A new Gainclone is born, but......... - diyAudio
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Old 7th June 2003, 01:19 AM   #1
Prof is offline Prof  United Kingdom
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Default A new Gainclone is born, but.........

I`ve just "thrown" together my first Gainclone. It is based on the Peter Daniel inverting design, simply a 220K feedback resistor, a 10K,2.2uf and 100k pot on the input. I`ve used the usual 160VA 18v toroid, 2 x bridge rec + 2 x 1000uf caps. I paid attention to short(ish) lead lengths and I was careful with my grounding (2 separate star grounds, connected together by a short piece of wire).

The sound is quite good, there is a slight hum with no signal input, but I haven`t got a metal box to shield it yet. The problem that I have is with a slight distortion or "crackle", noticeable when certain bass notes are playing, especially solo bass guitar. It isn`t very bad, but can be heard with some of the music that I play. It also seems worse when playing at low levels, turn up the volume and it`s less noticeable. Some tracks seem to sound OK, with little or no audible problem, but I`m sure there is some sort of trouble. Is it oscillation ? The chip is bolted to a smallish (about 10 x 7cm)aluminium heatsink that gets very slightly warm with no input signal and gets very warm when playing at moderate levels.

Where do I start ?

IC : LM 3875T

Bridges : 4 x 1N5822 (3A Schottkys).

Caps : 2 x 1000uf/35v (Rubycon YXF).

Resistors : 1/8 watt Carbon Film.
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Old 7th June 2003, 04:55 AM   #2
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What pot(s) did you use (as in brand/material)?
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Old 7th June 2003, 05:07 AM   #3
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the potentionmeter is my first belief in trouble also
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Old 7th June 2003, 07:40 AM   #4
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Default Pot type

Its a fairly cheap job. Made by Omeg, Dual Gang 100K Lin, conductive plastic track, plastic case. I will try to temporarily replace it with a a couple of fixed resistors. If this cures the problem, I can see me building a switched attenuator. I`ll report back with the results.
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Old 7th June 2003, 07:46 AM   #5
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Default Also............

The "crackle" isn`t because I`m listening to the latest Rush CD (Joe and Leadbelly, being from Canada, may know what I mean)
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Old 7th June 2003, 08:42 AM   #6
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Talking Aha !!!!!!!

A good night`s sleep seems to have cured the problem ! Well, actually I`ve just put a 4.7uf electrolytic between the +/- rails and it`s cured the annoying crackle. On the diagram it`s supposed to be a 1uf, I`ll fit one as soon as poss. I did try a "fixed resistor pot", but this didn`t solve the trouble, but thanks for the help guys. Now to order another transformer and some better quality bits (I`ll have to go hungry for a week to get some Black Gates !)

Just a thought, why is this 1uf left out on some designs and used in others, and what exactly does it do ? Peter Daniel`s circuit leaves it out, but it is used in the Thor amp.
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Old 7th June 2003, 10:31 AM   #7
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i belive peter daniels reason for leaving it out was that it took up too much room, and i am not sure if he has tested one with the 1uf between the rails, as his post caused the mass movement of people to make gainclones thay mostly decided to leave out the 1uf as well.

When i finally finnish my gainclone (after my exams) i will put the 1uf (or 0.68uf mkp in this case) in bweteen the rails and see if, like other people say it does, if it makes a difference.
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Old 7th June 2003, 11:07 AM   #8
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Why would you want to couple the B+ and B- rails ?.
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Old 7th June 2003, 01:43 PM   #9
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Quote:
Why would you want to couple the B+ and B- rails ?.
This and much more about Op-amp. decoupling is explained for example in the Analog Devices Application Note AN-202:

An IC Amplifier User’s Guide to Decoupling, Grounding, and Making Things Go Right for a Change

(Refer in particular to Figure 7. and the corresponding description)
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Old 7th June 2003, 01:55 PM   #10
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Default It now has a twin !

I`ve built the other channel, slightly better quality of build. WOW ! Nice sound, seems better than my NAD C350, when I`ve built the other power supply and boxed things up, I reckon it will be excellent. Thanks guys.
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