Decreasing Quad 303 input sensitivity

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21-May-2017 12:53 PM

hummph

hummph

Posts: 38

To change the input sensitivity of the 303, is it just a matter of changing R101 or is there more to it? I have read somewhere(probably on this forum) that the formula is R101/44000. So for a sensitivity of about 1V R101 would be 47k. (47000/44000= 1.06). Is this correct?

Thanks

Ken

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22-May-2017 03:48 AM

EJP

EJP

Posts: 1712

I don't find this clear, but the service manual states that the AC gain is set by the ratios of R113/R111 (~40) and R101/R108 (=1) (and which surely should be R108/R101). It seems to me you should double R111. If you double R101 you will also double the input noise, which isn't desirable.

EJP
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22-May-2017 09:07 AM

Joost Plugge

Joost Plugge

Posts: 971

On our HE boards we increased the feedback by lowering R108 and increase C103 with the same factor to maintain the HF and stability characteristic.

So for 1V sensitivity, solder a 22k in parallel with R108 and a 100pF C in parallel to C103 on the copper side.

Joost Plugge

DaDa Engineering

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23-May-2017 12:43 AM

hummph

hummph

Posts: 38

Thanks EJP and Joost for your advice and wisdom! I will try one of those two methods (probably paralleling R&C to R108/C103 which would be easy to reverse back if need be).

Many thanks

Ken

 

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23-May-2017 02:24 AM

EJP

EJP

Posts: 1712

Yes, do it Joost's way. My way you would also shift the turnover point of the DC servo down an octave so it has a compound effect. Nothing actuallywrong with that shift in my opinion, but strictly speaking to preserve the factory specification you should also halve the series capacitor if you do it my way.
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26-May-2017 09:19 AM

hummph

hummph

Posts: 38

EJP- I'm afraid that's way over my head, but thanks for the explanation!

 I have replaced R108 with 10k and C103 with a 220pf wima, are these values ok?.

Thanks

Ken

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26-May-2017 10:00 AM

EJP

EJP

Posts: 1712

Sure, that will halve the sensitivity to 1V.

The point is that where a capacitor is involved you have to double it if you halve the associated resistor. That applies to both modifications suggested here, mine and Joost's.
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