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Rod Eliot project 88 question
Rod Eliot project 88 question
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Old 25th July 2018, 09:19 AM   #21
Mixi is offline Mixi  Europe
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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?
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Old 26th July 2018, 08:28 PM   #22
Mixi is offline Mixi  Europe
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Quote:
Originally Posted by Dirttracker73 View Post
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.
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Old 27th July 2018, 02:11 AM   #23
Dirttracker73 is offline Dirttracker73  United States
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I used the schematic in your link. The gain is at 9.02dB total as shown in the gain setting table. The source is typically a small Sansa Clip digital music player, so it needs a fair amount of gain for this.
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Old 27th July 2018, 02:29 AM   #24
Dirttracker73 is offline Dirttracker73  United States
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Quote:
Originally Posted by Mixi View Post
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.
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Old 27th July 2018, 05:40 AM   #25
ivanlukic is offline ivanlukic  Serbia
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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.
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File Type: jpg P88 alernative.jpg (599.3 KB, 865 views)
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Old 9th August 2018, 09:47 AM   #26
Mixi is offline Mixi  Europe
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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? :-)

Click the image to open in full size.
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Old 9th August 2018, 10:17 AM   #27
ivanlukic is offline ivanlukic  Serbia
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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.
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Old 9th August 2018, 12:34 PM   #28
Mixi is offline Mixi  Europe
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thanks! this is very helpful! may i ask what the resistance value of your pot is?
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Old 9th August 2018, 12:59 PM   #29
ivanlukic is offline ivanlukic  Serbia
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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.
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Old 9th August 2018, 03:20 PM   #30
ivanlukic is offline ivanlukic  Serbia
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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.
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File Type: png P88 al.png (6.4 KB, 791 views)
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