Rod Elliot Project 88 question

I have built two p88 preamps on strip-board. Very simple opamp implementation that sounds totally clear and provides volume and balance controls. I have tried several opamps including OPA2134, TL072, 4558, 4560, 4580, LM833, 2068, 5532, and MC33078. All except OPA2134 and TL072 have volume pot noise. Some are extremely noisey with low frequency (DC) effects and visible speaker cone excursion when turning the volume pot. Definitely needs a capacitor between the balance and volume pots if using bjt opamps. I can't hear any difference between OPA2134 abd TL072, both sound great. Just my $.02

may i ask which schematic you used? the one pictured here does seem to differ from the pictured pcb.
 
that's fantastic information, thanks.
do the boards usually come with additional information / documentation that is not on the website? could you post pictures of your preamps?

I purchased guitar amp and preamp P27 & P27b from ESP. The board quality is very good and yes, there was additional information provided in his build guide. I would definitely recommend buying his boards. If I'm remembering correctly (5? years ago), after purchasing a board, you will be given access to the secure webpage for the build guide. P88 is pretty simple, so I don't know how much of an update his boards have over the generic schematic but it does appear to have dip switches to select the desired gain.
 
It shouldn't be pot noise even with bipolar opamps if input stage opamp is configured as unity gain buffer. In that case DC offset is very small and should not make problems.

This is P88 on the boards that I designed to have pot onboard. I don't like cables.
 

Attachments

  • P88 alernative.jpg
    P88 alernative.jpg
    599.3 KB · Views: 1,274
I decided to give this project a try. I ordered all the parts and will be setting it up on a breadboard first.

It seems there are a few different versions of using caps (electros and ceramic) in the power supply. I attached a drawing of the versions i found.

A) is according to recommendations on the ESP site for one opamp
B) seems the prototype pictured on the ESP site is set up like this
C) that's the setup ivanlukic used in the above post (the ceramic cap --I I-- are obviously by-polar - i was too eager with the +/- markings)
D) another version of the original PCB i found somewhere on the net.

So, which one to use? :)

Qmpxegs.jpg
 
I used the same method of bypassing opamp power supplies as used in original ESP P88 boards because it's the easiest way to route the board. One 100nF non polar cap close to each opamp PSU pins and one elco per supply rail few centimeters away that serves the whole preamp. Doug Self claims that for TL072 and NE5532 one non polar between supply rails close to opamp is enough. The real question is what kind of non polar to use? It seems that different kinds give somewhat different sound. I use MLCC for these positions but there are claims that polyester is better choice.
 
What you see in the photo is my test P88 board. Therefore I used what was at hand at the moment of assembly. I used 47K pot but I also used 680K resistor to ground at the noninverting input of output opamp. The value of the pot should be 5 to 1o times less than input resistor to ground. I do not recommend that you use the pot and input resistor that I used for my test build. Why? Because the values I used are to high compared with values I used for the feedback and gain setting resistors, so I ended up with high DC offset at the output of P88. If you do not include balance pot I suggest that you use values suggested in ESP article. At the output of first opamp stage you should have 50-100R series resistor, than 10K log pot and 100K input resistor to ground at the input of second opamp stage.
 
Here is schematic for my P88 boards. Please note that I used jumper instead of R3 and left unpopulated R4. Input cap should be at least 680nF and output cap should be 10uF bipolar elco or left unpopulated if you power amplifiier has input cap.
 

Attachments

  • P88 al.png
    P88 al.png
    6.4 KB · Views: 1,236
PCB from my photo is my second try for P88 and it's called P88 alternative. My first try was much more like Prasi's pcb, opamps side by side, not like my second try where one opamp is behind the other. But Prasi's pcb is much better since he has much more experience in pcb design.
 
I do not use 7,5mm lead spacing for 0207 resistors in my pcb designs because it is dangerous practice to bend copper leads at the joint of resisitor body. Although it saves pcb space the joint could be broken or damaged and made unreliable. I red a few warnings not to bend wire leads at the joint. If it is necessary due to pcb space restrictions do it very carefully.
 
I do not use 7,5mm lead spacing for 0207 resistors in my pcb designs because it is dangerous practice to bend copper leads at the joint of resisitor body. Although it saves pcb space the joint could be broken or damaged and made unreliable. I red a few warnings not to bend wire leads at the joint. If it is necessary due to pcb space restrictions do it very carefully.

Hello Ivan,
I have my created my own package where the lead spacing is 8.64mm and gently bend the leads. i havent faced a problem till now.
regards
Prasi
 
P88 Performance

I have built several P88s over the last 15 yrs. The sound is excellent and less tiring than the sound from a B1 buffer preamp. The P88 takes a little time to break in and so don't expect top performance straight away. Of course the break in time depends on the capacitors used in the signal path so that time will vary. I contemplated using a PSU with a basic capacitance multiplier to boost supply clarity but haven't tried that.

The design is fairly bulletproof, just keep the bypass caps close to the OPAMPs and use good quality input caps and good pots. I use ALPs pots and Auricaps for the input capacitors. All resistors are metal film 1% types and the PSU uses a small toroidal transformer with lots of supply capacitors in the PSU and on the preamp board to deliver +/- 15 VDC.
 

Attachments

  • P88.jpg
    P88.jpg
    82.5 KB · Views: 878