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Old 20th March 2016, 03:33 PM   #1
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Default Noise in Adcom GFA5200

How to cure noise in Andom GFA5200 amp?
Noise is not form power (50 Hz and harmonics) but high frequency hum from about 1 kHz and up. Very annoying for good sounding amp if using sensitive speakers.
Can be that input FETs IRFD210 are noisy and only way to get noise down is to replace this with better ones. What can be better?
Attached is GFA5300 schematic, GFA5200 is basically same only power and protection is different and it has only one pair output FETs.
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File Type: png Adcom5300Capture.PNG (113.7 KB, 105 views)
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Old 21st March 2016, 02:21 AM   #2
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You need to isolate where the noise is coming from, otherwise we only have guesswork to rely on for troubleshooting this unusual Mosfet/BJT design. For starters, before looking inside, is the noise continuous and the same in both channels? the same with no input source and inputs grounded/not grounded ?

Also (and sorry to raise trivial matters) is the amplifier case grounded (3 pin cord) and do you have any switch mode power supplies or flourescent lamps, CFLs, halogen desklamp etc near the power, signal or speaker leads?
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Old 21st March 2016, 02:40 AM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by Ian Finch View Post
do you have any switch mode power supplies or flourescent lamps,
CFLs, halogen desklamp etc near the power, signal or speaker leads?
Yes, try listening very late at night (or very early in the morning), with all your other appliances turned off,
and see if the noise is still there. Are you in a single family building? Light dimmers can make such noises,
for example. Also try listening with both inputs shorted, to verify that other components are not to blame.

Last edited by rayma; 21st March 2016 at 02:43 AM.
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Old 21st March 2016, 12:58 PM   #4
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I am pretty sure it is amplifier noise, not external noise.
It is continuous and same in both channels.
I made some measurements with sound card, amp load is 7 ohms, sound card input sensitivity is adjusted so that -3 dB level is 1 V RMS on amp output, amp input is grounded. I not find any difference in noise, is input grounded or open.
First image is base noise level of sound card and load, all connected but amp tuned off, second is amp tuned on.
Actually it seems quite pink noise, but from speakers it will heard/feel like high frequency noise.
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File Type: png ADCnoiseBase1.PNG (76.4 KB, 86 views)
File Type: png ADCnoiseShorted1.PNG (77.5 KB, 77 views)
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Old 21st March 2016, 02:52 PM   #5
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The spectra are good to see but as you say the noise is the same from either channel, it is most likely to have a common source. In other words, it is less likely to come from the amplifiers themselves. The noise levels and content would show some inconsistencies between channels if that were the case.

Since noise it is not entering via the signal inputs, it must be entering via the power supply if you remove the speakers when using a dummy load. This still does not eliminate external noise sources. Noise on the mains supply, nearby RFI generating equipment etc? As Rayma suggested, take another look at the noise in quiet periods when people are not using PCs, appliances, Low voltage lighting etc.

The alternative is simply take the amplifier to another location such as friend's place where there is already a sound system, different player, preamp, speakers etc and retest for consistent measurements there.
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Old 21st March 2016, 05:04 PM   #6
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If noise is from external source or power supply it must have some dominant frequencies seen in FFT analysis, but FFT is very even, noise is distributed equally on all bandwidth.
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Old 23rd March 2016, 04:04 PM   #7
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I partly find reason of noise, it is zener D005 in the input pair current source.
When I connected 20uF capacitor parallel to zener, I get 5-10dB lower noise in high frequencies.
Type of installed zener can not be identified, also is not any clear type given in specifications, just note "DZ 1/2 W 8.03-8.34V ROHM"
Now question what zener had lowest noises?
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Old 23rd March 2016, 08:39 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by kaameelis View Post
Type of installed zener can not be identified, also is not any clear type given in specifications,
just note "DZ 1/2 W 8.03-8.34V ROHM"
Adding a large capacitor across this Zener might cause an output transient upon power on/off,
so check carefully if you keep it there.
TFZFHTR8.2B - ROHM - Zener Single Diode, AEC-Q101, 8.2 V, 500 mW, SOD-323HE, 2 Pins, 150 C | Farnell element14

Last edited by rayma; 23rd March 2016 at 08:53 PM.
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Old 23rd March 2016, 09:03 PM   #9
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No,this is not right type, original is from year 1995 and in DO35 glass case.
Yes I know I can not leave capacitor across zener I used it only temporarily for testing, it can also cause more problems like generation.
What can be other ways to eliminate zener noise?

Last edited by kaameelis; 23rd March 2016 at 09:16 PM.
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Old 23rd March 2016, 09:17 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by kaameelis View Post
No,this is not right type, it is from year 1995 and in DO35 glass case.
What can be other ways to eliminate zener noise?
You could use a low noise adjustable voltage reference at 1mA current, instead of an 8.2V Zener.
http://www.ti.com/lit/ds/symlink/tl431.pdf

You could buy a bunch of 8.2V Zeners, and select one for low noise at the 1mA bias current.
1N5237BTR Fairchild Semiconductor | Discrete Semiconductor Products | DigiKey

Now the Zener is running with about 1mA of bias current. It should have less noise
if operated with more current, up to the 60mA maximum rating. I'd try around 25mA
bias current for the best DC stability, if the amp can be modified to do this.

Last edited by rayma; 23rd March 2016 at 09:46 PM.
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