Humming and Transmission issues

[Speedskater]

While phone patch mixers and phone hybrid mixers are always find of strange circuits, the above one is a lot stranger!

[Speedskater]

In the past Ramsey Electronics made a "Phone Patch Microphone Mixer) kit (PPM3) but looking on their forum pages, it seems that many builders had lots of problems.

[The conversation]

You don't understand my circuit.

[tINY]

A few things:

The schematic is not drawn well - clean it up so it's easier to follow....

It looks like you are driving the center tap of the handset side of the transformer. With the right settings of the headphone mixer pots, you could get a complete null and nothing would be transmitted to the central office.

The "connection to PC speakers" appears to be an input - are you trying to get a signal from the speakers themselves?




-tINY

[The conversation]

Originally Posted by tINY

A few things:

The schematic is not drawn well - clean it up so it's easier to follow....

It looks like you are driving the center tap of the handset side of the transformer. With the right settings of the headphone mixer pots, you could get a complete null and nothing would be transmitted to the central office.

The "connection to PC speakers" appears to be an input - are you trying to get a signal from the speakers themselves?




-tINY

Your questions are answered in the thread

[tINY]

Originally Posted by The conversation

Your questions are answered in the thread

That's nice.

I'm not going to read threads on other boards.




-tINY

[The conversation]

According to your kind of questions you are overwhelmed by the projects requirements, so that's no loss for me

[tINY]

No, this looks like a simple project (unless your schematic is totally inadequate for the task).

But, I'm not getting paid here and prefer to limit myself to a couple of message boards. This makes it possible to follow multiple threads in a short amount of time while taking a break from my real job, which pays me for my electronics skills.

I'm willing to help, but it'll be on my terms. If you want to commission a design, send me a PM at this board...




-tINY

[The conversation]

Originally Posted by tINY

No, this looks like a simple project (unless your schematic is totally inadequate for the task).

It is - but not for you; my schematic is perfectly fine - for the trained eye.

[Speedskater]

After 50 some years in electronics, now I need to train my eye?

[Richard Crowley]

The circuit is a hodgepodge of cut-n-paste, ill-fitting pieces. And the schematic is also a mess.
You have a great deal more to learn about transmission circuits before you can "roll your own" circuit. Stick to using an already-proved design.
It is considered bad form on most forums to simply point to a thread on another forum. If you had more experience using online forums you would know that. Many forums would simply delete your posting completely for a stunt like that.
Your attitude isn't doing you any favors, either. Try to avoid pissing-off people who could help you.
Good luck.

[tINY]

Originally Posted by Speedskater

After 50 some years in electronics, now I need to train my eye?

Apparently.

I see this at work too. Schematics are hard to read because people are lazy.

You need to train your eye to follow the lines that cross multiple times, ignore the long lines around schematic elements to tie two identical ground symbols, and understand why resistors are in the schematic when they are shorted (without any notes).

This one is a little worse than what I see at work, usually...



-tINY

[doulos30]

my schematic is perfectly fine - for the trained eye.

then it has no problems and you don't need anyones help awesome thanks for wasting our time

your schematic as well as circuit skills are a joke I can't even make out everything in it.

this is the same schematic

[doulos30]

now I'll try an discuss issues you are having 1 I see no gain on the opamps and your running resistance on them. That would mean less then unity gain being sent through. You need to amplify your signal at least greater then the resistance on the inputs. Your 470uf filter caps are to large I'd bring that down to around 220uf which is almost double what you need really. lot of this I cant make out the values. but your impedance looks off as well you want the impedance to be 10x or more lower then the input at the wiper.

the power supply needs to be more then just 2 batteries cause they drain unevenly which causes dc offset.

you should really use a opamp buffered virtual ground something like this is fine but I'd use a opamp with higher drive current



or you could do this in a dip 8


you can read more about virtual ground circuits here

http://tangentsoft.net/elec/vgrounds.html

[Matt Syson]

Hi
The schematics shown are all 'incorrect' and have nonsensical elements (resistors linked out for example).
Secondly, having unequal power rails for op amps does NOT cause DC offset unless one of the output approaches one of the rails at which point it will clip assymmetrically.
If the original was humming then it could either have been magnetic pickup by the transformer, or actually present on the telephone line. There is a good reason why telephones had a sharp filter below 300Hz.
Matt S

[doulos30]

Secondly, having unequal power rails for op amps does NOT cause DC offset unless one of the output approaches one of the rails at which point it will clip assymmetrically.

he was going to use a pair of 9 volts and yes having one 9 volt battery 1 volt lower then the other will cause dc offset to rise unless its built in the design of the circuit to deal with it.

[mjrippe]

Hey guys, the OP has dropped out of this conversation because he already got what he wanted - he pissed people off with a nonsense circuit and arrogant behavior. As they say on the interwebs "Don't feed the trolls!"

[Matt Syson]

Hi MJ
Yes it seems so but for now the assertion that unequal power rails will cause a DC offset must be challenged because in the circuit shown there is a 'centre ground' of 2 supplies which the inputs of the op amps are referred to. Thus the output (in this configuration) will always follow the INPUT DC conditions and NOT the actual voltages on the power rails with the condition that there is sufficient supply available so that the op amp is actually working as an op amp.
This is one of the basic 'laws' of an op amp.

Matt S