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Old 23rd March 2020, 10:38 AM   #2371
tyu is offline tyu  United States
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Bolserst….Can you give us one more, time what a standard DVM with 10Meg input impedance reads in the older MK121 bias output is?..
At the Red bias wire out put on the board..I can not find your post I have seen before...is it a reading of 85-90 volts =5k?
Thanks so much for your time
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Old 23rd March 2020, 04:45 PM   #2372
AcoustatAnswerMan is offline AcoustatAnswerMan  United States
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Quote:
Originally Posted by bolserst View Post
No, I don't think your probe is loading down the ultrasonic bias circuit enough to significantly change the reading. A change from 4.3 to 4.2 is about all I would anticipate. Only on the older 60hz supplies would you see much change. There it would read something like 2.5 instead of 4.3

I don't understand why you suggest that a HV probe will load the older 60Hz supplies more than the ultrasonic supplies. The voltage quintuplers and outfeed 500M-ohm resistors are the same for both supplies. Therefore, for a given measurement method and device, I would expect identical readings on either style of bias supply. The only difference is that the ultrasonic supply is both adjustable and regulated (but note that regulation is in the low-voltage front end only, and does not compensate for HV droop).
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Old 23rd March 2020, 07:50 PM   #2373
bolserst is offline bolserst  United States
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The difference is because the ultrasonic bias supplies are replenishing the multiplier chain with HV pulses at a ~25kHz rate whereas the older supplies are working at 60Hz. This results in the effective source impedance of the multiplier being much lower for the ultrasonic bias supply. To put some rough order of magnitude numbers to it, the 60Hz supply behaves like a 5kV source with impedance of ~100Meg. The ultrasonic supply behaves like 5kV source with impedance of ~1Meg. So if you are measuring with a HV probe upstream of the 500Meg bias resistor, the 75Meg vs 1000Meg HV probe will show much less difference with the ultrasonic supply than with the 60Hz supply.
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Old 23rd March 2020, 07:53 PM   #2374
bolserst is offline bolserst  United States
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Quote:
Originally Posted by tyu View Post
..
what a standard DVM with 10Meg input impedance reads in the older MK121 bias ...At the Red bias wire out put on the board.
It should read in the 75-80V range.
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Old 23rd March 2020, 07:59 PM   #2375
AcoustatAnswerMan is offline AcoustatAnswerMan  United States
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I follow your logic, but not sure it translates into a real world difference. At the factory, there was no change in bias measurement standards when we converted over to the ultrasonic supply. I suspect the high effective output impedance of the multiplier + constant-charge resistor more than swamps-out any difference in the charge rate. Note that my instructions on bias measurement make no distinction between 60 Hz and ultrasonic supplies, other than the adjustability of the latter.
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Old 24th March 2020, 04:10 AM   #2376
bolserst is offline bolserst  United States
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I was a bit puzzled by your comment until I located the factory measurement method you described in another thread (How to measure bias voltage on Acoustat). It has you measuring downstream of the 500Meg bias resistor. This would make the source impedance ~600Meg for the 60Hz supply and ~500Meg for ultrasonic supply. With a 1000Meg HV probe, the measured voltage would only change about 5% and easily fall into the 3000-3500 range you mention for both the 60Hz and ultrasonic supplies.

I was describing what to expect when measuring upstream of the 500Meg resistor(Method 1 in attached *.pdf). At this measuring point the source impedance is varying by a factor of 100, from ~100Meg(60Hz) to ~1Meg(ultrasonic). This is why switching between a 1000Meg probe to a 75Meg probe would affect the reading significantly with the 60Hz supply, but hardly at all for the ultrasonic supply.
Attached Files
File Type: pdf Andy Szabo - Ultrasonic Bias Power Supply.pdf (313.6 KB, 14 views)
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Old 25th March 2020, 04:56 PM   #2377
hhhh is offline hhhh  Canada
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Originally Posted by bolserst View Post
Comparisons between the MK-121 and MK-121C electrical response posted here: All Acoustat panels can give
*** Note the correction to attachments #4 & 5 in the subsequent post


You may also be interested to see electrical response with varying mixer resistance and bass tap positions.
Open CLS II stators - the best way to do it?
Thanks bolserst. If I'm understanding it correctly, the plots compare the frequency responses for the different interface configurations C-mod vs. earlier but at a fixed hi frequency pot setting. I was hoping to see the frequency response curves at different settings of the hi freq pot.
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Old 26th March 2020, 03:09 AM   #2378
bolserst is offline bolserst  United States
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Correct. Hi frequency settings were similar for C-mod vs. Pre-Cmod.
I thought the comparison would address the ~2dB difference in the mid-range mixer region that you were asking Tyu about in Post #2360.
But, looking at your post again I see you were actually interested in the resistor settings.

I'll see if I can locate the LTspice files this weekend and run some HF pot setting trends for both versions to post.
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Old 29th March 2020, 05:16 PM   #2379
malthuse is offline malthuse  Canada
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I have a pair of model 3s. Once in a while I cn hear a buzzing in one speaker. It's very slight but definitely there. Are there any suggestions on how to diagnose?
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Old 29th March 2020, 05:28 PM   #2380
mraudioguru is online now mraudioguru  United States
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Originally Posted by malthuse View Post
I have a pair of model 3s. Once in a while I cn hear a buzzing in one speaker. It's very slight but definitely there. Are there any suggestions on how to diagnose?
Could be just dust or dirt in the panel. Use an air compressor to blow it out or you could try to vacuum the panel. I did both, several times, when I rebuilt my 2+2's.
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