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Old 8th January 2014, 12:07 AM   #3141
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Hi guys

Just before starting the First One thread I have to get the info of delivery time for most important parts like output transistors, capacitors etc., to present you all an accurate information about waiting time to get the First One on the table.

In the mean time here's 19+20 kHz intermodulation distortion measurement

1. 30 ppm THD19+20k 80 Vpp on 4 Ohm load

2. Setup
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File Type: png First One 80 Vpp on 4 Ohm, 19+20 kHz.png (89.9 KB, 379 views)
File Type: jpg First One IM 19+20 kHz.jpg (402.9 KB, 383 views)
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Old 8th January 2014, 04:46 AM   #3142
Luke is offline Luke  New Zealand
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Quote:
Originally Posted by Lazy Cat View Post
Both channels plays distorted?

VAS 12 mA bias OK (120 mV voltage drop between test points)?

Check if R19, R20 measures 10 Ohm

Check sinewaves scope plots with speakers (load) connected.

Check input cabling, GND.
Hi LC, one channel plays fine the other not.

I went through the setup process again for the faulty channel. VAS 12mA is fine, offset voltage is fine. Bias current can not be adjusted at all, its at 190mA.

R19 and R20 are bypassed as per your instructions. Grounding is good, heatsink is grounded too. All cabling is fine.

I need to go to a friends house to use an oscilloscope as mine died last night:-( I will report back and post pics of sinewaves under load.

BTW, I tested that is was not the speaker or source. It is definitely the one channel of VSSA.

Thanks
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Old 8th January 2014, 09:05 AM   #3143
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Hi Luke,

Definitely you should find out what is wrong with the BIAS setting and fix. If something is wrong there, not being able to set the bias might not the only side effect. I would check the voltage between collectors of the VAS transistors (or between the gates of the ALF, pins 2 and 4); how it reacts when you rotate the bias trimmer? If that is constant, then I would check the voltages around the TLVH431 chip. You should have a constant voltage between REF and Anode around 1,25V and a varying voltage between Anode and Cathode, depending on the trimmer position.

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Old 8th January 2014, 09:49 PM   #3144
Luke is offline Luke  New Zealand
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Quote:
Originally Posted by metallicus69 View Post
Hi Luke,

Definitely you should find out what is wrong with the BIAS setting and fix. If something is wrong there, not being able to set the bias might not the only side effect. I would check the voltage between collectors of the VAS transistors (or between the gates of the ALF, pins 2 and 4); how it reacts when you rotate the bias trimmer? If that is constant, then I would check the voltages around the TLVH431 chip. You should have a constant voltage between REF and Anode around 1,25V and a varying voltage between Anode and Cathode, depending on the trimmer position.

Cheers!
Thanks for your reply Metallicus. Heres what I get.

The voltages between bases varies from 1.6V to 1.9V from one end of pot to the other.

The Vref is 1.244V from anode to cathode of shunt.

The varying voltage from anode to cathode is 1.995V hard clockwise, and 1.612V ant clockwise.

Whats going on?
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Old 8th January 2014, 10:22 PM   #3145
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Hi Luke

If the bias voltage between the gates changes according to trimmer rotation than the output bias current should follow that too. Please check PCB and output mosfet transistor for some shorts. Also scope test is necessary to see the test signal or possibly some artefacts present. According to your measurement data all is OK. What about PSU?

Regards L.C.
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Old 9th January 2014, 01:18 AM   #3146
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Hi LC, I just reflowed the solder around the vas and output transistors and it now seems to work fine. Problem now is something has happened to the other channel. It hasn't been touched so Im a bit confused what's going on. I only noticed this as it was making a clicking noise on the speaker, fade in and stop then again. R21 and are open circuit and brown. BTW these are not in your schematic.
Can anything be done?

BTW I am using a 500VA torroid and PMI capacitance multiplier.


Thanks guys for all your help so far, I'll get there:-(

regards Luke
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Last edited by Luke; 9th January 2014 at 01:34 AM.
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Old 9th January 2014, 01:51 AM   #3147
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Hi Luke

Replace burnt zobel resistors before proceeding. It looks you've got an oscillation on previously good channel. Since you don't have a scope to see what's the problem, I suggest you to use one of these (pic) on the output. It looks like yours speaker cables having highly capacitive character, causing problems. Or maybe you feed unintentionally some RF into input because of bad GND connection, etc.
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Old 9th January 2014, 02:05 AM   #3148
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LC, what can I do with the blown R21 and R22 resistors?

My speaker cable is capacitive. I made these and they need to be long.

DIY Cat5 Speaker Cables

OK, so any idea where to get these coils from? The first thing is to replace the resistors, this does not look an easy task, you think it can be done?

regards Luke
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Old 9th January 2014, 02:15 AM   #3149
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Of course it can be done hehe, just get them out and solder new ones, they're big enough so there should be no problem.

Output inductor from the pic has around 1 uH inductance and will prevent the VSSA to oscillate because of highly capacitive speaker cable you have. No worries since VSSA originally doesn't have inductor on the output, now the problem is solved.

Inductor from the pic can be made in no time from 1,5 mm Cu enameled wire, procedure how to make it is described in VSSA installation manual.
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Old 9th January 2014, 02:47 AM   #3150
BYRTT is offline BYRTT  Denmark
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Quote:
Originally Posted by Luke View Post
LC, what can I do with the blown R21 and R22 resistors?
Hi, the old pre BOM says "R21, R22 20ohm 0,75 W 1%, SMD 2010 (5025) 2 pcs".
If not a part you have, guess a 1-?w TH in vertical position would do .

Last edited by BYRTT; 9th January 2014 at 02:51 AM.
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