From: u9dmlb@ohm.york.ac.uk (Duncan McL Barclay)
Newsgroups: rec.audio
Subject: Update: Home brew pre-amp...
Date: 16 Aug 92 21:32:39 GMT
Organization: Electronics Department, University of York, UK

This is the second update on me building a pre-amp. Im posting
instead of mailing to those who were interested in futher info because
Ive lost some of your addresses, and some other people may be
interesed as well. I'll do this as a 'followup' to my own article, with
the >xxx being the stuff I posted a month or so back.

>Some people asked for some more information on what I am doing, so here goes...
>(Most of the below are thoughts etc, and not yet finalised).

Well a lot is now...
>
>The actual hardware is to be mounted in a 19inch equipment rack, to give
>me a modular approach. There will be:
>
>	1) A balanced turntable input stage.
>	2) RIAA equalisation.
>	3) Line input module for 4 sources.
>	4) Line input module for 3 sources.
>	5) Record output selector.
>	6) Record output buffer.
>	7) Main output selector.
>	8) Tone controls.
>	9) Main output buffer (with volume and stereo balance)
>	10) Headphone.
>
>A lot of the ideas have come from Ben Duncan's AMP-01/AMP-02 designs, but
>changed...
>
>	1) The balanced input stage: balanced to give a little better signal
>to noise ratio but more importantly to remove any induced drivel on the cables.
>Currently my deck is wired with 'Starquad' microphone cable. The balanced input
>will comprise a standard three op-amp instrumentation amplifier, the
>input buffers will be something like an OP37/LT1037 bringing the input signal
>to -30dBU, followed by a high spec unity gain differential amp. from the
>likes of Burr-Brown/Analog Devices (these companys produce this type of
>mono-lithic amp with resistors matched to 0.1% etc).
>	I havent done all the noise calculations yet but:
>  I have a moving magnet cartridge so need 47K load, now above about 15K
>the good FET input op-amps (eg AD711) give lower noise power, however they
>have a higher 1/f noise corner than an OP37. As the RIAA stage slaps 20dB of
>gain on a low frequencies the FET may come out noiser...Im going to
>have to slog this out!!!!! Unless anyone already knows?
>
Well a little different, the op-amps will all be LT1028s or OP37s depending
on how much money I have when I buy them! The noise specs are less than
10nV/rtHz at 1KHz for the good grades of the devices, this translates to
a noise figure refered to the noise on an LP of 0.0006 or something.
The second part, refering to the loads etc is a load of tosh becuase the 
op-amps see the 47K in parallel with the cartridge, I just wernt thinking!!!

>	2) RIAA stage, using OP37s probably in the standard fast poles provided
>by feedback around an op-amp, followed by passive slow pole, followed by
>buffer amp.

A bit changed, Ive implemented this as a 'buffered passive' network,
ie each part of the transfer function is passively implemented and then
buffered, as there are only two stages this is not too bad. I decided on
this topology after reading Lipschitz's paper on RIAA eq. networks
Lipschitz discusses eq. networks based around op-amps. He
only discusses inverting topologies (which are noisyer than a non-inverting
topology). However the only mention he makes of non-inverting systems
is that they do not work and should not be considered (interesting to note
that a lot of designs cite this article and then use an non-inverting
eq network.) Sorry but I dont have the reference to hand at the mo.
The performance of this (as simulated by SPICE) gives an error of
+-0.01dB to the RIAA curve, unfortunately this is for perfect values, using
0.1% resistors and 1% capacitors I acheive +-0.1dB maximum error.

>
>	3/4) Line stages, not thought about too much but will bring
>all inputs to -10dBU, and provide channel balancing.
>
>	5/7,6) Not sure yet!
>
>	8) The tone controls: switchable of course and semi-parametric,
>which I have at the moment, very useful for tweaking the poorly EQd records.
>

	9) The output card: This is slightly novel but decided upon after
weighing up varius factors. This stage is the balance and volume controls
with an output buffer. It was easy designing the volume control and output
buffer: an ALPs 10k pot, and a op-amp!!! However the balance control
was looked at carefully. The normal practise of hanging a pot with its
wiper grounded across the signals is okay, BUT the null and balance
depend on the wiper's contact resisance, which will require another
ALPs spec. pot. Another scheme, in my current amp is to make the
balanace control active, ie a dual pot controls the gain of a pair of buffer
amps, this is quite nice apart from the fact that the pot. again has to
be a high quality one because it is in the signal chain. So I have
decided upon using a string of fixed pi-pads (attenuators) seperately
switched in line with the signal and feeding in to the volumen control pot.
This is probably just as costly as a second pot, but will
be more impressive and The pads are all designed to keep the load impedance
of the whole string to 10K, this means that mods for a passive stage are
easy.

>	10) Headphone amp to be a QUAD style 'current dumping' idea for
>the hell of it. In this case with a suitable input attenuator this could
>be used for driving small speakers as well as headphones.
>        
>Hmm think thats all I have at the moment, I will keep people updated
>as I sus more stuff out if they want. Also I am receptive to any ideas/comments
>you may have.
>
>Oh yeah just as a background, Ive just finished by honours degree in electronic
>engineering and will be starting work on a PhD in October, currently unemployed
>and spending all my time doing this!!!

Well not much more to say apart from contact me for more info etc...
Also if anyone wants to buy the plans, etc. please, please contact me. I
need some money to build this!!!!

Raggy


-- 
Raggy                 | God smiles upon the little children, the alcoholics, and
u9dmlb@ohm.york.ac.uk | the permanantly stoned. S.King.
Newsgroups: rec.audio