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Help: what's the difference between the input stages of JLH1996 & JLH2000?

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xiongyw

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Hi, all,

The input stages (particularly the divider, marked in RED box in the diagrams enclosed) of JLH1969 and JLH2000 is a bit different, but I don't know the exact difference between the two, especially the purpose of C1/R3 of JLH2000.

I am wondering that may be the introduce of C1/R3 in JLH2000 is related to an acurrate control of frequency range of the input filter? But I am not very clear on the idea or calculations for the values involved.

Btw, the schematics are taken from http://www.electro-tech-online.com/custompdfs/2009/09/jlh1969.pdf and http://www.electro-tech-online.com/custompdfs/2009/09/jlh2000.pdf respectively.

Sorry, "JLH1996" in the title should read "JHL1969", but it seems I am not able to edit it once posted.

Thanks for any hints,
/bruin
 

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audioguru

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Most Helpful Member
These are extremely old class-A very hot amplifiers.

In the second one, C1 is a filter for the input bias voltage and R3 is the input resistor.
 

xiongyw

New Member
These are extremely old class-A very hot amplifiers.

In the second one, C1 is a filter for the input bias voltage and R3 is the input resistor.
So can C1 be ommited, given that the power supply is stable (e.g., regulated)?

I was wondering, will C1 has effect in AC analysis, e.g., C1 is to bypass/shortcut RV2 at audio frequencies somehow? And how the value of C1 is determined, etc...

Thanks,
/bruin
 

audioguru

Well-Known Member
Most Helpful Member
C1 is always used even if the supply is regulated.
R2 and the pot are effectively in parallel and are a total of about 43k ohms. The RC network of them being filtered by the 64uF capacitor can be easily calculated.
 

xiongyw

New Member
C1 is always used even if the supply is regulated.
R2 and the pot are effectively in parallel and are a total of about 43k ohms. The RC network of them being filtered by the 64uF capacitor can be easily calculated.
Thanks.
I just DIYed a JLH2000 (using LM338K regulated supply at 30VDC output), and found out that I missed the C1, so the question :)



I do not have a case yet, so just assembly them in a paper box at the moment:


As sufficient heatsink is not available at the moment, I adjust the quiescent current to less than 0.2A for testing:



The speaker shown above is a modified one, but may not match the amplifier (in terms of quality). Need to test the amplifier with a better speaker.

Though it works fine currently, I will try to add C1 to see what's the difference.

Thanks again,
/bruin
 
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