Hello all I am new to the forum and am going to be joining the QA family soon but I wanted to see if some of the super smart folks here can help me with a “Dummy Load” question, Yes I know not another dummy load question LOL!
I have a somewhat rare Hi power and super accurate load from the late 70’s it’s the BPI-250. This load is made up with (8) 7.87 ohms 1% 250W each using Arcol power resistors that must have been made custom for BPI. This load measures exactly 4-8-16 ohms and can handle up to 1000W!
This was built to test 4 Ch and 2 Ch amps. Quadraphonic
This is a 2 piece unit there is a control head with a heavy multi-pin cable that connects to the load plate.
I mostly work on 2ch Hi-Fi and pro audio stuff and typically the most powerful amp I test is around 500w 8ohm’s but normally most amps I work on are 100w or less.
So with that in mind I need help finding the best way to tap this load to work with the QA403 and QA451.
I think MAX 250wpc 8@ohms & 500wpc @4/16ohms should be my target power levels I don’t need any higher.
Unfortunately, the design of it is not conducive to measuring ultra low distortion like the QA403 is capable of.
You have a few issues:
The monitor out is common ground with the negative. Not a problem with low-medium powered single ended amplifiers, but it rules out all bridged/balanced/BTL output amplifiers.As soon as you connect the generator out on the QA and the monitor to the analyzer, you will short a BTL amp.
Also, the monitor out is basically straight through/un-attenuated, which means you will have to have your attenuators on in the QA-403 and potentially overload the inputs and be operating on ranges where the performance is not the best.
It is also set up to only monitor one channel at a time.
It was designed back in the day when amplifers were single ended and THD analysers were single channel common ground (no balanced inputs). That’s fine, I have several vintage analyzers I love that are like that, but with those caveats, you can use it with the QA, but you’ll have to consider all of the above.
Unless I am reading this incorrectly, I will check with meter later, I don’t think all the grounds are tied together, I believe the ground symbol represents the Sheild in the cable that goes from the switch box and is not connected at load end probably to lower noise at monitor output? BTW take note of the “OPEN” switch position.
From what I can see there is 4 independent banks or 4 channels of 2 resistors with switch to change from 4,8,16 ohms. The monitor just taps off each of the load banks selected. BTW I don’t have to use the Monitor output I can tap off each load channel independently.
I should be able to find a point where I add the power divider tap with appropriate padding for the QA inputs keeping in mind, I would not be using the internal loads in the QA but still would have use of the built-in attenuator in the QA for more range flexibility. I’m just not sure where to tap and not good with the math to come up with the optimal component values
This load is made of very high-quality resistors and don’t see why it can’t be made to work with the QA.
I will take some measurements later and see if all the grounds are tied together.
When BPI designed this load, they even calculated the resistance drop of the short cable from the load to the box, so it reads within 1% of selected load at the banana jacks! BPI was not playing around back then!
Maybe I can make some pad’s that plug into the banana jacks on the switch box for different power levels? OK now I’m getting ahead of myself.
You mention pro-audio…be very aware of what modern class D topologies look like. Most pro-audio equipment that I have seen uses split rails and fully custom modulators. But there are also class D running from single supply where the outputs idle at rail voltage divided by two. Fortunately these are modern and and very well protected. But the first time you encounter it can be very confusing.
This is great info! I totally forgot about ground ref. Most of the stuff I work on is old SE SS stuff from the 70’s 80’s almost all class a/b.
Got to read though that post lots of info there.
