[sdiy] LXP-1 mod?

[Magnus Danielson]

From: Michael Zacherl <sdiy-mz01 at arsprototo.at>
Subject: Re: [sdiy] LXP-1 mod?
Date: Sat, 10 May 2003 17:39:19 +0200

> Hi Peter,

Hi Peter and Michael,

> Peter Grenader wrote:
> > I just picked up an Lexicon LXP-1 for a good price on ebay and I notice it's
> > one of the ones made in Hong Kong, and I hear there's some mod for them
> > that's supposed to be especially kewl on the Hong Kong ones.
> 
> I just became aware of that I don't know some of
> the stuff  I have. I'm the original owner of two
> LXP-1, one LXP-5 and this MRC.
> What's the deal with the LXPs made in Hongkong?

I've never heard that there is any major difference between LXP-1s so I too
would be interested to learn about this.

> > I know, I probably have this all backward...but if any of you have the
> > slightest idea of what I'm speaking about, please share the wealth as it
> > were.
> 
> For more then 11 years I obviously just didn't
> care how the guts of this gadgets look like.

Which is 180 degree the oppositve view I had at the time, I *really* cared
about the inner guts of the Lexicon stuff at that time. Since then I've lost
track.

> A couple of months ago, the LXP-5 died.
> Just not doing a peep. Only the overload LED stays
>   on. Until now I didn't find the time to take
> care of it.  :-(

Hmm... it should be rewakeable.

> > I did find something on the web about upgrading the D to A and the amps for
> > 'true' 16 bit and better sig to noise....is that THE mod?
> 
> I now have a very small bit of a slight idea, and
> I googled around and found this for instance.
> While I'm pretty sure that you found yourself 
> those informations I post it anyway, just in case:
> 
> http://edanders.home.attbi.com/diy/lxp_mod.txt
> (also
> http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&th=f9e59647f3c36ce9&rnum=14 
> 
> for the original posts)
> 
> "The DAC chip used for the A/D and D/A converters
> is the Burr Brown PCM-54HP which can be replaced
> by the PCM-54KP.  This yields 10dB less
> distortion and is probably the best single thing
> to do to the device."

A look in the BB catalog confirms tha the KP is better than the HP device.

> and further on:
> 
> "If one were looking for a way to "soup-up" an
> LXP-1, one might well start by adding a pot to
> adjust the MSB erro on the PCM 54, in order to
> lower harmonic distortion.  This would be a very
> easy mod, involving one pot and three fixed resistors.
> The Burr-Brown data book shows the recommended
> circuit.

I quick look in the schematics gives that you can apply this patch without
any cuts. You take a 100k trimpot with one end wired to pin 1 through a 470k
resistor, the other end is connected to pin 28 through another 470k resistor
and finaly the center tap is wired to pin 27 through a 1M resistor. This would
complete Note 1 in BBs datasheet (Note 2 and Note 3 is already applied BTW). 

>  The next are of investigation might be
> the sample-and-hold circuitry, since I rather
> doubt the CD4053/LF353 combination is really good
> to sixteen bits.  Sample-and-hold design is not
> for the faint-of-heart, however!"

The S/H is done according to the book, so if you look in the far back of the
BB PCM54 datasheet you will learn how it should be done. The LXP-1 implements
the S/H exactly as given. There is even a special comment about how the switch
operates which points out that the Rds(on) is compensated for so that
distorsion of the signal is reduced. Also, it's only a LF353 on the input S/H
but 4570's on the output S/H.

> Currently I'm lurking around in the usenet
> archives and I found things like this:
> 
> http://groups.google.com/groups?q=lxp-1+mod+group:rec.audio.pro&hl=en&lr=&ie=UTF-8&group=rec.audio.pro&selm=49nemh%24o8o%40newsbf02.news.aol.com&rnum=1
> 
> "Their LXP-1 mod ("replace op-amps, sample & hold
> amp, caps and demultiplexer, new converter and
> comparator" and which costs $150 plus shipping &
> insurance both ways) results in a superior
> reverb sound--one more like that produced in $2000
> reverbs"
> 
> Well - THIS would be a thing I would be interested
> too (although I doubt this $2000 improvement of
> quality ;-)
> But I don't like to see my Lexicons on a two times
> 6000 mile travel.

Why don't experiment with 20-bit DACs?

If you toss another 4464, a 4-bit register and replaces the PCM54 with a 20-bit
device with parallel load (or you have to make a workaround) you should be
able to lift things quite a bit. The Lexichip is a 20-bit device, but it's
full potential where not put on the LXP-1 board. It was extended in the LXP-5
by adding another Z80 (for real-time parameter calculations) and onto
LXP-15 and 300L. Anyway, in order to unleach the 20-bit ability more things
needs to be fixed, since I think the SAR is only in 16-bit mode and this would
possibly require a higher clock etc. I could get messy. Somebody who has a
spare LXP-1? ;O)

> Also this (which is obviously the original thread
> of the first link)
> http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&th=2c5265bc080376d5&seekm=4s69a7%24u7%40chass.utoronto.ca&frame=off
> 
> Another thing about LF hum:
> http://groups.google.com/groups?q=lxp-1+mod+group:rec.audio.pro+group:rec.audio.pro&start=10&hl=en&lr=&ie=UTF-8&group=rec.audio.pro&selm=eist-1609960321410001%40rs4-annex5.sfu.ca&rnum=11

Thanks for the links. I will look them through.

> I'm looking for the service manuals for LXP-1/5
> but couldn't find them until now.

Let's put it this way, the PCM54 is on page 1 of 5 sheets ;O)

I am lacking the EPROM image... so if somebody has it to share... ;O)

Cheers,
Magnus