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From: rdp@teddy.UUCP
Newsgroups: net.audio
Subject: Re: Pad line level to phono level
Message-ID: <1397@teddy.UUCP>
Date: Tue, 8-Oct-85 08:33:12 EDT
Article-I.D.: teddy.1397
Posted: Tue Oct  8 08:33:12 1985
Date-Received: Sat, 12-Oct-85 21:05:51 EDT
References: <281@kepler.UUCP>
Reply-To: rdp@teddy.UUCP (Richard D. Pierce)
Organization: GenRad, Inc., Concord, Mass.
Lines: 43

In article <281@kepler.UUCP> mojo@kepler.UUCP (Morris Jones) writes:
>I'd like to be able to play my CD through my friend's Onkyo receiver/amp.
>Problem is the receiver has no other inputs besides the phono preamp
>inputs.  (Grrrrr.)
>
>I figure I can either cannibalize the receiver and look for a line level
>input to bring out to the case, or I can pad the output from the CD 
>down to phono preamp input level.  I realize this is a bit of a crock, 
>but it'll be nice for her to be able to record some CDs, and to listen
>to some of my favorites at her house.
>
>Could one of you with spare time and access to the standards for stereo
>input levels give me values for an H-pad or L-pad that would drop the 
>level and match the impedances?
>
>-- 

Dropping the level to match the phone input is quite impossible. While
we may be able to get the correct voltage levels, we must remember the
fact that phono inputs (presumably this is a magnetic phono input) are
heavily equalized to compensate for two things:

    1)	The output of a magnetic phono cartridge is velocity-dependent.
	This means that the output doubles for every doubling of frequency
	presuming the amplitude is kept constant. So built into every
	phono preamp is a 6 db/octave rolloff starting at about 20Hz or so.

    2)	Imposed upon the above curve is the RIAA equalization, a "shelf"
	starting at about 500 Hz or so and ending at about 2200 Hz. This
	is to reduce low frequency amplitudes on the records (allowing
	longer selection in a given amount of space due to the fact that
	you can pack more grooves/inch).

Both of these factors prevent the use of an attenuator. It is possible to
build a passive version of the pre-equalization network to do this all
for you, and I have done such for testing preamp frequency response and
transient characteristics, but you end up generating quite a bit of noise,
as well as having to deal with whatever problems the unit's phono preamp
presents you with.

In short, the project you describe is possible, but quite impractical.

Dick Pierce