Re: Op-amp mystery

piglet <erichpwagner@hotmail.com> wrote:

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

john larkin <jlarkin_highland_tech> wrote:
 

On Tue, 10 Sep 2024 10:31:37 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

It's always the stupid things that cause the most trouble:
 
I was working on a fairly straightforward experimental audio circuit
using TL074 op-amps laid out on Veroboard, which is prone to inter-track
shorts unless you are very careful with your soldering.   The gain of a
virtual-earth stage was too high, so I altered some resistor values to
cut it down. 

 
Show us a pic of your breadboard!

 
I've posted circuit diagrams and a picture at:
 
http://www.poppyrecords.co.uk/other/Turntables/parallel-tracker.htm
 
It was the headphone amplifier that caused the mystery.  So simple, yet
so confusing when you aren't expecting simple things to go wrong.
 
 

 
Thanks, interesting headphones volume setting.

I was originally just going to copy the circuit of the headphone
amplifier in a Marantz CD player (why bother to re-invent the wheel?),
then I spotted that they used a 100-ohm resistor in series with the
headphone socket.  This would remove the damping, so that headphones
with any unwanted resonances wouldn't give as good sound quality as they
would have been capable of if they had been fed from a low impedance.

To keep the output impedance low, I took the feedback from the output
terminals.and put the 100-ohm resistors in the collector circuit, where
they would still limit the output current to a safe value that wouldn't
destroy the headphones.

The volume control circuit was derived from a Doug Self design in which
he used a unity-gain buffer to minimise the loading on the pot wiper.  I
had a bagfull of 5k twin-gang pots, so I decided that I could omit the
buffer as long as the feedback resistors around the op-amp were high
compared with the pot itself.  The pot is linear but the scale is
approximately in dB, which works a lot better than trying to make two
logarithmic pots track accurately together (or buying some exotic and
expensive controls).

Originally I was aiming for similar gain characteristics to the line
driver amplifier (pot half-way up = 0dBm output) but I found that was
far too much gain and the volume control never needed to go beyond a
quarter of a turn.  The input signals going into all the output stages
are around -10dBu with a cartridge giving around -45dBu on an 'average'
record; this allows 30dB headroom before the volume control, which is
necessary to avoid clipping the crackles from a damaged 78.

The circuit I finished up with actually generates a loss, rather than a
gain, but that gives a more favourable pot setting for comfortable
listening.   Despite not workoing quite as Doug Self intended, it
manages to produce a smooth transition across the middle of the scale,
without 'crowding' near the top ot the bottom.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk