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I am a bit out of date with electronics since my heydays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

TIA






--
Mint 20.04, kernel 5.4.0-42-generic, Cinnamon 4.6.7
running on an AMD Phenom II X4 Black edition processor with 8GB of DRAM.

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.

The chances of an Elektor design of that era working as drawn were
marginally better than those in Wireless World but not by much. I
suggest comparing what you have got against one of the old application
notes for that chip in the hope that you can spot what is wrong with it.

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I am a bit out of date with electronics since my heydays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

Is it drawing any current? Is the output more or less mid way between
the two power supplies or stuck against one of the rails.

There are much nicer low noise preamp chips available now.


--
Regards,
Martin Brown

On 06/01/2021 20:24, Martin Brown 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.Â* I then checked the power links
throughout the board and they were fine.

The chances of an Elektor design of that era working as drawn were
marginally better than those in Wireless World but not by much. I
suggest comparing what you have got against one of the old application
notes for that chip in the hope that you can spot what is wrong with it.

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.

Is it drawing any current? Is the output more or less mid way between
the two power supplies or stuck against one of the rails.

There are much nicer low noise preamp chips available now.

That was my thought too.

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

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.

--
Mint 20.04, kernel 5.4.0-42-generic, Cinnamon 4.6.7
running on an AMD Phenom II X4 Black edition processor with 8GB of DRAM.

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

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

--
Mint 20.04, kernel 5.4.0-42-generic, Cinnamon 4.6.7
running on an AMD Phenom II X4 Black edition processor with 8GB of DRAM.

pinnerite wrote:
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


If you could manage it, you should put a new op amp in
the order too, like a dual op amp in an 8 pin DIP. Then,
get out your breadboard and cobble together a quick
circuit similar to your original. You can then
test out how well it works.

This is my favorite tool, an ACE236 breadboard. If I wanted
to easily fiddle with op amps, there's plenty of room on
one of these. The "RF characteristics" of this thing are rubbish
and not to be believed. But in its class, it's... better than
nothing. It has copper plane power distribution, but still
isn't designed well enough for HF work. You can't run 74S
on here and get good results.

https://www.bmisurplus.com/product/a...or-breadboard/

I put an entire computer on there once. Took a thousand
wires. And, because the computer was so slow, the thing
was stable and didn't crash. Not many of my hardware
projects at the time, were as obliging as that computer.

Paul

On 07/01/2021 18:11, 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.


I've only just noticed your reply.

One thing is of note. For rails of +/-15V the common mode input range is
+/-10V min to +/-11V typ.

Since all input pins are either the same as -V pin or within ~1V, that
suggests you're operating the device out of spec. Some old op-amps would
exhibit strange behaviour in these conditions which could explain the
behaviour on the 'b' side.

This op-amp looks designed for split power rails and not 0V and 24V. Can
you check your power supply side and check if this was meant to be +/-12
with a ground; where the inputs (and output) are ground referenced?

If this power setup is intended then you need an op-amp that includes
ground in the common mode input range. Is there any way of posting your
circuit on some sharing website?

On 07/01/2021 18:11, 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.

OK. Looks like the frequency compensation is plausible.
Assuming that the thing is being used as a stereo amp the two channels
should be very similar on corresponding nodes and they are not.

1-6 . Output is hard against the positive rail
inputs are equal voltage and zero which is plausible. In many OPAMP
designs there is a virtual earth at the feedback summing junction.

8-13. Output is close to OV but not exactly
inputs are somewhat away from OV but similar?
Pin 9 seems to be flapping around so odd that the output doesn't.

You have (at least) two independent faults one in each channel and they
are different. My suggestion would be make yourself a simple
multivibrator signal injector and apply to the input. A wet finger and a
crystal earpiece might do if any of the gain stages are working.

Big questions a

Why is output pin 1 so high
Why isn't output pin 13 closer to 0V
Why are input pins 8 & 9 not nearer 0V.

I think it must be the components around the chip or a solder bridge
somewhere that is preventing it from working. May have damaged the chip
itself if you are unlucky.

--
Regards,
Martin Brown

Is it docketed? Could it be just dodgy connections? Otherwise I think you
are screwed. You can use your el cheapo scope if you have the pin outs and
see what is going on, ie where its getting to, also you could measure
voltages to make sure its OK, Is it a cmos device with that age? You might
just have zapped it with static and its a piece of silicon junk now. I mean
op amps are hardly complex, two inputs one output and that is about all.

Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"pinnerite" wrote in message
news:20210106201634.7978823a120bfc4822767851@gmail .com...
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.

TIA






--
Mint 20.04, kernel 5.4.0-42-generic, Cinnamon 4.6.7
running on an AMD Phenom II X4 Black edition processor with 8GB of DRAM.

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.

TIA

This is a bit more modern than the TBA231/LM739.

https://www.ti.com/lit/ds/symlink/lme49720.pdf

That's a dual op amp which seems to take high rails
voltage and is intended for driving 600 ohm or 2K ohm loads.
The 600 ohm value, I gather, was a telephony headphone
value of long ago (suited to the vacuum tube era perhaps).
I might have had such a headphone at home when I was a kid.

The lme49720 has a GBW of 55MHz. Which means if you built a gain of 1000
preamp, the bandwidth of the circuit would be 55KHz. Compare
that to an LM741, where a gain of 1000 circuit has a bandwidth
of 1KHz and you can't even make out human speech properly in
such a situation. The LM741 is compensated for unity gain,
and has no external compensation network. The LME49720 is the
same way (there's no pins at all assigned for compensation
networks on it).

Op amps, in the canonical configuration, work via virtual grounds.
The input pins should be very very close to being the same voltage.
You can connect one input to a mid-rail ref ("ground" on your
dual rail supply), and the feedback sent around the feedback path
will bring the second input pin very close to the voltage of its mate.

Consequently, just building an amplifier and putting a small signal
into it, should be more than sufficient for verifying parametrics.
There's no distortion, because feedback removes it. Since your
preamp has tremendous gain, you'll need to take a resistive divider
and divide down your bench level test signal, to a suitably low
value for test. A phono cartridge might have 2mV p-p, so a
1mV test signal is suitable.

Look at how much lower the offset voltage is on the
op-amp above (not that this matters for audio amp
circuits). To get that low a decade or two ago, you
almost had to go with chopper stabilized amplifiers.
The offset voltage has to be that low, because of the
gains involved.

*******

On your current circuit, I would start by verifying
pin 14 and pin 7 have the V+ and V- values you expect
from your dual rail supply. The reference pin should
be at a mid-rail voltage (between V+ and V- values).
The output should not have an excessive load. If the
datasheet keeps referring to 600 ohms, then the circuit
is not going to be happy with 32 ohm headphones to drive.
Whereas a power amp with a 10K ohm input impedance,
is a sufficiently light load for a 600 ohm output and won't
upset it.

If you place a heavy enough capacitive load on an op amp,
it will "motor boat" and make a put-put-put sound at
a few hertz. One of my colleagues at work tried that.
Being a hobbyist after my own heart, he attempted to
"squash the erroneous behavior" of his op amp circuit,
by putting a large cap on the output. The app notes
will instruct you to place a 50 ohm resistor between
the output of the op amp and the capacitor, if you
really really want to be doing that :-) That's an example
of load-isolation to maintain gain/phase margin.

Paul

In article ,
Paul wrote:
That's a dual op amp which seems to take high rails
voltage and is intended for driving 600 ohm or 2K ohm loads.
The 600 ohm value, I gather, was a telephony headphone
value of long ago (suited to the vacuum tube era perhaps).
I might have had such a headphone at home when I was a kid.

It's the output impedance of a cathode follower valve. The lowest
impendence then without using a transformer.

--
*Why can't women put on mascara with their mouth closed?

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Dave Plowman (News) wrote:
In article ,
Paul wrote:
That's a dual op amp which seems to take high rails
voltage and is intended for driving 600 ohm or 2K ohm loads.
The 600 ohm value, I gather, was a telephony headphone
value of long ago (suited to the vacuum tube era perhaps).
I might have had such a headphone at home when I was a kid.

It's the output impedance of a cathode follower valve. The lowest
impendence then without using a transformer.

Thinking back to my BBC training many many years ago, it's also the
nominal impedance of a pair of overhead telephone wires reaching to
infinity. ;-)

--
*Why isn't 11 pronounced onety one? *

Dave Plowman London SW
To e-mail, change noise into sound.