Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp - Page 10 - diyAudio
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Old 14th October 2012, 03:15 PM   #91
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by jitter View Post
Looks good.
Personally, when finished, I would recheck and, if necessary, readjust the bias with the amp in normal working conditions; i.e. without bulb and with both channels connected to power supply.
We think alike

I'll look in later...
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Old 14th October 2012, 03:32 PM   #92
dtm1962 is offline dtm1962  Canada
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Quote:
Originally Posted by Mooly View Post
That all sounds good. I would probably leave the bulb in place after removing the short on Q617. You just never know... and its a good safeguard for an initial power up with the bias generator active. If the bulb is still dim when the short is removed and then begins to glow as the preset is turned you know its looking good. At the first sign of the bulb glowing as the preset is turned, you should then turn it back down, switch off and remove the short and set it up for real.

Once the bulb begins to light then the filament starts to increase in resistance rapidly and that causes the rails to collapse. So its just used for initial testing only. Never attempt to set the bias with the bulb present because the supplies will be a lot lower than normal. Also do not have any speakers connected during any tests or adjustments.

I'm not sure what transistors you actually have fitted at the moment but if they are suitable then there is nothing stopping you testing it out now. As long as the devices fitted are adequately rated then the amp should all work as intended.

Are you happy with Q619/621 that you have and all the rest of the output stage or would you like some more simple tests to see all that part of the circuit is OK first ?
Hi Mooly:

I have 2 qty SC2910 (Q619) & 3 qty SA1208 (Q621) that measure the same during the hfe test & measure correctly for diode test so I believe these will work.

Here goes.... I will update once complete..... I am assuming its late afternoon in UK/Europe at this time....

Thanks...
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Old 14th October 2012, 05:22 PM   #93
Mooly is offline Mooly  United Kingdom
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Nearly 6.30pm... so we're going for it

Take it slow and check everything along the way.

Good luck
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Old 14th October 2012, 05:32 PM   #94
dtm1962 is offline dtm1962  Canada
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Quote:
Originally Posted by Mooly View Post
Nearly 6.30pm... so we're going for it

Take it slow and check everything along the way.

Good luck
Thanks....

Results so far....

With Q617 shorted everything looks good...

VERY IMPORTANT NOTE (or clue) : even before powered up the DC offset is - 180mV once the meters are hooked up BUT no power....

Powered down and removed short.....

With bulb still in the AC line the bias was must set to 4.0mV just for a start @ +/-50V rail voltages versus +/-71 VDC without bulb...

Light is dim and varies (obviously) with the adjustment of pot....

DC offset stays fairly constant at -180mV which as Mooly noted is too high...

Next step is to take the bulb out of AC line and do the bias correctly.

Concerned about the -180mV DC offset (with or without power)! Is this a transistor matching for the input differential 1016's/2362's or ????

Let me know what you think

Have to do a few errands prior to last test tonight.... 4-5 hours away...

In the meantime.... please add any comments in regards the reasoning for
-180mV of DC ooffset.

Thanks in advance,
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Old 14th October 2012, 06:06 PM   #95
dtm1962 is offline dtm1962  Canada
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Just a note ... In regards to the -180mV ... I tried 2 different DVM to confirm
That it wasn't a meter issue. Again unpowered or
Powered it reads -180mV.

Thanks in advanced....
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Old 14th October 2012, 06:43 PM   #96
Mooly is offline Mooly  United Kingdom
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This is looking good...

This might sound a silly question but have to ask

The 180mv offset. You should now be measuring the offset by placing the meter directly across the speaker terminals on the amp. Just asking in case you were still measuring to Q617 or from some slightly non standard ground point... nah, thought not ...

I'm not 100% sure how input stage transistor matching would affect this, or should I say to what extent because it will affect the balance. If there really is an offset of 180mv you could try swapping just one pair of either the input NPN's or PNP's over. Q601 for Q603

That said lets put the offset into context. 180mv DC will dissipate only 8 milliwatts in a 4 ohm speaker. So in reality there is certainly no operational worries... just that 180mv is considered to high. A few years ago I think 100mv was deemed an upper limit.

It would be worth comparing with the other channel too.

When you set the bias current for real, follow the procedure in the manual (I haven't looked at that yet) and always recheck when the amp is hot and has been playing loud. If the bias isn't stable and seems to drift with temperature then always aim low rather than high to avoid problems.
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Old 14th October 2012, 06:51 PM   #97
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As a quick test I threw changing Q601 in spice and this is the result. The two figures in red are the offsets with Q601 as something different (as in not matched) So yes, matching plays a big part in this configuration... but even if your offset is 180mv its not detrimental to the amp or speaker. Just nice to get it lower
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Old 14th October 2012, 06:57 PM   #98
dtm1962 is offline dtm1962  Canada
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Hi Mooly:

Yes it is at the speaker terminals....

Could it be caused by the 8.2kohm & 6.19kohm
as indicated (in the schematic) in the bias circuit as the actual value
On the original circuit was changed to 6.8kohm & 3.9kohm working
in unison with the 2kohm pot? Just a thought?
I will confirm that there is no voltage differential
between the main ground (star grounding) at the 4 caps in power supply
and the speaker ground

Let me know

Thanks
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Old 14th October 2012, 07:00 PM   #99
dtm1962 is offline dtm1962  Canada
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Quote:
Originally Posted by Mooly View Post
As a quick test I threw changing Q601 in spice and this is the result. The two figures in red are the offsets with Q601 as something different (as in not matched) So yes, matching plays a big part in this configuration... but even if your offset is 180mv its not detrimental to the amp or speaker. Just nice to get it lower
Just noticed your simulation... I will consider this a potential path
forward.

Thanks again

Stay tuned
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Old 14th October 2012, 07:03 PM   #100
Mooly is offline Mooly  United Kingdom
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Good thinking but in practice the offset is independant of the bias settings and component values. It is all down to the balance of the input stage. Some amps would include a trimmer between the emitters of the input transistors to trim the offset out.
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