Dual Mono fnshed, but what to do about CHG, and DC offset - diyAudio
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Old 10th February 2014, 08:44 AM   #1
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Default Dual Mono fnshed, but what to do about CHG, and DC offset

Hi , I'd thought I'd ask two questions before sealing up the amp.
I built the dual mono amp, but have two issues, firstly, what to do with the CHG terminal on the amplifier boards, (I haven't wired them to anything at the moment, and assume they need to go to the earth post on the chassis where the incoming ac power cable is earthed) and secondly, I measured at the speaker terminals, 80mv DC on one and 120mvDC on the other, this seems unacceptably high especially as I have little or no DC going into the amp boards, would this go once they're earthed? Thanks
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Old 10th February 2014, 10:03 AM   #2
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What is the schematic? Is it a kit?
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Old 10th February 2014, 11:37 AM   #3
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Hi Mr Wagner, thanks for your reply, it as a dual mono kit from BrianGT @ Chipamp.com.
I have just read about 300 posts from the sticky "understanding star grounding" and have realised that I haven't connected any ground to the chassis earth post (to be made a "star" point).
As I understand it, I have to connect the CHG, the PG+ and PG- (from the psu's, there are two psu's, and two transformers 25v sec's) as well as the speaker grounds and RCA grounds? I should say that the speaker posts and the RCA posts are both insulated from the chassis. Is this correct?
I can post a photograph if it helps.
I was wondering why I had no hum, now I know-nothing grounded so no loops.
Hopefully once it's grounded I'll still have no hum, but doubt it reading these posts, though I was careful to have at least 25mm separation of signal to power cables by buying a largish chassis to put it all in.
If I do, I'll build two of those ground loop isolation circuits (with 35a bridge rectifiers), one for each psu, I'm led to believe.
Still don't know where the Dc offset is coming from.

Last edited by semmyroundel; 10th February 2014 at 11:39 AM. Reason: spealling
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Old 10th February 2014, 01:01 PM   #4
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If it is the 3886 kit, then did you use the optional Ci 47uF?

I don't connect everything to the safety ground, but it is my option. Usually it is suggested through the isolator thing.
Can you make a pic of your amp?
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Old 10th February 2014, 01:25 PM   #5
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Is your power transformer labeled with Safety Agency markings?, if yes and you built with properly rated fusing and took care with all primary wiring / insulated. Also consider that your power switch, fuse holder, and even wire can be approved for primary circuits. I would just call it Class II and be done (fixed 2 wire AC cord only). The danger is connecting primary circuits to the secondary side or chassis. IMO the biggest danger I see around here , is undersized cheap XFMRs (rare for DIY) together with flammable materials, like wooden boxes etc.
Dual Mono is the most difficult to get right after the fact , once you start adding extra connections to grounds and chassis connections IE creating new ground loops.

DC offset is a feature of minimal design chip amp kits. what does the vendor say about it? around 100mV is borderline for most stuff
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