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Old 30th July 2010, 10:44 AM   #8501
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Franz Gysi View Post
I dont know, how much VA it is good for, but I guess more than 500VA. ....................
The inrush current may be a problem that must be carefully handled to not blow the apartment's 10A fuses
if you need ~500VA and you are on 220Vac then you should be able to close rate the mains fuse to ~T2A or at worst T2.5A. Allowing 10A from the breaker as your safety limit is nonsense.
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Old 30th July 2010, 11:00 AM   #8502
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I think, this 1kVA tranny is oversized.

As an alternative, I have two similar 2 x 9V 6A (108VA) trannies available.

Maybe I go with this two trannies in a double mono configuration.

While I am writing this, the postman delivered the needed prints and parts:

Click the image to open in full size.

Thanks for the comments!

Should someone (nearby) want the 1kVA tranny for free: just visit me

Franz
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Old 30th July 2010, 11:13 AM   #8503
AndrewT is offline AndrewT  Scotland
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a 1kVA EI should work well as the PSU for a big ClassA output.

I can see the queue at your door from Scotland.
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Old 30th July 2010, 11:24 AM   #8504
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Quote:
Originally Posted by Franz Gysi View Post
Weight 11 kg, how much VA it is good for.
www.amveco.com/pdf/Amveco_Catalog.pdf
See page 10 of the pdf for a VA estimate.
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Old 30th July 2010, 12:05 PM   #8505
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Default F-5 bias and offset

Just wondering how stable this amp should be, how much use is necessary before it gets stable, and the best way to measure offset and bias.

Upon first fireup, it will show from 17 to 25 mv, which quickly begins dropping.
Many times it will, after an hour or so, still be as high as 15 mv on the left channel, which seems to drift a little more than the right one. Last night after quite a session, (2-3 hours) one channel, right after the music stopped was around 20 mv, but came back down to around 15 mv in a few mins. I set it back to 0mv, with in a few mins it rose to 5 mv, then 8 mv, adjusted back down to 0, begins to creep back up, at 10mv I reset to 0, then it stayed around 5-7 mv.

The music playing makes it jump around quite a bit, I assume this is normal, I've noted that bias moving if music starts on my tube stuff.

Would appreciate any insight on how others amps behave in terms of offset and bias drift.

This morning I turned it on, and its sat for an hour and a half, results are:

Right channel= 13 mv Left channel= 19 mv.

What makes my bias continue to drift? Others tell me after 2 months of playing the off set remains around 5 mv.

Russellc
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Old 30th July 2010, 12:13 PM   #8506
AndrewT is offline AndrewT  Scotland
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Hi,
noting the polarity of the output offset will reveal more about the behaviour with variations of use and ambient temperature.

I think this amplifier would gain much from using a good and well implemented DC correction servo/s.

It would also benefit from an alarm that activates on detecting excess output offset and, if severe, automatically shuts down the mains power input.
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Old 30th July 2010, 12:15 PM   #8507
sangram is online now sangram  India
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Offset an bias also depends on the location of the thermistors. The amp behaves differently depending on where those specific components are. If you are using a multimeter to measure offset, it should be only done at idle and with a shorted input. while playing music you are monitoring output voltage, not the offset

The way the thermistors work is reducing the bias level to its associated output transistor as the temperature rises. The sensitivity of the change depends on how the thermal connection to the output transistor is made - the less the coupling, the slower the change, but also longer the amp takes to warm up. There is also a bit of 'hysteresis' when adjusting with the thermistors in circuit - once the level is set, it changes a bit as the transistor warms up then you may have to back it off a bit, then up again as it may cool below the set level. Of course the offset changes with the bias levels of the two devices relative to each other, and the issue is a little more complicated by that/

I never managed to really get a result that I liked (less than +/-5mV) with the thermistors, so I removed them and now the amp does take an hour or a little more to reach operating temperature in a 25degree ambient, but the offset is rock-stable, and less than 2mV output offset with shorted input.
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Old 30th July 2010, 12:16 PM   #8508
udailey is offline udailey  United States
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While it can be annoying not to have 0mV... 13 and 19 are quite low and will not be a problem for you Russell.
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Old 30th July 2010, 12:35 PM   #8509
AndrewT is offline AndrewT  Scotland
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the thermistors are there to temperature compensate the bias current.
They are not there to compensate output offset.

These two requirements are quite different.
Changes in ambient temperature will change the bias. As ambient rises the devices will run hotter. That is where the Thermistors come in and attenuate the rise in device temperature with rise in ambient temperature.

The difference between the upper and lower bias currents determines the offset current and this in turn the output offset voltage.
Adding thermistors does not automatically attenuate the difference in bias currents particularly due to the delays that must be built into the feedback system.
A DC servo or combination of servos that is specifically designed to measure and correct output offset is what should be used to control offset variations, not thermistors, which as described above are for a completely different purpose.
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Old 30th July 2010, 01:09 PM   #8510
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Quote:
Originally Posted by AndrewT View Post
the thermistors are there to temperature compensate the bias current.
They are not there to compensate output offset.

These two requirements are quite different.
Changes in ambient temperature will change the bias. As ambient rises the devices will run hotter. That is where the Thermistors come in and attenuate the rise in device temperature with rise in ambient temperature.

The difference between the upper and lower bias currents determines the offset current and this in turn the output offset voltage.
Adding thermistors does not automatically attenuate the difference in bias currents particularly due to the delays that must be built into the feedback system.
A DC servo or combination of servos that is specifically designed to measure and correct output offset is what should be used to control offset variations, not thermistors, which as described above are for a completely different purpose.
I see what you are saying and appreciate the input. As to the thermistors compensating bias and not offset, I seem to find you cant adjust one without altering the other. The bias does seem to be fairly stable. sometimes just a touch high, i.e. .61 or so. I guess maybe its not a problem.
then?

Russellc
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