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Old 5th April 2009, 04:15 PM   #741
pooge is offline pooge  United States
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Quote:
Originally posted by Sheldon


The schematic in post 8 is a clear as it can be, 39v frontend, 34v output. Either way, the calculation is straightforward.

Sheldon

OK. That's DC. I was posting transformer AC. And you posted 34vac in post 738
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Old 5th April 2009, 04:19 PM   #742
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by pooge



OK. That's DC. I was posting transformer AC. And you posted 34vac in post 738

My bad. Yes, 34V DC, as I stated in my original post. I made it 34vac in my reply to your post, brain on autopilot. Sorry for the confusion.

Sheldon
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Old 9th April 2009, 11:43 PM   #743
Sheldon is offline Sheldon  United States
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Up and running. Makes music too. Now time to get the enclosure worked out.

One question: The transformer has dual windings. So choice of one or two bridges per channel. any strong opinions either way?

Sheldon

Edit: Don't know that it's necessarily a problem, but it doesn't like to be messed with. I left the input open. But if I touch it with a probe or lead it likes to oscillate and blow fuses (5A rail). A little buzz though the speaker when I do this, but nothing crazy, so it must be HF.
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Old 10th April 2009, 06:34 AM   #744
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Quote:
Originally posted by Sheldon
One question: The transformer has dual windings. So choice of one or two bridges per channel. any strong opinions either way?
Quote:
Originally posted by Nelson Pass
Most customers want total silence from the amplifier, including
mechanical noise. If there is not complete matching between the
secondary coils and only 1 rectifier bridge, any net DC imbalance
between the current of the + supply and the - will tend to
saturate the core of the transformer and create noise. This is
seen for quite low current differences and can also show up with
low frequency output. Using two bridges eliminates the problem.
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If you've always done it like that, then it's probably wrong. (Henry Ford)
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Old 10th April 2009, 01:06 PM   #745
roender is offline roender  Romania
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Quote:
Originally posted by Sheldon

Edit: Don't know that it's necessarily a problem, but it doesn't like to be messed with. I left the input open. But if I touch it with a probe or lead it likes to oscillate and blow fuses (5A rail). A little buzz though the speaker when I do this, but nothing crazy, so it must be HF.
It is a problem! I suspect occasional HF oscillations in CFP drivers.
How much current do you flow through CFP driver, master and slave transistors? Also, what transistors do you use?
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Old 10th April 2009, 03:09 PM   #746
Sheldon is offline Sheldon  United States
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Thanks pacificblue. Two bridges it is.

Quote:
Originally posted by roender
It is a problem! I suspect occasional HF oscillations in CFP drivers.
How much current do you flow through CFP driver, master and slave transistors? Also, what transistors do you use?
Thanks Mihai.

Voltage drop over R17 is 0.358V (2.4mA)

R5 is 12R, voltage drop is 1.275V (106mA).

Voltage drop over R6 is 0.596, R41 is 0.574

Transistors are 2SC3428Y:, 2SA1360O(couldn't get Y, so matched as close as I could).

Drivers are MJE15035,15034

R32, etc. are 0.1R, bias is 17mA (varies in a range from 16 to 19). Offset is about 7mA. Similar for all four boards.

OPT are MJL3201,1302

Sheldon
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Old 10th April 2009, 03:53 PM   #747
roender is offline roender  Romania
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I saw in your previous picture that you have an oscilloscope. Use it to trace any spurious oscillations. Pass through the amplifier square waves, with 8 ohm loads.
If you find something suspicious you could decrease R6/R41 to 150...120R
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Old 10th April 2009, 05:33 PM   #748
Sheldon is offline Sheldon  United States
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A 10kHz square wave at about 4V out looks good. At 1kHz and 100Hz, just a little overshoot, but no ringing. 100kHZ is clean, with some rounding of the leading and trailing edge.

Sheldon

Here's the 100Hz image.
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Old 10th April 2009, 06:50 PM   #749
roender is offline roender  Romania
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Sheldon,

That's good!
What is the scope's bandwidth? Could you check for local oscillations in every cascode transistors?
In any case, it is a good measure of precaution to decrease R6/R41 at 150ohms

Cheers,
Mihai
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Old 10th April 2009, 07:41 PM   #750
Sheldon is offline Sheldon  United States
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It's an old Tektronix 454, 150MHz at 20mV/div. I'll check out the cascode transistors (1kHZ SQW OK?), and swap out R6/41. If I get anything interesting, I'll post it.

Thanks Again
Sheldon
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