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Old 21st May 2014, 10:01 AM   #1
damped is offline damped  India
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Default How to calculate safe impedance level

I bought a ready-made amp board with 6 pairs of TTC5200 and TTA1943 giving 6X100W, (5.1 channel) output. The board has 30-0-30 V ac input, with on-board rectifier and filter. This, I presume, will give a 43V dc supply on the rails.

I have a set of 5.1 speakers from a defunct HTS. As of now, I don't remember their impedances but I can get the data in a day or two. However, can you help me calculate whether they can be used safely with the said amp? I need to know whether
a. anything can get fried
b. the complementary power stages are sensitive to impedance match.

Any rule of the thumb will do for now. Maybe a little formula or something to get started.

For a more detailed analysis, I will have to trace out the circuit. I hope to do that soon. I will post that too.

But meanwhile, any little help is appreciated
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Old 21st May 2014, 10:54 AM   #2
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Quote:
Originally Posted by damped View Post
Any rule of the thumb will do for now.
A rule of thumb is the total Pd of the devices, divided by the continuous power level.
In your case, it's a pair of 150W devices for 100W, 300/100 makes a factor 3.
Not recommended for 4 ohm use.

2nd rule of thumb is to calculate the max dissipation level of the output stage. For that, you need an estimate of the heatsink C/W rating.
Combined with the rail voltage level, it gives an indication of the lowest impedance level.
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Old 21st May 2014, 11:25 AM   #3
damped is offline damped  India
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Quote:
Originally Posted by jacco vermeulen View Post
...In your case, it's a pair of 150W devices for 100W, 300/100 makes a factor 3.
Not recommended for 4 ohm use.
...
.
@jacco vermeulen. Thanks
Need some clarification. Should the impedance be less than 4 ohms or greater than 4 ohms? How does this work please?
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Old 21st May 2014, 12:00 PM   #4
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A lower impedance load translates to a higher output current.

Output current times the voltage across the output stage devices, is the heat dissipated in the power transistors.
Lower impedance => higher current => higher dissipation
=> a 2-pair output stage (with 42Vdc rails) is not able to drive loads of 4 ohm or less.

The heatsink is also of importance. If it's relatively small, more likely that the minimum load is in the neighborhood of 6 ohm.

(a car transmission can handle much more torque if it has better cooling, by adding an intercooler. same deal for a power amp)
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Old 21st May 2014, 12:37 PM   #5
damped is offline damped  India
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Ok that sums it up, nice and clean. Thank you.
Will get back as soon as I have more data.
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Old 21st May 2014, 04:30 PM   #6
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I'd say you should be fine there assuming cooling is OK (and the circuit doesn't have any major flaws like tendency to thermal runaway). Some commercial receivers run 5200s or even 5198s at +/- 56 V! Unsurprisingly, secondary breakdown becomes a real threat if the output is accidentally shorted.

Satellites usually are 6 ohms up anyway. The sub is more likely to be a 4 ohm affair, depending on whether the HTS was supply voltage or current limited.
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Old 23rd May 2014, 04:23 AM   #7
damped is offline damped  India
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Well then, here is the bad news! All the six speakers are rated 3ohms

But then, is there a possible workaround or a compromise solution? Like, can I get away with limiting the input drive to the power stages? I don't mind losing some audio power as long as the fidelity is OK. (My neighbours would probably approve of this option )

Am I right in assuming that the speakers are dc coupled in these complementary power stages? I don't see any large coupling capacitors on the board.

A little experimentation from my side could save you the bother of a detailed explanation, but right now, a bunch of kids on holiday have locked me out of my hobby room . So I thought i could use this time to catch up on my homework. Thanks for your time.
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Old 23rd May 2014, 07:44 AM   #8
AndrewT is offline AndrewT  Scotland
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the speakers will almost certainly be DC coupled when the Power Supply is a dual polarity.

The amplifier is very likely to be AC coupled, using DC blocking capacitors in both the Input and the Negative FeedBack leg.
Check for no DC output when powered up with the inputs shorted and with the inputs connected to your sources.

For 3ohms duty you could run the amplifiers at a much lower voltage. Maybe around +-24Vdc.
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Old 23rd May 2014, 09:43 AM   #9
damped is offline damped  India
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Oh! OK. Can I just lower the dc supply voltage without making any other changes? Generally speaking, (as i do not have the actual ckt of the amp yet), will I need to change some biasing resistors?
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Old 23rd May 2014, 10:49 AM   #10
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