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Old 6th August 2012, 07:32 PM   #101
Bensen is offline Bensen  Belgium
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Smile Evertything finished except front and back

Hi,

Amp is running perfect
-bias is lightly decreasing when temperature is goiing. (drops very little: from 180mA/15C to 175mA/40C)
-Protectionboard with the TA7317 is doiing his job properly.
-DC-servo is keeping output offset well below 2mv.
-Static DC control is funcioning perfect: this makes sure that DC-offset is already very low (+-10mv) when the output relay kicks in.

Must admit that I maybe hat to put small heatsinks on the HFA25PB60 rectifier diodes.

The topside will be made out of 6mm glass.
Front and back panel is something else, I'm looking for nice wood, which I want to varnish with a lot of layers to make it very, very smoot.
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Old 6th August 2012, 07:34 PM   #102
Bensen is offline Bensen  Belgium
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Old 21st August 2012, 05:10 PM   #103
Bensen is offline Bensen  Belgium
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Default Upgrade or downgrade soundwise??

Hi,

I've clearly made a mistake by using resistive loading on the VAS to increase openloop bandwith. Some members pointed me this out. Douglas Self and Bob Cordell are telling the same in their literature.

Instead of resistive loading I would like to use local feedback arround the VAS stage.

Please have a look at the PDF document and tell me this is the right way to go??

Thanks in advance
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Old 27th August 2012, 07:26 PM   #104
Bensen is offline Bensen  Belgium
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Unhappy Upgrade results + oscillation

Hi,

The change from above is an upgrade! The sound hase more detail! The hights sounds much better.



From the start on I heard some buzzing throught the speakers. First I thought it had to do with the messy wiring in the begin. But even now I hearing buzzing throught the speakers.
I did a bit invesigation with the scope:

==> I'm having +-10Mhz oscillation with 50mV amplitude! It looks as if the 10Mhz is running on top of a triangle like signal with a frequency of +-2Mhz.

What have I tried:
1) grounding the heatsinks => NOK
2) played with the zobel capacitor ==> 10R +10N/100N/1 ==> NOK
3) played with the OPS BIAS ==> Nothing changes
4) disconnected 1 amplifer in total (no ground loops) ==> Nothing changes


Can someone give me a tip?

See the pictures below.
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File Type: jpg SAM_0767.JPG (178.9 KB, 189 views)
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File Type: pdf Final schematic.pdf (28.9 KB, 93 views)
File Type: pdf Final PCB layout - tracks + components.pdf (103.3 KB, 75 views)
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Old 28th August 2012, 06:41 AM   #105
AndrewT is offline AndrewT  Scotland
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c3, 4, 6, 7, 8 & 9 are all compensation as are r8 & 9.
Try increasing Miller comp around the VAS. That is the simplest and the results may give a clue to where you aim next.

Why did you choose 10n for the output Zobel?
How is it terminated? Does it have a long loop back into the output stage? If the loop is too long the inductance may be creating an impedance that prevents the Zobel doing it's HF job.
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Old 28th August 2012, 07:22 PM   #106
Bensen is offline Bensen  Belgium
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Thanks Andrew,


I will play arround a bit with the Miller comp the next days.


Concerning the zobel network. Both speaker output, zobel connection and feedback to IPS is only max 10mm from each other on the PCB. The 10n zobel cap is choosen without stability testing what so ever, I use 10n because it is a common value used for the zobel.

In general I've learned more things from this amp than I expected, and that's good.
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Old 29th August 2012, 08:11 AM   #107
AndrewT is offline AndrewT  Scotland
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100nF is more generally adopted for a Zobel, whether with or without the following R//L.

The bigger cap allows the amp output to see the damping resistor load from lower frequencies.
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Old 4th September 2012, 08:04 PM   #108
Bensen is offline Bensen  Belgium
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Luckily I swapped my matched output pairs with a testing pair. Because with testing other freq comp caps I managed to kill a pair of 2SK1530/2SJ201.

I increased the Miller comp from 33pf to 68pf, at the output of the drivers I measured +- the same oscillation. Also with connecting a speaker at the output of the drivers I heard exactly the same as with the 33pf miller comp.

I increasedthe gate stoppers from the drivers 2sk2013/2sj313 from 100R to 470R ==> no change.

I have no idea what is causing the hum/oscillation.

I think I'm gonna swap the driver FET to BJT's, jus to see what will happen.
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Old 29th September 2012, 07:49 PM   #109
Bensen is offline Bensen  Belgium
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Question Amp is still humming

After some trial and error testing, I still haven't found what is is causing the humming through the speakers.

The hum is a sinus signal with a frequency of +-160Hz, measured on a scope.
Whe I keep my ear close to the transformer (amplimo 625VA), the transformer is making the same noise/hum as the speakers?

Any suggestion
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Old 30th September 2012, 07:36 AM   #110
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Try something like this the man whom has done this had the same problem
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