Salas hotrodded blue DCB1 build - Page 221 - diyAudio
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Old 15th September 2012, 06:36 PM   #2201
Salas is offline Salas  Greece
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15 and 20dB is times 5.6 and times 10. So not that high offset multiplication. GC is 1+(Rf/Rfi). If it is something like 22K from output to other input and 220R to ground, then its 101 times or ~40dB. That one can take it to -450 mV. Not sure it may be a real problem still or if it even offsets some already positive amp's own. If you will make a mutual swap between the two audio section ''matched'' JFETs on your -4.5mV DCB1 channel, it can score lower if lucky. Or higher if unlucky.
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Old 15th September 2012, 06:40 PM   #2202
Marra is offline Marra  United Kingdom
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Thanks Salas I've been rereading the thread and saw that swapping the jfets over had been suggested.I think that would be the thing to try.Thanks again.
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Old 16th September 2012, 08:23 AM   #2203
AndrewT is offline AndrewT  Scotland
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Swapping the top jFET to the bottom is not likely to change the value of the offset.
It will change the polarity of the offset.

I strongly recommend you fit some form of output offset monitoring to your DC coupled amplifiers. Use that monitor to trigger some form of protection.
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Old 16th September 2012, 04:02 PM   #2204
Marra is offline Marra  United Kingdom
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Thanks Salas I swapped the jfets over and now have +1.2mv offset.Strangely enough the offset on the other channel increased slightly from 1.5 to 1.9mv.
Thanks for the advice Andrew I have two dc speaker protection boards but squeezing them into the power amp chassis might be a problem.
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Old 16th September 2012, 07:37 PM   #2205
AndrewT is offline AndrewT  Scotland
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+1.2mV output offset tells you the upper device is biased at very slightly more than Idss. This is not usually the recommended way to bias a jFET follower.
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Old 17th September 2012, 10:33 AM   #2206
Marra is offline Marra  United Kingdom
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Do you recommend that I put them back as was Andrew.
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Old 17th September 2012, 10:38 AM   #2207
AndrewT is offline AndrewT  Scotland
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the -ve output offset (if you measured correctly) indicates that the Vgs is -ve. That tells you the Id is less than Idss.

You achieve this by using the lower Idss device for the CCS and the higher Idss device for the Follower.
BUT. You cannot measure the Idss accurately enough to ensure that two almost identical devices are the correct way around. The "test" is to measure the actual operating conditions and identify the -ve offset at the output.

There was one Member that reported inserting them the wrong way around gave better sound quality.
I cannot recall any corroboration.

Looks like you have not read the Thread.
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Old 17th September 2012, 01:19 PM   #2208
Salas is offline Salas  Greece
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John Curl had given his approach on this once: Variation on the JC-2 preamplifier
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Old 17th September 2012, 01:58 PM   #2209
AndrewT is offline AndrewT  Scotland
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JC is referring to the DC bias being less than Idss.
And takes us through the story explaining that the AC (signal) allows +ve Vgs on the alternate half cycles.

I read that as meaning that DC bias should not be set to greater than Idss and that short term signal excursions into the Id>Idss are allowed. The B1 and the DCB1 are exactly that Id=Idss and signal produces +ve Vgs on alternate half cycles.

There is a proviso mentioned elsewhere. Be careful with high Id >Idss and gate leakage, particularly if Vds & Tj are also high.
As an aside, Tj will be high, if Id bias >= Idss and Vds is high as in a follower.
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Old 29th September 2012, 09:50 PM   #2210
atupi is offline atupi  Romania
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Hi, i plan to use a digital attenuator PGA2310. Any suggestion where to put the DCB1, before or after PGA2310 ?
PGA2311 is having a buffer inside chip on output side if i understood well.
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