On Thu, 07 Jan 2021 14:38:20 -0500
Paul wrote:

pinnerite wrote:
On Wed, 6 Jan 2021 20:37:27 +0000
Fredxx wrote:

On 06/01/2021 20:16, pinnerite wrote:
Ten years ago I bult a low-noise pre-amp based on a 1970's article in Elektor magazine.

It used a TBA231 dual op-amp IC.

It didn't work but I did not have time to test all the copper runs etc before I wa over-whelmed with 'work'.

This week I took it out and started fault-finding.There was one open connection, poorly soldered. I then checked the power links throughout the board and they were fine.

Next i connected in a little waveform generator to the input and one of those timy LCD screened chinese oscilloscopes to the output. Nothing.

So with no shortcircuits, I concluded that the capacitors were OK whch only leaves the IC.

I do not have another TBA231 or equyivalent but have no idea how to test the chip.

I am a bit out of date with electronics since my heydays

So any suggestions will be welcome.
I suggest you measure all the voltages on each pin and come back here
for analysis.

For Info the datasheet:
http://www.bg-electronics.de/datenbl...ise/TBA231.pdf

Although the example test circuit doesn't include the use of Output Lag.
This circuit does:

https://www.vinylengine.com/turntabl...719d&mode=view


Thank you. That focussed the one or two brain cells that I have left.

The DC voltages on the chip with both no external connections (audio oscillator and oscilloscope) were virtually the same:

Pins Voltage Pins Voltage
1 14.3 14 23.6
2 22.5 13 0.43
3 21.0 12 22.9
4 21.1 11 21.0
5 0 10 21.0
6 0 9 0.9-1.6
7 0 8 0.9

Thanks too for the links. I downloaded the spec sheet.


Pins Voltage Pins Voltage
1 14.3 OutA 14 23.6
2 22.5 13 0.43 OutB
3 21.0 12 22.9
4 21.1 11 21.0
5 0 InA+ 10 21.0
6 0 InA- 9 0.9-1.6 InB+
7 0 8 0.9 InB-

So we can see your A side is kinda bonkers.

Your B side looks "lively", almost like you haven't
connected your surrogate signal source to it yet
and screwed up the bias. 8,9,13 all have similar
voltages, as if the feedback is working.

Since it's a photo preamp (for a dynamic moving coil
cartridge with 1-2mV output, not a ceramic cartridge
with 70mV output), the signal source is likely floating,
and the amp needs to be differential (so it can amplify
the difference, but reject any common-mode 50Hz hum).
You need to craft a signal source, such that it doesn't
upset any of the assumptions about the circuit design.
For example, taking an unbalanced signal source and
running it through an audio transformer, would give
you a floating output with no bad DC habits.

Paul

I have given up after copious testing and ordered a new TBA231. It will take about a week to get here.
I shall don my anti-martian, anti-covid-19 Faraday cage and utter the engineers blessing before touching it.

Thanks for all your advice. I will report back in due course.

Alan

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