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8 ohm dummy load across speaker terminals
8 ohm dummy load across speaker terminals
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Old 30th June 2020, 11:51 AM   #11
Mark Tillotson is offline Mark Tillotson
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Well if you want to accurately measure the clipping point into 8 ohms, you need an accurate 8 ohms. The behaviour of an amp is not the same for different amps here.
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Old 30th June 2020, 12:57 PM   #12
freebee is offline freebee  Netherlands
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Any idea which resistors are best to use for adjusting the quarter-A in the PM-94? (type, wattage etc.)
Attached Files
File Type: pdf Adjustment procedure PM-94.pdf (144.3 KB, 12 views)

Last edited by freebee; 30th June 2020 at 01:00 PM.
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Old 30th June 2020, 03:50 PM   #13
amplidude is offline amplidude  Denmark
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According to the service manual , you should inject 1khz and raise output to 17 volt p/p, ohm 8 ohm,
That's roughly 36 watts, so an non inductive power resistor 8 ohm and above 36 watt.
Plz correct me if I'm incorrect.
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Old 30th June 2020, 04:44 PM   #14
rayma is offline rayma  United States
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Originally Posted by freebee View Post
I bought two no-brand wire wound resistors on AliExpress to set the offset on my Marantz PM-94.
If you ever use these for high power testing (more than a few watts), they must be
mounted on a large heat sink.

Last edited by rayma; 30th June 2020 at 04:47 PM.
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Old 30th June 2020, 06:16 PM   #15
bucks bunny is offline bucks bunny  Germany
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Originally Posted by amplidude View Post
According to the service manual , you should inject 1khz and raise output to 17 volt p/p, ohm 8 ohm,
That's roughly 36 watts, so an non inductive power resistor 8 ohm and above 36 watt.
Plz correct me if I'm incorrect.

17Vpk, not pp
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Old 30th June 2020, 08:15 PM   #16
indianajo is offline indianajo  United States
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Originally Posted by freebee View Post
I bought two 8 ohm 150W wirewound resistors which measure resp. 8.4 and 8.6 ohm. Will this do the adjust the bias or do I need to buy resistors which are within 1%?
Note also the performance of cheap (<$40) dvm or even analog vom below 10 ohms is hit and miss, and the state of the battery and cleanliness of the probes has a lot to do with the result. Check another time and note the differences you get.
Accurate 1 ohm ohmmeters cost about a kilobuck.
Rayma's response of 1% accuracy may have to do with registering power to legal agencies. To communicate with one of those you also need a calibration program certifying your meter back to the national bureau of standards.
For full power checks I use inductive 10 ohm 225 w logs from ohmmite, with tap set to what I measure as 8 ohms. Speakers are inductive, why not? You'll get better power measurements with non-inductive resistor of the nominal rating value, but you won't hear anything from those resistors.
Second JM Fahey, I have some 225 W 5 ohm dale logs I use for full power tests, too.
For setting output transistor idle bias current, you don't need load resistors at all, on a solid state amp.
Output DC at idle setting will probably be worse with no resistor on output compared to 8 ohm or 4 ohm, but may as well check both ways. Correct result is 200 mvdc or lower.
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Last edited by indianajo; 30th June 2020 at 08:20 PM.
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Old 1st July 2020, 07:56 PM   #17
hpro is offline hpro  Switzerland
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Default Nominal

Originally Posted by JMFahey View Post
Also remember an "8 ohm" speaker has anything but 8 ohm 90% of the time.

Click the image to open in full size.

Datasheet calls 8 ohm "nominal" impedance, now you see why

But as you make guitar Amps, for a guitar 4 - 6 Khz is more than a good response no need to go up to 20K..
BTW your thick line shows this just beautiful. Drop starts at about 3.8khz..

The high E-String will not reach that if the player doesn't bend the String on the 22nd Fret..and there he has to bend it up to the 4th to reach 4khz or more..\
I also stick with PRR and also Mark Tillotson.
If you want accurate measurements for output stage, then you need to make sure that what your are using between the terminals, is the correct Value,.

And also if you measure with 7.5Ohm as PRR stated you just can very easy calculate it for 8 Ohms.. To measure something you will need EXACT MEASURING TOOLS. So 5% =J is NOT GOOD ENOUGH

But I would suggest something else, a Dummy Load is not a real Speaker and it always will be the same Resistance for the output as far the Dummy can handle Bandwidth... So you will need the SPECS OF THAT RESISTOR what's the linearity of the R. Wire-wound tend to eat up High frequency. But it always also depends on how those Wire-wound resistors are build.. and this doesn't count only for the wire, but also for the Body and solder lug and more and more and more

The Graph of JimFahey is a real good Example and makes show reality what actually happen. And it's worth to look at..

Now I would suggest for all who haven't done that yet, remove the woofer out of your cabinet if you use a Passive Network or just disconnect the NETWORK. Because we do not want to measure the NETWORK but the WOOFER instead.
Then use a Analog Volt - Ohm meter and set it to 1 Ohm Range.
Clip the Probe of the Analogue Meter to Plus & Minus of the Loudspeaker terminals and then measure it and note down how much Ohm it displays..

But instead of being satisfied, now carefully push the cone downwards/inwards slowly and check the meter then you will see the change of OHMS. And do this in opposite direction as well. Note the numbers down when it reaches max or minimum. This will be DC RESISTANCE for the Loudspeaker and is a good referral to AC response and you can see the working point of the Loudspeaker. You even can calculate how much Power it takes to drive that speaker to it's best responds.
This will reveal things about your amp you don't expect.
One more thing, hook a AMP AC METER into one of the wires of the DUMMY LOAD - For woofer use 50hz or more but not over 125 hz and then Drive the amp from low to loud in slow steps and check the Amps running through the Dummy load, this also explains a lot about your amp. Hook up a OSC and check the SINE WAVE DISPLAYED. Make sure that you have a original Sine Wave of the one you are feeding that Amp displayed on the second Scope Channel, so you can compare many things. Near clipping note down how much AC AMPS are passing, because this will help you to calculate the output inclusive the PSU for that kind of Speakers,.
It's always better to have 20% more than you need.. Just to be on the safe side.
It not a big need to go higher than 200 HZ because the low frequency asking for most of the power on almost any Amplifier. But if you like to try this one mid and high then BE CAREFUL NOT TO connect the speakers. you can blow them very fast and easy.!

All this has nothing to do with adjusting the BIAS.
@thread starter Freebee, in my amps there are RE in the Value of 7.5 ohm @ 400 WATTS!! And just 2.5 Ohms higher = 10Ohm, the max Value for the Resistor it's only 170 WATTS..
Here the Resistor adjust the BIAS itself, nothing to turn.
My Question to you:
Why do you want to adjust Bias, what is wrong with that amp? Marantz usually were very user friendly. If this is the Transistor version and it's not broken I would not change.. If it's the CHIP Version* already made in Japan or Asia* not much you can change. Increasing Bias current on AB versions increase usually only heat,., Before any amplifier Manufacturer gives the OK for sale, he, and this I believe very strongly, has checked that amp and adjusted that amp for best performance, otherwise this amplifier manufacturer would be Bankrupt long time ago.. Releasing none working amplifier is about the same as selling rotten food in a Gourmet Restaurant. AND I OWN A RESTAURANT. There is no Way to release crap or bad food.. Of course it can happen on both sides that something is going wrong. But never do that for a long time, one time is more than enough.. it will be the death of that business. I can not talk for USA but here in swiss you will be wiped off.
So find the reason why do you want to adjust the BIAS.
Sorry for that long post.. it's my nature..

There are no dumb questions, there are only dumb answers.Accepting criticism is not for everyone..
Your criticism helps me to improve
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Old 1st July 2020, 10:27 PM   #18
tonskulus is offline tonskulus  Finland
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Even tho speaker impedance is not constant, we need standard method for measuring amplifier output power to make different amplifiers comparable with each other.

And it is usually 2, 4, 8 and even 16ohms, tested with RESISTIVE load. Continuous sine wave @ 1kHz. This is the old standard method.

Real life situations has more to it indeed, music signal is almost never pure sine wave.
Some manufacturers has moved to burst RMS power, something like sine wave 20ms burst or different kinds of crest factors which is closer to typical music material. What a mess of power ratings!
It makes sense. There are many high power class-D/SMPS amplifiers, rated for lets say 2x5kW "music power". But this is short term or peak RMS (unclipped) power. Such an amplifier will limit the continous output power down to around 2x1kW after a second or less to prevent overheating and to limit the mains input current.

Those old heavy "iron" amplifiers had no input current limiting or nothing like that, giving the full rated output power continuously even 247.

Not to forget marketing watts!
I'm still using that old continuous sine wave method (usually 40Hz and 1kHz) when I measure amplifiers "brutality" but as well I'm using 10-20ms burst testing.
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Old 1st July 2020, 11:11 PM   #19
esgigt is offline esgigt  Netherlands
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Originally Posted by tonskulus View Post
Even tho speaker impedance is not constant, we need standard method for measuring amplifier output power to make different amplifiers comparable with each other.
The case of TS however is not about power measurement but adjusting bias and offset.


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