Pioneer QX-8000A questions

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Monroe L King Jr

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Joined
Jun 13, 2016
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Hi guy's I'm new to old gear. I just got a QX-8000A a friend just gave me one that had been sitting in front of a non working car parked in his yard for who knows how long. This is my second post by the way and I got no response on an old thread I commented on so I figured I needed to make a new post about it.

Anyway in this deal he wanted me to replace a belt in an Akai GX-4000-D in exchange for the Pioneer. He also threw in a Teac X-1000-R that needed a new belt and a Panasonic 8-Track player. I'm a pretty good fixer upper kinda guy and all the gear is back in working order now.

I was interested in getting another QX-8000A and using both to run a 7.1 Theater because the QX just sounds so good!

Anyway the question is about strapping the amps in the QX by something I noticed in this other thread. The guy said "He then said that he could "jump" the two amplifiers with an RCA cable at the tape record and monitor plugs and keep the selector in "2 channel" mode"

Can someone explain this set up?

Is this really strapping the amps?
 
I am not really sure what "Strapping" the amp means, but if you are trying to bridge 2 amps into a twice the power mode, then it won't work with the QX-8000A. Yes, with all of the options this receiver has: 2 tape monitor loops and a pre-out/main-in loop, you could jump the front stereo pair to the rear. This will be of no benefit since you can't tie the amps together at the speaker terminals without damage. Also if the "A" version is like my non "A" unit, then putting the selector switch to "2 CH Stereo" will route the the front right signal to the rear right and the same for the left channel.

A little study on bridged amps. Some receivers allow you to select a mode that will bridge 2 amps for twice the power (Sansui QRX-9001 comes to mind since I have one). In a quad receiver this means a twice power stereo receiver. The power supply and amps themselves have to be designed for this purpose.
The way this works is this, when you select that mode then the pre-amp that feeds the power amps will feed one amp with a "IN Phase" signal while the other amp gets an "Out of Phase" signal. You connect your speakers to both "+" terminals of the 2 amps.
What happens is when one amp is going positive the other amp is going negative. This doubles the voltage the speaker sees thereby doubling the power. The 2 "-" terminals of the amps are internally connected.

Now a well healed hobbyist could build a op-amp based in phase/out of phase buffer and place it this way on the receiver: You would select "separated" on the back panel. You would connect the input of the circuit to the front pre-out terminals. Then you would connect the "in phase" output to the front main in terminals and the "out of phase" output to the rear main input terminals. You then connect your speakers to the right front "+", and the right rear "+" speaker terminals. Repeat for the left channel. Now you have a stereo bridged amp from a quad receiver. Depending on the design overhead the designers gave the amps and power supply you may enjoy a song or two before catastrophic failure occurs.

An interesting side note: The QX-8000 uses a capacitor output coupled power amp design Ala the old Dynaco ST80 and ST120. They do have advantages, but man they have a lot of issues. Everyone one in the market got away from that design topology.

Bottom Line, hook up you 2 receivers as 2 quad receivers with no amp bridging or anything abnormal. 8 signals in, 8 power amp signals out feeding 8 speakers.

Enjoy merging the old with the new.
 
"Strapping" would use the same input signal for the front on the rears (Stereo) and then YOU DO tie the speaker terminals together at the speaker. This is "parallel" operation that does not double the voltage as in "bridging". Strapping half's the load or impedance and doubles the current instead of voltage.

To do this you need a common power supply power rails and identical amplifiers (which the QX-8000A and the QX-8000 have)

If you wanted to double your stereo output power this would work however you would want your DC offset to be the same on both amps and you would want a small resistance in the output to balance the amplifiers. It should be very easy to modify the QX-8000 or the A to have double the power in stereo by strapping the amps.

The original post was incorrect however because that was not "strapping" because he was powering 4 speakers which meant the outputs where not tied together. What they where doing there was bypassing the rear preamp and feeding the rear the same signal as the front (Which would do no harm at all).

I think "strapping" these little quad amps would be fun but not really worth the effort unless you never use quad mode and you want that extra power in stereo.

I am looking for QX-8000's to play with or amplifier sections if anyone's interested in giving them up.
 
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