In article , Jonathan
Ives
wrote:
"Mike Coatham" wrote in message
...
"Jonathan Ives" wrote in message

Yes, I have converted an early Radio module to the CD module. I had a
genuine CD/Aux module from Quad and just changed the older module
components to suit. The component changes a Reduce R200 & R201 from
1M to 470K (you've done this already). Reduce R202 & R203 from 2K2 to
1K5 Reduce R204 & R205 from 3K3 to 820R


Hmmm, maybe I got the 202/204 & 203/205 the wrong way round but I tried
this as this is the arrangement on the CD/Aux TL072 300mv/500K input. I
found I was getting very little volume range i.e it was too lund at 4 on
the Quad 44 dial. This was worse than I was getting on the CD/Aux input
which I have left alone

The 'radio' board I have the circuit for is M12511 - ISS1. This shows a
feeback pair of 2k2 and 3k3. The voltage gain of the actual op-amp stage is
therefore 5.5/3.3 = 1.66 (4.4 dB). If you changed the 2k2 to a 0 resistor
(wire link) and took out the 3k3 (open circuit) the voltage gain would fall
to 1.0 (0dB). For a non-inverting amp of this type this is the lowest gain
you can get from the op-amp stage.

Most of the steps on the 44's detent pot are 2dB apart. However below '7'
the steps rise and you start to lose the ability to set the loudness you
want with good precision. Hence you'd need to alter the gain overall of the
44 to get the same output from settings around '8' to '10' as you now get
from '4'.

Setting '9' corresponds to a volume pot attenuation of -26dB.

Setting '4' corresponds to a volume pot attenuation of -42dB

i.e. You probably need to reduce the total gain by about 14dB. About 10dB
more than you can expect to get by altering the feedback resistors.

I'd say that you best bet was a mix of:

1) altering the *input* resistors and perhaps caps, leaving the feedback
ones alone.

2) Altering the *output* resistors of the preamp.


So, for example, with 'radio' board M12511 ISS1 you could change C200/C202
into a couple of 22K resistors, and R200/R201 into a couple of 10K
resistors. These would drop the board gain by 10/32 = 0.31 (10dB).

The above would mean you'd have to ensure no d.c. input, but the board also
has a d.c. break cap on its output, so a small level of d.c. would not
matter. The input impedance would be 32K, but I doubt most sources would
care about this value.

Changing the input as above (i.e. method (1)) may mean you can then hear
some background noise. So (2) might be better if this is the case.


Maybe my CD player (ISOmagic DAC) or Sony DVD player sources are too
high ? I know some peak at 2.4v in order to sound "better" in an A-B
comparison in your average retailer.

2.4V is nominally only 1.5 dB bigger than 2.0V. Hence it would mean less
than one step on the 44's volume control even in its 2dB-per-step regions
above '9'.

How much volume range do you have using the Radio as a CD input ? I'm
using the 1.5v outputs (as opposed 0.5 or 5v) into a Radford STA15 with
very inefficient LS3/5a speakers.

Ah. Have you tried the 0.5V outputs? :-)

Using the 0.5V outputs instead of the 1.5V outputs would drop the level by
about 10dB. i.e mean you could use volume settings around '8' rather than
around '4'.

Slainte,

Jim

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