The Plexus Parasound P/PH-100 Modifications


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PPH-100 Phono Preamp Features:

Works with all line drive preamplifiers
RIAA curve accurate to 0.25 dB
Ultra low distortion
Internally selectable adjustment for moving coil or moving magnet
Analog Devices operational amps
Audiophile grade resistors and capacitors
Computer grade glass epoxy circuit board
Gold-plated RCA jacks
9 1/2" width front panel
Removable IEC AC cord
1 rack space height front panel
Two unit fastening hardware available for side-by-side rack mounting


Specifications: 

Input Sensitivity: moving magnet: 2 mV; moving coil: .5 mV
Input Impedance: 47k ohms
Input Overload: 220 mV, I kHz mm
RIAA Accuracy: 0.25 dB, 20 Hz - 20 kHz
S/N Ratio: 72 dB, ref 5 mV input
Dimensions: w 9 1/2" x h 1 3/4" x d 7", h 2 3/8" with feet
Net Weight: 3 lb.

Special thanks to Klaus Böning for his help with some of the calculations and the finer details of opamp design. Klaus has designed a preamp which I urge you to explore. His page was an inspiration for this page and I couldn't have made this preamp as good as it is without his help. http://www.klaus-boening.de

The Parasound P/PH-100 is an inexpensive phono preamp that has an unusually good sound in it's price range. Taking a look at the circuit, there is some good engineering here, with two gain stages, a passive RIAA network and a little active before the second gain stage. Measuring the values of the components in the RIAA section, it seems as if Parasound matched these because they are very accurate. This is one big reason why this preamp sounds so good.

There are many ways to improve on this circuit. In these mods, many refinements take place but keep to the original circuit topology. Namely:

replace the stock opamps with faster, quieter, FET input burr-brown op amps NE5534's with OPA637BP and the AD712 with OPA2134
added 4700uF per each rail after the regulators, added 0.1uF HF bypass WIMA caps on all the opamp power rail pins (prevents oscillation)
replaced all the RIAA parts with hand matched parts to 0.5% to spec and 0.1% across channels
added the 4th time constant 50kHz cutoff in the RIAA network to conform to unpublished standards in cutting lathe specs (ie. cutting lathes cut off at 6db/oct at 50kHz to prevent head stress, this same cutoff is not used generally in RIAA networks but it helps to clean up imaging lots of research on this out there)
the final gain stage, now using a FET opamp, needs to have it's non-inverting input, which was normally to ground, be impedance matched with the inverting input. a guy in germany helped me with this he calculated the impedance of the RIAA network and feedback loop to be around 18k. so i added an 18k resistor in to ground. this really made the sound warmer by reducing high-order distortion produced by the imbalance.
remove the RF filter on the input by removing R1/2 and C1/2 and jumping the R1/2 pads. the holes for C1/2 will become the input from the load module.
i added in a cartridge loading feature so i could adjust the load on the cartridge. i used a 10PST dip switch per channel and cascaded resistors so that loading can be adjusted from 50 to 50k in 50 ohm increments. there is also a resistor per channel that is responsible for the gain of the first stage i brought that resistor out to a socket so that i can change the resistor to suit various gains. again, important when changing the load because more load reduces the output of the cartridge.
replace the two 22uF electrolytics with PP; I am using Solen Fast Caps for this application - matched pair. NON-POLAR!
moved the feedback resistors right to the pins on the dip socket to eliminate oscillations with the 637's. not required for use with 604's.
added a switchable external battery power supply based on 8 6V/4aH sealed lead acid cells providing +-12V at 8Ah. FYI: each supply rail draws 24mA.
a faraday cage around the transformer

I can do these modifications for you for $500US which includes all parts and labour. These modifications truely transform this preamp. The mods do not include the switchable loading block however I will set the load to match your cartridge.

Some Notes:
The gain of the second stage (AD712 replaced with OPA 2604/2134) is 90. The gain of the first stage is calculated to be 1+(R5 / (1/((1/R7) + (1/R23)))) or 1+(R5/(sum of the two gain sink resistors which are in parallel)). so by replacing R23 with any value the gain of the first stage is 1+(2700/(1/((1/470)+(1/R23)))). gain in dB is 20*log(gain). so the gain for the second stage is 20*log(90) = 39dB. the gain for the first stage, using stock values (R23=0 for MM and 47 of MC) is MM=16dB and MC=32db for a total gain of MM=55dB and MC=71dB.
by replacing R23 you can change the gain of the first stage which will change the total gain of the preamp to suit your needs.
i have tried OPA637BP's in the preamp and they work pretty well with only a minimal amount of oscillation. before you go an invest in the almost $30US per 637, check here for further details. it would be nice to completely eliminate the oscillations. I am working on that.

UPDATE 1 March 2002

R5 and R6 are the first stage feedback resistors. The oscillations when using OPS637BP can be completely eliminated by moving these resistors directly to the pins of the opamp (soldered underneath). Simply remove them and resolder between pins 2 and 6 on the sockets.

Hum can be significantly reduced by using a bettery supply. The use of sealed lead acid cells will result in lower maintenance. It is fine to use two 12V cells for a +-12V supply. These will be easier to charge as 12V charges are readily available. Put the battery supply after the voltage regulators. In my unit, I left the internal AC supply intact and installed a switch that allows switching between the internal AC supply and the external battery supply. When there is no AC connected, the switch acts as an on/off switch.

The 4th time constant: the RIAA spec doesn't officially mention the 4th time constant which is another eq filter which is a cutoff from 50kHz up at 6db/octave. This was used to relieve stress on the cutting lathe. There are many links on the web about this time constant. Here are a couple comments:

Extract from: The Analog Addicts Phono Preamp

By Thorsten Loesch

The RIAA equalisation shall have the best accuracy that can be obtained using of-the-shelf parts and MUST implement a replay curve which mirrors the curve used when cutting a record, NOT THE documented RIAA Curve, which is incorrect. The Neumann Cutting Amp Manual states that the boost in high frequencies is being rolled off at about 50kHz. Neumann cutting lathes and amps are pretty much the Industry Standard, we can assume this as being a de-facto standard. Any kind of warp-filtering is bound to introduce low frequency phase shifts. Most decent record players use clamps to minimize warp related effects, and no-one should need attenuation of any rumble, so the misguided IEC amendment to the RIAA curve must be avoided, and the absolute lower cutoff point of the phono headamp should be as low as possible. The lower cutoff should be a single dominant pole at 5Hz or less. With current off-the-shelf parts of moderate cost but high quality, an RIAA accuracy of +/-0.1db or better can be achieved in the midband. Due to the additional high frequency breakpoint we expect an error (with respect to the standard RIAA curve) of about +0.25db @ 20kHz.<

Extract from:The secrets of the phono stage

By Allen Wright4

Add the missing Time Constant!

The 75ÁS networks in all these designs (except mine) fall at 6 dB/oct forever... OK, this may be the RIAA spec but if you think of the record cutting process-can they really boost at 6dB/Oct. from 2122Hz on up forever? Back in the 70's we called some cutting equipment service departments and found they do roll off this boost with achicane at around 50kHz (3.18ÁS)-so as to keep cutter head warranty claims to a minimum or whatever. And when this is done (in reverse) in a preamp, it flattens out that 75ÁS drop to hell and restores much of the air and naturalness that's on the master tape. This is the purpose of R3 on the FVP5 map, and it's pretty easy to try yourself: a/ Find the cap used for the 75ÁS roll off (i.e. 820 or 1000 pF in Diego's) b/ Calculate what R you will need to get 3.18ÁS in conjunction with this cap (= 3K87 or 3K18 in Diego's) c/ Fit it in series with the cap-and tweak for sonic satisfaction and exact upper octave ch to ch balance. N.B. It's phase accuracy across all four bands that gives you that real life image the bottom feeders say don't exist!

Now, how do we add this into the Parasound? Very simple! Add a 1000 ohm resistor after the point where C5/C7 connect. You will have to cut a trace. This will put the cut off at around 43.7kHz which is almost ideal. Make sure to match the resistors across the channels at least withing 0.1%.

The Load/Gain Module

Uses resistors 50, 100, 200, 400, 820, 1600, 3200, 6400, 12k, 25k soldered across each switch and then joined staggered to facilitate a series resistor of aroun 50k. Turning a switch ON, shorts that's resistor taking it out of the chain allowing values from 50 to 50k in 50 ohm increments.

Completed Unit

Cartridge Loading

For an excellent description of cartridge loading and calculators to help you figure out the optimal loading go here http://www.hagtech.com/loading.html. You will have to know as closely as possible the capacitance of your arm cable including the arm wire as well as the impedance of your cartridge. If you have a capacitance meter you can measure the capacitance by disconnecting the cartridge at the headshell and measure from the rca jacks on one channel.