Earphone O/P to PA amp input -- circuit ideas wanted 3

[otaio]

I wish to take the output from tape recorder earphone socket and feed the signal into the input of PA amplifier.

The PA input is 600 ohm balanced.

Welcome ideas -- direction how to go about matching the impedances and attenuating the signal.

B

 

[cbarn24050]

you dont need to match it, jsut use a resistive divider.

 

[otaio]

cbarn

Thank you for this suggestion. I would be very grateful if could give a bit more detail e.g. possible values of resistances, number and circuit arrangement.

Again many thanks for taking the time to respond.

Bob B

 

[BrianG]

Hi,
The resistor divider idea is fine except you need to be careful about the "balanced 600-ohm input" to the PA. This may use a centre tapped input transformer to give high common mode rejection of noise (e.g. mains "hum" pickup). If so the centre tap will be connected to ground.

Using an unbalanced output earphone socket from your tape recorder may mean one side is connected to ground at the tape recorder (if it's floating from a transformer secondary that's fine).

If the output is not floating then when you then provide the resistive divider/attentuator one end must also be at ground, and connecting one input of the PA to ground to complete the circuit would would also short circuit half of the input transformer winding. This would cause severe, if not complete, signal loss to the PA, probably with high distortion as well.

 

[otaio]

BrianG

Thank you for this very informative and very interesting analysis. I will reflect on what you say -- and do some background reading on balanced circuits.

The tape recorder is battery operated so that simplifies things -- doesn't it?

Again thank you for your comments

Bob B

 

[BrianG]

Hi Bob B,
Yes you are quite right, if the tape recorder is battery operated then it should not be a problem as the case (plastic?) will insulate it from ground.

In an earlier posting you asked about resistor values. Do you know what signal level the PA requires? It may be relatively insensitive needing about 0.5V signal to get full power.

If your earphone output is of the type which diconnects the internal speaker when you plug it in then the signal level could be of the order of a few volts. In this case a simple divider could consist of a series resistor of perhaps 2200 ohms (2K2) and a shunt resistor across the PA input of 620 ohms (620R).

If you don't get enough signal to the PA then reduced the value of the series resistor a little at a time. The best thing to do is experiment! You could alway use a potentiometer (say 5K) wired as a simple volume control to get the optimum setting.

Have fun!

 

[otaio]

BrianG

What a pleasure to hear from you again and once more you are a bearer of riches.

The microphone normally used with this PA is a dynamic type -- from memory it's a Shure SM57.

I note your comment regarding experimentation -- the basis of all discovery.

Once more thank you for your support and guidance.

Best wishes


Bob B

 

[BrianG]

Hi again Bob B,

So I see that the input you intend to use is actually intended for a dynamic microphone. (This fact means that it is very likely to have the centre-tapped transformer that I mentioned earlier as this is very common on microphone inputs)

I don't know the particular model of Shure mic (makers of an excellent range of audio products) but the input sensitivity is going to be millivolts - not volts. It would be better if the PA had a less sensitive input - is there one marked "Line" or "Aux" ?

If you have to use the Mic input you will need to attenuate the output from the tape player a lot more than I first suggested, so increase the series resistor to at least 22,000 ohms (22K). It may require to go even higher , say 47K, to avoid overloading the input stage of the PA. (This would be evident by lots of distortion regardless of the setting of the PA volume control, as this is usually after the Mic pre-amp stage)

If you get the signal level to be acceptable without distortion, then on a purist note you will find the frequency response a little odd: excessive bass due to the Mic input compensation circuit and loss of treble due to our external resistive divider. This can be corrected, however it will need a few more components!

 

[otaio]

BrianG

Thank you for your continued interest and suggestions in regard of this matter.

The PA is a transistor mixer amplifier with three inputs:

mic 1
mic 2
cartridge (don't know if this is magnetic? piezo?)

The amplifier has no auxillary input.

The amplifier is 25-30 years old.

SM57 microphone moving coil dynamic sensitivity:

minus 75.5 dB (0dB=1v per mirobar)

Your comments regarding frequency response are interesting.

Could you expand on these?

Aso your comment regarding '...a few more components' is intriguing.

Regards

Bob B

 

[BrianG]

Hi Bob B

Regarding frequency response, from what you say about the amplifier it probably has just a simple "flat" Mic input with no frequency compensation, so ignore my previous comment about excessive bass response.

The "cartridge" input may be a better bet if it for a general purpose piezo-electric (crystal) type as some of these can have quite high output voltages (250-500mV). If it's for a magnetic cartridge this will be less useful as it will be like the Mic input for low level signals, and the RIAA frequency compensation will be wrong.

The reduced treble response I referred to could arise if you need to use very large values of series resistor to get sufficient attenuation of the signal for feeding the Mic input. This is because the various "stray" capacitance to ground, including the input capacitance of the Mic input, make an unintended simple low pass filter (i.e. a "treble-cut" tone control)with the series attenuator resistor.

This problem can be corrected easily by adding a compensating capacitor in parallel with your series attenuator resistor which is equal in value to the stray capacitor values. As these are likely to be unknown try a few values between perhaps 100 and 500pF until it sounds right!

Hope that helps

 

[otaio]

BrianG

I award you:

The Thomas Edison Award for Elucidation

(In other words for bringing a great deal of light to bear on a subject which hitherto was shrouded in darkness).

Armed with your insights I feel confident of getting a positive outcome to my problem.

It is people like you who are willing to share their knowledge and experience who make Eng-Tips such a great site.

I'll let you know how I get on.

Regards
Bob B