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Two problems: One, keeping
feedback constant (not totally possible but acceptable) when we still have a
pot directly in circuit. Actually this did not turn out as troublesome as
was thought.
The second problem is major: How
do we employ the low pass filter to eliminate HF brightness when component
values are shifting about and hence also the turn-over frequency of the
filter. This will be covered in this installment.
So this is our REAL PROBLEM, the
filtering/bandwidth limiting. How do we get our target of –1.25dB at 20KHz?
Oh yes we can get it but only when the pot is set at one discrete position
(all you need is adjust 1n3* to get it, simple) but as soon as we move the
pot position the filter shifts turn-over frequency and our target is lost.
Clearly it has to come down to a
compromise. Please note the suggested compromise works better with lower
value pot, so 10K is better than 25K and likewise 25K should be better than
50K (max).
Here is the basic recommended
circuit using a 25K Pot:
Below is the basic circuit using
a 50K Pot:
Below is the basic circuit using
a 10K Pot:
They are much the same except
using a different value pots means adjusting the cap value after the 2K2
resistor. Look again and you will see what I mean.
This cap value is the most tricky
part of this NON-Tube IGC and what I’ve shown is based on getting the
desired –1.25dB value when the pot is set at its –20dB setting.
I ran some simulations (aren’t
computers great?) for the three different pot values and measured each at
–14dB, –20dB and –26dB. This gives us an indication what happens to the HF
response over a window of 12dB adjustment in an area (pot position) where
you will most like be when listening to a 2V RMS output CD Player.
Caption for all graphs: The middle line in each graph is the desired
minus 1.25dB with the cap value shown and pot in –20dB position. The lines
above and below is varying -6dB and +6dB, a total of 12dB variation of
the volume control/pot.
They all look
similar but the 10K is more stable whereas surprisingly not much difference
between 25K and 50K, provided cap value is made. Please note this is only a
guide but also a good starting point when building it first time. Just
choose the pot and its recommended value.
Here is the FINAL
suggestion, find the volume at which you generally listen and make a note of
the Volume pot setting. Find an average setting from a reasonable range of
CDs (assuming that is your source). Now vary 1n3 (or whatever matches the
pot) cap value by listening carefully – or if you have the instrumentation,
adjust to minus1.25dB @ 20KHz. In which case you will still be able to
further experiment by ear.
Getting Stable Gain:
Let’s examine
GAIN variation with the three different pot values. I made the
calculations/measurements and came up with these results:
25K: Circuit Gain
variation between +31.2dB and +33.3dB. Quite a good result. Over-all
recommendation?
10K: Circuit Gain
variation between +32.2dB and +33.3dB. This is close to the ideal BUT with
one BIG drawback: The 10K Log pot is not really suitable for a Tube source,
but if from a direct coupled (also sometimes called DC coupling) like most
CD Players, should be OK. Again vary the 1n3* by ear.
50K: Circuit Gain
variation between +29.5dB and +33.3dB. This is just acceptable but loads the
source the least. Otherwise I don’t see much problem here.
I would not use
100K.
This then sorts
out having too much feedback, much lower than the original IGC. Because of
this we now revert to normal Log operation. All three pots are acceptable,
but 10K load needs to be kept in mind.
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