMFahey for posting on my other thread Advice/guidance on reusing the power supply & power amp module from the Onkyo TX-875 being the only 2 people to actually post any advice and provide support helping me reuse this high quality amplifier stage

i'll be eternally grateful as 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 37mV

Now i've just got to finnish the power supply modification and order the speaker protection boards and enclosure from amazon UK

any advice on decent speaker protection board's i can buy from amazon UK please ?

Looking at this case for it...
https://www.amazon.co.uk/Aluminum-Chassis-Amplifier-Enclosure-Cabinet/dp/B073WVQTJ8/ref=pd_day0_hl_107_7?_encoding=UTF8&pd_rd_i=B073WVQTJ8&pd_rd_r=19909f6a-e9de-11e8-9060-4b622bef9fcb&pd_rd_w=swhxS&pd_rd_wg=nIVDY&pf_rd_i=desktop-dp-sims&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_p=b082d07b-aaea-4f40-9ff3-d27463f747d7&pf_rd_r=BBJZYGXJQSA7KWYACJDM&pf_rd_s=desktop-dp-sims&pf_rd_t=40701&psc=1&refRID=BBJZYGXJQSA7KWYACJDM


thanks

 

[paulflan]

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 post

would this be used when you set the speaker's ohm's rating's on the receiver or for required/demanded power output to the speakers

once i've sorted this final hurdle I'll go ahead and order the speaker protection boards and enclosure from amazon UK to finish the project

any advice on decent speaker protection board's i can buy from amazon UK please ?

Looking at this case for it...
https://www.amazon.co.uk/Aluminum-Chassis-Amplifier-Enclosure-Cabinet/dp/B073WVQTJ8/ref=pd_day0_hl_107_7?_encoding=UTF8&pd_rd_i=B073WVQTJ8&pd_rd_r=19909f6a-e9de-11e8-9060-4b622bef9fcb&pd_rd_w=swhxS&pd_rd_wg=nIVDY&pf_rd_i=desktop-dp-sims&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_p=b082d07b-aaea-4f40-9ff3-d27463f747d7&pf_rd_r=BBJZYGXJQSA7KWYACJDM&pf_rd_s=desktop-dp-sims&pf_rd_t=40701&psc=1&refRID=BBJZYGXJQSA7KWYACJDM


any advice or suggestions gratefully received

 

[paulflan]

in fact with more deeper investigation with a test meter it appear's to be different tap's of the same secondary winding providing different voltages controlled by relay's,

any idea's why ? benefit's of doing this please as it's the first time i've come across it in a project where i need a basic understanding of why and how best to use it if at all ?

would it serve at providing different wattage output's into different load's ? 4-8-16 ohms

any advice or suggestions gratefully received

 

[richie00boy]

It's either a class-g amp or they use different rails for 4 and 8 ohms. It enables the camp to delivered maximum advertised power into 8 ohms and 4 ohms, usually it's just 4 ohms.

 

[paulflan]

Thanks, it was documented as providing the rated wattage into 8 & 4 ohm's load when using the settings in the menu, that's both what I thought and handy to know as I can just use a normal rotary switch on back of the case to select between the 8 and 4 ohm's speakers for the main 4 channels 🙂

And tested the output transistors I'd modified to run in parallel and they didn't get hot or anything after a couple of hours of normal usage into a 4 ohm's load 🙂

 

[richie00boy]

Ok so you checked the driver transistors Q6106/Q6116 temperature?

 

[paulflan]

I'll get the temperature probe on them next weekend and see exactly what they are doing, would it be possible to use the other pair of driver transistors on the other driver board my modifying the second board?